NH 4 BF 4 -assisted conversion of alkaline surface species enables air-stable Li 5 FeO 4 cathode prelithiation additives

Li5FeO4 is a promising cathode prelithiation additive for compensating initial lithium loss in high-energy lithium-ion batteries, but its alkaline and air-reactive surface causes rapid degradation under humid air. Herein, we report a simple NH4BF4-assisted surface fluorination treatment that converts reactive Li-containing surface species into a LiF-rich fluorinated region while preserving the antifluorite Li5FeO4 bulk structure. This surface chemical conversion reduces LiOH-like surface species and suppresses the subsequent formation of hydrated hydroxide, carbonate, and LiFeO2-related degradation products during humid-air storage. As a result, the NH4BF4-treated Li5FeO4 (denoted as LFO-BF) maintains a high lithium-donating capacity after exposure to humid air, even under harsh humidity conditions, while pristine Li5FeO4 suffers severe capacity decay. Structural and gas-evolution analyses further show that LFO-BF retains the typical bulk delithiation pathway of active Li5FeO4 and substantially suppresses CO2 evolution and associated interfacial side reactions. When used in lithium nickel manganese cobalt oxide (NMC)//graphite full cells, LFO-BF provides effective lithium compensation and improves capacity retention. This work presents alkaline-surface conversion into a LiF-rich region as a non-carbon approach for developing humid-air-tolerant Li-rich antifluorite prelithiation additives.