Abstract: High-voltage LiCoO₂ (LCO) offers a prelude to breaking the bottleneck of the energy density of lithium-ion batteries, however, LiCoO₂ is subject to serious structural and interfacial degradation above voltages >4.55 V (vs. Li/Li⁺). Herein, an in-situ Li_6.25La₃Zr₂Al_0.25O₁₂ (LLZAO) layer is constructed on the LCO surface to achieve operating voltage at 4.6 V. The detailed characterizations (ex-situ XRD, ex-situ Raman, DFT, etc.) reveal that the LLZAO layer greatly enhances Li⁺ conductivity attributed to the ion-conducting layer on the surface/interface, and closely combines with LiCoO₂ particle to ensure stable cathode/electrolyte interface, thus suppressing the highly reactive Co⁴⁺ and O^- triggered surface side reactions at high-voltage. Moreover, the introduction of La³⁺/Zr⁴⁺/Al³⁺ with a larger ionic radius (La³⁺/Zr⁴⁺ are larger than Co³⁺) and weaker electronegativity (La/Zr/Al are weaker than Co) into Co³⁺ sites readjusts the electron cloud density between Co-O-Li, which reinforces the Co-O bond and widens the band-center gap of Co 3d and O 2p, thus restraining the detrimental phase transition (from H3 to H1-3 phase) and the formation of Co₃O₄ spinel phase (attributed to lattice oxygen release), subsequently alleviating the particle cracking and structural collapse during repeated Li⁺ de/intercalation. Therefore, after 100 cycles at 3.0-4.6 V, LCO@1.0LLZAO exhibits a superior discharge capacity of 188.5 mA h g^-1, with a capacity retention of 85.1%. The above research has brought about meaningful guidance for the evolution of cathode materials with high voltage.

Key words: LLZAO layer, High-voltage LiCoO₂, Irreversible phase transition, Band-center gap, Lattice oxygen release