In the pursuit of sustainable energy storage solutions, the limitations of fossil fuels and the rising demand for renewable integration have propelled the development of advanced battery technologies. Among these, sodium-ion batteries have emerged as a promising alternative to lithium-ion systems due to the abundance and lower cost of sodium resources. However, the commercialization of sodium-ion batteries faces significant challenges, particularly concerning anode materials. Hard carbon, while offering a reasonable reversible capacity for sodium storage, suffers from low initial Coulombic efficiency and a relatively high operating potential, which curtails the overall energy density of sodium-ion batteries. To address these issues, we present a novel design for a quasi-solid-state sodium-ion battery that incorporates a pre-sodiated solvent-free hard carbon anode and a polyethylene-polyphenylene sulfide based composite solid-state separator. This approach aims to enhance the cycle life and energy density of sodium-ion batteries, making them more viable for large-scale energy storage applications.
Our work focuses on two key innovations: first, the pre-sodiation of hard carbon anodes to mitigate irreversible sodium loss and lower the electrochemical potential; and second, the implementation of a solid-state separator to suppress sodium dendrite growth and ensure uniform sodium deposition. The pre-sodiation process involves a simple thermal infusion of molten sodium into the hard carbon matrix, which not only improves the initial Coulombic efficiency but also addresses the issue of polytetrafluoroethylene binder consumption of sodium in solvent-free electrodes. Meanwhile, the composite separator, derived from polyphenylene sulfide, exhibits high ionic conductivity, a wide electrochemical window, and excellent stability, enabling robust performance even under high areal capacity conditions. Through comprehensive material characterization and electrochemical testing, we demonstrate that this quasi-solid-state sodium-ion battery configuration achieves remarkable cycling stability and energy density, paving the way for next-generation energy storage systems.
The development of sodium-ion batteries has gained momentum as researchers seek to overcome the resource constraints associated with lithium. Sodium shares similar chemical properties with lithium but is far more abundant in the Earth’s crust, potentially reducing the cost and environmental impact of battery production. However, the electrochemical behavior of sodium differs from that of lithium, necessitating tailored materials and designs. Hard carbon anodes have shown promise due to their disordered structure, which facilitates sodium ion intercalation. Yet, the initial cycles often involve substantial irreversible capacity loss due to solid electrolyte interphase formation and pore filling. Pre-sodiation strategies, such as chemical or electrochemical methods, have been explored, but they often involve complex procedures or unstable reagents. Our thermal pre-sodiation method offers a straightforward and effective solution, integrating metallic sodium directly into the electrode architecture.
Furthermore, the use of metallic sodium anodes in sodium-ion batteries can lead to dendrite formation, especially at high current densities or areal capacities, resulting in short circuits and capacity fade. Solid-state electrolytes or separators are considered a key enabler for safe and efficient sodium metal batteries. Polyphenylene sulfide based materials have attracted attention for their ionic conductivity and mechanical strength. In this study, we engineer a composite separator by combining polyphenylene sulfide with a thin polyethylene layer to protect against reduction at the anode side. This quasi-solid-state configuration not only enhances safety but also improves the interfacial stability between the electrode and electrolyte. We systematically evaluate the material properties and electrochemical performance of our sodium-ion battery components, providing insights into the mechanisms underlying their improved behavior.
The fabrication of materials for our sodium-ion battery began with the synthesis of the polyphenylene sulfide based solid-state separator. Polyphenylene sulfide powder, synthesized via a conventional reaction from sodium sulfide and p-dichlorobenzene, was mixed with sodium chloride as a byproduct. To enhance ionic conductivity, tetrachlorobenzoquinone was added as a chelating agent, and the mixture was heated under argon atmosphere at 210°C for 2 hours. This process facilitated the formation of sodium-sulfur bonds, as confirmed by subsequent analyses. The powder was then washed to remove excess sodium chloride and blended with polytetrafluoroethylene at low temperatures to induce fibrillation. The fibrillated mixture was hot-rolled into a thin film approximately 50 μm thick, resulting in the polyphenylene sulfide based solid-state separator. To prevent reduction by sodium metal, a thin polyethylene porous membrane was laminated on the anode side, creating the polyethylene-polyphenylene sulfide based composite solid-state separator.
For the anode, we employed a solvent-free dry electrode technique. Hard carbon derived from starch was mixed with polytetrafluoroethylene binder in a 9:1 mass ratio and fibrillated using a supersonic dry jet air process. The fibrillated composite was then hot-pressed into electrodes approximately 30 μm thick, supported on carbon-coated aluminum foil current collectors. Pre-sodiation was achieved by melting metallic sodium in an inert atmosphere and allowing it to infiltrate the hard carbon electrode pores. The sodium content was controlled by varying the infusion time and temperature, with optimal performance observed at around 31% sodium by mass. The cathode was similarly prepared using a dry process, with Na3V2(PO4)3 as the active material, super P carbon black as conductive additive, and polytetrafluoroethylene as binder, achieving an areal capacity of approximately 1 mAh cm-2.
