Revealing the Roles of CuF ₂ /NiF ₂ Incorporation in the Electrochemical Performance of FeF ₃ Cathodes in Solid‐State Batteries

Mixed metal fluorides have been considered as a promising candidate to lower the voltage hysteresis of conversion‐type iron fluoride cathodes, but their cycling stability is limited due to transition metal dissolution and interphase growth in liquid electrolyte batteries. Here, we study the role of incorporating CuF 2 and NiF 2 in the electrochemical performance of FeF 3 cathode in halide‐based solid‐state batteries to test whether we can transfer the kinetic benefit of low voltage hysteresis to solid‐state batteries while using solid electrolyte to eliminate transition metal dissolution and stabilize the interphase. Synchrotron X‐ray absorption spectroscopy results indicated the redox reactions are attributed to Cu 0 /Cu + and Fe 0 /Fe 2+ in 25CuF 2 ‐75FeF 3 and Ni 0 /Ni 2+ and Fe 0 /Fe 3+ in 10NiF 2 ‐90FeF 3 . While no apparent improvement in electrode kinetics can be observed, the incorporation of CuF 2 and NiF 2 can largely improve the cycling stability of FeF 3 cathodes. The results demonstrate the advantages of using solid‐state concept to improve the cycling stability of conversion‐type cathodes.