Solid-state sodium-ion batteries (SSSBs) are attracting significant research interest as a safer and potentially more energy-dense alternative to conventional lithium-ion batteries. The replacement of flammable liquid electrolytes with solid electrolytes (SEs) offers enhanced safety and enables the use of higher voltage cathodes, metal anodes, and advanced stacking architectures to improve energy density. The abundance of sodium also makes sodium-ion batteries a more cost-effective option, especially for large-scale grid storage applications. However, the development of SSSBs faces challenges in finding suitable SEs that meet several stringent requirements: high ionic conductivity, low electronic conductivity, and excellent electrochemical, chemical, and mechanical compatibility with the electrodes. While sulfide-based SEs have shown promising liquid-like ionic conductivity, their poor interfacial stability with common electrodes hinders their practical application. Recent studies have explored halide SEs in lithium-ion batteries, demonstrating their electrochemical and chemical stability and compatibility with high-voltage cathodes. This has motivated the investigation of halide-based SEs for sodium-ion batteries as well, aiming to overcome the limitations of sulfide-based electrolytes and achieve improved performance in SSSBs. This paper focuses on addressing these challenges by developing a novel halide-based solid electrolyte for SSSBs and demonstrating its effectiveness in enhancing battery performance and stability.

Previous research on solid-state electrolytes (SSEs) for sodium-ion batteries has largely focused on sulfide-based materials, which exhibit high ionic conductivity. However, these materials suffer from poor electrochemical stability at high voltages, limiting their compatibility with high-energy-density cathodes. In contrast, halide-based SSEs, such as Li3YCl6 and Li3YBr6, have shown promise in lithium-ion batteries due to their electrochemical stability and reasonable ionic conductivity. These studies highlight the potential of halide-based materials in SSE applications. However, their sodium counterparts have received less attention, with limited reports indicating low room-temperature ionic conductivities. The limited exploration of halide SEs for sodium-ion batteries presents an opportunity to explore new compositions and improve the overall performance and cycle life of SSSBs. This research builds upon these earlier findings, leveraging the potential of halide-based materials while aiming to improve their conductivity and stability for sodium-ion battery applications.

This research employed a combined computational and experimental approach. Density functional theory (DFT) calculations were used to predict the effect of aliovalent doping of Na3YCl6 with various ions (Ti4+, Zr4+, Hf4+, and Ta5+) on its ionic conductivity, electrochemical stability window, and chemical stability. The calculations assessed the formation energies of dopants, enthalpies of mixing, and electrochemical stability using the grand potential phase diagram approach. Ab initio molecular dynamics (AIMD) simulations were performed to study Na+ diffusivity at elevated temperatures. Machine learning interatomic potentials (ML-IAPs), based on the moment tensor potential formalism, were developed using snapshots from AIMD trajectories and DFT calculations for more accurate prediction of diffusivities at lower temperatures. The ML-IAP enabled simulations with larger supercells and longer timescales, allowing for a more comprehensive understanding of the Na+ diffusion mechanism at various temperatures. Experimental synthesis of Na3-xY1-xZrxCl6 (NYZCx) compounds was carried out using stoichiometric amounts of NaCl, YCl3, and ZrCl4. X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), and DC polarization measurements were used to characterize the crystal structure, ionic conductivity, and electronic conductivity of the synthesized NYZCx compounds. Solid-state nuclear magnetic resonance (NMR) spectroscopy was employed to study local Na environments and structural disorder. X-ray photoelectron spectroscopy (XPS) analyzed the chemical states of elements in the electrode materials before and after cycling. Solid-state batteries were assembled using NaCrO2 as the cathode, Na3PS4 as the electrolyte, and Na-Sn as the anode, with NYZC0.75 incorporated into the cathode composite. Electrochemical cycling tests were performed at various temperatures and rates to evaluate the battery performance and long-term stability. Temperature-dependent XRD was used to assess the chemical stability of the composite cathode materials. Scanning electron microscopy (SEM) images provided insights into the morphology of the synthesized materials.

DFT calculations predicted that aliovalent doping of Na3YCl6 with Zr4+ would significantly enhance Na+ conductivity while maintaining a wide electrochemical window and good chemical stability. AIMD simulations and ML-IAP MD simulations confirmed the increased Na+ diffusivity with Zr doping. Experimental measurements showed that Na3-xY1-xZrxCl6 (NYZCx) with x = 0.75 (NYZC0.75) exhibited the highest ionic conductivity of 6.6 × 10-5 S cm-1 at room temperature, significantly higher than Na3YCl6. The electrochemical stability window of NYZC0.75 was determined to be up to 3.8 V vs. Na/Na+, compatible with the NaCrO2 cathode. Solid-state sodium-ion batteries (SSSBs) using a NaCrO2 + NYZC0.75 composite cathode, Na3PS4 electrolyte, and Na-Sn anode showed an exceptionally high first-cycle Coulombic efficiency (97.6%) at room temperature. Cycling tests at 40 °C and 1C rate demonstrated excellent long-term stability with over 1000 cycles and 89.3% capacity retention. XPS analysis revealed that NYZC0.75 remained chemically stable after cycling, while Na3PS4 showed signs of oxidation. The superior performance of the SSSB with NYZC0.75 is attributed to the high ionic conductivity, wide electrochemical window, and chemical stability of NYZC0.75, which prevents oxidation of the Na3PS4 electrolyte by the NaCrO2 cathode.

The findings demonstrate the effectiveness of aliovalent substitution in halide-based electrolytes for improving the performance and stability of SSSBs. The combination of high ionic conductivity, wide electrochemical stability window, and chemical compatibility of NYZC0.75 with oxide cathodes resulted in a significant improvement in the battery's first-cycle Coulombic efficiency and long-term cycling stability. The observed high capacity retention at elevated temperatures and high rates highlights the potential of halide-based electrolytes for high-performance SSSB applications. The superior performance of NYZC0.75 compared to Na3PS4 underscores the importance of electrolyte selection for achieving stable and long-lasting SSSBs. These results provide valuable insights into the design principles for high-performance SSEs and pave the way for further exploration of halide-based materials in energy storage applications.

This work successfully demonstrated a novel halide-based solid electrolyte, Na2.25Y0.25Zr0.75Cl6 (NYZC0.75), for high-voltage, long-cycle-life solid-state sodium-ion batteries. The exceptional electrochemical performance, including a high first-cycle Coulombic efficiency and extended cycle life exceeding 1000 cycles with significant capacity retention, highlights the promise of NYZC0.75 for next-generation energy storage. Future work could focus on optimizing the cathode composite to increase the active material loading, exploring other halide compositions for further improvements in ionic conductivity and stability, and investigating alternative anode materials to enhance overall battery performance.

While the study achieved remarkable results in terms of cycle life and stability, some limitations exist. The amount of active material (NaCrO2) in the cathode composite (39%) is currently relatively low and needs improvement for practical applications. The relatively thick electrolyte layer (800 µm) could also limit the rate capability at lower temperatures. Further optimization of the cell design and fabrication techniques is needed to enhance the rate capability and energy density.