Z-Type Interstice Leap Migration Driving High Ionic Conductivity in Monoclinic LiBiBr₄ Solid State Electrolyte

Halide solid electrolytes receive much attention due to their electrochemical properties, such as high ionic conductivity, oxidative stability, and ease of preparation. In this work, a bromide solid electrolyte LiBiBr₄, exhibiting ease of processing and high ionic conductivity, is designed for the first time and investigated through a comparative investigation with monoclinic LiAlCl₄ and LiAlBr₄ for the migration path. The processing pressure for LiBiBr₄ with annealing at 120 °C is less than one-tenth that of other chloride electrolytes (≈5 MPa). Computational analyses unveil crucial mechanistic insights into the three migration mechanisms and the factors that influence them within the monoclinic structure. The distribution and distance of non-Li polyhedrons to the migration pathways are pivotal for the migration. The strategic positioning of the Bi polyhedron in LiBiBr₄ is far from the Li⁺ pathway. The unique leap migration within the LiBiBr₄ has a lower energy barrier and facilitates an interconnected migration that forms a 3D interstice network. This interconnected leap migration network within LiBiBr₄ constitutes a Z-type interstice leap migration along the ab-axis. Thus, the LiBiBr₄ obtains a high ionic conductivity of 0.19 mS cm^-1 with the 0.349 eV low activation energy. This discovery and research methods provide significant impetus and support for the development of halogen-based electrolytes.