Elucidating the diffusion pathway of lithium ions in superionic halide solid electrolytes Li2+xHf1−xInxCl6 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 Li 2+ x Hf 1− x In x Cl 6 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 Li 2 HfCl 6 are provided. Importantly, with the increased amount of aliovalent substitution in Li 2+ x Hf 1− x In x Cl 6 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 In 3+ -substituted Li 2 HfCl 6 . Additionally, due to the oxidative stability of In-substituted Li 2 HfCl 6 , the bulk-type ASSBs with bare LiCoO 2 deliver distinguished electrochemical performance.