salt water battery research paper

Journal of Materials Chemistry A

Saltwater as the energy source for low-cost, safe rechargeable batteries †.

* Corresponding authors

a School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Republic of Korea E-mail: [email protected] , [email protected]

The effective use of electricity from renewable sources requires large-scale stationary electrical energy storage (EES) systems with rechargeable high-energy-density, low-cost batteries. We report a rechargeable saltwater battery using NaCl (aq.) as the energy source (catholyte). The battery is operated by evolution/reduction reactions of gases (mostly O 2 , with possible Cl 2 ) in saltwater at the cathode, along with reduction/oxidation reactions of Na/Na + at the anode. The use of saltwater and the Na-metal-free anode enables high safety and low cost, as well as control of cell voltage and energy density by changing the salt concentration. The battery with a hard carbon anode and 5 M saltwater demonstrated excellent cycling stability with a high discharge capacity of 296 mA h g hard carbon −1 and a coulombic efficiency of 98% over 50 cycles. Compared with other battery types, it offers greatly reduced energy cost and relatively low power cost when used in EES systems.

• This article is part of the themed collection: 2016 Journal of Materials Chemistry A HOT Papers

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Saltwater as the energy source for low-cost, safe rechargeable batteries

S. Park, B. SenthilKumar, K. Kim, S. M. Hwang and Y. Kim, J. Mater. Chem. A , 2016,  4 , 7207 DOI: 10.1039/C6TA01274D

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• Published: 11 January 2021

Stable, high-performance, dendrite-free, seawater-based aqueous batteries

• Huajun Tian 1 ,
• Zhao Li 1 ,
• Guangxia Feng 2 ,
• Zhenzhong Yang 3 ,
• David Fox 1 , 4 ,
• Maoyu Wang 5 ,
• Hua Zhou   ORCID: orcid.org/0000-0001-9642-8674 6 ,
• Lei Zhai 1 , 4 ,
• Akihiro Kushima   ORCID: orcid.org/0000-0001-5166-4198 1 , 7 , 8 ,
• Yingge Du   ORCID: orcid.org/0000-0001-9680-1950 3 ,
• Zhenxing Feng   ORCID: orcid.org/0000-0001-7598-5076 5 ,
• Xiaonan Shan 2 &
• Yang Yang   ORCID: orcid.org/0000-0002-4410-6021 1 , 7 , 9  

Nature Communications volume  12 , Article number:  237 ( 2021 ) Cite this article

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Metal anode instability, including dendrite growth, metal corrosion, and hetero-ions interference, occurring at the electrolyte/electrode interface of aqueous batteries, are among the most critical issues hindering their widespread use in energy storage. Herein, a universal strategy is proposed to overcome the anode instability issues by rationally designing alloyed materials, using Zn-M alloys as model systems (M = Mn and other transition metals). An in-situ optical visualization coupled with finite element analysis is utilized to mimic actual electrochemical environments analogous to the actual aqueous batteries and analyze the complex electrochemical behaviors. The Zn-Mn alloy anodes achieved stability over thousands of cycles even under harsh electrochemical conditions, including testing in seawater-based aqueous electrolytes and using a high current density of 80 mA cm −2 . The proposed design strategy and the in-situ visualization protocol for the observation of dendrite growth set up a new milestone in developing durable electrodes for aqueous batteries and beyond.

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Introduction.

The strong safety concerns caused by the decomposition of organic electrolytes are challenging non-aqueous lithium-ion battery (LIB) communities, posing formidable barriers to reliable electric vehicles (EVs) and personal electronics 1 . Alternatively, emerging metal-anode-based aqueous batteries are attracting increasing attention due to the high-safety of nonflammable electrolytes 2 , 3 , 4 and environmental benignity 5 , 6 , 7 . More importantly, when coupled with earth-abundant elements (e.g., O 2 and S) at the cathodes, high-energy-density are possible, leading to cutting-edge technology for the advanced battery systems that exceed the energy density of 500 Wh kg −1 required for the future EVs 8 . However, inhomogeneous metal plating and electrochemical instability at the liquid-solid (electrolyte/metal anode) interface severely jeopardize the performance and life span of aqueous batteries 9 , 10 , 11 , 12 , 13 . The inhomogeneous metal plating incurs uncontrollable dendrite growth on the anode surface during charge/discharge cycling, inevitably leading to low Coulombic efficiency (CE), poor cyclability, and even operating failure caused by short-circuit 14 , 15 , 16 . In recent years, various strategies have been suggested to resolve the aforementioned interfacial instability issues of metal anodes in aqueous batteries from the perspectives of materials science and surface chemistry, including structural optimization 17 , surface modification 18 , artificial solid-electrolyte interphases (SEI) 19 , understanding the metal-based battery chemistry, and controlling metal plating 15 , 20 . Nevertheless, progress in stabilizing metal anodes is still in early infancy, which encourages the aqueous battery communities to explore more efficient and universal strategies for addressing the issues of inhomogeneous metal plating and interfacial instability.

On the other hand, from the perspective of electrolyte chemistry, the solvents and salts used in aqueous electrolytes are among the most important components in aqueous batteries that determine their performance 21 . In practice, deionized (DI) water and high-purity water are commonly used solvents 16 , 21 in aqueous batteries to achieve well-controlled battery chemistry by eliminating the interference of hetero-ions (e.g., Ca 2+ , Mg 2+ , Na + , SO 4 − , Cl − , NO 3 − , F − , etc.) on the battery stability 21 . Besides, blended salts have been used in the electrolytes to improve the electrochemical performance of aqueous batteries 22 , 23 by tuning the composition of cations and anions in the electrolyte, thereby achieving high ionic conductivity 22 , 24 , 25 . However, the complexity of the electrolyte components used in those strategies makes them economically less competitive than current rechargeable battery technologies for industrial-level applications.

Herein, a three-dimensional (3D) alloy anode has been proposed and demonstrated to resolve the interfacial instability issues and improve the electrochemical performance of aqueous batteries using low-cost seawater-based electrolytes. Different from the strategies using surface passivation layers to prevent dendrite growth in non-aqueous lithium electrochemical systems 26 , 27 , we propose a strategy that will efficiently minimize and suppress the dendrite formation in aqueous systems by controlling: (1) the surface reaction thermodynamics with the favorable diffusion channel of Zn on the Zn 3 Mn alloy, and (2) the reaction kinetics through the 3D nanostructures on the electrodes, at the same time. The relatively higher binding energy on the surface of the Zn 3 Mn alloy could help to guide and regulate Zn nucleation and growth and minimize the dendrite formation at the early stage of the deposition. The porous 3D nanostructure will be favorable for controlling the Zn 2+ ions diffusion kinetics, further minimizing the dendrite growth throughout the entire deposition process. We designed an optical in-situ visualization protocol that could exactly mimic the actual electrochemical conditions in the aqueous systems. Using this protocol, we observed reversible metal plating and stripping processes within the 3D Zn-Mn anode under different aqueous electrolytes including seawater. Also, theoretical (density functional theory, DFT) and experimental (microscopic and spectroscopic) studies proved that the proposed 3D alloy anode has outstanding interfacial stability achieved by the favorable diffusion channel of Zn on the alloy surface. As a proof-of-concept, the proposed Zn-Mn alloy anodes were demonstrated to be ultra-stable during the Zn plating and stripping processes, leading to durable and dendrite-free electrodes for aqueous battery even under a high current density of 80 mA cm −2 . This work presents a big step towards high-performance, high-flexibility, and reliable rechargeable batteries using seawater-based electrolytes. This work also provides a further understanding of aqueous battery chemistry that will advance the use of aqueous batteries in the renewable energy field and beyond.

Preparation and characterizations of alloy anode

An alloy electrodeposition approach was developed to prepare 3D structured Zn-Mn anodes as proposed in this work. This method can be used as a universal strategy for synthesizing various alloy anodes by adjusting the composition of deposition solution, applied deposition current or voltage, and deposition time. In this work, we focused on validating the proposed concept of 3D alloy anode by studying the electrochemical performance of Zn-Mn anode. Compared with Zn 2+ /Zn, the standard equilibrium potential of Mn 2+ /Mn is much lower (Supplementary Table  1 ), enabling the Zn deposition on the surface of Zn-Mn alloy unfavorable for Zn dendrite formation due to the electrostatic shield effect 10 , 28 . We also demonstrated the potential extension of this alloy electrodeposition strategy by showcasing another anode − Zn-Cu alloy at the end of this paper. We further suggested other alloys beyond Zn-Mn and Zn-Cu, such as Zn-Ni, Zn-Co, Zn-Fe, Zn-Mg, etc., based on their high corrosion resistance among the typical Zn-based alloys 29 , which will inspire more follow-up works from the battery and materials science communities. The electrodeposition of 3D Zn-Mn alloy was performed in a two-electrode electrochemical cell by a galvanostatic method (more experimental details in the Methods section). Continuous hydrogen (H 2 ) bubbles were observed during the alloy electrodeposition because of water dissociation incurred by the extremely high current density of 0.3 A cm −2 used in this work. We varied the electrodeposition time from 10 min to 40 min and found that the evolved H 2 bubbles served as gaseous templates for the 3D structure formation following the Stranski-Krastanov mechanism (Supplementary Fig.  1 and Supplementary Discussion  1 ) 30 . The morphologies of the Zn-Mn alloy changed from an isolated island-like structure to an interconnected 3D structure with a cauliflower-like surface (Supplementary Fig.  2 ). Based on the microscopic characterizations, the proposed alloy electrodeposition processes mainly include: (i) co-electrodeposition of various ions (Zn 2+ and Mn 2+ ); 31 (ii) H 2 bubbles evolution at the solid-liquid interface leading to the formation of the 3D structure (Fig.  1a and Supplementary Fig.  3 ). Meanwhile, the hierarchical pores on the surfaces of the cauliflower-like 3D structures (Supplementary Fig.  4 ) are beneficial for the facilitated mass transfer during charge/discharge cycling 32 , 33 . XRD pattern (Fig.  1b ) and energy-dispersive X-ray spectroscopy (EDS, Supplementary Fig.  5 ) elemental mapping confirm the formation of Zn-Mn alloy. The main peaks in the XRD pattern primarily correspond to the phase of P63/mmc(194)-hexagonal Zn 3 Mn (note: in the following discussion Zn-Mn alloy and Zn 3 Mn denote the same material). The topography of the Zn-Mn alloy was observed with atomic force microscopy (AFM, Fig.  1c and Supplementary Fig.  6 ) over a 20 × 20 μm area. The cauliflower-like 3D structures show a hierarchical roughness due to the co-existence of both micro- and nanoscale pores on the surface (Supplementary Fig.  7 ). After Zn plating, the hierarchical roughness does not show a significant change with the root mean squared (RMS) of 25 nm and 32 nm calculated for the Zn-Mn anodes before and after Zn plating, respectively. In contrast, the AFM topographies of the pristine Zn after Zn plating indicate that the dendrites formation would occur easily even in a range of area capacities from 1.0 mAh cm −2 to 5 mAh cm −2 (Supplementary Fig.  8 ). The AFM topographies prove that the inhomogeneous dendrite growth is suppressed in the 3D Zn-Mn alloy by the Zn plating primarily into the hierarchical pores. High-resolution transmission electron microscopy (HRTEM, Fig.  1d ) of Zn 3 Mn shows a well-crystallized alloy structure. In addition, atomic resolution high angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images (Fig.  1d–f ) show the unique structure of Zn 3 Mn, which provides the fast diffusion path for the cations.

a SEM image. The scale bar: 10 μm. b XRD pattern. c AFM image. d HRTEM image of Zn 3 Mn viewed along [001] direction. The scale bar: 10 nm. e , f HAADF-STEM image and the corresponding atomic crystal structure. The scale bars: 2 nm. The purple balls in the crystal structure model represent the co-occupied Zn/Mn atoms.  g Atomic structure and the surface ad-atom energy landscape of Zn 3 Mn. h Schematic illustration of Zn plating processes on Zn anode (top) and Zn-Mn anode (bottom).

Hydrophilic surfaces on metal anodes are essential for homogeneous Zn plating in aqueous electrolytes 34 . The surface wettability of Zn 3 Mn and Zn anodes with different solvents (DI water and seawater) was characterized by the contact angle (CA) goniometry (Supplementary Figs.  9 – 11 ). It is noteworthy that the 3D Zn 3 Mn anodes with unique 3D structure and nature of alloy possess superhydrophilic characteristics (CA: 0°; Supplementary Discussion  2 ) compared with the pristine Zn anode (CA: 103 ± 0.5°), enabling the considerably enlarged electrode/electrolyte contact, facilitating the mass transfer, and thus achieving homogeneous Zn plating. After the calendering process, the surface structure and morphology of the Zn-Mn alloy electrode remain barely changed even under high pressure of 80 MPa, indicating excellent mechanical stability (Supplementary Fig.  12 ). Even under much higher pressures of 160 MPa and 200 MPa, only the top-surface structure of Zn-Mn alloy was squeezed. The basic shape of the 3D structured Zn-Mn alloy with a large number of voids and trenches remains stable, which provides free space for depositing Zn metal. Furthermore, density functional theory (DFT) calculations were employed to understand the role of the alloy phase in regulating Zn nucleation and growth in the plating process. The calculated binding energy of a Zn atom on the surface of Zn 3 Mn is 1.42 eV, a higher value than the Zn surface (1.12 eV), indicating that the Zn 3 Mn phase could be an ideal matrix to guide the Zn plating because of a stronger interaction between Zn and Zn 3 Mn. In contrast, the pristine Zn shows a weaker interaction with Zn atoms resulting in a greater tendency for dendrite growth. The theoretical understanding of Zn diffusion on the Zn 3 Mn surface is demonstrated in Fig.  1g , showing the surface structure of Zn 3 Mn and the Zn-ad-atom energy landscape. The energy landscape clearly shows two channels with lower energy and small ripples separating the local minima located at the surface lattice points. The activation barrier for Zn diffusion is 0.24 ± 0.025 eV, comparable to that of Li diffusion inside graphite and graphene 35 , 36 , 37 , indicating that Zn 3 Mn could be a promising host for Zn diffusion. Particularly, the fast Zn diffusion channel inside the Zn-Mn alloy with stronger binding contributes to a homogeneous Zn coverage on the electrode surface and therefore suppresses dendrite growth (Fig.  1h ). In contrast, the Zn plating/stripping behaviors on the surface of the Zn anode are inhomogeneous, and subsequently favor the dendrite growth, also known as the “tip effect” 16 .

