Interfacial anionic competition-driven electrochemical evolution in FeF3 conversion electrodes

Iron trifluoride demonstrates poor performance as a multi-electron conver-
sion 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
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evolution and degradation pathways of iron-based fluoride conversion
electrodes.