Oxyfluoride Cathode for All-Solid-State Fluoride-Ion Batteries with Small Volume Change Using Three-Dimensional Diffusion Paths

All-solid-state fluoride-ion batteries (FIBs) are expected to become the next generation of battery systems owing to their outstanding energy storage characteristics. However, the volume expansion of the cathode that accompanies the insertion of fluoride ions remains an urgent issue to be addressed. Even if an intercalation-type cathode is applied in FIBs, fluoride-ion insertion into the interstitial sites of two-dimensional materials such as LaSrMnO4 still leads to non-negligible volume expansion. Here, we report a novel intercalation-type material, Sr3Fe2O5F2, possessing the Ruddlesden–Popper structure as a cathode material for FIBs that features not only interstitial sites but also anion vacancies as three-dimensional (3D) percolation sites to accommodate fluoride ions. This material exhibits a relatively high capacity of 118 mAh g–1 and good cycling stability over 70 cycles. Fe2+/Fe3+ redox reactions are responsible for charge compensation during the charging and discharging processes. The crystal structure during the charging process changes from Sr3Fe2O5F0.46 to Sr3Fe2O5F2 by using the 3D percolation sites with an extremely small volume change of approximately 0.17% and thereafter changes to Sr3Fe2O5F3 with a P4/mmm symmetry by using interstitial sites with a large volume change of approximately 11%. Our findings should pave the way for the design of new cathodes with excellent cycling stability and minimal volume expansion based on the percolation mechanism.