Rate-Determining Process at Electrode/Electrolyte Interfaces for All-Solid-State Fluoride-Ion Batteries

Developing high-performance solid electrolytes that are operable at room temperature is one of the toughest challenges related to all-solid-state fluoride-ion batteries (FIBs). In this study, tetragonal β-Pb_0.78Sn_1.22F₄, a promising solid electrolyte material for mild-temperature applications, was modified through annealing under various atmospheres using thin-film models. The annealed samples exhibited preferential growth and enhanced ionic conductivities. The rate-determining factor for electrode/electrolyte interface reactions in all-solid-state FIBs was also investigated by comparing β-Pb_0.78Sn_1.22F₄ with representative fluoride-ion- and lithium-ion-conductive materials, namely, LaF₃, CeF₃, and Li₇La₃Zr₂O₁₂. The overall rate constant of the interfacial reaction, k⁰, which included both mass and charge transfers, was determined using chronoamperometric measurements and Allen-Hickling simulations. Arrhenius-type correlations between k⁰ and temperature indicated that activation energies calculated from k⁰ and ionic conductivities (σ_ion) were highly consistent. The results indicated that the mass transfer (electrolyte-side fluoride-ion conduction) should be the rate-determining process at the electrode/electrolyte interface. β-Pb_0.78Sn_1.22F₄, with a large σ_ion value, had a larger k⁰ value than Li₇La₃Zr₂O₁₂. Therefore, it is hoped that the development of high-conductivity solid electrolytes can lead to all-solid-state FIBs with superior rate capabilities similar to those of all-solid-state Li-ion batteries.