Fluoride ions as charge carriers in electrochemical energy storage.

Lithium-ion batteries are commonplace. However, the large-scale production of lithium-ion batteries also raises concerns related to material criticality and safety. Anion charge carriers are intriguing alternatives that invert the current paradigm of battery development and hold promise for high theoretical energy densities that surpass those of lithium technologies at a markedly lower cost, while mitigating challenges in light metal electrodeposition. This Review outlines the design principles for conversion- and insertion-type electrodes and examines their use in conjunction with liquid and solid electrolytes to create devices. The path of 'hard' fluoride ions is traced across the varying components of possible energy storage constructs. Scale-bridging materials design concepts, such as atomic diffusion pathways in periodic solids, ion diffusion through solid and liquid electrolytes and (electro)chemical reactivity at interfaces, are emphasized, as well as systems considerations at the full-cell level. In particular, issues such as materials compatibility, electrode crosstalk and degradation mechanisms are examined. Finally, the potential pitfalls and critical research fronts that must be considered to realize fluoride-ion batteries as a next-generation energy storage technology are discussed.