Interfacial anionic competition-driven electrochemical evolution in FeF₃ conversion electrodes

Iron trifluoride demonstrates poor performance as a multi-electron conversion positive electrode in conventional ester-based electrolytes, yet exhibits higher capacity retention in ether-based electrolytes. This contrast has long remained unresolved in high-energy-density metal fluoride-lithium batteries. Here, we show that the electrolyte-dependent behavior originates from interfacial anion competition within the cathode-electrolyte interphase. Spectroscopic and electrochemical analyses reveal that anions derived from lithium salts preferentially interact with iron species, governing interfacial conversion pathways and phase evolution. In ether-based electrolytes, lithium sulfide and lithium oxide react with iron to form electrochemically active iron sulfide species, sustaining reversible cycling. In contrast, lithium carbonate formed in ester-based electrolytes leads to inert carbonate-rich iron species, resulting in surface passivation and capacity decay. With prolonged cycling, iron trifluoride gradually evolves into thermodynamically stable iron-based phases, highlighting that interfacial anion chemistry dictates long-term phase evolution and degradation pathways of iron-based fluoride conversion electrodes.