HCl‐Free Mechanochemical Synthesis of Lithium Niobium Oxychlorides

Halide‐based oxychloride electrolytes, including LiNbOCl ₄ , are promising candidates for all‐solid‐state batteries (ASSBs) due to their high ionic conductivity and compatibility with oxide cathodes. Common synthesis using LiOH and NbCl ₅ produces corrosive HCl gas, raising environmental and safety concerns. Here, we report an alternative route using mixtures of oxides and chlorides that eliminates HCl byproduct. LiNbOCl ₄ synthesized via this method forms a crystalline phase without postannealing and exhibits an ionic conductivity of 11.8 mS cm ⁻¹ at 25 °C. The soft lattice and flexible [NbOCl ₄ ] ⁻ chains facilitate rapid Li ⁺ diffusion with low activation energy, enhancing ionic transport. Electrochemical tests demonstrate that LiNbOCl ₄ functions effectively as a catholyte with high‐nickel cathodes (LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) in a dual‐electrolyte architecture, underscoring its practical applicability. This work establishes a safe and environmentally friendly, HCl‐free mechanochemical synthesis pathway and highlights the critical role of precursor selection in achieving high‐conductivity oxyhalide solid electrolytes without postannealing for next‐generation ASSBs.