Fluorine‐Rich Catalyst‐Induced Interphase Engineering to Enable the First Ah‐Level FeF ₃ Conversion Solid‐State Batteries

ABSTRACT Conversion‐type FeF 3 cathodes promise ultrahigh energy density but suffer from sluggish reaction kinetics and interfacial instability in solid‐state battery systems. Here, we designed a fluorine‐rich NaBiF 4 @Bi 2 O 3 catalyst‐initiated polymer electrolyte via in situ ring‐opening polymerization of 1,3‐dioxolane. The NaBiF 4 phase initiates polymerization and serves as a fluorine reservoir, while Bi 2 O 3 participates in regulating the fluorine environment and contributes to the formation of a Li 3 Bi alloy clusters during cycling. This electrolyte enables the construction of LiF/NaF/Li 2 O‐reinforced solid electrolyte interface with embedded Li 3 Bi domains, delivering the homogeneous Li + flux and dendrite‐free Li deposition, enabling the stable Li‖Li symmetric cell cycling for 9700 h. The electrolyte demonstrates broad compatibility with both intercalation and conversion cathodes, achieving excellent cycling stability (800 cycles) in LiFePO 4 and high areal capacity (6 mAh cm −2 ) in LiNi 0.8 Co 0.1 Mn 0.1 O 2 . The Bi 2 O 3 component further catalyzes the interfacial dissociation of LiF at FeF 3 cathode and promotes the dynamic evolution of fluorine‐rich cathode electrolyte interphase, enabling the remarkable reversibility in FeF 3 conversion chemistry (641 mAh g −1 at 0.2 C and 300 cycles at 1 C). A 20‐layer FeF 3 ‐based pouch cell is demonstrated with a discharge capacity exceeding 1 Ah for the first time, marking a critical milestone toward practical high‐energy FeF 3 batteries.