Alloy anode stability under harsh electrochemical environments

Traditional metal anodes used in aqueous batteries have poor stability under harsh conditions because of the accelerated corrosion, hetero-ions interference, and unexpected side-reactions. To further examine the electrochemical stability of Zn 3 Mn anode under harsh environments, seawater-based electrolytes consisting of complex compositions (3.5% saline water containing Na + , Mg 2+ , Ca 2+ , SO 4 − , Cl − , etc.) were adopted in this work. Another benefit of using seawater-based electrolyte is attributed to its earth abundance and almost free of charge (Supplementary Table  2 ) 38 , 39 , providing gigantic economic interest and competitiveness in the increasing energy storage markets. To systematically compare seawater-based electrolytes with conventional DI water-based electrolytes, we prepared nine kinds of aqueous electrolytes using DI water and seawater (Supplementary Fig.  13 ) as solvents for different metal salts (ZnSO 4 , MgSO 4 , NaSO 4, and MnSO 4 ). In general, seawater-based electrolytes have higher pH levels than DI water-based electrolytes (Fig.  2a ), making seawater a viable solvent for the naturally mild aqueous electrolytes. We used a three-electrode electrochemical cell with Pt as the working electrode and Zn 3 Mn alloy as the counter and reference electrodes to test the reversibility of Zn plating/stripping behaviors and electrochemical window in an electrolyte composed of 2 M ZnSO 4 in seawater (Fig.  2b ). The chronocoulometry curves show that the Zn plating/stripping is highly reversible with a nearly 100% CE (initial CE: 99.92%). A stable and wide electrochemical window up to 2.6 V was achieved by using a Zn 3 Mn anode in the seawater-based electrolyte without any electrolyte decomposition (Supplementary Fig.  14 ). The electrochemical stability window of aqueous electrolytes was explored by testing water dissociation potentials (Supplementary Fig.  15a ), e.g., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), in a three-electrode system. The seawater-based electrolyte (2 M ZnSO 4 in seawater) has a wider electrochemical window increased from 2.4 V to 2.6 V as compared with DI water-based electrolyte (2 M ZnSO 4 in DI water). When using seawater as a solvent, the content of free water molecules decreases, which has been proven to be an effective strategy to expand the electrochemical stability window 21 , 40 . Moreover, the Zn 3 Mn electrode shows a significantly improved anti-corrosion ability in the seawater-based electrolyte as compared to the Zn electrode (Supplementary Fig.  15b ), due to the synergistic effects as reported in the previous reports 41 . On the contrary, a vigorous electrolyte decomposition and much narrower electrochemical windows were detected by using pristine Zn anode in both DI water-based and seawater-based electrolytes (Supplementary Fig.  16 ). The CE of Zn plating/stripping processes was further evaluated via Cu//Zn (or Cu//Zn-Mn) cells using different aqueous electrolytes. A higher and more stable CE for Cu//Zn-Mn cells using different aqueous electrolytes was obtained (Supplementary Figs.  17 and 18 ). For the cycling performance of CE, the Zn-Mn alloy appears to have an average CE above 99.6% over 2500 cycles at a current density of 10 mA cm −2 (Fig.  2c ), demonstrating the long-term durability of Zn 3 Mn anode in the seawater-based electrolyte. Furthermore, electrochemical impedance spectra (EIS) of Zn//Zn and Zn-Mn//Zn-Mn symmetric cells were examined to understand the charge transfer kinetics in different electrolytes. In seawater-based electrolyte, a remarkably reduced charge transfer resistance was achieved with a Zn-Mn//Zn-Mn symmetric cell (Supplementary Fig.  19a ), which was much lower than that of Zn//Zn symmetric cell, indicating the facilitated reaction kinetics of Zn-Mn alloy. Similarly, the improved reaction kinetics was observed in the Zn-Mn alloy symmetric cells using DI water-based electrolytes (2 M ZnSO 4 in DI water, Supplementary Fig.  19b ) compared with pristine Zn. The nucleation and plateau overpotentials indicate the formation and growth thermodynamics of critical Zn atoms/clusters in the plating process 42 , 43 . The nucleation and plateau overpotentials (27 mV and 19 mV, respectively) for the Zn-Mn alloy are much lower than those of pristine Zn anode (47 mV and 30 mV, respectively), further confirming the regulated Zn plating dynamics for Zn-Mn alloy anode (Supplementary Fig.  19c ). Moreover, the outstanding stability of Zn-Mn anode was further proved by galvanostatic cycling in the symmetric Zn-Mn//Zn-Mn cell under an extremely high current density of 80 mA cm −2 , showing ultra-stable plating/stripping behaviors for over 1900 cycles. Whereas, the short-circuit of the symmetric Zn//Zn cell was observed only after 80 cycles within <30 h (Fig.  2d and Supplementary Fig.  20 ). The achieved great improvements in the electrochemical stability of metal anode under harsh environments validate our concept of using a Zn-Mn alloy for durable aqueous batteries. To further confirm the significance of Zn 3 Mn in the stabilized electrochemical performance, we prepared a 3D Zn@Zn anode (Zn foil coated with 3D Zn particles, Supplementary Fig.  21 ) as a control sample for electrochemical tests. The Zn plating/stripping profiles and cycling performance of symmetric 3D Zn@Zn cell (Supplementary Fig.  22 ) exhibit a large overpotential and failure caused by the dendrite growth and the corresponding internal short-circuit within <100 cycles at a low current density of 5 mA cm −2 and <250 h at a high current density of 80 mA cm −2 . Ex-situ SEM observations (Supplementary Fig.  23 ) were performed to diagnose the Zn plating processes under different current densities from 1 mA cm −2 to 80 mA cm −2 . The dendrites were observed from the surface of pristine Zn anode, while a smooth surface without dendrite growth was achieved on the 3D Zn-Mn alloy anode even under harsh conditions such as high current densities up to 80 mA cm −2 . The demonstrated homogenous Zn plating double confirmed the favorable binding energy of Zn atoms and the fast Zn diffusion channel in the Zn-Mn alloy as suggested by the DFT calculations.

a pH values of different electrolytes prepared using DI water and seawater as solvents. b Cyclic voltammetry curves of seawater-based electrolytes. Scan rate: 1 mV s −1 . Working electrode: Pt. Reference and counter electrodes: Zn-Mn alloy. c Long-term galvanostatic cycling performance of Cu//Zn and Cu//Zn-Mn cells at a current density of 10 mA cm −2 . d Long-term galvanostatic cycling performance of symmetric Zn-Mn and pristine Zn cells at a current density of 80 mA cm −2 (areal capacity: 16 mAh cm −2 ; Electrolyte: 2 M ZnSO 4 in seawater). 3D COMSOL simulation: e Morphology of 3D Zn-Mn alloy in 3D COMSOL model before Zn plating. f Morphology of 3D Zn-Mn alloy after 50 s of Zn plating. g Thickness change after 50 s of Zn plating on the Zn-Mn alloy surface. Scale bars in e – g : 20 µm.

In-situ visualization of Zn plating/stripping processes

The Zn plating and stripping processes in the aqueous batteries were in-situ visualized via a specially designed optical protocol (Fig.  3a ). We imaged and compared the Zn plating and stripping processes on Zn-Mn alloy and pristine Zn electrodes under different current densities of 5–80 mA cm −2 in DI water and seawater-based aqueous electrolytes. Since the proposed Zn batteries were operated in 2 M ZnSO 4 salt-based aqueous electrolytes, the refractive index mismatch between the air and electrolyte could cause severe distortion and blur the image due to the optical aberration. To minimize this effect, a ×20 water immersion objective was employed. We also constructed 2D and 3D finite element analysis models of electrochemical cells using COMSOL multiphysics (see Supplementary Discussion  3 – 6 ) and simulated the plating profile and thickness change on the Zn-Mn alloy. Concretely, Fig.  2e–g , Supplementary Fig.  24 , and Supplementary Movie  1 illustrate the Zn plating process over the entire electrode surface at a current density of 80 mA cm −2 . The results clearly show that initially, the trenches between the Zn-Mn particles filled up rapidly compared to the protruding areas in the early stage. After the initial stage, as the plating proceeded the surface became smoother and the deposition rate in the trenches decreased and resembled deposition in other regions (Supplementary Fig.  25 ). This process resulted in a uniform electrode surface, which was consistent with the ex-situ SEM characterizations (Supplementary Fig.  23 ).

a Schematic illustration of the experimental setup. b SEM image of 3D Zn-Mn alloy. c – e The early stage of Zn plating. Images were taken with a ×20 water immersion objective at 25 frames per second, and the experiment was performed at a current density of 80 mA cm −2 . c shows the 3D Zn-Mn alloy before the experiment. d , e show the differential images at 10 s and 30 s, respectively, after the start of the experiment. f – h Zn plating on 3D Zn-Mn alloy. The experiment was performed at a current density of 80 mA cm −2 for 320 s. f , g are the images of 3D Zn-Mn alloy before and after Zn plating. h was calculated by ( g – f )/ f  = ( ∆ I / I ). i – q Evolution of Zn plating on the 3D Zn - Mn alloy. i – q are from the three different regions of interest labeled in f , where i – k , l – n , and o – q correspond to regions E ,  F , and G in f , respectively. The images were taken at 0 s ( i , l , o ), 160 s ( j , m , p ), and 320 s ( k , n , q ). The black dashed lines in ( i – q ) circle out the trench regions ( i – n ) and the protruding regions ( o – q ). Scale bars: 10 μm.

For the unique in-situ visualization (Fig.  3a ), the objective, top, and bottom electrodes were all immersed in the electrolyte. Since the visible light cannot penetrate through the thick Zn electrodes, we extended the bottom substrate and imaged the morphology changes by the reflected light intensity on the extended area. Note that the observation area did not directly face the top electrode, which could decrease the electric field and the current density in the area and cause the non-uniform electric field distribution. To minimize this effect, the observation region was chosen as close as possible to the projection of the top electrode. The distance from the edge of the top electrode’s projection to the center of the observation area was 200–400 μm which was significantly smaller than the separation of the electrodes (~2 mm), and the current densities in the observation area were similar to those inside the area of the electrodes. Comparing with other imaging techniques that have been utilized to study the metal deposition process, our in-situ optical system provide the direct top view that will clearly illustrate the deposition dynamics at different locations and is easy to use comparing with X-ray imaging which requires the synchrotron beamline 44 . To verify our assumption, a COMSOL model was developed to simulate the current density distribution across the bottom electrode (Supplementary Fig.  26 ). The current density at the observation area was at 95% of the maximum values between two electrodes, and this result further verified that the observation area can reflect the changes inside the battery electrodes.

The corresponding SEM and optical images of the 3D Zn-Mn alloy are shown in Fig.  3b , c , respectively. The optical image clearly shows the 3D structures of the Zn-Mn alloy with hierarchical pores on the surface. The contrast of the optical image originated from the morphologies and variations in reflectance at different locations. The brighter areas represent material protruding from the surface and high reflectivity, and the darker areas correspond to the trenches and low reflectivity. The shapes and sizes of the 3D structures match very well with those in the SEM images, demonstrating the feasibility of using optical microscopy to study the dynamic process of electrode reactions. To understand the Zn plating process on the 3D Zn-Mn alloy, a constant current was applied through the electrodes while the optical images were obtained at a certain framerate. The experimental conditions used in the in-situ optical microscopy study were identical to those in the Zn battery test. The entire Zn plating process was recorded, and the optical signal reflected the morphological changes during the charging and discharging processes. The spatial resolution in the optical imaging system could not resolve the initial nucleation sites that are smaller than the diffraction limit, however, it allows us to image and measure the entire Zn deposition process. This information provides critical evidence that we have utilized the morphology of 3D Zn-Mn alloy to control the reaction kinetics and minimize the dendrite formation.

The pristine Zn anode was studied first using the in-situ optical microscope (Supplementary Fig.  27a ). The Zn dendrites nucleated on the electrode surface after 60 s of plating and continued to grow through the entire process (Supplementary Fig.  27b , c and Supplementary Movie  2 ). The results showed that the Zn plating on the pristine Zn surface was inhomogeneous, leading to both vertical and lateral growth of dendrites at a certain location (bottom right in Supplementary Fig.  27 ). To completely understand why the dendrite growth can be suppressed on the surface of this Zn-Mn alloy, we carefully studied the Zn plating dynamics using the in-situ optical microscopy and COMSOL simulation. In addition to the outstanding interfacial stability achieved by the favorable diffusion channel of Zn on the alloy surface, we found two additional reasons that are responsible for the suppressed dendrite growth.

First, in the early stage of deposition, the nano-voids embedded in the 3D Zn-Mn alloy structure helped to control the nucleation sites, leading to the random distribution of nucleation sites. Such a structure allows Zn to deposit easily inside the nano-voids. We found that Zn plating on the 3D Zn-Mn alloy showed completely different dynamics. In the early stage of Zn plating on the 3D Zn-Mn alloy, the Zn was mostly deposited inside the 3D structure with hierarchical pores (white dashed regions in Fig.  3c ). Figure  3d , e and Supplementary Movie  3 show the differential optical images after 10 s (Fig.  3d ) and 30 s (Fig.  3e ) of Zn plating at a current density of 80 mA cm −2 on the 3D Zn-Mn alloy. Besides, bright spots corresponding to big nucleation sites (marked by white arrows in Fig.  3c ) were observed in the trenches of the 3D structure. These phenomena were caused by the enhanced electric field and the high current density in the nano-voids of the 3D Zn-Mn alloy. To verify this hypothesis, a 2D COMSOL model was established to simulate the plating rate and the current density inside and around the nano-void (Supplementary Fig.  28 ), and the results showed that the Zn plating rate inside the nanostructure was much faster than that outside.

Secondly, the trenches in the 3D Zn-Mn alloy structures grew faster initially and formed a uniform electrode surface after plating. After the initial nucleation process, Zn started to deposit over the entire surface. However, the deposition rate varied with location: Zn deposited much faster in the trench compared with the deposition on the original structures (i.e., protrusions). Figure  3f shows the initial profile of 3D Zn-Mn alloy before Zn plating. After a long deposition time (320 s under a current density of 80 mA cm −2 ), the surface became smoother (Fig.  3g and Supplementary Movie  4 ). To further illustrate this effect, we chose three regions of interest (regions E, F, and G in Fig.  3f ), and plotted their changes over time (Fig.  3i–q ). The regions marked by the dashed black lines in Fig.  3i–n indicate the trenches on the electrode. These images at different time points (Fig.  3i–n ) clearly show that the trenches were filled up quickly, and the final surface became much smoother. Furthermore, protruding regions (original structures), circled by the black dashed line in Fig.  3o–q also grew during the Zn plating process at a relatively slower rate. The percent intensity change over the entire surface is demonstrated in Fig.  3h . The changes in the trench were much bigger (40–60%) than that of the original structures (20%). This phenomenon was due to the uneven distribution of the electric field and the current density of the 3D alloy structure. Note that the color map represents the image intensity, and the bigger intensity corresponds to the higher structure altitude. We have also obtained 3D morphology using our in-situ optical microscope by taking pictures at a different focus plane of entire 3D structures and reconstruct the 3D morphology (Supplementary Fig.  29 ). The results further verify our conclusion that the deposition in the trench will be much faster than that on the protruding region which minimized the dendrite formation. Supplementary Movie  5 – 8 illustrate the Zn plating and the corresponding stripping processes on the same electrode (see Supplementary Discussion  7 ). We have also quantified the amount of Zn deposited onto the 3D Zn-Mn alloy electrode with no obvious dendrites formation. Supplementary Movie  9 – 13 shows that the Zn can be continuously deposited onto the substrate for >8200 s with 80 mA cm −2 without dendrites, and further demonstrates the superior performance of 3D Zn-Mn alloy substrate.

COMSOL models in 2D (Supplementary Fig.  30 and Supplementary Movie  14 ) and 3D were built to further understand the Zn plating processes over the 3D Zn-Mn structure as mentioned above. The half-spheres were used to mimic the 3D Zn-Mn alloy structure (Fig.  2e and Supplementary Fig.  24 ). The trenches on the 3D Zn-Mn alloy were filled in much faster than the protrude regions (Fig.  2e, f ) and became smooth during the plating process. The deposition thickness changed much faster in the trenches (Fig.  2g ) which perfectly reproduced the experimental results (Fig.  2e–g vs Fig.  3f–h ). During the stripping process, the deposited Zn was removed and the original 3D surface almost completely recovered (Supplementary Fig.  31 ). This observation directly proves the absolute reversibility of the plating/stripping process by using Zn-Mn alloy, which has never been achieved by other metal anodes. Furthermore, in-situ optical microscopy was used to study the Zn plating in other aqueous electrolytes to further prove the stability of Zn-Mn alloy: (1) 2 M ZnSO 4 in seawater; (2) 2 M ZnSO 4 and 0.1 M MnSO 4 in seawater; and (3) 2 M ZnSO 4 and 0.1 M MnSO 4 in DI water. No obvious difference was observed (Supplementary Movie  15 – 17 for electrolyte (1), (2), and (3), respectively), confirming the dendrite-free and ultra-stable nature of 3D Zn-Mn alloy anode for aqueous batteries. Pristine Zn was also tested with seawater-based electrolyte (electrolyte 2) to compare with the 3D Zn-Mn alloy anode (Supplementary Movie  18 and 19 ). The movies show that the pristine Zn has a much faster dendrite formation rate. In addition, Zn deposited unevenly leading to quickly formed dendrites on the electrode surface, which further demonstrated the advantages of 3D Zn-Mn alloy over pristine Zn metal anode for aqueous batteries.

