Recently, supply chain pressures tied to the geopolitics and scarcity of Li, Co, and other metals have led to the first increase in Li-ion cell price ever, with the cost of essential precursors skyrocketing to new highs. This kind of volatility makes deploying Li-ion batteries at grid scales challenging, despite their many advantages in terms of large operating potentials and slow self-discharge rates. While a tremendous amount of work has explored replacing Li with more abundant cations like Na or Mg the possibility of harnessing negatively charged anions like fluoride, has largely been overlooked because trying to move anions around through closely packed lattices is tantamount to playing a game of atomic Jenga®.

This talk will present our group’s recent work that reports the first example of a reversible battery based on the electrochemical (de)insertion of F-ions at room temperature. We demonstrate that after three cycles, one full equivalent of F-ions can be reversibly cycled in CsMnFeF₆. Electrochemical impedance spectroscopy and Mössbauer spectroscopy suggests the formation of fluoride vacancies in early cycles generates mixed-valent Fe that enhances the electrical conductivity of the electrode. Furthermore, ex situ powder diffraction reveals a subtle expansion and contraction of the cubic lattice during oxidation (insertion) and reduction (removal) respectively, that eventually leads to the evolution of new reflections corresponding to a closely related orthorhombic polytype in later cycles. This topotactic transformation suggests that structural derivatives of the fluorite structure offer a promising class of materials for creating high-performance F-ion insertion electrodes.