As a researcher focused on next-generation energy storage systems, I have been investigating the potential of aqueous sodium-ion battery technology for large-scale applications. The abundance of sodium resources and the inherent safety of water-based electrolytes make this system highly attractive. However, the narrow electrochemical stability window of water, typically limited to 1.23 V due to hydrogen and oxygen evolution reactions, poses a significant challenge for achieving high energy density in aqueous sodium-ion battery designs. In this work, I explore a strategy to expand this window using a high-concentration “water-in-salt” electrolyte composed of low-cost acetate salts, specifically ammonium acetate and sodium acetate. This approach aims to enhance the viability of aqueous sodium-ion battery systems for cost-effective and safe energy storage.
The development of efficient energy storage solutions is critical for integrating renewable energy sources into the grid. Among various options, the sodium-ion battery has emerged as a promising candidate due to the natural abundance of sodium, which contrasts with the limited and geographically concentrated lithium resources. Traditional sodium-ion battery systems often employ organic electrolytes, which raise concerns about flammability, toxicity, and sensitivity to moisture. In contrast, aqueous electrolytes offer improved safety, lower cost, and environmental benignity. However, the energy density of an aqueous sodium-ion battery is constrained by the narrow voltage window where water remains stable. Overcoming this limitation is essential for advancing aqueous sodium-ion battery technology towards practical deployment.
My research builds upon the “water-in-salt” concept, where high salt concentrations reduce water activity by forming solvation structures that inhibit electrolysis. Previous studies have used expensive salts with fluorinated anions, but here I focus on acetate-based salts, which are cost-effective and environmentally friendly. I formulated a mixed aqueous electrolyte containing 25 mol·L−1 ammonium acetate (CH3COONH4) and 5 mol·L−1 sodium acetate (CH3COONa). This combination leverages the high solubility of ammonium acetate to achieve a high total salt concentration, while sodium acetate provides sodium ions for battery operation. The goal is to achieve a wide electrochemical stability window while maintaining high ionic conductivity, thereby enabling a high-performance aqueous sodium-ion battery.
To evaluate this electrolyte, I paired it with compatible electrode materials. For the positive electrode, I synthesized a composite of layered manganese dioxide and multi-walled carbon nanotubes (MnO2/CNTs), which exhibits capacitive behavior and good stability in aqueous media. For the negative electrode, I prepared carbon-coated NaTi2(PO4)3 (NTP/C) with a NASICON structure, known for its reversible sodium ion insertion/extraction. The full cell assembly and comprehensive electrochemical characterization were conducted to assess the performance of this aqueous sodium-ion battery system. In this article, I present detailed findings on the electrolyte properties, material characteristics, and battery performance, supported by tables and mathematical formulations to summarize key data.
Electrolyte Formulation and Characterization
The preparation of the high-concentration acetate electrolyte involved dissolving appropriate amounts of CH3COONH4 and CH3COONa in deionized water to achieve final concentrations of 25 mol·L−1 and 5 mol·L−1, respectively. For comparison, a single-salt solution of 25 mol·L−1 CH3COONH4 was also prepared. The ionic conductivity of these electrolytes was measured using electrochemical impedance spectroscopy with a two-electrode cell. The resistance Rs was obtained from fitting the impedance spectra, and the conductivity κ was calculated using the formula:
$$ \rho = R_s \frac{A}{L} $$
$$ \kappa = \frac{1}{\rho} $$
where ρ is the resistivity, A is the electrode area, and L is the distance between electrodes. The results for different electrolyte compositions are summarized in Table 1.
| Electrolyte Composition | Total Concentration (mol·L−1) | Ionic Conductivity (mS·cm−1) | pH |
|---|---|---|---|
| 25 M CH3COONH4 + 5 M CH3COONa | 30 | 28.2 | ~7 (neutral) |
| 25 M CH3COONH4 | 25 | 24.5 | ~7 (neutral) |
| Traditional Organic Electrolyte (reference) | ~1 | 9.0 | N/A |
The mixed acetate electrolyte exhibited an ionic conductivity of 28.2 mS·cm−1, which is over three times higher than that of typical organic electrolytes used in sodium-ion battery systems. This high conductivity is advantageous for rapid ion transport, potentially enhancing the rate capability of the aqueous sodium-ion battery. The neutral pH of the acetate solutions also broadens the choice of current collectors and electrode materials, unlike alkaline electrolytes that may cause corrosion.
To understand the molecular interactions in the electrolyte, I performed Raman spectroscopy analysis. The spectra in the O-H stretching region (3000–3500 cm−1) showed a significant reduction in peak intensity with increasing salt concentration. This indicates that water molecules are strongly coordinated with cations (NH4+ and Na+), forming solvation shells that disrupt the hydrogen-bonding network of bulk water. The decrease in free water activity is critical for suppressing water decomposition, thereby expanding the electrochemical stability window. The effect can be described by a simplified model where the water activity aw is related to the salt concentration C:
$$ a_w = \gamma_w x_w $$
where γw is the activity coefficient and xw is the mole fraction of water. At high salt concentrations, γw decreases due to ion-water interactions, leading to a lower aw and higher overpotentials for water electrolysis.