Dendrite suppression strategy: simultaneous control of thermodynamics and reaction kinetics for Zn plating

In recent years, the study of cathodes for aqueous Zn-air and Zn-ion batteries (ZABs and ZIBs) has been at the forefront of aqueous battery research 45 , 46 , 47 . Different strategies have been suggested and demonstrated to improve the interfacial stabilities. However, the critical issues of Zn metal anodes, such as dendrite growth, surface passivation and corrosion, etc., have been insufficiently addressed and continue to significantly challenge the development of high-performance and fully-rechargeable aqueous Zn batteries 48 .

In this paper, we have proposed and demonstrated a strategy that will efficiently minimize and suppress the dendrite formation by controlling: (1) the surface reaction thermodynamics with the favorable diffusion channel of Zn on the Zn 3 Mn alloy, and (2) the reaction kinetics through the 3D nanostructures on the electrodes, at the same time. The relatively higher binding energy on the surface of Zn 3 Mn alloy indicates that the alloy phase is an ideal matrix to guide and regulate Zn nucleation and growth and minimize the dendrite formation at the early stage of the deposition. On the other hand, the porous 3D nanostructure will help to control the Zn 2+ ions diffusion kinetics, and further minimize the dendrite formation throughout the entire deposition process. The combination of both Zn 3 Mn alloy and 3D nanostructure provides the 3D Zn-Mn alloy electrode the superior performance on dendrite suppression and corrosion prevention.

A series of experiments have been conducted to demonstrate that the high-performance dendrite-free 3D Zn-Mn alloy is the result of both (1) Zn 3 Mn alloy which will control the surface reaction thermodynamics; and (2) the 3D nanostructure will control the 3D reaction kinetics. We have fabricated the flat Zn 3 Mn electrode by mechanically pressing the 3D Zn-Mn alloy and imaged the Zn deposition with our in-situ optical microscope. The results show that the Zn deposition happens on the flat Zn-Mn alloy area immediately and there is no obvious dendrite formed within 900 s at 80 mA cm −2 (Supplementary Fig.  32 and Supplementary Movie  20 ). Comparing with the pristine Zn electrode, which starts to show dendrite formation after just 100 s (Supplementary Fig.  27 and Supplementary Movie  2 ), the flat Zn-Mn alloy shows the good capability to control the surface reaction and suppress the dendrite formation. On the other hand, the 3D Zn-Mn alloy does not show obvious dendrite formation after 8200 s of Zn deposition (Supplementary Fig.  33 and Supplementary Movie  9 – 13 ) under the same experimental conditions. Besides, we have fabricated a 3D Zn substrate without Mn, and the result shows improved performance comparing with the pristine Zn surface but still outperformed by the 3D Zn-Mn alloy (Supplementary Fig.  22 ). These results indicate that by coupling with the Zn-Mn alloy composition, the 3D nanostructures help to control the deposition kinetics and further minimized the dendrite growth.

Electrochemical performance of Zn-Mn anode in aqueous Zn batteries

To demonstrate the practical performance of the Zn-Mn anode in aqueous batteries, we assembled ZABs using commercial Pt/C@RuO 2 as the cathode and Zn-Mn alloy as the anode (Supplementary Fig.  34a ). A control battery was assembled using the pristine Zn as the anode for a comparison. The ZABs using Zn-Mn anodes showed excellent charge/discharge cycling stability for over 6000 min test without degradation at a current density of 10 mA cm −2 . In contrast, the ZABs using Zn anodes failed quickly after 2760 min test with a huge hysteresis (Fig.  4a ). The galvanostatic discharge capacities of ZABs using different anodes were recorded (Fig.  4b and Supplementary Fig.  34b ). Note that ZABs (Zn 3 Mn) and ZABs (Zn) are used to represent the batteries using Zn 3 Mn and Zn anodes, respectively, to reduce the wordy description. At a high current density of 30 mA cm −2 , the ZABs (Zn 3 Mn) delivered an extremely high discharge capacity of 816.3 mAh g Zn −1 corresponding to an energy density of 798.3 Wh kg Zn −1 , higher than those of ZABs (Zn; 784 mAh  g Zn −1 and 657 Wh kg Zn −1 ) and superior to the recent benchmarking ZABs 49 , 50 , 51 . The significantly improved performance of the ZABs (Zn 3 Mn) is ascribed to the sufficiently exposed active areas in the hierarchically porous 3D architectures via this surface/interface engineering 52 . To further demonstrate the outstanding ZABs (Zn 3 Mn) performance, we used our most recently developed materials composed of the co-incorporated platinum (Pt) and fluorine (F) in the PtCo nanosheets as a cathode to replace commercial Pt/C@RuO 2 53 . As a proof-of-concept, a high peak power density of 196 mW cm −2 (Fig.  4c ) was achieved by ZABs (Zn 3 Mn), which was much higher than that of ZABs (Zn) (130 mW cm −2 ). Besides, the Zn-Mn alloy is also mechanically robust and can be used for flexible ZABs. The flexible ZABs (Zn 3 Mn) in tandem cells exhibited nearly doubled voltages under different current densities. Under repeated twisting, the flexible tandem ZABs (Zn 3 Mn) retained a stable voltage and sustained an electric fan without any malfunction (Fig.  4d and Supplementary Movie  21 ). At the same time, the voltages of tandem ZABs (Zn 3 Mn) at high current densities were quite stable, confirming the outstanding performance for the Zn-Mn anode (Supplementary Fig.  35 ). Moreover, we assembled ZIBs full cells using MnO 2 cathodes, Zn-Mn alloy anodes, and seawater-based electrolyte (2 M ZnSO 4 and 0.1 M MnSO 4 in seawater) to evaluate the electrochemical performance of Zn-Mn anode for aqueous ZIBs (Supplementary Fig.  36 ). The addition of Mn 2+ in the electrolytes would improve the reversibility, greatly enhance the utilization of MnO 2 active material, and suppress the dissolution of MnO 2 in aqueous Zn//MnO 2 batteries 2 , 54 . The ZIBs (Zn 3 Mn) using seawater-based electrolytes presented a higher capacity (373.2 mAh g −1 , Fig.  4e ) at 0.5 C and higher discharge voltage plateaus than that of ZIBs (Zn) (262.5 mAh g −1 ), confirming a more efficient charge transfer dynamics based on the Zn-Mn anode.

a Cycling performance of ZABs (Zn 3 Mn) and ZABs (Zn). b Discharging plateaus of ZABs (Zn 3 Mn) and ZABs (Zn) at a current density of 30 mA cm −2 . c Discharging and power density plots of ZABs (Zn 3 Mn) and ZABs (Zn). d Photograph of an electric fan powered by two flexible ZABs (Zn 3 Mn). e Typical charge/discharge profiles of ZIBs (Zn 3 Mn) at 0.5 C (electrolyte: 2 M ZnSO 4 and 0.1 M MnSO 4 in seawater). Cycling performance of ZIBs (Zn 3 Mn) at f 1C and g 4C, respectively. h High-resolution HAADF-STEM image of a fully discharged MnO 2 cathode for ZIBs (Zn 3 Mn) using Mg 2+ -containing aqueous electrolyte. The yellow and pink dots represent Mg and Mn atoms, respectively. Scale bar: 1 nm. Wavelet transform of Mn K-edge EXAFS for i pristine Zn-Mn anode, j fully discharged Zn-Mn anode, and k fully charged Zn-Mn anode.

We also investigated the anti-interference property of Zn-Mn anode against hetero-ions such as Na + and Mg 2+ in the seawater-based electrolyte. As a control experiment, the ZIBs (Zn 3 Mn) using Na + -containing electrolyte (2 M Na 2 SO 4 in seawater) showed a noticeable capacity of 30 mAh g −1 , indicating a considerable storage capability in the ZIBs (Zn 3 Mn; Supplementary Fig.  37 ). Besides, we used a Mg 2+ -containing electrolyte (2 M MgSO 4 in seawater) to test the Mg 2+ anti-interference property in the ZIBs (Zn 3 Mn). A distinct intercalation behavior was observed in the ZIBs (Zn 3 Mn) with a high initial capacity of 110 mAh g −1 (Supplementary Figs.  38 and 39a, b ) compared with the pristine Zn anode (Supplementary Fig.  40 ). We also investigate the impact of hetero-ions (Na + and Mg 2+ ) on the electrochemical performance of Zn-Mn alloy in the symmetric Zn-Mn//Zn-Mn cells (Supplementary Fig.  39c, d ). And the anti-interference property of Zn-Mn anode against the other hetero-ions, including Ca 2+ and Cl − , has also been investigated as shown in Supplementary Fig.  41 , confirming the insignificant effect of hetero-ions (e.g., Ca 2+ and Cl − ) on the electrochemical performance of Zn-Mn alloy. The results also confirmed the highly anti-interference behaviors of the Zn-Mn anode. Furthermore, the ZIBs (Zn 3 Mn) using seawater-based electrolyte exhibited a stable capacity of 300 mAh g −1 at 1 C, whereas the ZIBs (Zn) delivered a much lower capacity of 130 mAh g −1 (Fig.  4f ), demonstrating the superior electrochemical performance of ZIBs based on Zn-Mn anode in the seawater-based electrolyte. In particular, the self-discharge test of ZIBs (Zn 3 Mn) using the seawater-based electrolyte showed no drop in open-circuit voltage for 120 h (Supplementary Fig.  42 ). Furthermore, at a high rate of 4 C (Fig.  4g ), the ZIBs (Zn) failed quickly after 368 cycles due to dendrite growth and short-circuit. In sharp contrast, the ZIBs (Zn 3 Mn) could keep a very stable performance over 2000 cycles without any dendrite growth and short-circuit (Supplementary Fig.  43 ), suggesting outstanding stability under harsh conditions far surpassing those of other benchmarking Zn anodes (Supplementary Table  3 ). The slow activation of the Zn//MnO 2 batteries as shown in Fig.  4g could be caused by: (i) the diffusing paths of Zn 2+ ion are gradually constructed due to the continuous infiltration of electrolytes after cycling; (ii) during the electrode activation process, more reactive sites could be exposed and the ionically conductive network of Zn 2+ ion is greatly improved at the electrolyte/electrode interface 55 , 56 . To further demonstrate the broader impacts of the proposed concept in the battery field, we electrodeposited 3D Zn-Cu alloy (Supplementary Fig.  44 ), which could be another materials for high-performance aqueous batteries. Note that the 3D Zn-Cu anode is identified here as a potential extension of the proposed strategy for anode stabilization. We will fully discuss the battery performance of the Zn-Cu anode in our future work.

To understand the reaction mechanism and confirm the structural changes of the electrodes during the charge/discharge processes for ZIBs (Zn 3 Mn), we performed ex-situ X-ray absorption spectroscopy (XAS) measurements 53 , 57 , 58 on the Zn-Mn anodes and MnO 2 cathodes at pristine, fully charged, and fully discharged states. For MnO 2 cathodes, we tested the intercalation behaviors of Zn-Mn/MnO 2 , which existed in the seawater-based electrolyte, by using the Mg 2+ -containing electrolyte. X-ray absorption near edge structure (XANES, Supplementary Fig.  45 ) on MnO 2 cathode shows distinct edge shifts. At the fully discharged state for MnO 2 cathode, the XANES spectrum at Mn K-edge moves to the lower energy compared to that of the pristine MnO 2 cathode, suggesting the lower oxidation state of Mn and the successful intercalation of hetero-ions in the bulk structure. Furthermore, XANES of charged MnO 2 cathode overlaps with the pristine one, implying good reversibility during charge/discharge processes. We also performed the HAADF-STEM analysis for the fully discharged MnO 2 cathode, as shown in Supplementary Fig.  46 . The low magnification image (Supplementary Fig.  46a ) shows that the MnO 2 nanowire maintained a good structure with a smooth surface, indicating the stability of the MnO 2 cathode during the charge/discharge process. From the atomic resolution HAADF-STEM images (Fig.  4h and Supplementary Fig.  46b ), Mg 2+ -insertion was observed in the structure of MnO 2 . These spectroscopic and microscopic characterizations suggested a completely reversible storage capability of hetero-ions (e.g. Mg 2+ ) in MnO 2 cathodes of ZIBs (Zn 3 Mn). The ex-situ XPS spectra of the cycled Zn-Mn alloy-based electrodes in Mg- and Na-containing seawater-based electrolytes were characterized as shown in Supplementary Fig.  47 . XPS spectra demonstrate that the existence of the adsorption and/or binding of Mn with cations/metal 59 , 60 . To further confirm the local structure change on the Zn-Mn anode during charge/discharge processes, the one-dimensional (1D) Fourier transform of the extended X-ray absorption fine structure (EXAFS) spectra of Mn K-edge for the Zn-Mn anode at three states of charge/discharge process (Supplementary Fig.  48 ) was first applied. Although some changes are found, this 1D EXAFS does not have a good resolution to distinguish the broad peak ~2.2 Å 61 , 62 . Then the two-dimensional (2D) wavelet transform of the EXAFS spectra was used, as shown in Fig.  4i–k 58 . Clearly, the 2D spectra that combine the R-space and the k-space can distinguish the differences in three states. The single peak found in pristine (Fig.  4i ) and fully discharged (Fig.  4j ) Zn-Mn anode suggests the existence of Mn-Mn scattering only. In contrast, the fully charged (Fig.  4k ) Zn-Mn anode has two well-splitting peaks, suggesting the co-existence of Mn-Mn and newly formed Mn-X (X = Mg, etc.) scattering that could be due to the adsorption and/or alloying of Mn with cations/metal. All these characterizations confirmed the success of the rationally designed Zn-Mn alloy anode and the benefits of using seawater-based electrolytes for aqueous Zn batteries.

In conclusion, we report a universal strategy for designing 3D Zn-Mn alloy anodes with a potential extension to other alloy-based anode materials for stable, high-performance, dendrite-free, seawater-based aqueous batteries. Equally important, we built an in-situ protocol to mimic the actual electrochemical environments of aqueous batteries and directly observe the metal plating/stripping processes on the electrode surface. The 3D Zn-Mn alloy anode, even under harsh electrochemical environments (hetero-ions interference from the seawater-based electrolyte and high current density of 80 mA cm −2 ), maintained controllable Zn plating/stripping with robust structural stability and absolute reversibility for aqueous batteries. As a proof-of-concept, the seawater-based aqueous ZIBs and ZABs using Zn-Mn alloy anodes delivered outstanding performance towards energy storage, which proved the novelty and significance of this work. The concept demonstrated in this work will bring a paradigm shift in the design of high-performance alloy anodes for aqueous/non-aqueous batteries and beyond, therefore, revolutionizing the battery industries.

