Super Chloride Ionic Conductivity in CsSnCl₃‐Based Perovskite Compound and Its Application for Solid‐State Chloride Batteries

A CsSnCl 3 ‐based solid electrolyte in which 5% of Sn sites are occupied by Y ions, CsSn 0.95 Y 0.05 Cl 3.05 (CSYC), is developed using a one‐step mechanical ball‐milling method. CSYC exhibits a stable cubic perovskite crystal structure and a high chloride ion conductivity of 4.9 mS cm −1 . The relative density of a CSYC pellet following cold pressing without heat treatment is 97.5%. The high relative density and ionic conductivity are considered to originate from the high mechanical plasticity of the pellets, as evidenced by a low Young's modulus of 8.2 GPa. An all‐solid‐state chloride ion battery cell using CSYC as the electrolyte, BiCl 3 as the active cathode material, and Sn as the anode is confirmed to operate at room temperature. The cell shows a large initial discharge capacity of 178 mAh g −1 , ≈70% utilization, and good capacity retention with a reversible capacity of 100 mAh g −1 after 40 cycles. The mechanical plasticity of CSYC is considered to be a major contributor to its high ionic conductivity and good battery cycling performance.