Influence of Cation Doping on the Ionic Potential-Induced Concerted Migration of Ions in Halide Solid Electrolytes

Li3InCl6 and Li3ScCl6 are the focus of research as solid electrolytes in all-solid-state lithium batteries with high ionic conductivity and low activation energy. However, due to the mixed occupancy of In3+/Sc3+ and Li+ cations in the framework of Li3InCl6 and Li3ScCl6, there is no significant difference in the potential energy between octahedral and tetrahedral sites for ion transport, which suppresses the concerted migration effect between ions during the transport process. Here, we demonstrate that doped metal cations occupy the central ion sites of the polyhedra in the transport channel, thereby enhancing the local ionic potential during migration. Changes in ionic potential provide the necessary conditions for the concerted migration of high- and low-energy octahedra and tetrahedra in solid electrolytes. We find that the regulation of the ionic potential difference not only reduces the migration energy barrier of individual Li+ ions but also activates the concerted effect between ions with low migration energy barriers, including homogeneous ions (Li+–Li+) and heterogeneous ions (Li+–Mg2+/Li+–Ca2+), in a lithium bimetallic halide (Li–BiMe–Cl) solid electrolyte. The Li2.5InMg0.25Cl6 compound has a low migration barrier of 0.123 eV due to concerted Li+–Li+ migration. This discovery proves that regulating the ionic potential is an effective strategy for promoting concerted effects in improving the ion transport performance of solid electrolytes and designing novel fast ion conductors.