Galvanostatic alloy electrodeposition of Zn-Mn alloys

All three-dimensional (3D) structured Zn-Mn alloys were electrodeposited on Zn substrates (99.95% metals basis, 0.25 mm thick, Alfa Aesar TM ). In all, 100 mL deionized (DI) water was pre-heated at 80 °C as the solvent to dissolve 0.2 M zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O, Fisher Chemical), 0.2 M sodium citrate dihydrate (Granular/Certified), and 0.6 M ethylenediaminetetraacetic acid disodium salt dihydrate (Crystalline/Certified ACS, Fisher Chemical) under continuous stirring for 30 min (noted as Solution A). Then, 0.6 M manganese (II) sulfate monohydrate (MnSO 4 ·H 2 O, 99+%, extra pure, ACROS Organics™) was added to Solution A and stirred for another 30 min until a transparent solution was obtained (noted as Solution B). The Zn-Mn alloys were then deposited on Zn substrates using a two-electrode setup with platinum mesh as the counter electrode at a current density of 0.3 A cm −2 in Solution B.

Potentiostatic alloy electrodeposition of Zn-Cu alloys

In total, 100 mL DI water was pre-heated as the solvent to dissolve zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O, Fisher Chemical), copper (II) sulfate pentahydrate (Fisher Chemical), and boric acid (Powder/Certified ACS, Fisher Chemical) under continuous stirring for 20 min until a transparent solution was obtained (noted as Solution C). The Zn-Cu alloys were deposited on Zn substrates using the two-electrode setup in Solution C.

Zn@Zn anode fabrication

The Zn@Zn anode was electrodeposited in Solution A using the same conditions as those for the deposition of Zn-Mn alloy.

Seawater-based aqueous electrolytes

Nine kinds of aqueous electrolytes were prepared: Electrolyte 1 (2 M ZnSO 4 and 0.1 M MnSO 4 in DI water); Electrolyte 2 (2 M ZnSO 4 in DI water); Electrolyte 3 (2 M ZnSO 4 and 0.1 M MnSO 4 in seawater); Electrolyte 4 (2 M ZnSO 4 in seawater); Electrolyte 5 (1 M ZnSO 4 and 1 M MgSO 4 in seawater); Electrolyte 6 (1 M ZnSO 4 and 1 M MgSO 4 in DI water); Electrolyte 7 (2 M MgSO 4 in seawater); Electrolyte 8 (2 M Na 2 SO 4 in seawater); and Electrolyte 9 (2 M MgSO 4 in DI water). The seawater was taken from Florida’s nearshore zone, physically filtered to remove the suspended particles, and directly used in this work without any other treatment.

Cathode preparation for rechargeable Zn aqueous batteries

MnO 2 cathode materials were prepared for Zn-ion batteries (ZIBs) full-cell testing by a hydrothermal method. Typically, 0.5 g MnSO 4 ·H 2 O and 2 mL 0.5 M H 2 SO 4 were added to 100 mL DI water under continuous stirring until a clear solution (noted as Solution D) was obtained. After that, 25 mL 0.1 M KMnO 4 aqueous solution was slowly added to Solution D and stirred for 5 h. The as-prepared solution was transferred to a Teflon-lined PTFE autoclave vessel and heated at 120 °C for 8 h. Then, MnO 2 powder was collected, washed by DI water, and dried at 60 °C overnight in a vacuum oven. The ZIBs cathodes were prepared by a doctor-blade method. First, MnO 2 powder, polyvinylidene fluoride (PVDF) binder, and super P carbon were mixed in N-methyl pyrrolidinone (NMP) solvent in a weight ratio of 7:1:2 to get a homogenous slurry. Then, the obtained mixed slurry was coated onto carbon paper (CP) and dried at 80 °C overnight in the vacuum oven.

Pt/C@RuO 2 and F-doped PtCo nanosheets on the nickel foam (PtCoF@nickel foam) were prepared as cathodes for Zn-air batteries (ZABs) testing according to our prior work 53 . The Pt/C@RuO 2 cathode was prepared in the following procedure: (1) 3.2 mg Pt/C powder was mixed with 3.2 mg RuO 2 in the 3.2-ml Nafion/isopropanol solution (98:2, v/v), and then ultrasonicated for 20 min. The obtained suspension was disposed on 4 × 4 cm 2 carbon paper and dried at 60 °C. The single-atom PtCoF@nickel foam was prepared by fluorine (F)-plasma treatment using carbon tetrafluoride as a source in a plasma etcher (Trion MiniLock II RIE-ICP) using the PtCo@nickel foam as a precursor.

Electrochemical tests

Symmetric cells were assembled using Zn (or Zn-Mn alloy) foils as both cathode and anode, which were separated by a glass fiber membrane saturated with different aqueous electrolytes. For Cu//Zn (or Cu//Zn-Mn) cells, Cu and Zn (or Zn-Mn alloy) foils were used as cathode and anode, respectively, for the plating/stripping tests in the aqueous Zn batteries. The active areas of electrodes were 1 cm 2 (1 cm × 1 cm) in coin cells. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data were measured by CHI 600E electrochemical workstation. The electrochemical performance of aqueous electrolytes was tested in a three-electrode setup (Pt mesh as the working electrode, Zn (or Zn-Mn alloy) foil as both counter and reference electrodes) at a scan rate of 1 mV s −1 .

Zn (or Zn-Mn alloy) anodes and MnO 2 @Carbon Paper (MnO 2 @CP) cathodes were assembled in CR2032 coin cells for the ZIBs full-cell testing. The mass loading of MnO 2 was 2–3 mg cm −2 . Pt/C@RuO 2 (or PtCoF@nickel foam) cathodes and Zn (Zn-Mn alloy) anodes were assembled with an electrolyte consisting of 6 M KOH and 0.2 M zinc acetate for ZABs full-cell testing. Gel electrolytes were also prepared by mixing polyvinyl alcohol (PVA) powder with 6 M KOH and 0.2 M zinc acetate at 80 °C to assemble the flexible ZABs.

Materials characterizations

X-ray diffraction patterns (XRD) were obtained on a film XRD system (Panalytical X’celerator multi-element detector with Cu Kα radiation source, λ = 1.54056 Å). The surface topographies were characterized by atomic force microscopy (AFM, Veeco Dimension 3100) using tapping mode. The contact angles were measured with an OCA 15EC goniometer and analyzed with the SCA 20 module from DataPhysics Instruments. A droplet volume of 3 µL was used for each measurement. The morphologies of the materials were characterized by scanning electron microscopy (SEM, ZEISS ultra 55) with EDS mapping. Transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray spectroscopy (EDS) were performed using a probe corrected FEI Titan 80–300 microscope operating at 300 kV. Mn K-edge X-ray absorption spectroscopy experiments were carried out at beamline 12BM, Advanced Photon Source (APS), Argonne National Laboratory (ANL). Data reduction, data analysis, and EXAFS fitting were performed with the Athena, Artemis, and IFEFFIT software packages.

In-situ optical imaging

To realize in-situ imaging of Zn plating/stripping dynamics, a two-electrode system was used in which the pristine Zn foils were employed as counter and reference electrodes, and 3D Zn-Mn alloy was used as a working electrode. To realize the in-situ imaging in the aqueous electrolyte, a special electrochemical cell was designed using polydimethylsiloxane (PDMS, prepared by the mixing of base elastomer and curing agent with a ratio of 10:1 and then cross-linking for 3 h at 75 °C) to hold the electrolyte. In all, 800 μL electrolyte was applied to the cell with a size of 1 cm × 1 cm × 3 mm. Images were recorded with a CCD camera (FLIR Blackfly S USB 3, 720 × 540 pixels) on an Olympus BX60 upright microscope. To minimize the refractive index mismatch between the air and high concentration saline electrolyte, a ×20 water immersion objective (working distance: 2 mm, N.A. 1.0, Thorlab) was submerged into the electrolytes and the reflected images of the electrode surface were obtained. The setup diagram is shown in Fig.  3a . The imaging area was 200–400 μm away from the top electrode’s projection. Different current densities (5–80 mA cm −2 ) were applied, and four electrolytes (Electrolyte 1: 2 M ZnSO 4 in seawater; Electrolyte 2: 2 M ZnSO 4 with 0.1 M MnSO 4 in seawater; Electrolyte 3: 2 M ZnSO 4 with 0.1 M MnSO 4 in DI water; Electrolyte 4: 2 M ZnSO 4 in DI water) were tested to investigate the Zn plating process. To compare with the 3D Zn-Mn alloy, a pristine Zn foil anode was also tested in Electrolyte 2. After the Zn plating, the current density of 80 mA cm −2 was used to analyze the stripping process of the 3D Zn-Mn anode.

DFT calculations

Density functional theory (DFT) simulation was conducted to analyze the adsorption and kinetics of Zn ad-atoms on the experimentally confirmed Zn 3 Mn (110) surface. For the simplicity and the efficiency of the calculation, the simulation was conducted on the cubic cell to extract the effect of Mn substitution alone. The simulation model was constructed with 40 Zn and 16 Mn atoms in a unit cell of 1.08 × 0.76 × 2.08 nm. The x , y , and z directions of the cell correspond to [001], [1–10], and [110] crystal orientation, respectively. A vacuum layer of 1.2 nm was included in the z -direction to avoid the interaction from the neighboring cells in a periodic boundary condition. Vienna Ab-initio Simulation Package (VASP) was used in the calculation 63 , 64 , with projector augmented wave (PAW) pseudopotential 65 , 66 and generalized gradient approximation by Perdew-Burke-Ernzerhof 67 . The plane wave energy cut off was 400 eV and 3 × 5 × 1 k -points were selected based on the Monkhorst-Pack method 68 . First, the conjugate gradient structure optimization was performed while fixing the atoms in the bottom two layers. Then, a Zn atom was placed on the surface at 10 × 10 grid points and structural optimization was performed. Here only the ad-atom was relaxed in z -direction while fixing the x and y coordinates. Also, a Zn ad-atom was placed on a surface lattice point and structure optimization was conducted to calculate the binding energy. The binding energy calculation was also performed on the Zn (110) surface for comparison. The binding energies were calculated by the (total energy of the system w/o Zn ad-atom) + (isolated Zn atom) – (total energy of the model w/ Zn ad-atom).

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was primarily supported by the National Science Foundation under grant no. CBET-1949840, CMMI-1851674, CBET-1949870, CBET-2016192, and the startup grant from the University of Central Florida (UCF). H.T. thanks for the Preeminent Postdoctoral Program (P3) at UCF. Z.F. thanks for the startup funding from Oregon State University. This research used resources of the beamline 12-BM of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02–06CH11357. TEM and data analysis were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Early Career Research Program under award #68278. A portion of the research was performed using EMSL, a DOE User Facility sponsored by the Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory.

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Y.Y. led the team and supervised the experiments. H.T. prepared the materials and performed the electrochemical analyses. Z.L. prepared the materials and performed the Zn-air batteries tests. X.S. and G.F. built the in-situ visualization protocol for the Zn plating/stripping imaging. H.Z., M.W., and Z.F. performed the XAS analyses. Z.Y. and Y.D. performed the TEM characterizations and data analysis. D.F. and L.Z. carried out the surface wettability and AFM investigations. A.K. conducted the DFT calculations. All authors contributed to the discussion.

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Tian, H., Li, Z., Feng, G. et al. Stable, high-performance, dendrite-free, seawater-based aqueous batteries. Nat Commun 12 , 237 (2021). https://doi.org/10.1038/s41467-020-20334-6

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Sodium-ion battery from sea salt: a review

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The electrical energy storage is important right now, because it is influenced by increasing human energy needs, and the battery is a storage energy that is being developed simultaneously. Furthermore, it is planned to switch the lithium-ion batteries with the sodium-ion batteries and the abundance of the sodium element and its economical price compared to lithium is the main point. The main components anode and cathode have significant effect on the sodium battery performance. This review briefly describes the components of the sodium battery, including the anode, cathode, electrolyte, binder, and separator, and the sources of sodium raw material is the most important in material synthesis or installation. Sea salt or NaCl has potential ability as a raw material for sodium battery cathodes, and the usage of sea salt in the cathode synthesis process reduces production costs, because the salt is very abundant and environmentally friendly as well. When a cathode using a source of Na 2 CO 3 , which was synthesized independently from NaCl can save about 16.66% after being calculated and anode with sodium metal when synthesized independently with NaCl can save about 98% after being calculated, because sodium metal is classified as expensive matter.

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Introduction

Researchers have established renewable resources usage as part of the Paris Agreement [ 1 , 2 ]. The agreement aims to reduce global greenhouse gas emissions to restrict the increment in global temperatures in 2 °C above per-industrial levels while pursuing ways to limit the increment in 1.5 °C [ 3 ]. To achieve this goal, the obligation for transition from fossil energy to clean energy is conducted to get a better life. As a result, energy research and development has become a well-known topic and a major goal in world achievements.

The development of renewable power generation is inseparable from the importance of reliable and efficient energy storage technology [ 4 ]. Energy storage devices convert electrical energy into several forms that can be stored and released when they are used [ 5 ]. The energy storage system increases the reliability of the electricity supply by storing electricity during off-peak hours and releasing during on-peak hours [ 6 ]. Several types of these devices include secondary batteries, compressed air energy storage (CAES), electrochemical double-layer capacitors (EDLC), flywheels, superconductive magnetic energy storage (SMES), fuel cells, and thermometric energy storage (TEES) [ 5 ]. Secondary batteries have a long-life cycle, flexible power, high round-trip efficiency, and easy maintenance among these energy storage devices. Secondary batteries will be good energy storage technology when integrated with renewable resources. Furthermore, the compact size of the battery is suitable to be used in distribution network locations [ 7 ]. The challenge is that some portable devices, i.e. mobile phones, laptops, digital cameras, and drones, turn out more expensive because of the batteries [ 8 ]. To get the desired specifications, selecting the type of battery is the main point. The type of battery chosen is undoubtedly related to the production cost, which is dominated by the material, about 70% of the total cost [ 9 ].

The Lithium-ion battery (LIB) contains the most expensive material but has many advantages [ 10 ]. High power density, high energy efficiency, and being environmentally friendly are the main advantages of this battery [ 11 , 12 , 13 ]. The research on LIB was conducted in 1970–1980s and Sony became a successful pioneer in the commercialization in 1991 [ 14 ]. The most important component of the LIB is the electrode (cathode and anode), the separator and the electrolyte. Commercial anodes in commercial LIB are generally made of graphite, which can easily diffuse Li ions over thousands of cycles. Since this battery is used widely, especially for electric vehicles and consumer use, its production increases every year [ 15 ]. As shown in Fig.  1 , the application of batteries has grown rapidly and is expected to increase simultaneously. The applications, which include portable devices such as video cameras, PCs, mobile phones, and a variety of other electronic gadgets, are designated as “consumer use” and contain features and functionalities that were previously inaccessible.

Global battery demand from 2020 to 2030 [ 1 ]

Electric vehicles will progressively use lithium-ion batteries as an environmentally acceptable form of transportation, which is predicted to grow dramatically [ 16 ]. However, the main source of batteries, namely lithium, is a challenge for the future because it is a finite metal source. According to US geological surveys, it is estimated that worldwide lithium resources can meet market demand by 2100 [ 17 ]. Sodium is found in the form of brine and seawater, counted for 61.8% of the world's total (26.9 Mt), especially in the USA, Bolivia, Chile, Argentina and China. The remainder is in mineral form about 16.7 Mt [ 18 ]. However, actual demand may exceed this forecast demand coupled with hard-to-find lithium sources [ 17 ]. New sources of lithium are being explored, both primary sources from mining and secondary sources from recycled active materials. It is possible that the LIB will not be able to cross the growing demand [ 19 ].

The new battery source that is more readily available in nature and less expensive is the sodium-ion battery (SIB). Several studies have been carried out, therefore SIB can be an alternative to LIB for large-scale production [ 20 ]. From an economic point of view, SIB can compete with LIB in terms of price, as explained by Peters et al. [ 21 ]. In the same cell, such as the 18,650-round cell, the SIB is cheaper than the LIB with an LFP/NMC cathode.

