Superionic Alkali Diffusion in Amorphous Oxyhalide Solid-State Conductors

One of the main concerns with lithium-ion batteries is the flammability of their organic liquid electrolyte, and replacing it with a solid electrolyte (SEs) is considered the most promising solution for improved safety. In the last years, halide SEs have gained substantial attention due to their high room-temperature ionic conductivity, good electrochemical stability and compatibility with electrode materials. It has been recently shown that amorphous phases of mixed-anion halide SEs (oxyhalides) can exhibit superionic conductivity, in some cases remarkably higher than their crystalline counterparts [1]. Although study of ionic transport in these materials has been attempted, it is still not yet fully established which are the factors responsible for the enhancement of ionic mobility in the amorphous phase. By considering the representative example of LiAlCl 2.5 O 0.75 , using a combination of first–principles calculations and machine-learning molecular dynamics simulations, we systematically study the alkali diffusion in amorphous halides. We find that the superionic conductivity exhibits a remarkable chemical independence upon isovalent substitution - both cationic and anionic - in good agreement with experimental results. By analyzing the local atomic structure, residence time, and distinct particle correlations, we provide fundamental insights about microscopic transport mechanisms occurring in this class of materials. Our findings confirm and support recent experimental observations, and show that amorphous conductors represent a promising structural prototype for materials to be used in solid state batteries as an alternative to conventional Li-ion batteries. [1] Dai, T., Wu, S., Lu, Y. et al., Nat Energy 8 , 1221–1228 (2023).