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.