Transient phase change of Ruddlesden-Popper type perovskite on fluoride-ion intercalation reaction

Ruddlesden-Popper type perovskites are recognized as a promising active material for fluoride-ion batteries (FIBs) due to the intercalation of fluoride-ions into the crystal structure. However, the precise reaction mechanism has not been thoroughly elucidated. In this study, we examined the phase transition mechanism of electrochemical F- intercalation into La1.2Sr1.8Mn2O7 oxide by several analytical techniques and by comparing the phase transition of chemical F- intercalation into La1.2Sr1.8Mn2O7 oxide. The phase transition behavior during Fintercalation into La1.2Sr1.8Mn2O7 follows the two stages. In the first stage from x = 0 to x = 1.0 of La1.2Sr1.8Mn2O7Fx, the fluorination proceeds in a two-phase reaction between the La1.2Sr1.8Mn2O7 phase (space group; I4/mmm) and the La1.2Sr1.8Mn2O7F phase (space group; P4/nmm). In this stage, the lattice constant c of the La1.2Sr1.8Mn2O7F phase gradually increased with increasing x amount. In the second stage from x = 1.0 to x = 2.0 of La1.2Sr1.8Mn2O7Fx, the fluorination proceeds in a two-phase coexistence reaction of the La1.2Sr1.8Mn2O7F phase and La1.2Sr1.8Mn2O7F2 phase. In the second stage, the lattice constants of the La1.2Sr1.8Mn2O7F phase and La1.2Sr1.8Mn2O7F2 phase are independent of the x amount of La1.2Sr1.8Mn2O7Fx. The apparent diffusion coefficients of fluoride-ions in the first stage are larger than those in the second stage maybe because the lattice mismatch between the two phases is reduced by the gradual change in the lattice constant of the La1.2Sr1.8Mn2O7F phase in the first stage. This finding is useful for understanding the reaction mechanism of intercalation-type cathode materials for FIBs and improving the electrochemical performances.