Toward room temperature fluoride-ion batteries: quaternary ammonium-bearing fluoride-exchange membrane as a quasi-solid polymer electrolyte

Fluoride-ion batteries (FIBs) are promising alternatives to lithium-based energy storage systems, offering superior electrochemical stability, efficient ion transport, and high elemental abundance. However, the practical application of FIBs is hindered by the lack of efficient solid-state electrolytes with adequate ionic conductivities at room temperature. In this study, we present a novel quasi-solid polymer electrolyte (SPE) in the form of a fluoride-exchange membrane (FEM) based on a perfluorosulfonic amide ionomer with tetraalkylammonium fluoride functional groups. The fluoride-ion conductivity of the polymer electrolyte is enhanced through tailored ionomer swelling using a solvent mixture of 1,3-dioxolane (DOL) and dimethyl sulfoxide (DMSO). Molecular dynamics simulations support the role of DMSO as a polar FEM swelling agent. Electrochemical impedance spectroscopy (EIS) reveals ionic conductivities of up to 1.3 × 10⁻3 S cm⁻¹ at room temperature, with stable cycling performance exceeding 50 cycles in a coin-cell configuration. The electrolyte stability and reversibility are demonstrated by galvanostatic charge-discharge tests using Pb/PbF₂ electrodes, in which reversible fluorination/ defluorination at ±0.8 V with a current density of 0.1 C is achieved. These results pave the way for roomtemperature applications of FIBs and their future commercialization.