As the main source of SIB, sodium is the lightest metal and the second smallest after lithium [ 22 ]. Geographically, compared to the limited lithium, the availability of sodium is more abundant. Sodium can be resourced from both seawater and the earth’s crust. In seawater, the sodium concentration is 10,800 ppm compared to the lithium concentration of only 0.1–0.2 ppm [ 23 ]. Similarly in the earth’s crust, 2.8% sodium is provided [ 24 ], whereas lithium is only 0.002–0.006% [ 25 ]. The high gap in availability further strengthens sodium as a raw material for new battery materials. Sodium sources can be reached from various compounds such as Na 2 CO 3 , NaCH 3 COO, NaCl and NaNO 3 . Most of the sodium salt can be produced using NaCl or saline salt. As the main SIB source, sea salt (NaCl) from seawater can be the main candidate [ 26 , 27 ] due to its abundant compounds and not geographically limited. NaCl can be activated into an electrode by inducing it electrochemically in a crystalline structure [ 26 ]. NaCl has been used in several components of sodium-ion batteries, including electrolyte [ 28 , 29 ], as a raw material for electrodes and electrode doping [ 30 ]. Nowadays, the challenge in developing SIB is selecting the suitable electrode type.

This review examines SIB as an alternative to LIB for the future secondary battery using NaCl potential as a raw material especially from seawater. Seawater is processed in such a way into sea salt (NaCl) which is ready to use for production. NaCl can be used directly as raw material for SIB or processed into intermediate raw materials for SIB, such as Na 2 CO 3 or NaNO 3 . Details on the seawater potential for the SIB component, its challenges, and future projections are discussed in the next section.

Sodium-ion battery

A sodium-ion battery (SIB) is one of the options for LIB. Because of the comparatively high amount of sodium sources in the earth's encrustation and seawater, as well as its relatively inexpensive manufacturing costs, SIB has recently gained a lot of interest as a promising commercial choice for large-scale energy storage systems [ 20 ]. Furthermore, because sodium belongs to the same periodic table group as lithium and has similar physicochemical qualities, SIB's operating mechanism is extremely similar to LIB's [ 7 ].

Lithium and sodium are parts of the periodic table's elements in group 1. They are known as alkali metals, because their valence shell has one loosely held electron. As a result, alkali metals are extremely reactive, hardness, conductivity, melting point, and initial ionization energy fall as their progress through the group [ 31 ]. Table 1 summarizes some of the characteristics of sodium and lithium that interest in their development. The redox potential of the two alkali elements is one of the most important things to compare. The standard Na + /Na reduction potential vs. SHE is − 2.71 V, which is roughly 330 mV higher than Li + /Li, which is − 3.04 V. SIB's anodic electrode potential will always be greater than LIB's since this potentially specifies a thermodynamics minimum for the anode. However, because the ionic radius of Na (1.02) is much larger than that of Li (0.76), finding suitable crystalline host materials for Na + with sufficient capacity and cycling stability may be more difficult [ 19 ].

Similar as LIB, an SIB cell consists of a cathode, anode, and electrolyte. The cathode in SIB is made of a substance that can absorb Na cations reversibly at voltages significantly higher than 2 V positive for Na metal. The best anodes are those with low voltages (less than 2 V vs. Na). The active cathode material commonly used is NaFeO 2 and the negative electrode or anode is hard carbon. Throughout charging, the cathode (NaFeO 2 ) will donate electrons to the external circuit, which can cause oxidation for the transition metal. Some of the added sodium atoms dissolve as ions in the electrolyte to maintain charge neutrality. They travel to the anode (hard carbon) and are incorporated into the structure to restore charge neutrality to the site, which was disrupted by electrons transmitted and absorbed from the cathode side. During discharge, the procedure is iterated in the opposite direction. This complete cycle of reactions happens in a closed system. Each electron produced during oxidation is consumed in the reduction reaction at the opposite electrode [ 32 ]. Figure  2 shows the entire procedure diagrammatically.

Working Mechanism of Sodium-Ion Batteries [ 2 ]

The specific capacity, cyclic stability, and rate performance of the SIB need to be improved for commercialization. The electrochemical effect is influenced by the electrode material used in cell manufacturing. The primary challenge is discovering an electrode material with a high and stable specific capacitance, minimal volume change during charge/discharge cycles, and adequate current performance [ 33 ]. Increasing the energy density of SIB can be achieved by enhancing the cathode’s working voltage or decreasing the anode’s working potential, increasing the capacity of specific electrodes, and generating solid-particle materials [ 19 ]. Another major challenge related to plated cathode materials is their hygroscopic properties after exposure to air, leading to poor cell performance and ultimately increasing transportation costs [ 34 ]. Due to some of these challenges, it is necessary to have a stable material towards the air exposure to produce good cell performance.

Na-containing anode materials

Many types of anodes have been developed by researchers, including sodium metal, oxide-based, carbon-based, alloys, and convention anodes. They have a relatively low irreversible capacity, and most of their capacities are potentially close to that of sodium metal. The metal insertion mechanism of sodium is nearly identical to that of lithium [ 35 ].

Sodium metal has been studied by many researchers as the negative electrode in sodium-ion batteries [ 36 , 37 ]. Due to its high density, it has a good anode for energy storage applications in the post lithium-ion battery era because of its large capacity (1166 mAhg −1 ), availability on earth, and inexpensive cost. However, sodium metal anodes suffer from inconsistent plating, stripping and, therefore, it cause a low Coulombic efficiency [ 37 ]. The large reactivity of sodium metal with organic electrolyte solvents and the production of dendrites during Na metal deposition are even more troublesome, and the low melting point of Na (98 °C) poses a considerable safety hazard in devices intended for operation at room temperatures [ 19 ]. Tang et al. devised a method of "sodiophilic" coating an Au-Na alloy onto a Cu substrate that works as a current collector to drastically minimize the propensity for nucleation over abundant to address this challenge. This coating significantly increases the coulombic efficiency of Na coating and stripping. NaCl allows to be used as a source of Na in this layer [ 37 ]. NaCl has been used in several components of sodium-ion batteries, including anode component and sodium metal. Sodium metal is usually produced by electrolysis of sodium chloride (NaCl) in the liquid state at the cell, by sticking up a steel gauze diaphragm between the anode and cathode. The function of the diaphragm is for reducing the mixing of the anode and cathode products when traverse the electrolyte [ 38 ].

David S. Peterson in 1966, got sodium metal by electrolysis in a mixture of molten salt which consist of 28–36% sodium chloride (NaCl), 23–35% calcium chloride (CaCl 2 ), 10–25% strontium chloride (SrCl), and 13–30% barium chloride (BaCl 2 ) [ 26 ] takes pure NaCl as the SIB electrode. NaCl is a non-metallic compound, so when it is used as an electrode, it must be metalized, i.e. it must be electromagnetically active for a reversible cycle. The metallization process can be led at high temperatures, as described above, or by induction. During the induction process, an electrochemical before filling up to 4.2 V was brought out as an activation cycle. This process generates a partial transition from phase B1 to B2-NaCl. Furthermore, the release process of about 0.1 V was done, therefore, Na + was intercalated into the active compound. During the release process, the B2-NaCl phase can accommodate Na + and form Na x Cl compounds, x  > 1. The reactions that occur are as follows

The phase change from B1 to B2 is the key to the reversible process, therefore it can intensify the ionic and electrochemical conductivity. Na metal in NaCl can reversibly intercalate/deintercalate up to a discharge capacity of 267 mAhg −1 [ 26 ]. The success of NaCl as an electrode is shown in Fig.  3 .

Galvanostatic profile of NaCl electrodes a intercalation and deintercalation of sodium through the NaCl structure with and without an activation cycle at 0.03 C. b Charge–discharge profile of activated NaCl electrodes at 0.05 C. c Performance of the NaCl cycle. d Voltammetry curve of the NaCl electrode at 0.1 mV / s in a sodium half cell [ 5 ]

Oxide-based materials have also been developed as well, as anodes in sodium-ion batteries, such as (NTP), NaTi 2 (PO 4 ) 3 , Na 2 Ti 3 O 7 and its composites with carbon, which have been studied by several researchers [ 29 , 39 ]. The three-dimensional structure of NTP, which creates an open framework of large interstitial spaces modified with NMNCO, with rate capability and cycle stability is increasing, because a better structure can ensure stability between the phases [ 40 ]. A similar study was conducted by Hou using NTP / C composites with water-based electrolytes to achieve an energy density of 0.03 Wh / g and a colombic efficiency close to 100%, due to the three-dimensional structure of NTP with an open framework and uniform nanoparticle shape [ 41 ]. NTP / C composites were also studied by Nakamoto using several types of electrolytes and Na 2 FeP 2 O 7 cathodes [ 42 ]. Chen et al. (2018) succeeded in manufacturing a desalination battery consisting of a NaTi 2 (PO 4 ) 3 anode and a silver cathode in an aqueous NaCl electrolyte for the deionization of seawater as an aqueous energy storage system. During the charging process, the sodium ions in the electrolyte are electrochemically caught into the NaTi 2 (PO 4 ) 3 electrode while the chloride ions are captured and interacted with the silver electrode to generate AgCl. The discharge causes the release of sodium and chloride ions from the corresponding electrode. This will greatly contribute to more energy-efficient seawater desalination technology in the future [ 29 ].

SIB anodes have been reported to feature three different types of energy storage mechanisms such as intercalation reactions, conversion reactions, and alloying reactions. This mechanism occurs during sodiation and desodiation [ 31 ]. In SIB, carbon compounds are similarly subjected to the intercalation mechanism. Graphitic, hard carbon, and graphene are three types of carbon materials that are one of the most potential anodes for SIBs due to their excellent charge/discharge voltage plateaus and low price [ 43 ]. Graphite is the most widely used anode material in LIB, with a capacity of 372 mAhg −1 . Graphite is not suitable for sodium-based systems, because Na almost does not form gradual graphite intercalation compounds and the radius size of Na is larger than Li, which means Na ions cannot enter the graphite [ 44 ]. To overcome this limitation, Yang et al. created expanded graphite (EG) with a 4.3 interlayer lattice spacing by oxidizing and reducing some of the graphite. The long-range layer structure of EG is similar to that of graphite. They showed that Na + may be absorbed into and removed from EG in a reversible manner [ 45 ].

Hard carbon has a lot of potential as SIB anodes because of its substantial intercalation capacity, low charge/discharge voltage plateaus, and low-cost methods of preparation [ 44 ].The application of hard carbon as a sodium battery anode was investigated by Alcantara. Sodium can be inserted reversibly in amorphous and non-porous hard carbon, resulting in a high irreversible capacity in the first cycle due to the carbon surface area [ 46 ] . Furthermore, due to its wide middle layer, adequate operating voltage, and inexpensive cost, hard carbon can be utilized as anode material for sodium-ion batteries. However, poor efficiency and initial colombic performance remain a problem [ 47 ]. NaCl can also be combined as an additive on hard carbon to become the anode. The electrochemical properties of hard carbon can be improved by intercalating NaCl. Na + and Cl − can intercalate into the coating at high temperature and pressure, therefore the load transfer resistance can decrease sharply. With the addition of this NaCl, the capacity of the SIB can be increased to 100% [ 48 ].

The capacity of graphene-based carbon materials is higher than that of hard carbon. However, the density of tap graphene is often less than 1.0 g cm −3 , far lower than that of hard carbon, lowering graphene’s volumetric capacity. Due to the substantially higher outer surface area compared to hard carbon, these low ICEs are almost unsolvable. SIB anodes have been developed that combine graphene with conversion/alloy anodes to generate bi-functional electrodes. Due to the synergistic effect, graphene can be combined with other electroactive materials, such as metals (or metal oxides), to supply much greater storage capacities and better cycle stability than metal samples (or metal oxides) [ 49 ].

Sodium can form alloys with elements such as Si, Ge, Pb, Sn, Sb, P, and Bi so that it can be used as a SIB anode. Single atoms of these elements can form an alloy with more than one Na + at an average working potential with less than 1 Volt for Na/Na + . Alloying anodes have large specific capacities, and advanced composite nanostructure alloying anodes offer good capacity and cycle stability [ 50 ]. During the sodiation phase, however, there is a significant volume growth. Furthermore, during long-term cycling, they are constantly pulverized, which creates a considerable obstacle to commercialization [ 44 ]. Their stability must also be increased by the development of advanced structures or interfacial electrode/electrolyte adjustment. Alloying anodes with advanced composite and nanostructures have been shown to have high capacity and cycling stability [ 51 ].

Conversion anodes are a typical way to make a high-capacity SIB anode. P, S, O, N, F, Se, and other conversion elements are examples. The components must be paired with metal or non-metal materials as a partial SIB anode, where the alloy metal's discharge product can give high conductivity while also shielding the alloying discharge product from agglomeration [ 40 ]. Conversion anodes has obstacles in its implementation such as volume expansion, poor cycle stability, and crushing during the charge–discharge process, all of which are significant impediments to practical implementation [ 44 ]. Fortunately, nano-engineering of the alloying anode material can solve this volume change. Nano-engineering approaches can help to increase the cycling performance of alloying anodes. Combining alloying and conversion elements is another key method for improving the capacity and stability of these materials. These anodes have good cycling performance thanks to the synergism of conversion and alloying products. Combining alloying or conversion anodes with carbonaceous materials has been proven to be an excellent strategy for producing an extremely stable alloying or conversion anode [ 52 ].

Na-containing cathode materials

One of the needful components in a sodium battery is the cathode. However, its development is relatively slow. Therefore, developing suitable cathode materials with high capacities and voltages is essential to develop the energy density of the SIB. Several cathode materials have been developed by many researchers such as layered transition metal oxide, sodium poly anion compound, prussian blue, sulfur and air, and other organic compounds. Nowdays, layered metal oxides (Na x MO 2 , 0 <  x  < 1, M = Fe, Mn, Co, Cu, Ni, etc.) and poly anion-type materials have been the most important thing for studying cathodes in SIB [ 31 ]. In the 1970s, Delmas et al. found the electrochemical characteristics and structural of Na insertion in NaCoO 2 as a viable cathode material for SIB, which prompted more research [ 53 ]. Other layered oxides of 3d transition metals such as Na x CrO 2 [ 54 ], Na x FeO 2 [ 55 ], and Na x MnO 2 [ 56 ], were studied further in the early 1980s. These investigations were limited to 3.5 V versus Na +/ Na during that period and due to the electrolyte's instability in the beginning cycles [ 57 ]. Based on the nomenclature suggested by Delmas et al. in 1980., the layered TMOs prototype can be described in terms of Na 1- x MO 2 (0 <  x  < 1, M is a transition metal), and has two distinct types of structures which are presented in Table 2 about types of cathode structures. The structural form of the layered TMO consists of P2-type or O3-type which is depicted in Fig.  4 . The letters “P” and “O” stand for prismatic and octahedral, respectively, denoting the lattice site inhabited by alkali ion, while the numbers “2” and “3” denote the number of layers or stacks in a repetition unit of the TMO crystal structure [ 58 ].

Structure of a O3- b P2 TMO cathode is sodium coated (sodium atom is yellow, the oxygen atom is red, the transition metal is blue) [ 3 ]

NaCoO 2 is the oldest form of TMO cathode insertion material for SIB, having been investigated in the 1980s [ 53 ]. Research on NaCoO 2 was conducted by Ding et al. showed a poor cycle life of around 100 cycles [ 59 ]. In their investigation of P2-Na 0.7 CoO 2 , Fang et al. increased the electrochemical performance from 300 cycles and 86% initial capacity retention. [ 60 ]. In general, NaCoO 2 has a excellent voltage stability,high rate capability, and a large range of reversible sodium contents in its many polymorphs [ 60 ]. Despite this, it has a oblique voltage profile and has a inclination for reacting with electrolytes containing NaPF 6  [ 61 ]. The high cost of Co, on the other hand, is a major impediment to the widespread use of all Co-based electrode materials.

