Enhanced Interfacial Conduction in Low-Cost NaAlCl4 Composite Solid Electrolyte for Solid-State Sodium Batteries

All-solid-state sodium batteries offer the advantage of both sustainability and safety. Solid-state electrolytes play a key role, and an oxygen-incorporated NaAlCl₄ composite electrolyte is presented with a high ambient-temperature ionic conductivity of > 0.1 mS cm⁻¹. The electrolyte synthesized with a mechanochemical reaction consists of in situ-formed Al₂O₃ nanoparticles that provide enhanced conduction through an oxychloride phase at the interface. Magic angle spinning nuclear magnetic resonance spectroscopy confirms the formation of Al₂O₃ and the oxychloride phases at the interface and sheds insights into the origin of the enhanced ionic conductivity of the composite electrolyte. Additionally, simply adding Al₂O₃ nanoparticles to NaAlCl₄ before mechanochemical synthesis is investigated, and a relationship between Al₂O₃ surface area and composite electrolyte ionic conductivity is identified. All-solid-state sodium batteries assembled with the composite electrolyte demonstrate a high specific capacity of 124 mA h g⁻¹, clearly outperforming the baseline NaAlCl₄ electrolyte. Furthermore, X-ray photoelectron spectroscopy is utilized to understand the origin of capacity fade and obtain insights into electrolyte decomposition products. This work provides a deeper understanding of methods for boosting the ion transport in a low-cost halide solid electrolyte for practical viability of all-solid-state sodium batteries.