Regulating Li Ion Migration Pathway through Structural Distortion in Monoclinic Halide Solid Electrolytes

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Traditional Li-ion batteries that utilize liquid electrolytes are limited in their applications due to safety concerns, such as flammability risks. As an alternative that is both safe and efficient, considerable attention is being directed towards all-solid-state batteries that use inorganic materials. Notably, sulfide-based solid electrolytes have garnered extensive research focus due to their high lithium-ion conductivity (approximately 10 -2 S/cm at room temperature). These electrolytes are easily deformable, enabling cell manufacturing at low temperatures through compression, although they present practical application challenges due to their narrow electrochemical stability voltage windows.In pursuit of overcoming these limitations, there is vigorous ongoing research into halide-based solid electrolytes. These materials maintain stability at high voltages exceeding 4V, preserve the ductility characteristics similar to sulfides, and demonstrate high ionic conductivity. Numerous previous research studies have focused on halide solid electrolytes materials doped with cations of a similar radius to regulate Li concentration. In our study, we developed the halide solid electrolytes materials, which are doped with the larger radius Ca 2+ cation into monoclinic cubic close-packed (CCP) structures to induce structural distortion. We assessed the structural distortion in the materials using the continuous symmetry measures (CSM) value and observed the presence of enlarged tetrahedral sites in the metal layer. This indicates that the energy landscape of Octahedral site-Tetrahedral site-Octahedral site pathway for Li-ion diffusion in the structures has changed. At the optimal point, the composition with a larger unit cell volume can accommodate structural distortion. As a result, percolation in the metal layer becomes possible, leading to an enhancement for ionic conductivity of ab-plane. This increase ab-plane as the rate-determining step for overall ionic conductivity in the Ca 2+ doped material structure, as supported by ab initio molecular dynamics (AIMD) simulation results. Our study proposes a design rule for modifying ab-plane diffusion through structural distortion in halide solid electrolytes of CCP structure.

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