fig1

Figure 1. (A) Schematic illustration of the NH₄BF₄-assisted surface fluorination treatment of Li₅FeO₄, in which reactive alkaline Li-containing surface species are proposed to be converted into a LiF-rich surface region after calcination at 430 °C for 10 h under Ar; (B) TEM and HRTEM images of LFO-BF. The orange square marks the region used for local lattice-fringe analysis of the LFO phase, with a spacing of approximately 0.27 nm assigned to the (023) plane of LFO. The outer surface-modified region has a locally observed thickness of approximately 25 nm; (C) F 1s and B 1s XPS spectra of fresh LFO-BF. TEM: Transmission electron microscopy; HRTEM: high-resolution transmission electron microscopy; XPS: X-ray photoelectron spectroscopy; LFO-BF: NH₄BF₄-treated Li₅FeO₄.