Naked F − Provided by Imidazolium Fluoride‐Based Electrolytes Promotes Low Overpotential in Fluoride‐Ion Batteries

As an alternative to lithium‐based batteries, fluoride‐ion batteries (FIBs) offer a promising pathway owing to their high theoretical energy density. However, their progress has been hindered by the lack of room‐temperature electrolytes, either solid or liquid, that exhibit sufficiently wide electrochemical stability windows and that allow F‐ion shuttling between electrodes with low overpotential. In this work, we identify 1‐butyl‐3‐methylimidazolium fluoride (BMimF) dissolved in acetonitrile (MeCN) as a particularly promising electrolyte candidate. This system exhibits a high room‐temperature ionic conductivity of 15.3 mS cm −1 and an electrochemical stability window ranging from −0.37 to +1.59 V vs. Pb/PbF 2 . 19 F NMR reveals that BMimF provides a “naked” F − that enables the stable cycling of a symmetric Pb + PbF 2 cell with a very low overpotential of 75 mV (at 6 mA g −1 ) and 160 mV while paired with BiF 3 . XRD, AES, and SEM‐EDX reveal that, compared with the widely used TBAF·3H 2 O in THF electrolyte, BMimF in MeCN provides a more stable chemical environment for CsMnFeF 6 , a previously reported insertion‐type electrode material, by effectively limiting cation dissolution.