Manipulating cation-anion coordination in fire-retardant electrolytes to enable high-areal-capacity fluoride conversion batteries

Abstract

Resource-abundant and multi-redox iron fluorides are considered promising cathodes for large-scale battery systems. However, existing research often overlooks the critical issues at the fluoride-electrolyte interface that cause voltage plateau blurring and capacity degradation. Here, we propose an interfacial engineering strategy for the conversion-type FeF₃ cathode enabled by manipulating the cation-anion coordination in a fire-retardant electrolyte. Lithium difluoro(oxalato)borate has strong electron affinity and induces an anion-rich inner solvation sheath, thereby dominating the construction of the cathode-electrolyte interphase (CEI). The inorganic-enriched CEI layer features electron insulation and facile mass transport, which could suppress interfacial parasitic reactions and promote fluoride structural reversibility. The Li-FeF₃ cell enables well-preserved voltage plateaus and a high capacity of 412 mAh g⁻¹ with inspiring cycle durability. The superior electrolyte wettability further contributes to a reversible areal capacity as high as 2.94 mAh cm⁻² for fluoride cathode under high FeF₃ mass loading of ∼7.0 mg cm⁻² and lean electrolyte conditions.