The pursuit of sustainable energy storage solutions has placed aqueous sodium-ion batteries at the forefront of research for grid-scale applications. The abundance of sodium resources, coupled with the intrinsic safety and low cost of water-based electrolytes, presents a compelling case for this technology. However, the journey towards commercialization is paved with challenges, primarily centered on achieving long-term cycle life without compromising on energy density or cost. A critical bottleneck lies in identifying robust electrode materials that can withstand the rigors of reversible sodium (de)insertion in an aqueous environment.
Among the pantheon of potential anode materials, NASICON-structured NaTi2(PO4)3 stands out. Its open three-dimensional framework facilitates rapid Na+ ion diffusion, while its redox activity centered on the Ti4+/Ti3+ couple offers a theoretically favorable capacity and a suitable operating potential versus the standard hydrogen electrode (SHE). Yet, in my investigation, a glaring issue emerged: the material suffers from pronounced structural degradation in conventional, dilute aqueous electrolytes like 1 M Na2SO4. This degradation manifests as irreversible capacity fade, severely limiting the practical viability of aqueous sodium-ion batteries employing this otherwise promising anode. The root cause, as elucidated by prior studies and confirmed through my initial experiments, is the chemical and electrochemical instability of the NaTi2(PO4)3 surface. Parasitic reactions, including partial dissolution of Ti4+ ions and the reduction of water at the low operating potential, lead to the formation of electrochemically inert surface phases (e.g., amorphous TiOx or Ti(SO4)2) and an increase in local pH. These side products impede ion transport, increase polarization, and ultimately cause rapid performance decay.
Addressing this stability challenge became the central focus of my work. While strategies like carbon coating and cation doping have been explored, they often come with trade-offs—modest improvements at the expense of capacity or increased complexity. My approach pivoted towards a more holistic solution: engineering the electrolyte itself. The concept was to design a functional electrolyte system that could actively suppress the degradation pathways. This led to the development and comprehensive evaluation of a novel sulfate-based electrolyte: 2 mol·L-1 Na2SO4 with a 0.3 mol·L-1 MgSO4 additive. This work details the synthesis, electrochemical analysis, and mechanistic investigation that demonstrates how this simple yet effective electrolyte formulation dramatically enhances the cycling stability of carbon-coated NaTi2(PO4)3 (NTP/C), paving a new route for durable aqueous sodium-ion batteries.
Experimental Methodology: Synthesis and Characterization
The synthesis of electrode materials followed established wet-chemical routes to ensure phase purity and controlled morphology. For the NaTi2(PO4)3/C anode, a citric acid-assisted sol-gel method was employed. Stoichiometric amounts of sodium acetate (NaCH3COO), ammonium dihydrogen phosphate (NH4H2PO4), and citric acid (as a chelating agent and carbon source) were dissolved in deionized water. Titanium isopropoxide, dissolved in anhydrous ethanol, was then slowly added under vigorous stirring. The mixture was evaporated at 80°C to form a viscous gel, which was subsequently dried and calcined in an argon atmosphere. The thermal profile involved a pre-calcination step at 450°C followed by a high-temperature treatment at 750°C to crystallize the NASICON phase, with the in-situ decomposed citric acid providing a uniform carbon coating. The carbon content was determined to be approximately 4.3 wt.% via a simple combustion method in air. The cathode material, Na2Ni[Fe(CN)6] (a Prussian blue analogue, PBA), was synthesized via a facile co-precipitation reaction between NiCl2 and Na4Fe(CN)6 solutions, followed by thorough washing and drying.
Material characterization was integral to linking structure with performance. X-ray diffraction (XRD) using a PANalytical X’Pert Powder diffractometer with Cu Kα radiation confirmed the phase purity and crystal structure of the as-synthesized and cycled materials. The diffraction patterns were refined to obtain lattice parameters. Surface chemistry and elemental states were probed using X-ray photoelectron spectroscopy (XPS) on a KRATOS AXIS Supra instrument, with particular attention paid to the Ti 2p and Mg 1s regions for cycled electrodes. Prior to XPS analysis, cycled electrodes were carefully rinsed with deionized water to remove residual salts and dried at ambient conditions.
