Terminally Fluorinated Polyimide Interphase for Durable Lithium-Rich Layered Oxide Cathodes

Lithium-rich layered oxides (LLOs) deliver high energy density via coupled cationic/anionic redox, but high-voltage oxygen activation generates radical-rich interfaces that accelerate electrolyte decomposition, surface reconstruction, and mechanical failure. Although polyimide serves as an oxidation-resistant interphase on LLO cathodes, its durability is limited by labile termini vulnerable to reactive oxygen species, whereas backbone fluorination compromises ionic transport. In this study, a terminally fluorinated polyimide (FPI) interphase is constructed on LLOs (LLO-FPI) by introducing electron-withdrawing ─CF₃ termini to elevate the interfacial electronic barrier, thereby mitigating oxidative attack and oxygen release while preserving backbone integrity. Terminal fluorination suppresses radical-mediated degradation and parasitic oxidation, while the high-modulus FPI interphase constrains stress-driven particle cracking without sacrificing Li⁺ kinetics. Consequently, LLO-FPI exhibits exceptional long-term cycling stability, retaining 80.8% capacity after 1000 cycles. A Si/C||LLO-FPI pouch cell delivers 409 Wh kg^-1 with 90.1% capacity retention over 100 cycles. These findings identify terminal fluorination as a versatile strategy for durable high-energy-density batteries.