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

Abstract All‐solid‐state sodium batteries offer the advantage of both sustainability and safety. Solid‐state electrolytes play a key role, and an oxygen‐incorporated NaAlCl 4 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 Al 2 O 3 nanoparticles that provide enhanced conduction through an oxychloride phase at the interface. Magic angle spinning nuclear magnetic resonance spectroscopy confirms the formation of Al 2 O 3 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 Al 2 O 3 nanoparticles to NaAlCl 4 before mechanochemical synthesis is investigated, and a relationship between Al 2 O 3 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 NaAlCl 4 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.