Synergistic Bulk/Interface Engineering Enables Stable 4.6 V Cycling of LiCoO₂ in Quasi-Solid-State Li-Ion Batteries

Integrating high-voltage LiCoO₂ (LCO) cathode materials with self-polymerized gel polymer solid-state lithium-ion battery systems could simultaneously achieve high industrial maturity and energy density targets. However, LCO cathodes face severe challenges in high-voltage solid-state systems: cathode-electrolyte interface instability from interfacial degradation and parasitic reactions coupled with anisotropic lattice changes and heterogeneous Li⁺ diffusion-induced stress accumulation/microcracks that initiate interface contact loss, exacerbated side reactions, and impedance rise. To combat synergistic degradation, this work stabilizes LCO through multielement (Al/Mg/Ni/Mn) doping and LiZr₂(PO₄)₃ (LZP) conformal coating, enhancing bulk and interfacial stability. The LZP layer establishes three-dimensional Li-ionic diffusion pathways and a buffer interface, effectively improving cathode/electrolyte compatibility, suppressing surface side reactions, and accelerating reaction kinetics. Additionally, the lithium compensation mechanism of LZP alleviates lithium concentration gradients within particles, enabling uniform Li⁺ diffusion and eliminating interlayer dislocations. This integrated approach enables LCO to exhibit stable cycling in a 4.6 V high-voltage quasi-solid-state battery system by synergistically addressing structural degradation and interfacial instability. Consequently, the modified cathode demonstrates superior capacity retention (88.5% capacity retention after 500 cycles, 4.5 V) and excellent rate capability (126 mAh g^-1 at 5 C, 4.6 V) in self-polymerized gel polymer quasi-solid-state batteries.