In the 1980s, Tekeda et al. were prepared NaFeO 2 through solid-state method at 700 °C. In their examination of the effect of cutoff voltage on electrode performance, Yabuuchi et al. discovered that NaFeO 2 has capacity of 80–100 mAhg −1 . For a cutoff voltage of 3.4 V, the electrode barely manages excellent capacity. When the cutoff voltage exceeds 3.5 V, the material undergoes irreversible structural changes [ 58 ].

NaMnO 2 for cathode application was examined by Mendiboure et al. on NaMnO 2 premise an impractical low reversible capacity of 54 mAhg −1 [ 56 ]. Caballero et al. synthesize P2-Na 0.6 MnO 2 and gain a reversible capacity of 140 mAhg −1 and incredible thermal stability [ 62 ]. NaNiO 2 and NaCrO 2 are also appointed as postulant electrode materials for SIB. The electrode NaCrO 2 was synthesized by Komaba et al. by capacity of 120 mAhg −1 in the first cycle. Vassilaras et al. defined the electrochemical characteristics of O′3-NaNiO 2 as a capacity of 120 mAhg −1 . Substitution part of Fe with either Mn [ 58 ], Co [ 63 ], or Ni [ 63 , 64 ] has attested to be a successful technique for dealing with structural changes and increasing storage capacity while keeping material costs down.

Other important type of cathode material is poly-anionic compounds. The most popular learning of poly-anionic groups contain phosphate (PO4) 3− , sulfate (SO4) 2− ,and pyrophosphate (P2O7) 4− ions [ 65 ]. The most productive structures in the poly-anionic compounds family are the maricite, olivine, and NASICON [ 65 ]. In the case of SIB, maricite type NaFePO 4 is the thermodynamic stable. However, at temperatures above 450 °C, olivine-type NaFePO 4 change into maricite type NaFePO 4 . Analogous to carbonophosphates and Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), Na 3 MePO 4 CO 3 (where Me = Fe or Mn), has also been observed [ 66 ]. A promising poly-anionic carbonophosphate cathode material has been identified as the Mn compound. However, electrode tuning to reduce capacity loss by 50% in the initial cycles (from 200 to 100 mAhg −1 ) is required to attain its full potential. Besides, the rate performance is unimpressive [ 67 ]. However, the observed reasonable capacity storage is adequate to generate optimization in this material [ 31 ].

NaCl can be utilized as an additional raw material in another compound cathode. In its pure form, NaCl is not considered to work properly. Yang et al. [ 68 ] synthesizes Na 4 Fe(CN) 6 /NaCl as SIB cathode. NaCl has a part in increasing the Na + diffusion, therefore it improves the electrochemical performance of pure Na 4 Fe(CN) 6 [ 68 ]. Moreover, it [ 69 ] also synthesized the Na 2 MnFe(CN) 6 cathode with Na 4 Fe(CN), NaCl and MnCl 2 raw materials using the co-precipitation method using deionized water as a solvent. Using water-based electrolyte, such as 32K 8 Na (Acetate), electrochemical stability is gained and can use Al foil as a flux collector for the first time. Besides, the contact with NMHCF shows a clear dark yellow color shift from the production of iron hydroxide as a result of NMHCF decomposition, therefore, it is necessary to develop a transition metal based on cathode to make it more stable.

Besides being an additional raw material, NaCl can be formed in other compounds, as shown in Table 3 , and therefore, it can be used for SIB cathodes. The source of the raw material generally used is Na 2 CO 3 , which requires a process to alter NaCl into Na 2 CO 3 . Synthesis of Na 2 CO 3 according to the solvay process with table salt, therefore it is more environmentally friendly in the equation below [ 70 ]:

The recovered sodium bicarbonate can be altered to carbonate (soda-ash) by the arts skill method, through calcification (heating) at 170–190 °C.

Na 2 CO 3 is gained. After that, it ready to use as raw material for sodium-ion battery cathode.

The sources of sodium can turn into NaNO 3 as well and some compounds can be reacted with NaCl to form NaNO 3 with heterogeneous reactions at atmospheric pressure such as [ 71 ]:

The formation of NaNO 3 from NaCl and HNO 3 begins with the recrystallization of NaCl using an ethanol–water mixture. The solid formed is filtered and dried to remove surface-bound H 2 O. NaCl is formed from the precipitation process is not from porous. Dry HNO 3 steam was prepared by mixing HNO 3 and H 2 SO 4 . Furthermore, NaNO 3 − capping NaCl is produced by dry exposure and recrystallization of NaCl to dry HNO 3 in a closed container [ 72 ].

Na-containing electrolytes

As explained in the previous point, the presence of electrolytes is very important in sodium batteries. The usage of electrolytes is also build upon the resistance of the electrode material used. Regarding of performance, electrolytes specify the properties of ion transport and the solid electrolyte interface (SEI). The usage of electrolytes is also build upon the resistance of the electrode material used. Not only is ionic conductivity, electrochemical stability window, thermal and mechanical (for solid electrolytes) strength, safety, economics, and ecology important when choosing an electrolyte, as well as the mechanism of its interaction with electrode materials and the characteristics of the SEI produced [ 74 ]. For safety reasons, when it comes to thermal runaway or cell breakdown, the correct electrolyte can help reduce the chances of an explosion [ 73 ]. Highly modified intercalation compounds used as anodes likely need a customized SEI to allow stable for cycling in the cell, equal to the litigated graphite electrodes in Li-ion systems [ 19 ]. Organic electrolytes, ionic electrolytes, aqueous based electrolytes, inorganic solid electrolytes, and solid polymer electrolytes are among the electrolytes utilized in sodium batteries. Each of the electrolytes that are currently available is described below.

Organic electrolytes

Diluted sodium salt in organic solvent (ester based and ether based). First, linear carbonates (DMC, EMC, and DEC) and cyclic carbonates (EC and PC); second, ether-based organic solvents (diglyme and triglyme), which suppress dendritic formation, improve thermal stability, and lengthen the ESW [ 74 ]. Both sodium salt and solvent is important, the additive choice is important to improve the SIB performance (ester based). Organic solvents have several advantages, including a high dielectric constant (> 15) that promotes sodium salt dissociation, a low viscosity that promotes Na + migration, chemical and electrochemical stability to charged and discharged electrode materials within a particular voltage range, the formation of stable passivation films on electrode surfaces, and it is also cost effective for commercial production [ 75 ]. However, organic electrolytes' flammability and instability at high temperatures are major challenges to practical usage [ 76 ]

Nowadays, the electrolytes used widely for sodium batteries are NaClO 4 , NaPF 6 , dissoluble in carbonate-based organic solvents, either made from a single solvent, such as polypropylene carbonate (PC), or a mixture of solvents.

Ionic liquid electrolytes

Because some worse point of organic solvent, ionic liquid being used to solve. Cations and anions are the sole constituents of ionic liquids, often identified as room-temperature molten salts or ambient-temperature melts. Many studies on ionic liquid electrolytes have been led by their great thermal stability, no volatility, and broad Electrochemical Stability Windows (ESWs) (up to 6 V), which offer high-quality interfaces and good wetting qualities for separators. Furthermore, their uses have been limited to some extent because to their poor ionic conductivity and relatively high viscosity [ 65 ]. That is also because to the high costs of ingredients, manufacture, and purification, ionic liquid-based battery are sometimes too expensive for large-scale energy storage system [ 76 ].

Inorganic solid electrolytes

Solid electrolyte may be divided into two main categories that is inorganic and organic solid electrolyte. Inorganic solid electrolytes shown a diverse group of crystalline ceramics and amorphous glasses. The primary goal of inorganic solid electrolyte development is to solve safety concerns, while also increasing energy density and also the flammability case of previous electrolyte. This electrolyte began to be developed due to its non-flammability, strong thermal stability, mechanical qualities, and broad ESW. Sulfides, β-Alumina, NASICON, and Complex hydrides are examples of inorganic solid electrolytes that have been discovered. Sulfides are known for their strong ionic conductivities, as well as their good mechanical characteristics and low grain-boundary resistances [ 74 ]. Because of its layered structure with open galleries separated by pillars through which Na + ions may easily move, β-Alumina has been employed as a rapid Na + ion conductor. NASICON for their compositional variety and exceptional performance. Complex hydrides have a wide ESW and excellent thermal stability, although they have a poor ionic conductivity.

Besides of their good thermal stability and mechanical strength, there are some point that being major concern on developing inorganic solid electrolyte. Which include poor interface contact, unfavourable chemical/electrochemical reaction with alkali metal, and solid-state electrolytes with low thermodynamic and mechanical stability [ 77 ].

Solid polymer electrolyte

Organic solid electrolytes, which are mainly carbon-based polymers containing heteroatoms like oxygen or nitrogen capable of solvating embedded sodium ions, are another type of solid electrolyte. Solid polymer electrolytes are the subject of a lot of research because their mechanical strength and process ability efficiently increase ionic conductivity and expand ESW. Differ from liquid electrolyte that dissociated ions are easily move within the liquid, polymer electrolyte that contain no liquid, ions in polymer electrolytes migrate via the polymer network itself.

Some polymer that already developed is PEO, P(VDF-HFP), PMMA, PAN-based gel electrolytes. Liquid plasticizers, organic solvents, and salt solutions immobilized inside polymer host matrices make up solid polymer electrolytes [ 78 ]. Because the loss of quick Na + ion transfer medium and interfacial compatibility during cycling has a detrimental impact on SPE performance, high-viscosity solvents must be used to solve the problem. SPEs have low mechanical strength, which puts them at danger of dendritic penetration [ 76 ].

Aqueous-based electrolyte

The development of aqueous electrolytes for SIBs is essential from the standpoints of sodium storage and interfacial stability. Diluted sodium salt in aqueous solution. The electrolyte concentration is regarded as an important index for improving aqueous electrolytes because of its direct effects on ionic conductivity and rate performance. The oxygen solubility decreases and becomes more stable as the electrolyte concentration rises. Because self-discharging is generated by oxygen that is not soluble in the electrolyte, the higher the electrolyte concentration, the less self-discharging occurs.

Because of their low cost and great safety, aqueous electrolytes are widely studied. Because of the kinetic impact, the practical stability window of the aqueous electrolyte is greater than the thermodynamic limit, allowing for the use of a wider range of electrode materials in these systems. However, excessive salt concentrations cause corrosion. There is following aspect that can be considered to improve aqueous electrolytes: such as combination of adequate sodium salts with an appropriate anion to build a stable and low-resistance protective layer on the electrode surface; and inclusion of functional additives to increase interface stability and reduce side reactions in high-concentration electrolytes [ 76 ].

Aqueous-based electrolyte or water used as a salt solvent has been studied deeply. The salt used are NaCl [ 29 , 79 ], Na 2 SO 4 [ 80 , 81 , 82 ], and CH 3 COONa [ 69 , 83 , 84 ]. Water electrolytes are promising candidates for an environmentally friendly SIB system due to long life and good performance, safe and low-cost SIB system for grid-scale applications [ 73 ]. Because of its high dielectric constant, low viscosity, strong ionic conductivity, and low vapor pressure, water is an appealing choice as an ideal solvent for aqueous electrolytes, especially because of its inherent safety [ 79 ]. The majority of inorganic salts in aqueous electrolytes (such as Na 2 SO 4 , NaCl) have no influence on the electrochemical stability window [ 85 ]. Further development of NaCl as sodium salt electrolyte is really compromising, because the abundance of NaCl in the form of sea salt in the world.

NaCl from sea salt has been one of promising point for sodium-ion battery development. In fact, NaCl can be used separately as an electrolyte. As an electrolyte, NaCl is used as an additive in the NaPF 6 electrolyte. The concentration used is about 0.19% and was reported to improve cycling ability [ 86 ]. It can also be used as a raw material for the production of electrolytes by reacting with AlCl 3 at 300°C, and the electrolyte formed is NaAlCl 4 [ 87 ]. Furthermore, NaCl can be electrolyzed to become NaClO 4 and the electrolysis process occurs on the solid surface, in the case an electrode, and therefore, the solid acts as a catalyst. The reaction mechanism in the formation of chlorides in perchlorates is through the formation of hypochlorite, chlorite, and chlorate compounds first. More specifically, the reaction mechanism is shown below [ 88 ]:

On the anode surface:

On the cathode surface:

In an electrolyte solution:

NaClO 4 have been widely used as sodium battery salt electrolyte. NaClO 4 can be used in organic solvent or aqueous solvent. The impact of extending the aqueous electrolyte's limited working potential window has also been examined using aqueous Na-ion batteries with highly concentrated electrolytes [ 89 ].

For Na + in aqueous solutions below 5M, which the water molecules surpass the salt level, the solvation shell consists of at least two layers a closely related primary and a relatively loose secondary solvation shell, with the first Na + solvation layer usually contains 6 oxygen. However, when the salt concentration is above 9M, there are hardly enough water molecules available to form the "classic" primary solvation sheath, and bring out "water-in-salt" solution then it can be visualized as molten salt [ 90 ].

Application of aqueous sodium-ion batteries suffer from its poor cycling stability, lower voltage window and corrosion due to high salt concentration. One of the solution to widen the voltage window is focused on current collector. As a result, corrosion-resistant current collectors on the cathode and current collectors with a high hydrogen over potential on the anode should be used to build high-voltage and long-life sodium-ion batteries [ 79 ].

Na-containing other components

Separators, current collector, and binders are also important components for SIB. Fiber glass is used as a separator in cells with electrolyte dissolved in PC solvent. The usage of fiber glass can reduce the energy density lower build upon the weight or volume of SIB whole components [ 91 ]. Fiber glass that composed of nonmetallic fibers have a melting point more than 500 °C and excellent fire-resistance performance. But it also has disadvantages such as bad flexibility, mechanical strength and high cost. It makes the difficulty in the assembly process and also bring great safety hidden danger in large-scale application [ 92 ]. Here is the requirement of separator for SIBs [ 93 ] (1) minimal cost to fulfill the requirements of large-scale energy storage; (2) due to the high viscosity of SIB electrolyte, better chemical stability and wettability of separators are required; (3) Na dendrites have a higher reaction rate and risk than Li dendrites, and SIB separators should be more resistant to dendrites; (4) other safety indexes of SIB separators, such as thermal stability and mechanical strength, are also strongly. Several separator modifications have also been unfolded, including the Electrospun Hybrid PVDF-HFP/SiO 2 fiber-based separator, which is applied to sodium-ion batteries and the electrolyte absorption shows no swelling, and stable interface [ 93 ]. The present research on SIB separators is mostly focused on the modification of polyolefin separators and the manufacture of nonwoven separators to ensure that the separator can retain chemical stability in the electrolyte while also having a high affinity for the electrolyte.

Another major component for the development of practical SIB is the binder used for powdered electrode materials. A binder’s key functions may be described as follows: functioning as a dispersant or thickening agent to bind the active material together and provide consistent mixing of electrode components; maintaining the electrode structure's integrity by functioning as a conductive agent and fluid collector; allowing the electrode to conduct the requisite amount of electrons; increasing the electrolyte's wettability and encouraging ion transport between the electrode and the electrolyte contact [ 94 ]. The most used binders are PVDF and CMC. Furthermore, PVDF is used with NMP (N-methyl-2-pyrrolidone) solvent which is pestilent and volatile, while CMC is water-soluble, consequently, it is become easier and safer. Due to its great mechanical properties, high electrochemical stability, thermal stability, good interaction with electrolyte solutions, and the proven ability to make this feasible, PVDF has been the dominant binder in the battery industry for the transport of Li + . The main damage of using PVDF are the usage of toxic solvents during its processing and the poor adhesion/consistency of power collectors [ 95 ].

