Researchers have made notable progress in the ongoing effort to develop high energy-density batteries by demonstrating the room-temperature operation of fluoride-ion based (FIB) energy cells, a new study reports. FIBs offer an attractive alternative to other types of potential high-energy battery electrochemistries, such as those based on lithium or nickel-ion reactions. Due to the low atomic weight of fluorine, rechargeable batteries based on the element could offer very high energy densities - at least eight times greater than the theoretical values for lithium-ion technologies. However, they are limited by the need for solid state electrolytes that need to operate at temperatures above 150 degrees Celsius. According to the authors, these limitations in the electrolyte have presented a significant challenge for achieving low-temperature operating FIBs. Here, Victoria Davis and colleagues report on a method for creating a fluoride-ion electrochemical cell capable of operating at room temperature - a breakthrough made possible by a chemically stable liquid fluoride-conducting electrolyte with high ionic conductivity and a wide operating voltage. Davis et al. developed the electrolyte using dry tetraalkylammonium fluoride salts dissolved in an organic, fluorinated ether solvent. When paired with a composite cathode featuring a core-shell nanostructure of copper, lanthanum and fluorine, the researchers demonstrated reversible electrochemical cycling at room temperature.