Fluoride Ion Storage and Conduction Mechanism in Fluoride\nIon Battery Positive Electrode, Ruddlesden–Popper-Type Layered\nPerovskite La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub> Crystal

Structural characteristics on fluoride\nion storage and conduction\nmechanism in La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>, and its fluoridated materials, La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>F and La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>F<sub>2</sub>, for an all-solid-state\nfluoride ion battery positive electrode with a high volumetric capacity\nsurpassing those of lithium-ion ones have been revealed using the\nRietveld method and maximum entropy method. In La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>, once the F<sup>–</sup> ions are taken into the NaCl slabs in its crystal through the charging\nprocess, it forms two stable fluoride compounds, La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>F and La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub>F<sub>2</sub>, with the help\nof the Mn oxidation reaction. In these oxyfluorides, thermal vibrations\nof the F<sup>–</sup> ions inserted are much larger, especially\nin the <b><i>a</i></b>–<b><i>b</i></b> plane, than along the <b><i>c</i></b> axis.\nWhen surplus energy, such as an electric field for charging, is applied\nto these crystals at near room temperature or higher, the anions immediately\nbegin to jump to their neighboring lattice sites, resulting in sufficiently\nrapid and large ionic conduction. The MEM analyses and density functional\ntheory (DFT) calculations have revealed that the F<sup>–</sup> ions enable to easily travel along the ⟨110⟩ directions\nin the NaCl slabs of these crystals. These structural features thus\nmake La<sub>1.2</sub>Sr<sub>1.8</sub>Mn<sub>2</sub>O<sub>7</sub> and\nits fluorides possess both of two features incompatible with each\nother, ion storage and conduction, indispensable for rechargeable\nbatteries.