The fluoride ion
battery (FIB) is a promising post-lithium ion
battery chemistry owing to its high theoretical energy density and
the large elemental abundance of its active materials. Nevertheless,
its utilization for room-temperature cycling has been impeded by the
inability to find sufficiently stable and conductive electrolytes
at room temperature. In this work, we report the use of solvent-in-salt
electrolytes for FIBs, exploring multiple solvents to show that aqueous
cesium fluoride exhibited sufficiently high solubility to achieve
an enhanced (electro)chemical stability window (3.1 V) that could
enable high operating voltage electrodes, in addition to a suppression
of active material dissolution that allows for an improved cycling
stability. The solvation structure and transport properties of the
electrolyte are also investigated using spectroscopic and computational
methods.