The electrochemical evaluation was conducted in a three-electrode configuration for half-cell studies and in a 2032-type coin cell for full battery assembly. The working electrode for half-cell tests was fabricated by casting a slurry of NTP/C active material, Super P conductive carbon, and polyvinylidene fluoride (PVDF) binder (in a 7:2:1 mass ratio) onto a titanium current collector. The mass loading of active material was controlled at ~2 mg cm-2. An Ag/AgCl (saturated KCl) electrode served as the reference, and a large-area Na2Ni[Fe(CN)6] electrode acted as the counter. Three electrolyte systems were prepared for comparison: (E1) 1 M Na2SO4 (conventional), (E2) 2 M Na2SO4 (concentrated), and (E3) 2 M Na2SO4 + 0.3 M MgSO4 (functional). Cyclic voltammetry (CV) was performed on a CHI600E workstation within a potential window of -1.0 to 0 V (vs. Ag/AgCl) at a scan rate of 0.1 mV s-1. Galvanostatic charge/discharge (GCD) cycling was carried out on a LAND CT2001A battery tester at a current density of 100 mA g-1 over the same potential window. Full aqueous sodium-ion batteries were assembled with NTP/C as the anode, PBA as the cathode (active mass ratio ~1:1.8), and electrolyte E3. The full cells were cycled between 0.6 and 1.6 V.
Electrochemical Performance Enhancement
The impact of the electrolyte engineering strategy was immediately evident in the cyclic voltammetry responses. The CV curves provide a clear view of the redox kinetics and reversibility of the Ti4+/Ti3+ couple in the NTP/C electrode. In the conventional 1 M Na2SO4 electrolyte (E1), the first cycle showed a distinct cathodic peak near -0.88 V (Na+ insertion/Ti4+ reduction) and an anodic peak near -0.71 V (Na+ extraction/Ti3+ oxidation). However, upon successive cycling, a rapid decay in peak current intensity was observed, accompanied by a significant increase in the peak potential separation (∆Ep). This is quantitatively summarized in Table 1.
| Electrolyte | Anodic Peak Potential (V vs. Ag/AgCl) | Cathodic Peak Potential (V vs. Ag/AgCl) | Peak Separation, ∆Ep (mV) | Relative Peak Current Retention* |
|---|---|---|---|---|
| 1 M Na2SO4 (E1) | -0.712 | -0.876 | 164 | ~65% |
| 2 M Na2SO4 (E2) | -0.734 | -0.819 | 85 | ~85% |
| 2 M Na2SO4 + 0.3 M MgSO4 (E3) | -0.745 | -0.805 | 60 | >95% |
*Estimated from the decrease in cathodic peak current from the 1st to the 3rd cycle.
The data shows that simply increasing the Na2SO4 concentration to 2 M (E2) markedly improved the situation. The ∆Ep narrowed substantially to 85 mV, indicating reduced polarization and better reaction kinetics. The peak current retention also improved. The most striking enhancement was achieved with the functional electrolyte E3. Here, the ∆Ep narrowed further to an exceptionally low 60 mV, and the CV curves were nearly superimposable over multiple cycles, signifying superb electrochemical reversibility and stability for an aqueous sodium-ion battery anode.
The galvanostatic cycling data presented an even more compelling narrative for long-term stability. Figure 1 shows the charge/discharge profiles for the initial cycles, while the long-term cycling performance is summarized in Table 2.
| Electrolyte | Initial Discharge Capacity (mAh g-1) | Reversible Capacity (mAh g-1)* | Initial Coulombic Efficiency (%) | Capacity after 100 cycles (mAh g-1) | Capacity Retention (%) |
|---|---|---|---|---|---|
| 1 M Na2SO4 (E1) | 113.0 | 88.6 | 78.4 | 63.9 | 72.1 |
| 2 M Na2SO4 (E2) | 105.5 | 88.9 | 84.3 | 74.2 | 83.4 |
| 2 M Na2SO4 + 0.3 M MgSO4 (E3) | 102.1 | 93.4 | 91.5 | 90.1 | 96.5 |
*Average capacity from stable cycles 2-5.
