Steric Hindrance-Driven Closed-Loop Conversion of Acceptor Enables Long-Life and High-Capacity Fluoride-Ion Batteries

Due to the ultrahigh theoretical energy density, no dendrite issue, and abundant resources, fluoride ion batteries (FIBs) have received a lot of attention. Regarding the issue of dissolving inorganic salt CsF in aprotic organic solvents, some anion acceptor (AA) strategies have been proposed. However, the strong binding force greatly constrains F– ion desolvation, resulting in short lifespan, low specific capacity, and poor reversibility of FIBs. Herein, we propose a concept of steric hindrance-driven closed-loop acceptor to address these problems, by using tetraphenylphosphonium chloride (Ph4PCl) with appropriate Lewis acidity to dissolve CsF and prepare a dynamic fluoride ion electrolyte based on the F–Cl exchange reaction, with a high ionic conductivity of 4.1 mS/cm at room temperature. The steric hindrance effects of chlorine and phenyl can accelerate the desolvation kinetics of F– ions. The excellent kinetics of the Ph4PCl-based electrolyte endows FIBs with long-term cycling stability, and the Sn@SnF2 symmetric cells can cycle for 500 h at 100 μA/cm2 and tolerate a critical current density as high as 1250 μA/cm2. Due to the potential dissociation ability of five-coordinated acceptor central phosphorus for fluorides and the closed-loop conversion effect of chlorine, the fluorination and defluorination reaction proceeds in a dissolution-deposition mode. The CuF2//Sn@SnF2 cell (under a high cathode loading of 4.2 mg/cm2) exhibits the highest reversible capacity up to 717.7 mAh/g and remains 316 mAh/g after 65 cycles with a small voltage polarization of only 11 mV. This work points out the novel design concept of AAs for developing high capacity and long lifespan FIBs.