Accelerated fluoride-ion intercalation/deintercalation in a layered-perovskite cathode by controlling the interlayer distance for fluoride-ion batteries

Fluoride-ion batteries (FIBs) are promising candidates for next-generation batteries because of their high theoretical energy density. A Ruddlesden-Popper perovskite oxide, La1.2Sr1.8Mn2O7, is not only a known host material that undergoes fluoride-ions intercalation/deintercalation reactions but also exhibits a high capacity and good cyclability when used as a cathode material of FIBs. However, the kinetics of fluoride-ion intercalation/deintercalation in this compound must be further improved for practical application. In this study, we control the interlayer distance to improve the fluoride-ion diffusion by systematically changing x in La2-2xSr1+2xMn2O7. The interlayer distance shows a maximum at x = 0.35 despite a monotonous decrease of the c-axis length with increasing x. The expansion of the interlayer distance suppresses the coulombic repulsion between fluoride-ions and oxide ions, favoring good capacity retention even at higher current densities.