A Low-Strain Lithium Cathode Material Li_2–2xFe_1+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, Li 2–2 x Fe 1+ x Cl 4 (0 ≤ x ≤ 1/3). Among this series, Li 2 FeCl 4 experiences no phase changes upon the (de)lithiation process, contributing to exceptionally high reversibility and stable cycling. ASSBs utilizing Li 2 FeCl 4 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 Li 2–2 x Fe 1+ x Cl 4, a maximum lithium storage capacity of 155.2 mAh g –1 was achieved with Li 4/3 Fe 4/3 Cl 4, which is comparable to that of a commercial LiFePO 4 cathode. This study proves the advantage of halide cathode materials in halide-based ASSBs.