Near-Room-Temperature Quasi-Solid-State F-Ion Batteries with High Conversion Reversibility Based on Layered Structured Electrolyte

Solid-state fluoride ion batteries (SSFIBs) are anion-shuttle-driven and alkali-metal-free emerging energy storage systems, and have the advantages of low cost, high safety, and energy density. However, the current major challenges of SSFIBs or quasi-SSFIBs are the lack of high-conductivity fluoride electrolytes and the difficulty of mild-temperature operation of full cells. Here, a 2D fluoride conductor (KSn 2 F 5 ) synthesized by mechanochemical method is employed as the compatible solid electrolyte with a high F-ion conductivity of 10 −4 S cm −1 at 60 °C. The high concentration and frequent hopping of charge carriers in F-vacancy-rich KSn 2 F 5 contribute to the higher ionic conductivity compared to most reported fluoride structures. Integrating this layered electrolyte with high-voltage CuF 2 cathode and Sn/SnF 2 anode, a highly reversible cycling of conversion-type quasi-SSFIBs with a discharge capacity of 150 mAh g −1 for at least 70 cycles at 60 °C is achieved. The interface engineering aided by tetra- n -butylammonium fluoride salt facilitates the F-ion transfer between cathode and electrolyte and fluorination/defluorination conversion mechanism of CuF 2 cathode with optimized multiphase and nanodomain distributions.