Another important external component in the operation of a battery is the binder that holds the electrode material to the current collector. The electrochemically active mass must be fastened to the current collector unless the electrode material is naturally self-standing and may be used as a monolith anode. As briefly mentioned in the previous point, corrosion-resistant current collectors on the cathode and current collectors with a high hydrogen over potential on the anode should be used to develop sodium-ion batteries with high voltage and longer life [ 79 ]. As a result, strong electronic conductivity and long-term viability in a certain electrochemical environment are key parameters for the current collector. It also does not have to be overly thick or heavy to avoid lowering the system's gravimetric and volumetric energy densities [ 96 ]. There have been proposed for sodium batteries current collector such as prepatterned current collectors, porous Al and Cu current collectors, carbon felt, and conducting polymer paper-derived mesoporous 3D N-doped carbon [ 97 ].

Sodium source

Sodium can be found from sea salt and the earth’s crust. There is 2.8% sodium available in the earth’s crust [ 24 ]. Figure  5 demonstrated the relative abundance of the eight most abundant elements.

Relative abundance of the eight most abundant elements in the continental encrustation [ 4 ]

According to the 2013 Geospatial Information Agency, Indonesia has a coastline of 99.093 km, which is the second longest in the world after Canada. It shows that the country has a great potential for salt development.

The quality of saltwater, the procedure, and the technology which are have an impact on the salt production. The production of salt in Indonesia is in the middle class. Based on Ministry of Industry, Indonesia 2018, the average national salt production is 1,281,522 tons and the average national salt consumption is 3,185,194 tons that means the national salt production cannot meet national salt consumption [ 98 ]. It contains 85–90 percents sodium chloride. The sodium chloride level of those salts is still beneath the Indonesian National Standard (SNI) for human salt consumption (dry base) [ 99 ] and for salt industry is 98.5% [ 100 ].

A method is needed for the salt production to fulfill the needs based on the quality of the generated salt and the salt requirements for human salt consumption and salt industry. There are plenty methods for increasing salt quality, including physical and chemical approaches. A physical method is one method that improves salt quality without using chemicals, such as hydro-extraction and evaporation (re-crystallization) [ 101 ] and chemicals such as sodium carbonate (Na 2 CO 3 ), sodium hydroxide (NaOH), barium chloride (BaCl 2 ), calcium hydroxide (Ca(OH) 2 ), calcium chloride (CaCl 2 ), and others are used in the chemical procedure [ 102 ]. The hydro-extraction procedure is an extraction or separation of a solid-phase component using a liquid phase as a solvent. The salt is the solid phase, while the salt solution is the solvent in this context. The size of salt, the concentration of the salt solution as a solvent, and the extraction period all work on the performance of the hydro-extraction process.

The high abundance and an urge to develop Indonesian salt industries made sodium-ion batteries from sea salt is an interesting research field to develop. Sodium can be extracted from sea salt to be used for sodium-ion batteries. As explained before, desalination batteries also have been developed and use seawater in the case of NaCl. Besides of using seawater directly to the battery system, a lot of sodium salt have been used. Some of the sodium salts used by researchers to gain sodium batteries are Na 2 CO 3 [ 81 , 84 , 103 , 104 , 105 , 106 , 107 ], NaCH 3 COO [ 29 , 69 , 108 , 109 , 110 ], NaCl [ 30 , 69 , 87 , 111 , 112 , 113 ], NaNO 3 [ 57 , 59 , 114 , 115 ], Na 2 O [ 64 , 116 ], NaOH [ 83 ], NaI [ 82 ] as shown in Table 3 below. Na 2 CO 3 is the dominant salt in the manufacturing of sodium-ion battery cathodes.

For example, of the using of Na 2 CO 3 , Sauvage, 2007, synthesized single-phase Na 0.44 MnO 2 powder via a classic solid-state reaction using MnCO 3 and Na 2 CO 3 (with excess stoichiometry of 10 wt %). Billaud also performed a solid-state method with combining Na 2 CO 3 and Mn 2 O 3 ; then 15% excess sodium weight was used to obtain β-NaMnO 2 [ 117 ]. Another study conducted by Jo, 2014, synthesized a-NaMnO 2 by a conventional solid-state method using a mixture of Mn 2 O 3 and Na 2 CO 3 at a molar ratio of 2:1. This selection is due to the fact that nano-sized Na 2 CO 3 as a source of Na has a good size distribution, and Mn 2 O 3 as a source of Mn has polygon pieces with micro sizes. The synthesized A-NaMnO 2 shows an agglomerate stick shape with an average particle size of more than 10–50 mm, but due to the high surface activity, the small powder stick form having a 4–5 mm size can be agglomerated easily [ 105 ]. The β-NaMnO 2 sample was synthesized by the solid-state method. The solid-state route implicate mixing Na 2 CO 3 and Mn 2 O 3 ; then 15% excess sodium weight was used. The significant proportion of Na atoms at this intermediate site indicates that -NaMnO2 has a strong probability to produce planar breakdown. The compound shows a high discharge capacity of 190 mAhg −1 at a low C/20 level when it examined as a cathode in a sodium-ion battery [ 107 ].

The source of the raw material generally used is Na 2 CO 3 , which requires a process to alter NaCl into Na 2 CO 3 . Synthesis of Na 2 CO 3 according to the solvay process with table salt, therefore, it is more environmentally friendly in the equation below [ 70 ]:

Potential of NaCl for sodium-ion batteries

The various sources of sodium, sodium chloride (NaCl) or commonly considered as salt, mentioning above that they have been known for a long time and used widely in various industrial fields. Salt is easy to obtain from evaporation of seawater. This dominates mineral deposits as well, especially in the form of halite [ 118 ]. The physical and chemical properties of NaCl can be seen in Table 4 below.

NaCl is predicted to be thermodynamically stable by preserving its electronic properties and bond structure. The chemical properties of NaCl under ambient conditions can be understandable, but under a very high-pressure conditions, they can turn the NaCl bond properties. The turning can make differences in mechanical and electronic properties. NaCl runs into a phase transformation from F-centered B1 to primitive B2 phase in the NaCl to CsCl structure at pressures between 20 and 30 GPa, depending on the temperature [ 119 ].

NaCl has been used in several components of sodium-ion batteries, including electrolyte [ 28 , 29 ], as a raw material for electrodes and electrode doping [ 30 ]. NaCl as an electrolyte was examined by Liu and compared to NaNO 3 , it has a lower ionic strength with the same conductivity [ 28 ]. Chen utilizes NaCl as a template for the carbon airgel, with NaCl and flux, a carbon air-gel having a wave-like morphology with overflow micro-pores, large accessible surface area and strong structural stability. Furthermore, it can provide an increasing specific capacity and a better capability for sodium storage [ 66 ].

Future projection of SIBs obtained from cheap Na sources

There is something more interesting about the use of sea salt in batteries, namely the desalination batteries. Desalination batteries recover sodium and chloride ions from seawater and create fresh water using an electrical energy input, firstly developed by Pasta, 2012 [ 120 ]. They used Na 2-x Mn 5 O 10 as cathode for capture Na + ions and Ag electrode was used to capture Cl − ions with this following reaction.

The desalination battery is simple to build, employs commonly accessible materials, has a promising energy efficiency, runs at room temperature with less corrosion issues than conventional desalination technologies, and it has the potential to be Na + and Cl − selective, obviating the need for resalination [ 120 ].

Other desalination method also developed by Lee [ 121 ]. They created a novel way of desalting water by merging CDI and battery systems to improve the desalination performance of capacitive approaches, dubbed "hybrid capacitive deionization (HCDI)". A sodium manganese oxide (NMO) electrode, an anion exchange membrane, and a porous carbon electrode make up the HCDI asymmetric system [ 121 ]. The result is the HCDI system has successfully desalted high-capacity sodium chloride solution, faster ion removal rate, excellent stability and gotten higher specific capacity of the batteries.

Lately, Cao (2019) also developed HCDI system for desalination using Na3V2(PO4)3@C as sodium ions trapper while chloride ions are physically trapped or released by the AC electrode. From the result, conclude that the removal ion rates increased as the water concentrations increased [ 122 ].

NaCl as an electrolyte also examined by Liu and compared to NaNO 3 , it has a lower ionic strength with the same conductivity [ 28 ]. Another by Chen, Na + during charging is captured by the NaTi 2 (PO 4 ) 3 cathode, while Cl − is captured by the silver electrode to form AgCl, and vice versa [ 29 ]. The function of the NaCl is to provide better ion conduction. This research use 0.6 M NaCl that is equivalent to seawater and increase into 1 M NaCl to make sure that ion conduction is enough during charge process.

The charge process is described in the following reaction:

In contrast, the discharge process occurs as follows:

Therefore, the overall reaction occurs as follows:

The charge–discharge process shows good stability [ 29 ]. Besides conducted new battery system. This alternative also contributed to renewable energy storage from desalination process, because it is used natural seawater and collect the salt.

From an economic point of view, SIB can compete with LIB in terms of price, as explained by [ 21 ] in Fig.  6 . In the same cell, such as the 18,650-round cell, the SIB is cheaper than LIB with an LFP/NMC cathode. The changing of lithium salt into sodium salt, changing copper foil to aluminium foil for anodes and some additive can help decrease the material cost production [ 123 ]. Furthermore, the usege of NaCl as an electrode, additive and electrolyte is also a factor that give contribution in reducting production costs, especially in raw materials. When a cathode using a source of Na 2 CO 3 , which was synthesized independently from NaCl can save about 16.66% after being calculated and anode with sodium metal when synthesized independently with NaCl can save about 98% after being calculated because sodium metal is classified as expensive matter. Because of the congested areas of Na, developing aqueous Na-ion batteries is both valuable and possible (NaCl, Na 2 SO 4 , NaNO 3 , etc.).

Comparison of the 18,650 cells price [ 6 ]

As an alternative to LIB, SIB looks forward to have a low environmental impact. Peters [ 124 ] quantify the impact of SIB on the environment and compare it with LIB. Consequently, SIB has less impact due to the following reasons:

It is possible to use aluminium for both cathode and anode

When the anode uses hard-carbon, it may be possible to use hard-carbon from organic waste.

It can reduce the use of nickel, which is commonly used for LIB cathodes, therefore the impact of nickel mining on the environment can also be reduced. However, more in-depth research is needed.

The binder commonly used in LIB, such as PVDF, in its production, causes a high greenhouse gas effect, therefore, an alternative water-based binder is needed.

Bossche et al. [ 125 ] classify battery types with adverse environmental impacts, including lithium-ion, nickel–cadmium, lead-acid, sodium-nickel chloride, and nickel-metal hydride. The sodium-based battery type has the lowest environmental impact than others.

Some of researchers also have been conducted life cycle assessment (LCA) for sodium-ion batteries. LCA is a defined approach for calculating the environmental effects of commodities, products, and activities. It considers the entire life cycle, from resource extraction to production, usage, and end-of-life management, as well as waste recycling and disposal [ 124 ]. Except for LFP–LTO type LIBs, the examined SIB would already beat existing LIBs in terms of environmental performance with lifetimes of roughly 3000 cycles.

Sodium-ion batteries are still one of the most promising alternatives for next-generation stationary batteries, where the lack of volumetric limits and a focus on economy, safety, and extended life are well linked with SIB features.

Lithium-ion batteries (LIB) have long dominant energy storage technology in electric vehicles and hand-held electronics. The cost and limited quantities of lithium in the earth's encrustation may prevent LIBs from being used in future large-scale renewable energy storage. Lithium metal, if used simultaneously, is depleted easily. The searching for new sources of lithium batteries and recycling is not expected to meet the high demand. The increasing demand for LIB has led researchers to look for alternative batteries.

Because of their abundant sodium supplies and similar electrochemical principles, sodium-ion batteries (SIB) are being explored as a viable alternative to lithium-ion batteries in large-scale renewable energy storage applications. Na raw material base is much greater. The sources are more abundant in nature and cheaper than lithium Na thought that it has similar characteristics in batteries to Li. Many potential raw materials for Na. The sources of sodium in the form of NaCl (table salt) can be the primary source of SIB both as electrodes, doping, and electrolytes. NaCl can also be used in the form of other compounds such as Na 2 CO 3 which is the most commonly applied to SIB right now.

The use of NaCl as an electrode, additive, and electrolyte contributes to make production costs lower, particularly in terms of raw materials. Because sodium metal is classified as expensive matter, a cathode using a source of Na 2 CO 3 that was synthesized independently from NaCl can save about 16.66 percent after being calculated, and an anode with sodium metal that was synthesized independently from NaCl can save about 98 percent after being calculated.

The generation of solid-particle material cannot be separated from the selection of raw materials. Sodium sources from seawater can be chosen as raw materials because they can be processed into other forms of sodium salt or used directly. The source of this materials can meet the possible high demand for SIB in the future. As will be explained in the next section, NaCl from seawater can contribute to all SIB components, both cathode, anode, and electrolyte.

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Acknowledgements

This paper was funded by the Ministry of Research, Technology and Higher Education (KemenRistekdikti) through the “Penelitian Dasar” scheme with Grant Number 221.1/UN27.22/HK.07.00/2021.

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Anisa Raditya Nurohmah, Shofirul Sholikhatun Nisa, Cornelius Satria Yudha, Windhu Griyasti Suci & Agus Purwanto

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Nurohmah, A.R., Nisa, S.S., Stulasti, K.N.R. et al. Sodium-ion battery from sea salt: a review. Mater Renew Sustain Energy 11 , 71–89 (2022). https://doi.org/10.1007/s40243-022-00208-1

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This salt battery harvests osmotic energy where the river meets the sea

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“Decoupled Ionic and Electronic Pathways for Enhanced Osmotic Energy Harvesting” ACS Energy Letters

Estuaries — where freshwater rivers meet the salty sea — are great locations for birdwatching and kayaking. In these areas, waters containing different salt concentrations mix and may be sources of sustainable, “blue” osmotic energy. Researchers in ACS Energy Letters report creating a semipermeable membrane that harvests osmotic energy from salt gradients and converts it to electricity. The new design had an output power density more than two times higher than commercial membranes in lab demonstrations. 

Osmotic energy can be generated anywhere salt gradients are found, but the available technologies to capture this renewable energy have room for improvement. One method uses an array of reverse electrodialysis (RED) membranes that act as a sort of “salt battery,” generating electricity from pressure differences caused by the salt gradient. To even out that gradient, positively charged ions from seawater, such as sodium, flow through the system to the freshwater, increasing the pressure on the membrane. To further increase its harvesting power, the membrane also needs to keep a low internal electrical resistance by allowing electrons to easily flow in the opposite direction of the ions. Previous research suggests that improving both the flow of ions across the RED membrane and the efficiency of electron transport would likely increase the amount of electricity captured from osmotic energy. So, Dongdong Ye, Xingzhen Qin and colleagues designed a semipermeable membrane from environmentally friendly materials that would theoretically minimize internal resistance and maximize output power. 

The researchers’ RED membrane prototype contained separate (i.e., decoupled) channels for ion transport and electron transport. They created this by sandwiching a negatively charged cellulose hydrogel (for ion transport) between layers of an organic, electrically conductive polymer called polyaniline (for electron transport). Initial tests confirmed their theory that decoupled transport channels resulted in higher ion conductivity and lower resistivity compared to homogenous membranes made from the same materials. In a water tank that simulated an estuary environment, their prototype achieved an output power density 2.34 times higher than a commercial RED membrane and maintained performance during 16 days of non-stop operation, demonstrating its long-term, stable performance underwater. In a final test, the team created a salt battery array from 20 of their RED membranes and generated enough electricity to individually power a calculator, LED light and stopwatch. 