The progression is clear. In E1, the NTP/C anode suffered from significant first-cycle irreversible capacity loss (low ICE) and a continuous fade, retaining only 72% of its reversible capacity after 100 cycles. Concentrating the electrolyte (E2) improved the ICE and cycling retention to 83.4%. The functional electrolyte E3, however, delivered a transformative improvement: a high initial Coulombic efficiency of 91.5%, a competitive reversible capacity of 93.4 mAh g-1, and an outstanding capacity retention of 96.5% over 100 cycles. This level of stability is remarkable for an aqueous sodium-ion battery anode operating at such a low potential and represents a major step forward.
The full-cell demonstration cemented the practical relevance of this finding. The NTP/C || Na2Ni[Fe(CN)6] aqueous sodium-ion battery, using electrolyte E3, delivered an average operating voltage of ~1.3 V and a initial capacity of 84.2 mAh g-1 (based on anode mass), yielding a specific energy of approximately 110 Wh kg-1. Most importantly, it exhibited excellent long-term cycling stability, retaining 80% of its capacity after 500 cycles at a higher current density of 500 mA g-1. This performance underscores the viability of the functional sulfate electrolyte in a practical aqueous sodium-ion battery configuration.
Mechanistic Insights into Enhanced Stability
To unravel the mechanism behind the dramatically enhanced performance, a detailed post-cycling analysis of the NTP/C electrodes was conducted using XRD and XPS. The results pointed towards a synergistic effect between the concentrated salt and the Mg2+ additive in stabilizing the electrode-electrolyte interface.
The XRD patterns of the electrodes after 100 cycles provided the first clue. The electrode cycled in E1 (1M Na2SO4) showed the main NTP peaks but also featured a very broad, low-intensity hump around 17° 2θ, indicative of the formation of amorphous titanium-based side products (e.g., Ti(SO4)2 or TiO2·xH2O). The electrode from E2 (2M Na2SO4) showed sharper, distinct extra peaks, suggesting the growth of more crystalline parasitic phases under concentrated conditions. In stark contrast, the XRD pattern for the electrode cycled in the functional electrolyte E3 was virtually identical to that of the pristine NTP/C material. No extraneous peaks were detected, providing direct evidence of exceptional structural integrity retained during cycling in the Mg2+-containing electrolyte.
XPS analysis offered a chemical perspective on the surface state. The Ti 2p spectra for all cycled electrodes showed the characteristic doublet for Ti4+ (Ti 2p3/2 at ~458.5 eV), with no significant shift or appearance of Ti3+ signatures (which would be present if surface Ti was irreversibly reduced), confirming that the bulk redox process was reversible. The key difference emerged in the survey and Mg 1s spectra. A distinct Mg 1s peak at a binding energy of ~1303.5 eV was observed only on the surface of the electrode cycled in E3. This peak is attributed to Mg in a hydroxide/oxide environment, such as Mg(OH)2.
Based on these observations, I propose a coherent mechanism for the action of the functional sulfate electrolyte. The enhanced stability in the aqueous sodium-ion battery stems from two interconnected effects:
1. Effect of High Salt Concentration (2 M Na2SO4): A concentrated electrolyte alters the solvation structure of ions. The number of free water molecules is significantly reduced, as more water molecules are involved in the primary solvation shells of Na+ and SO42- ions. This reduces the overall electrochemical activity of water, effectively widening the electrolyte’s stable potential window. The reduced water activity mitigates the thermodynamics of the hydrogen evolution reaction (HER) at the anode’s low working potential. This can be conceptually related to a shift in the reversible potential for HER, as described by the Nernst equation under non-ideal conditions where water activity (aH2O) is less than 1:
$$E_{HER} = E^{0}_{HER} – \frac{2.303RT}{F}pH – \frac{2.303RT}{2F}\log(a_{H2O})$$
While a quantitative calculation is complex, a decrease in aH2O makes the HER less favorable, suppressing one major source of pH increase and associated material dissolution.
2. Effect of the MgSO4 Additive: Despite the suppressed HER, trace water reduction is inevitable at these potentials, generating hydroxide (OH–) ions locally at the electrode surface. In the conventional electrolyte, these OH– ions accumulate, raising the interfacial pH and directly attacking the NTP structure, leading to Ti dissolution. In the functional electrolyte, the freely available Mg2+ ions act as a “scavenger.” They instantaneously react with the nascent OH– ions to form an insoluble precipitate:
$$\text{Mg}^{2+}_{(aq)} + 2\text{OH}^-_{(surface)} \rightarrow \text{Mg(OH)}_{2(s)}$$
This in-situ precipitated Mg(OH)2 forms a thin, protective passivation layer on the NTP/C particle surface. This layer acts as a physical barrier, preventing direct contact between the active material and the bulk electrolyte, thereby drastically curtailing chemical dissolution. Furthermore, Mg(OH)2 is a poor electronic conductor but can allow selective Na+ transport, maintaining electrochemical function while inhibiting parasitic reactions. This process is self-limiting and stabilizes the interface.
