Origin of Fast Li+-Ion Conductivity in the Compressible Oxyhalide LiNbOCl4

Realizing solid state batteries with superior energy density and long cycle life requires fast-ion conducting solids that feature low density, (electro)chemical compatibility with electrode materials, as well as mechanical elasticity to retain paths for mobile ions across the evolving electrolyte/electrode interfaces during cycling. As the structural prerequisites for fast ionic conductivity typically also enhance glass-forming ability, a promising design strategy is to explore the relatively soft and light oxyhalide glass ceramics. Among these, LiMOCl4 (M = Nb, Ta) synergize competitive room temperature ionic conductivities of 10 mS cm−3 with a high oxidation stability. The Li-disordered tetragonal crystal structure of LiNbOCl4 revealed in this work is consistent with DFT simulations, yields plausible bond valence sums, ion transport pathways and mechanical properties. Ab initio and empirical molecular dynamics simulations reveal besides favorable compressibility the ion transport mechanism and its correlation with the onset of rotational mobility of the (NbOCl4–)∞ polyanion chains therein. These correlations also provide a deeper understanding of the non-Arrhenius temperature dependence of the fast ionic conductivity in these novel inorganic plastic crystal solid electrolytes that are compatible with high voltage cathode materials.