Pretreatment-derived LiF/LiPO₂F₂ species enable robust high-voltage NCM811 cathode interphases

Ni-rich NCM811 cathodes offer high energy density but undergo severe interfacial degradation above 4.5 V. Conventional electrolyte additives typically form protective cathode–electrolyte interphases (CEIs) through their in situ decomposition, often accompanied by continuous electrolyte consumption and heterogeneous interphase growth. Here, we develop a pretreatment-enabled carbonate electrolyte that preintroduces nano-LiF and LiPO₂F₂-related species as interphase-building components. Thermal pretreatment generates these inorganic species before cycling, while difluoroethylene carbonate (DFEC) is added to modulate Li⁺-PO₂F₂^- association and stabilize LiF-containing species near Li-deficient NCM811 surfaces. NMR, spatially resolved Raman mapping, theoretical calculations, DRT analysis, depth-profiled XPS, and post-cycling characterizations collectively show that this strategy suppresses excessive carbonate-solvent participation and promotes a stratified LiF/ Li_xPO_yF_z-enriched CEI. The resulting interphase lowers interfacial resistance, facilitates Li⁺ transport, and mitigates surface reconstruction and microcracking. Consequently, a 1 Ah NCM811||graphite pouch cell retains 87.8% of its capacity after 1000 cycles at 1 C with an upper cut-off voltage of 4.6 V. This work provides a practical route to direct high-voltage CEI chemistry through pretreatment-derived inorganic species and DFEC-mediated interfacial regulation.