Fluoride Ion Transport and Phase Evolution in the Cathode Material LaSrMnO4 within All-Solid-State Fluoride Ion Batteries

Fluoride ion batteries (FIBs) are an emerging energy storage technology with high gravimetric and volumetric energy densities along with the potential for material sustainability. In this study, we increase the fundamental understanding of the Ruddlesden-Popper-type cathode material LaSrMnO4. The layered structure of LaSrMnO4 provides interstitial sites that facilitate the reversible fluoride ion insertion and extraction. Structural and electrochemical analyses confirm its stability over multiple charge-discharge cycles, and structural changes within the defluorination process are highlighted within this study for the first time. Computational investigation of the LaSrMnO4Fx (0 < x < 2) system allowed us to identify stable intermediate compositions as well as the existence of monophasic and biphasic domains during cell cycling and helped us to further understand the experimental charging/discharging curves. Molecular dynamics simulations further reveal that fluoride ion migration pathways are limited to the interstitial anion sites within the compound, giving the first time evidence for the 2D nature of fluoride ion transport in LaSrMO4F.