The synergy is powerful: the concentrated base electrolyte suppresses the bulk generation of OH–, while the Mg2+ additive neutralizes any remaining local OH– by forming a protective shield. This dual-action mechanism is responsible for the unprecedented cycling stability observed in the aqueous sodium-ion battery tests.
Quantitative Perspectives and Material Properties
The performance improvement can also be framed in terms of fundamental electrochemical parameters. The reduced polarization (smaller ∆Ep in CV) suggests lower charge-transfer resistance and faster ion diffusion kinetics at the stabilized interface. The apparent chemical diffusion coefficient of Na+ (DNa) within the NTP framework, which can be estimated from galvanostatic intermittent titration technique (GITT) or low-scan-rate CV data, would likely show less degradation over cycling in electrolyte E3 compared to E1. While a full quantitative analysis of DNa is beyond this summary, the relationship is governed by equations derived from Fick’s law. For a semi-infinite diffusion process in a planar electrode, the current response in a potential step experiment relates to D as:
$$i(t) = \frac{nFAD^{1/2}C}{\pi^{1/2}t^{1/2}}$$
where \(i\) is current, \(n\) is electrons transferred, \(F\) is Faraday’s constant, \(A\) is area, \(C\) is concentration, and \(t\) is time. A more stable interface (less clogging by side products) maintains a higher effective D over time.
Furthermore, the role of the carbon coating in the NTP/C composite should not be overlooked. It provides an essential conductive network, enhancing electron transfer. Its contribution to the overall performance, especially rate capability, can be conceptually separated. The total electrode impedance (Z) in a simplified Randles circuit model can be expressed as:
$$Z = R_\Omega + \frac{1}{j\omega C_{dl} + \frac{1}{R_{ct} + Z_W}}$$
where \(R_\Omega\) is the ohmic resistance (from electrolyte and contacts), \(C_{dl}\) is the double-layer capacitance, \(R_{ct}\) is the charge-transfer resistance, and \(Z_W\) is the Warburg impedance related to diffusion. The functional electrolyte primarily acts to minimize the increase in \(R_{ct}\) (due to passivating layers) and \(Z_W\) (due to blocked pores) upon cycling, thereby preserving the kinetic advantages provided by the carbon coating.
Conclusion and Future Outlook
This investigation successfully demonstrates that electrolyte engineering is a potent and straightforward strategy to overcome the critical stability limitations of promising electrode materials for aqueous sodium-ion batteries. The developed functional sulfate electrolyte, comprising a concentrated 2 M Na2SO4 solution with a 0.3 M MgSO4 additive, fundamentally alters the interfacial chemistry at the NaTi2(PO4)3/C anode. By synergistically suppressing water reduction and in-situ forming a protective Mg(OH)2 layer, it effectively halts the dissolution-driven degradation pathway. The result is an aqueous sodium-ion battery anode with exceptional cycling stability—96.5% capacity retention over 100 cycles—and a full cell with promising longevity.
The implications of this work extend beyond this specific material system. The principle of using benign, low-cost multivalent cation additives (like Mg2+, Zn2+, or Al3+) to stabilize reactive interfaces in aqueous batteries is broadly applicable. Future work should focus on several fronts to advance this technology: 1) Exploring the universality of this approach with other anode and cathode materials for aqueous sodium-ion batteries; 2) Optimizing the additive type, concentration, and combination for further performance gains; 3) Investigating the formation mechanism and precise nanostructure of the protective interphase using advanced in-situ/operando techniques; and 4) Integrating this electrolyte with gel polymer or hydrogel matrices to develop leak-proof, high-energy-density quasi-solid-state aqueous sodium-ion batteries. By addressing the interface stability challenge through smart electrolyte design, the path towards low-cost, safe, and durable grid-scale energy storage using aqueous sodium-ion batteries becomes significantly clearer.