Ye, Qin and their team members say their findings expand the range of ecological materials that could be used to make RED membranes and improve osmotic energy-harvesting performance, making these systems more feasible for real-world use. 

The authors acknowledge funding from the National Natural Science Foundation of China.

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May 6, 2022

Rechargeable Molten Salt Battery Freezes Energy in Place for Long-Term Storage

The technology could bring more renewable energy to the power grid

By Anna Blaustein

Close-up of the freeze-thaw battery developed by the Pacific Northwest National Laboratory team.

Andrea Starr/Pacific Northwest National Laboratory

During spring in the Pacific Northwest, meltwater from thawing snow rushes down rivers and the wind often blows hard. These forces spin the region’s many power turbines and generate a bounty of electricity at a time of mild temperatures and relatively low energy demand. But much of this seasonal surplus electricity—which could power air conditioners come summer—is lost because batteries cannot store it long enough.

Researchers at Pacific Northwest National Laboratory (PNNL), a Department of Energy national laboratory in Richland, Wash., are developing a battery that might solve this problem. In a recent paper published in Cell Reports Physical Science , they demonstrated how freezing and thawing a molten salt solution creates a rechargeable battery that can store energy cheaply and efficiently for weeks or months at a time. Such a capability is crucial to shifting the U.S. grid away from fossil fuels that release greenhouse gases and toward renewable energy. President Joe Biden has made it a goal to cut U.S. carbon emissions in half by 2030 , which will necessitate a major ramp-up of wind, solar and other clean energy sources, as well as ways to store the energy they produce.

Most conventional batteries store energy as chemical reactions waiting to happen. When the battery is connected to an external circuit, electrons travel from one side of the battery to the other through that circuit, generating electricity. To compensate for the change, charged particles called ions move through the fluid, paste or solid material that separates the two sides of the battery. But even when the battery is not in use, the ions gradually diffuse across this material, which is called the electrolyte. As that happens over weeks or months, the battery loses energy. Some rechargeable batteries can lose almost a third of their stored charge in a single month.

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“In our battery, we really tried to stop this condition of self-discharge,” says PNNL researcher Guosheng Li, who led the project. The electrolyte is made of a salt solution that is solid at ambient temperatures but becomes liquid when heated to 180 degrees Celsius—about the temperature at which cookies are baked. When the electrolyte is solid, the ions are locked in place, preventing self-discharge. Only when the electrolyte liquifies can the ions flow through the battery, allowing it to charge or discharge.

Creating a battery that can withstand repeated cycles of heating and cooling is no small feat. Temperature fluctuations cause the battery to expand and contract, and the researchers had to identify resilient materials that could tolerate these changes. “What we’ve seen before is a lot of active research to make sure you do not have to go through that thermal cycle,” says Vince Sprenkle, a strategic advisor in energy storage at PNNL and a co-author of the new paper. “We’re saying, ‘We want to go through it, and we want to be able to survive and use that as a key feature.’”

The result is a rechargeable battery made from relatively inexpensive materials that can store energy for extended periods. “It’s a great example of a promising long-duration energy-storage technology,” says Aurora Edington, policy director of the electricity industry association GridWise Alliance, who was not involved with this research. “I think we need to support those efforts and see how far we can take them to commercialization.”

The technology could be particularly useful in a place such as Alaska, where near-constant summer sunlight coincides with relatively low rates of energy use. A battery that can store energy for months could allow abundant summer solar power to fulfill winter electricity needs. “What is so attractive about the freeze-thaw battery is that seasonal shifting capability,” says Rob Roys, chief innovation officer at Launch Alaska, a nonprofit organization that works to accelerate the deployment of climate technologies in the state. Roys hopes to pilot the PNNL battery in a remote part of his state.

Heating the battery may be a challenge, especially in cold places. Even under mild conditions, the heating process requires energy equivalent to about 10 to 15 percent of the battery’s capacity, Li says. Later phases of the project will explore ways to lower the temperature requirements and incorporate a heating system into the battery itself. Such a feature would simplify the battery for the user and could potentially make it suitable for home or small-scale use.

Right now the experimental technology is aimed at utility-scale and industrial uses. Sprenkle envisions something like tractor-trailer truck containers with massive batteries inside, parked next to wind farms or solar arrays. The batteries would be charged on-site, allowed to cool and driven to facilities called substations, where the energy could be distributed through power lines as needed.

The PNNL team plans to continue developing the technology, but ultimately it will be up to industry to develop a commercial product. “Our job at the DOE is really to derisk new technologies,” Sprenkle says. “Industry will make the decision whether they think that it’s been derisked enough, and they will take that on and run with it.”

The DOE is working to shrink the lag that usually occurs between initial research demonstrations and commercialization of energy technologies. Although scientists began developing lithium-ion batteries in the 1970s, for example, the batteries did not end up in consumer products until around 1991 and were not incorporated into electrical grids until the late 2000s. Artificial intelligence and machine learning may help expedite the validation and testing process for new technologies, Sprenkle says, allowing researchers to model and predict a decade of battery performance without needing 10 years to collect the data.

Whether adoption will happen quickly enough to meet decarbonization targets is unclear. “If we are truly trying to hit 2030, 2035 decarbonization goals, all these technologies need to be accelerated by about a factor of five,” Sprenkle says. “You’re looking at developments that need to come online, be validated and ready to hand off in the next four to five years to really, truly have an impact.”

Salt battery design overcomes bump in the road to help electric cars go the extra mile

Using salt as a key ingredient, Chinese and British researchers have designed a new type of rechargeable battery that could accelerate the shift to greener, electric transport on our roads.

Many electric vehicles (EV) are powered by rechargeable lithium-ion batteries, but they can lose energy and power over time. Under certain conditions, such batteries can also overheat while working or charging, which can also degrade battery life and reduce miles per charge.

To solve these issues, the University of Nottingham is collaborating with six scientific research institutes across China to develop an innovative and affordable energy store with the combined performance merits of a solid-oxide fuel cell and a metal-air battery. The new battery could significantly extend the range of electric vehicles, while being fully recyclable, environmentally-friendly, low-cost and safe.

A solid-oxide fuel cell converts hydrogen and oxygen into electricity as a result of a chemical reaction. While they are highly-efficient at extracting energy from a fuel, durable, low-cost and greener to produce, they are not rechargeable. Meanwhile, metal-air batteries are electrochemical cells that uses a cheap metal such as iron and the oxygen present in air to generate electricity. During charging, they emit only oxygen into the atmosphere. Although not very durable, these high-energy dense batteries are rechargeable and can store and discharge as much electricity as lithium-ion batteries, but much more safely and cheaply.

In the early research phases, the research team explored a high-temperature, iron-air battery design that used molten salt as a type of electrolyte -- activated by heat -- for electrical conductivity. Cheap and inflammable, molten salts help to give a battery impressive energy storage and power capability and a lengthy lifecycle.

However, molten salts also possess adverse characteristics. University of Nottingham study lead, Professor George Chen said: "In extreme heat, molten salt can be aggressively corrosive, volatile and evaporate or leak, which is challenging to the safety and stability of battery design. There was an urgent need to fine-tune these electrolyte characteristics for better battery performance and to enable its future use in electric transport."

The researchers have now successfully improved the technology by turning the molten salt into soft-solid salt, using solid oxide nano-powders. Professor Jianqiang Wang, from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences, who is leading this collaboration project has predicted that this quasi-solid-state (QSS) electrolyte is suitable for metal-air batteries which operate at 800 ºC; as it suppresses the evaporation and fluidity of the molten salts that can occur at such high operating temperatures.

Project collaborator, Dr Cheng Peng, also from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences, explains a unique and useful design aspect of this experimental research. The quasi-solidification has been achieved using nanotechnology to construct a flexibly-connected network of solid oxide particles that act as a structural barrier locking in the molten salt electrolytes, while still allowing them to safely conduct electricity in extreme heat.

Professor Chen, who is leading a molten salt electrolysis laboratory in Nottingham, hopes the team's "encouraging results" will help to establish a simpler and more efficient approach to designing low-cost and high-performance molten salt metal-air batteries with high stability and safety.

He adds, "The modified molten salt iron-oxygen battery has great potential applications in new markets, including electric transport and renewable energy which require innovative storage solutions in our homes and at grid-level. The battery is also, in principle, capable of storing solar heat as well as electricity, which is highly-desirable for both domestic and industrial energy needs. Molten salts are currently used at large scale in Spain and China to capture and store solar heat which is then converted to electricity -- our molten salt metal air battery does the two jobs in one device."

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• Shiyu Zhang, Yun Yang, Liwei Cheng, Jian Sun, Xiaomei Wang, Pengfei Nan, Chaomei Xie, Haisheng Yu, Yuanhua Xia, Binghui Ge, Jun Lin, Linjuan Zhang, Chengzhi Guan, Guoping Xiao, Cheng Peng, George Zheng Chen, Jian-Qiang Wang. Quasi-solid-state electrolyte for rechargeable high-temperature molten salt iron-air battery . Energy Storage Materials , 2021; 35: 142 DOI: 10.1016/j.ensm.2020.11.014

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Got tinnitus a device that tickles the tongue helps this musician find relief.

Allison Aubrey

After using the Lenire device for an hour each day for 12 weeks, Victoria Banks says her tinnitus is "barely noticeable." David Petrelli/Victoria Banks hide caption

After using the Lenire device for an hour each day for 12 weeks, Victoria Banks says her tinnitus is "barely noticeable."

Imagine if every moment is filled with a high-pitched buzz or ring that you can't turn off.

More than 25 million adults in the U.S., have a condition called tinnitus, according to the American Tinnitus Association. It can be stressful, even panic-inducing and difficult to manage. Dozens of factors can contribute to the onset of tinnitus, including hearing loss, exposure to loud noise or a viral illness.

There's no cure, but there are a range of strategies to reduce the symptoms and make it less bothersome, including hearing aids, mindfulness therapy , and one newer option – a device approved by the FDA to treat tinnitus using electrical stimulation of the tongue.

The device has helped Victoria Banks, a singer and songwriter in Nashville, Tenn., who developed tinnitus about three years ago.

"The noise in my head felt like a bunch of cicadas," Banks says. "It was terrifying." The buzz made it difficult for her to sing and listen to music. "It can be absolutely debilitating," she says.

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Tinnitus bothers millions of americans. here's how to turn down the noise.

Banks tried taking dietary supplements , but those didn't help. She also stepped up exercise, but that didn't bring relief either. Then she read about a device called Lenire, which was approved by the FDA in March 2023. It includes a plastic mouthpiece with stainless steel electrodes that electrically stimulate the tongue. It is the first device of its kind to be approved for tinnitus.

"This had worked for other people, and I thought I'm willing to try anything at this point," Banks recalls.

She sought out audiologist Brian Fligor, who treats severe cases of tinnitus in the Boston area. Fligor was impressed by the results of a clinical trial that found 84% of participants who tried Lenire experienced a significant reduction in symptoms. He became one of the first providers in the U.S. to use the device with his patients. Fligor also served on an advisory panel assembled by the company who developed it.

"A good candidate for this device is somebody who's had tinnitus for at least three months," Fligor says, emphasizing that people should be evaluated first to make sure there's not an underlying medical issue.

Tinnitus often accompanies hearing loss, but Victoria Banks' hearing was fine and she had no other medical issue, so she was a good candidate.

Banks used the device for an hour each day for 12 weeks. During the hour-long sessions, the electrical stimulation "tickles" the tongue, she says. In addition, the device includes a set of headphones that play a series of tones and ocean-wave sounds.

The device works, in part, by shifting the brain's attention away from the buzz. We're wired to focus on important information coming into our brains, Fligor says. Think of it as a spotlight at a show pointed at the most important thing on the stage. "When you have tinnitus and you're frustrated or angry or scared by it, that spotlight gets really strong and focused on the tinnitus," Fligor says.

"It's the combination of what you're feeling through the nerves in your tongue and what you're hearing through your ears happening in synchrony that causes the spotlight in your brain to not be so stuck on the tinnitus," Fligor explains.

A clinical trial found 84% of people who used the device experienced a significant reduction in symptoms. Brian Fligor hide caption

A clinical trial found 84% of people who used the device experienced a significant reduction in symptoms.

"It unsticks your spotlight" and helps desensitize people to the perceived noise that their tinnitus creates, he says.

Banks says the ringing in her ears did not completely disappear, but now it's barely noticeable on most days.

"It's kind of like if I lived near a waterfall and the waterfall was constantly going," she says. Over time, the waterfall sound fades out of consciousness.

"My brain is now focusing on other things," and the buzz is no longer so distracting. She's back to listening to music, writing music, and performing music." I'm doing all of those things," she says.

When the buzz comes back into focus, Banks says a refresher session with the device helps.

A clinical trial found that 84% of people who tried Lenire , saw significant improvements in their condition. To measure changes, the participants took a questionnaire that asked them to rate how much tinnitus was impacting their sleep, sense of control, feelings of well-being and quality of life. After 12 weeks of using the device, participants improved by an average of 14 points.

"Where this device fits into the big picture, is that it's not a cure-all, but it's quickly become my go-to," for people who do not respond to other ways of managing tinnitus, Fligor says.

One down-side is the cost. Banks paid about $4,000 for the Lenire device, and insurance doesn't cover it. She put the expense on her credit card and paid it off gradually.

Fligor hopes that as the evidence of its effectiveness accumulates, insurers will begin to cover it. Despite the cost, more than 80% of participants in the clinical trial said they would recommend the device to a friend with tinnitus.

But, it's unclear how long the benefits last. Clinical trials have only evaluated Lenire over a 1-year period. "How durable are the effects? We don't really know yet," says audiologist Marc Fagelson, the scientific advisory committee chair of the American Tinnitus Association. He says research is promising but there's still more to learn.

Fagelson says the first step he takes with his patients is an evaluation for hearing loss. Research shows that hearing aids can be an effective treatment for tinnitus among people who have both tinnitus and hearing loss, which is much more common among older adults. An estimated one-third of adults 65 years of age and older who have hearing loss, also have tinnitus.

"We do see a lot of patients, even with very mild loss, who benefit from hearing aids," Fagelson says, but in his experience it's about 50-50 in terms of improving tinnitus. Often, he says people with tinnitus need to explore options beyond hearing aids.

Bruce Freeman , a scientist at the University of Pittsburgh Medical Center, says he's benefitted from both hearing aids and Lenire. He was fitted for the device in Ireland where it was developed, before it was available in the U.S.

Freeman agrees that the ringing never truly disappears, but the device has helped him manage the condition. He describes the sounds that play through the device headphones as very calming and "almost hypnotic" and combined with the tongue vibration, it's helped desensitize him to the ring.

Freeman – who is a research scientist – says he's impressed with the results of research, including a study published in Nature, Scientific Reports that points to significant improvements among clinical trial participants with tinnitus.

Freeman experienced a return of his symptoms when he stopped using the device. "Without it the tinnitus got worse," he says. Then, when he resumed use, it improved.

Freeman believes his long-term exposure to noisy instruments in his research laboratory may have played a role in his condition, and also a neck injury from a bicycle accident that fractured his vertebra. "All of those things converged," he says.

Freeman has developed several habits that help keep the high-pitched ring out of his consciousness and maintain good health. "One thing that does wonders is swimming," he says, pointing to the swooshing sound of water in his ears. "That's a form of mindfulness," he explains.

When it comes to the ring of tinnitus, "it comes and goes," Freeman says. For now, it has subsided into the background, he told me with a sense of relief. "The last two years have been great," he says – a combination of the device, hearing aids and the mindfulness that comes from a swim.

This story was edited by Jane Greenhalgh

• ringing in ears
• hearing loss