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.