A Low-Strain Lithium Cathode Material Li2–2xFe1+xCl4 for Halide-Based All-Solid-State Batteries

Halide electrolytes are of particular interest due to their high ionic conductivity and ductile nature. However, halide-electrolyte-based all-solid-state batteries (ASSBs) are subject to degradation when conventional layer oxide cathodes are used. Herein we report a class of halide cathode materials, Li2–2xFe1+xCl4 (0 ≤ x ≤ 1/3). Among this series, Li2FeCl4 experiences no phase changes upon the (de)lithiation process, contributing to exceptionally high reversibility and stable cycling. ASSBs utilizing Li2FeCl4 as the cathode demonstrate good rate performance (122.4 mAh g–1 at 0.2C rate and 91.4 mAh g–1 at 10C rate) and cycling stability (i.e., a capacity retention of 85.1% is maintained upon 5000 cycles at 10C rate). By optimizing the Li/Fe ratio in Li2–2xFe1+xCl4, a maximum lithium storage capacity of 155.2 mAh g–1 was achieved with Li4/3Fe4/3Cl4, which is comparable to that of a commercial LiFePO4 cathode. This study proves the advantage of halide cathode materials in halide-based ASSBs.