Engineering Residual Rock-Salt Reconstruction through Near-Surface Fluorine Regulation for Stable 4.6 V Cycling of Regenerated Spent Layered Oxide Cathodes.

Direct regeneration enables high-value recovery of spent layered oxide cathodes, but their high-voltage reuse requires stabilization of both regenerated surface and lattice oxygen beyond bulk compositional restoration. Here, we report a high-voltage upcycling strategy for spent LiNi0.6Co0.2Mn0.2O2 (NCM) based on near-surface fluorine (F) regulation of residual rock-salt layers. Molten-salt relithiation recovers Li content and layered bulk framework, whereas subsequent fluorine treatment converts the remaining inert surface reconstruction into a thin F-enriched rock-salt interphase. Rather than impeding Li+ transport, this interphase stabilizes the reconstructed surface, reduces oxygen defects, and mitigates electrolyte-induced interfacial degradation. The resulting cathode with F-enriched rock-salt surface (RFNCM) delivers 208.7 mAh g-1 at 0.1 C within 3.0-4.6 V, and retains 80.28% capacity after 400 cycles at 1 C and 25°C, and 80.93% after 200 cycles at 50°C. In situ X-ray diffraction and gas-evolution measurements reveal more homogeneous high-voltage structural evolution and suppressed oxygen release, respectively. According to analysis results of XPS, in situ Raman and Density functional theory calculations, these improvements can be attributed to strengthened Ni─O interactions and increased oxygen-vacancy formation energies after fluorine incorporation. This work converts an unavoidable reconstruction product into a protective interphase for high-voltage reuse of spent layered oxide cathodes.