Room-Temperature, Rechargeable Solid-State Fluoride-Ion Batteries

Fluoride ion batteries (FIBs) are among interesting electrochemical energy storage systems that are being considered as alternatives to lithium-ion batteries (LIBs). FIB offers high specific energy and energy density, thermal stability, and safety. Despite the advantages posed by the FIBs, several challenges need to be addressed to realize its full potential. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Here, for the first time, we demonstrate room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF<sub>4</sub> as fluoride transporting solid electrolyte. The high ionic conductivity of tetragonal BaSnF<sub>4</sub> (3.5 × 10<sup>–4</sup> S cm<sup>–1</sup>) enables the building of RT FIB. We built fluoride ion batteries using Sn and Zn as anodes and BiF<sub>3</sub> as a cathode. We have investigated the electrochemical properties of two different electrochemical cells, Sn/BaSnF<sub>4</sub>/BiF<sub>3</sub> and Zn/BiSnF<sub>4</sub>/BiF<sub>3</sub> at various temperatures (25 °C, 60 °C, 100 °C, and 150 °C). The first discharge capacity of the Sn/BaSnF<sub>4</sub>/BiF<sub>3</sub> and Zn/BiSnF<sub>4</sub>/BiF<sub>3</sub> cells amounts to 120 mA h g<sup>–1</sup> and 56 mA h g<sup>–1</sup> at room temperature, respectively. Although Sn-based cells showed capacity fading, Zn-based cells provided relatively stable cycling behavior at low temperatures. High reversible capacities were observed at elevated operating temperature. EIS, ex-situ XRD, and SEM studies were performed on the cells to investigate the reaction mechanism.