Effect of moisture on the phase transition of β-PbSnF4 at ambient temperature as the fast fluoride ion conductor

The solid-state electrolyte β-PbSnF4 has been reported to have the highest fluorine-ion conductivity at room temperature among four different structures of the monoclinic (α), orthorhombic (o), cubic (γ), and tetragonal (β) at steady state. Therefore, finding an effective way to prepare β-PbSnF4 or to transform β-PbSnF4 from other phases is a research hotspot in FIBs. In particular, the transition mechanisms between γ-PbSnF4 and β-PbSnF4 are still unclear in the reported literature because the influence of trace moisture is ignored in previous research, which always leads to controversial conclusions about the phase transition mechanism. Previous articles have reported that γ-PbSnF4 can only be transformed into β-PbSnF4 by aging at a high temperature above 100 °C. However, this work shows that the transition from γ-PbSnF4 to β-PbSnF4 can occur at room temperature. Moisture is the critical factor in inducing the phase transition from γ to β-PbSnF4. The resulting β-PbSnF4 has a high fluoride-ion conductivity of 8.1 × 10−4 S cm−1, while the solid electrolyte for an all-solid-state FIB with BiF3 as the cathode and Sn as the anode has a maximum discharge capacity of 55.8 mAh g−1 at 120 °C.