Engineering a Fluorinated AlPO ₄ Interface: Toward High Initial Coulombic Efficiency and Stable Oxygen Redox in Li‐Rich Layered Oxide Cathodes

ABSTRACT

Li‐rich layered oxides (LLOs) are promising high‐energy‐density cathode candidates; however, their practical application is hindered by severe voltage decay originating from irreversible layered‐to‐spinel phase transformation and interfacial degradation under high‐voltage operation (>4.5 V). Herein, we report a fluorinated AlPO ₄ (AlPO ₄ ‐F) surface architecture for LLO cathodes designed to simultaneously stabilize the oxygen framework and modify the interfacial charge environment. Electrostatic force microscopy, zeta‐potential measurements, and density functional theory calculations reveal that fluorination induces localized charge redistribution and a more negative surface potential, accompanied by enhanced Li affinity at the cathode surface. In situ Raman spectroscopy and in situ X‐ray diffraction further demonstrate improved structural reversibility, suppressed c‐axis contraction, and mitigated phase‐transition behavior during electrochemical cycling. As a result, the fluorinated AlPO ₄ ‐coated cathode (LLO‐APF) delivers an initial Coulombic efficiency of 95.9% and stable cycling over 244 cycles at 1 C, compared with 57 cycles for pristine LLO under identical conditions, while limiting voltage decay to 0.23 V. These results suggest that fluorinated phosphate interfaces can provide a multifunctional platform for simultaneously regulating interfacial stability, Li ⁺ transport behavior, and structural reversibility in high‐capacity anionic‐redox cathode systems.