Electrode Materials Synthesis and Structural Analysis
For the positive electrode, I synthesized MnO2/CNTs via a redox reaction between potassium permanganate and carbon nanotubes in an acidic medium. The X-ray diffraction (XRD) pattern confirmed the formation of δ-MnO2 with a layered structure, as indicated by peaks at 2θ = 12.5°, 25.2°, 37.3°, and 64.5°. The incorporation of CNTs enhances electronic conductivity, which is crucial for high-rate performance in a sodium-ion battery. Thermogravimetric analysis revealed that the composite contained approximately 5.7% carbon, contributing to its structural stability.
The negative electrode material, NaTi2(PO4)3/C, was synthesized via a hydrothermal method followed by carbonization. XRD patterns matched the NASICON structure (JCPDS No. 85-2265), with no impurity phases. The carbon coating, accounting for about 3.3% by weight, improves electronic conductivity and mitigates volume changes during sodium ion insertion/extraction. Scanning electron microscopy images showed that the material consists of clustered particles with a uniform size distribution around 5 μm.
The structural properties of these materials are vital for their performance in an aqueous sodium-ion battery. The layered MnO2 allows for capacitive charge storage, while the NASICON framework of NTP provides stable pathways for sodium ion diffusion. The combination of these materials with the high-concentration acetate electrolyte aims to achieve a balanced system for efficient energy storage.
Electrochemical Stability Window of the Electrolyte
The electrochemical stability window was evaluated using linear sweep voltammetry with carbon-coated aluminum foil as the working electrode. The mixed acetate electrolyte (25 M CH3COONH4 + 5 M CH3COONa) exhibited an anodic limit of 2.1 V vs. Ag/AgCl and a cathodic limit of -1.8 V vs. Ag/AgCl, resulting in a wide window of 3.9 V. In comparison, the single-salt 25 M CH3COONH4 electrolyte showed a window of 3.5 V. The expansion beyond the thermodynamic water splitting potential (1.23 V) is attributed to the reduced water activity in the “water-in-salt” environment. The overpotentials for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can be expressed using the Tafel equation:
$$ \eta = a + b \log(i) $$
where η is the overpotential, i is the current density, and a and b are constants. In high-concentration electrolytes, the increased viscosity and altered interfacial structure lead to higher a values, effectively shifting the reaction potentials.
This broad window is essential for operating an aqueous sodium-ion battery at higher voltages, thereby increasing energy density. The stability window data for different electrolytes are summarized in Table 2.
| Electrolyte | Anodic Limit (V vs. Ag/AgCl) | Cathodic Limit (V vs. Ag/AgCl) | Total Window (V) |
|---|---|---|---|
| 25 M CH3COONH4 + 5 M CH3COONa | 2.1 | -1.8 | 3.9 |
| 25 M CH3COONH4 | 1.9 | -1.6 | 3.5 |
| Dilute Aqueous Solution (reference) | 0.6 | -0.6 | 1.2 |
Electrochemical Performance of Electrode Materials
I conducted cyclic voltammetry and galvanostatic charge-discharge tests on individual electrodes in a three-electrode cell. For the MnO2/CNTs positive electrode, the cyclic voltammograms showed rectangular shapes without distinct redox peaks, indicating dominant capacitive behavior. This is beneficial for fast charge-discharge cycles in a sodium-ion battery. The galvanostatic tests at 0.1 A·g−1 yielded an initial discharge capacity of 58.8 mAh·g−1, with a capacity retention of 67% after 50 cycles.
For the NTP/C negative electrode, cyclic voltammetry displayed a pair of reversible peaks at -0.71 V and -0.80 V vs. Ag/AgCl, corresponding to the Ti3+/Ti4+ redox couple. The galvanostatic charge-discharge profiles showed flat plateaus with low polarization, reflecting efficient sodium ion transport. At a current rate of 0.2C, the material delivered a discharge capacity of 120 mAh·g−1 and maintained nearly 100% capacity over 50 cycles. The excellent reversibility aligns with the stable NASICON structure, making it suitable for long-life aqueous sodium-ion battery applications.