Material characterization involved multiple techniques to elucidate the structure and composition of the components. X-ray diffraction was used to analyze the crystallinity of polyphenylene sulfide materials, showing that the linear structure remained intact after processing. Scanning electron microscopy and focused ion beam scanning electron microscopy revealed the morphology of the separator and electrodes, indicating dense packing with minimal porosity. Energy-dispersive X-ray spectroscopy provided elemental mapping, confirming the uniform distribution of sulfur and chlorine in the separator, with sodium and oxygen enriched on the surface. X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance spectroscopy were employed to investigate chemical bonding, particularly the formation of sodium-sulfur bonds and the presence of ether linkages from tetrachlorobenzoquinone incorporation. These analyses collectively confirmed the successful synthesis of the composite separator with enhanced ionic transport properties.
Electrochemical evaluations were conducted to assess the performance of the sodium-ion battery components. The ionic conductivity of the polyphenylene sulfide based solid-state separator was measured via electrochemical impedance spectroscopy over a temperature range from -30°C to 70°C. The conductivity followed an Arrhenius relationship, with activation energies of 0.54 eV in the high-temperature region (30–70°C) and 0.62 eV in the low-temperature region (-20–30°C). The sodium ion conductivity, denoted as σNa+, was calculated using the formula:
$$\sigma_{\text{Na}^+} = \frac{l}{R S}$$
where l is the thickness of the separator, R is the resistance obtained from impedance spectra, and S is the area of the stainless steel blocking electrode. The results indicated that the polyphenylene sulfide based separator maintained reasonable conductivity across a wide temperature range, making it suitable for diverse operating conditions. Additionally, the sodium ion transference number, tNa+, was determined using the Bruce-Vincent-Evans method based on polarization experiments with symmetric sodium metal cells. The transference number was calculated as:
$$t_{\text{Na}^+} = \frac{I_{\text{SS}} (\Delta V – I_0 R_0)}{I_0 (\Delta V – I_{\text{SS}} R_{\text{SS}})}$$
where I0 and ISS are the initial and steady-state currents, ΔV is the applied polarization voltage (10 mV), and R0 and RSS are the initial and steady-state resistances, respectively. The polyphenylene sulfide based separator exhibited a high transference number of approximately 0.82, suggesting that sodium ions are the primary charge carriers, which minimizes concentration polarization and promotes uniform deposition.
Linear sweep voltammetry was performed to evaluate the electrochemical stability window of the separator. Using stainless steel as the working electrode and sodium metal as the counter electrode, the potential was scanned from the open-circuit voltage to 5 V at a rate of 1 mV s-1. The polyphenylene sulfide based separator demonstrated stability up to 4.5 V, comparable to commercial polyethylene separators, indicating its suitability for high-voltage sodium-ion battery applications. To assess dendrite suppression capabilities, symmetric cells were assembled with sodium metal electrodes and cycled at a current density of 0.5 mA cm-2. The polyethylene-polyphenylene sulfide based composite solid-state separator enabled stable cycling for over 300 hours with minimal voltage hysteresis, whereas cells with commercial ceramic-coated polyethylene separators exhibited voltage fluctuations and early short circuits. Post-cycling analysis via scanning electron microscopy confirmed that the composite separator surface remained smooth without dendritic growth, while the polyethylene separator showed significant sodium dendrite penetration.
The performance of the pre-sodiated hard carbon anode was evaluated in half-cell and full-cell configurations. Cyclic voltammetry of the pre-sodiated electrode in a sodium metal half-cell showed a distinct oxidation peak around 0.6 V versus Na/Na+ during the first anodic scan, corresponding to the extraction of pre-inserted sodium. Subsequent cycles displayed high reversibility with overlapping curves, indicating improved electrochemical stability. In contrast, non-pre-sodiated hard carbon electrodes exhibited a large reduction peak near 0.7 V in the first cycle due to solid electrolyte interphase formation, along with significant capacity fading. Galvanostatic charge-discharge tests revealed that the pre-sodiated anode operated at a lower voltage platform, with a polarization gap of less than 0.15 V at 10 mA g-1, compared to over 2.5 V for the untreated anode. The initial Coulombic efficiency of the pre-sodiated hard carbon increased from 73% to 93%, as summarized in Table 1, highlighting the effectiveness of pre-sodiation in reducing irreversible sodium loss.
