Progress and Challenges in Li <i>M</i> OCl ₄ and Na <i>M</i> OCl ₄ ( <i>M</i> = Nb, Ta) Oxyhalide Solid Electrolytes for Solid‐State Batteries

Abstract The discovery of the Li M OCl 4 ( M = Nb, Ta) oxyhalides is exciting for the field of solid‐state batteries as they offer similar benefits to other halide solid electrolytes in terms of electrochemical stability, but with high ionic conductivities exceeding 10 mS·cm −1 . Analogous materials have also been synthesised with Na + as the mobile cation making them attractive for Na‐ion solid‐state battery applications as well. However, challenges remain, for example, their poor reduction stabilities necessitate additional separator layers when used with metal anodes. In this perspective, an overview is provided of the known compositions, their structures, stabilities, syntheses, and their suggested conduction mechanisms, with an outlook into what developments are needed to see their adoption into solid‐state batteries. A comparison will be made between the Li M OCl 4 and Na M OCl 4 materials, highlighting the experimental and theoretical challenges when working with this class of materials. While these oxyhalides are promising for use in solid‐state batteries, research in this class of materials is still in its infancy, and further research into understanding their structure, processability in air and with solvents, and their potential usage as cathode coatings needs to be explored in greater depth.