The performance metrics of these electrodes are summarized in Table 3, highlighting their compatibility with the acetate electrolyte.
| Electrode Material | Current Density | Initial Discharge Capacity (mAh·g−1) | Capacity Retention after 50 Cycles | Primary Charge Storage Mechanism |
|---|---|---|---|---|
| MnO2/CNTs (Positive) | 0.1 A·g−1 | 58.8 | 67% | Capacitive |
| NTP/C (Negative) | 0.2C | 120 | ~100% | Diffusion-controlled insertion |
Full Cell Assembly and Performance
I assembled a full aqueous sodium-ion battery using MnO2/CNTs as the positive electrode, NTP/C as the negative electrode, and the mixed acetate electrolyte (25 M CH3COONH4 + 5 M CH3COONa) as the electrolyte. The mass loading was optimized to balance capacity between electrodes. Cyclic voltammetry of the full cell showed symmetric redox peaks, indicating good reversibility. The galvanostatic charge-discharge tests were performed between 0.4 V and 1.8 V.
At a current density of 0.1 A·g−1 (based on positive electrode mass), the full cell delivered an average discharge voltage of 1.3 V and a capacity of 74.1 mAh·g−1. The energy density can be estimated as:
$$ E = \frac{C \times V}{3.6} $$
where E is in Wh·kg−1, C is capacity in mAh·g−1, and V is average voltage in V. For this cell, E ≈ 26.8 Wh·kg−1, which is competitive with other aqueous sodium-ion battery systems. The rate capability was evaluated at various current densities, as shown in Table 4.
| Current Density (A·g−1) | Discharge Capacity (mAh·g−1) | Capacity Retention Relative to 0.05 A·g−1 |
|---|---|---|
| 0.05 | 71.4 | 100% |
| 0.1 | 73.0 | 102% (slight activation) |
| 0.2 | 70.8 | 99% |
| 1.0 | 52.0 | 73% |
The high ionic conductivity of the electrolyte contributed to the excellent rate performance, with 73% capacity retention even at 1 A·g−1. Cycling stability was assessed over 500 cycles at 0.1 A·g−1. The cell retained 51.3 mAh·g−1 after 500 cycles, corresponding to a capacity decay rate of 0.062% per cycle. The coulombic efficiency remained around 99.8%, demonstrating high reversibility. This long-term stability is crucial for the practical application of aqueous sodium-ion battery technology.
To investigate degradation mechanisms, I performed electrochemical impedance spectroscopy on the full cell before and after cycling. The impedance spectra were fitted to an equivalent circuit consisting of series resistance (Rs) and charge transfer resistance (Rct). The fitted values are listed in Table 5.
| Cycle Number | Rs (Ω) | Rct (Ω) |
|---|---|---|
| Fresh cell | 4.12 | 295.8 |
| 1 | 5.57 | 360.5 |
| 5 | 3.60 | 425.5 |
| 10 | 5.53 | 451.3 |
| 20 | 3.69 | 627.5 |
| 50 | 2.67 | 709.5 |
The increase in Rct over cycles suggests growing interfacial resistance, possibly due to structural changes in the electrodes. Post-mortem analysis via scanning electron microscopy and X-ray diffraction revealed cracking and partial amorphization of the MnO2/CNTs positive electrode, while the NTP/C negative electrode remained intact. This indicates that positive electrode degradation is a primary factor limiting cycle life in this aqueous sodium-ion battery system.
Theoretical Insights and Future Perspectives
The success of this high-concentration acetate electrolyte can be explained by the unique solvation dynamics in “water-in-salt” systems. The formation of cation-water clusters reduces the number of free water molecules, thereby increasing the energy required for water splitting. This effect can be quantified using molecular dynamics simulations, but a simplified analytical model relates the stabilization energy ΔG to ion concentration:
$$ \Delta G = -RT \ln(a_w) $$
where R is the gas constant, T is temperature, and aw is water activity. As concentration increases, aw decreases, leading to a more negative ΔG for ion-water interactions and a positive shift in water stability.
For future work, optimizing the electrode materials to better withstand the aqueous environment could further improve the performance of aqueous sodium-ion battery systems. Exploring other low-cost salts and additives may also expand the electrochemical window while maintaining high conductivity. Additionally, scaling up the cell design and testing under realistic conditions will be essential for transitioning this technology from lab to market.
Conclusion
In this study, I developed a high-concentration acetate-based electrolyte for aqueous sodium-ion battery applications. The mixture of 25 mol·L−1 ammonium acetate and 5 mol·L−1 sodium acetate achieved an electrochemical stability window of 3.9 V, surpassing the limits of conventional aqueous electrolytes. This electrolyte exhibited high ionic conductivity (28.2 mS·cm−1) and neutral pH, enabling compatibility with a wide range of materials. When paired with MnO2/CNTs positive electrode and NTP/C negative electrode, the full aqueous sodium-ion battery delivered an average voltage of 1.3 V, a capacity of 74.1 mAh·g−1, and stable cycling over 500 cycles. These results demonstrate the potential of acetate “water-in-salt” electrolytes to enhance the energy density and longevity of aqueous sodium-ion battery systems, offering a cost-effective and safe pathway for large-scale energy storage. Further research into electrode stability and electrolyte formulations will continue to advance the field of aqueous sodium-ion battery technology towards commercial viability.