| Anode Type | Initial Coulombic Efficiency | Charge-Discharge Polarization (V) | Cycle Stability |
|---|---|---|---|
| Non-pre-sodiated Hard Carbon | 73% | >2.5 | Poor |
| Pre-sodiated Hard Carbon (HC/Na) | 93% | <0.15 | Excellent |
Full sodium-ion batteries were assembled with Na3V2(PO4)3 cathodes and pre-sodiated hard carbon anodes, using the polyethylene-polyphenylene sulfide based composite solid-state separator and a minimal amount of liquid electrolyte (0.6 M NaBF4 in diglyme with 2% fluoroethylene carbonate additive). The electrochemical performance was tested at various current rates from 0.1 C to 2 C (where 1 C = 120 mA g-1 based on the cathode active mass). At 0.1 C, the battery delivered an initial discharge capacity of 119 mAh g-1 with a Coulombic efficiency of 91%. Even at 0.5 C, the capacity remained at 116 mAh g-1, demonstrating excellent rate capability. Long-term cycling at 0.5 C showed a capacity retention of over 70% after 300 cycles, with stable charge-discharge plateaus. The energy density of the sodium-ion battery, calculated based on the cathode active material, reached up to 360 Wh kg-1, as derived from the following relationship:
$$E = \frac{C \times V}{3.6}$$
where E is the energy density in Wh kg-1, C is the specific capacity in mAh g-1, and V is the average discharge voltage in volts. For our sodium-ion battery, with C ≈ 116 mAh g-1 and V ≈ 3.3 V, the energy density approximates 360 Wh kg-1. This places our sodium-ion battery system between lithium-ion and nickel-metal hydride batteries in terms of energy density, and about two to three times higher than lead-acid batteries, making it competitive for grid-scale storage.
To understand the degradation mechanisms, we analyzed the evolution of the solid electrolyte interphase on the pre-sodiated hard carbon anode over multiple cycles. X-ray photoelectron spectroscopy spectra collected after 10, 30, 50, 100, and 300 cycles revealed changes in surface chemistry. Initially, the anode surface showed strong signals for C–F bonds from polytetrafluoroethylene and Na–F bonds from sodium fluoride, indicating that pre-sodiation led to the formation of an artificial solid electrolyte interphase. As cycling progressed, the intensity of C–O and C=O peaks increased, suggesting continuous decomposition of the liquid electrolyte. Concurrently, the polytetrafluoroethylene-related peaks diminished, and sodium fluoride content grew, implying that the solid electrolyte interphase thickened over time. This gradual thickening is likely responsible for the capacity fade and voltage polarization observed in long-term cycling, emphasizing the need for more stable electrolyte formulations in future sodium-ion battery designs.
Low-temperature performance is crucial for practical applications of sodium-ion batteries, especially in outdoor energy storage. We tested our quasi-solid-state sodium-ion battery at -20°C and found that it retained approximately 45% of its room-temperature capacity, delivering 47.5 mAh g-1 at a 0.1 C rate. This resilience can be attributed to the moderate temperature dependence of ionic conductivity in the polyphenylene sulfide based separator, as evidenced by the Arrhenius plot. The activation energy for ion transport remained below 0.65 eV across the tested range, ensuring operable conductivity even in cold environments. Such performance underscores the versatility of our sodium-ion battery system for use in diverse climatic conditions.
In summary, we have developed a high-performance quasi-solid-state sodium-ion battery that combines a pre-sodiated hard carbon anode with a polyethylene-polyphenylene sulfide based composite solid-state separator. The pre-sodiation process effectively boosts the initial Coulombic efficiency and lowers the operating potential of the hard carbon anode, while the composite separator suppresses sodium dendrite growth and enables uniform sodium deposition. Electrochemical tests demonstrate excellent cycling stability, high energy density, and good low-temperature performance. However, the gradual thickening of the solid electrolyte interphase remains a challenge that requires further optimization of electrolyte components. Future work will focus on enhancing the interfacial stability and exploring alternative binder materials to prolong cycle life. Our findings contribute to the advancement of sodium-ion battery technology, offering a pathway toward cost-effective and reliable energy storage solutions for a sustainable future.
The development of sodium-ion batteries is accelerating, and innovations like pre-sodiation and solid-state separators are key to overcoming existing limitations. By integrating these approaches, we have shown that sodium-ion batteries can achieve performance metrics that rival traditional lithium-ion systems in certain applications. Continued research into material synthesis, electrolyte engineering, and cell design will further improve the viability of sodium-ion batteries for large-scale deployment. As global energy demands grow, such technologies will play a critical role in enabling the transition to renewable energy sources, reducing reliance on fossil fuels, and mitigating climate change impacts.
Throughout this study, the term “sodium-ion battery” has been emphasized to highlight the focus of our research. The repeated mention of sodium-ion battery underscores its importance in the context of energy storage innovation. We believe that the strategies presented here—pre-sodiation of hard carbon anodes and the use of polyphenylene sulfide based solid-state separators—can be extended to other battery chemistries, potentially inspiring new developments in the field. The quasi-solid-state configuration, in particular, offers a balance between the high ionic conductivity of liquid electrolytes and the safety benefits of solid-state systems, making it an attractive option for next-generation sodium-ion batteries.
In conclusion, our work demonstrates a significant step forward in sodium-ion battery technology. The combination of a pre-sodiated hard carbon anode and a polyethylene-polyphenylene sulfide based composite solid-state separator results in a sodium-ion battery with enhanced cycle life, high energy density, and improved safety. These advancements address critical challenges in sodium-ion battery development and pave the way for their commercialization in energy storage markets. As research progresses, we anticipate further refinements that will unlock the full potential of sodium-ion batteries, contributing to a more sustainable and resilient energy infrastructure worldwide.