Elucidating the diffusion pathway of lithium ions in superionic halide solid electrolytes Li2+Hf1−In Cl6 for all-solid-state lithium-metal based batteries

All-solid-state batteries (ASSBs) with inorganic solid-state-electrolytes (SSEs) have been regarded as the promising candidate for next-generation energy storage due to their high energy density and outstanding safety performance. However, the representative oxide and sulfide electrolytes suffer from low ionic conductivity and poor (electro)chemical stability, respectively. Herein, we report a series of new halide superionic conductors Li2+xHf1−xInxCl6 with high ionic conductivity up to 1.05 mS cm−1 at 30 °C that are simultaneously stable to high voltage. By means of the characterization techniques and bond-valence site energy (BVSE) calculation, insights into the effect of the phase transformation and underlying ionic transport mechanism by In substitution for Hf in Li2HfCl6 are provided. Importantly, with the increased amount of aliovalent substitution in Li2+xHf1−xInxCl6 microcrystal framework, a gradual structure evolution from trigonal to monoclinic phase has been observed, which is accompanied by the redistribution of Li-ions to generate two dimensionally (2D) preferable diffusion pathways through octahedral-tetrahedral-octahedral sites in In3+-substituted Li2HfCl6. Additionally, due to the oxidative stability of In-substituted Li2HfCl6, the bulk-type ASSBs with bare LiCoO2 deliver distinguished electrochemical performance.