SnF2-based fluoride ion electrolytes MSnF4 (M = Ba, Pb) for the application of room-temperature solid-state fluoride ion batteries

Abstract

The fluoride ion batteries (FIBs) based on “fluoride ion shuttle” have received extensive attention due to their high energy density, safety and thermal stability. Although the FIBs show above advantages, several challenges still remain to be tackled, e.g., so far reported solid-state FIBs can only operate at the high temperature like 150 °C and above. Herein, the SnF2-based fluoride ion conductors MSnF4 (M = Ba, Pb) are prepared through high-energy ball-milling and annealing as solid electrolytes in solid-state FIBs. The morphology and structure of the solid electrolytes are studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Especially, the fluoride ion transporting characteristics of BaSnF4 and PbSnF4 solid electrolytes are investigated by impedance spectroscopy (EIS). The results show that the high RT ionic conductivities of BaSnF4 (2.02 × 10−4 S cm−1) and PbSnF4 (5.44 × 10−4 S cm−1) make it possible to prepare the RT solid-state FIBs with Sn as an anode and BiF3 as a cathode. The initial discharging capacities of the Sn/PbSnF4/BiF3 cell and the Sn/BaSnF4/BiF3 cell studied by galvanostatic charge/discharge cycling are 175 mAh g−1 and 125 mAh g−1 at RT, respectively. The structural and chemical characterization of the BiF3 composites and the Sn composites before and after discharge is examined in detail by X-ray diffraction (XRD) in order to explicate the charge/discharge behavior of the RT solid-state FIBs.