Enabling Conversion‐Type Iron Fluoride Cathode by Halide‐Based Solid Electrolyte

The practical application of low‐cost and energy‐dense iron fluoride cathodes is hindered by the first cycle electrochemical irreversibility, cycling instability, and large voltage hysteresis. Here, it is reported that these challenges may be overcome by the utilization of halide‐based solid electrolytes (SEs). The excellent electrochemical stability of halide‐based SEs enables a complete conversion and deconversion of FeF 2 that cannot be achieved with sulfide‐based SEs. Due to restricted and reversible decomposition of SE, prevention of Fe dissolution, mechanical confinement of active material, as well as improved electrode kinetics, solid‐state FeF 2 cathode with halide‐based SE demonstrates superior electrochemical performance compared with FeF 2 electrodes in liquid electrolytes, with a high 1st cycle coulombic efficiency (≈100%), high specific capacity (≈600 mAh g −1 ), long cycle life (>100 cycles), and high‐rate performance (up to 2 C). The results suggest solidifying the batteries is a viable approach to addressing the long‐standing key challenges of iron fluoride cathodes.