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.