Electrochemical, Thermal, and Structural Features of BaF<sub>2</sub>–SnF<sub>2</sub> Fluoride-Ion Electrolytes
Kazuhiro Mori, Atsushi Mineshige, Takuro Emoto, Maiko Sugiura, Takashi Saito, Kaoru Namba, Toshiya Otomo, Takeshi Abe, Toshiharu Fukunaga
Abstract
Fluoride-ion-conducting compounds are key materials for solid electrolytes in all-solid-state fluoride shuttle batteries (FSBs) and widely regarded as promising rechargeable batteries. However, their ionic conductivities are still insufficient to allow room-temperature operation. Particularly, the transportation of F ions through solid-state ionic devices is yet to be fully understood. We studied the electrochemical, thermal, and structural features of BaF2–SnF2 solid electrolytes by means of AC impedance, differential scanning calorimetry, X-ray diffraction, and neutron diffraction experiments. The substitution of Ba by Sn atoms increased the electrical conductivity of BaF2–SnF2 to 107–109 times that of BaF2; particularly, (BaF2)0.47(SnF2)0.53 exhibited the highest electrical conductivity (σ = 4.1 × 10–3 S/cm at room temperature) with the lowest activation energy (Ea = 17.9 kJ/mol). Structural analysis revealed that (BaF2)0.47(SnF2)0.53 consists of a tetragonal structure (T-phase) and residual amounts of the cubic structure (C-phase). The T-phase could be refined on the basis of a [−SnSnMMSnSn−]-layered structure (M = BaxSn1–x) with three nonequivalent fluorine sites: F1, F2, and F3. The anisotropic displacement of F3 was more pronounced toward F1; thus, the “–F1–F3–F1–” zigzag network between the M and Sn layers plays a key role in two-dimensional fast F-ion diffusion.