Co‐Engineering of In Situ Lithium Compensation and Oxygen‐Anchoring Modulation Enables Thermochemically Stabilized NCM811‐LATP Composite Cathodes for Ultra‐Long Cyclability

Abstract The development of high‐energy all‐solid‐state lithium‐ion batteries (ASSLIBs) is hindered by interfacial incompatibility between Ni‐rich layered oxides (LiNi 0.8 Co 0.1 Mn 0.1 O 2 , NCM811) and solid electrolytes (Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 , LATP), where co‐sintering‐induced Li + depletion and the subsequent oxygen release synergistically degrade electrochemical performance. Here, a dual‐functional interface engineering strategy is developed through thermochemical coordination of dynamic lithium compensation and oxygen‐anchoring modulation. Incorporation of 10 wt.% LiOH effectively suppresses cation interdiffusion during 700 °C co‐sintering, while La 4 NiLiO 8 lithium–oxygen conductor stabilizes lattice oxygen via vacancy passivation, thus eventually protecting NCM811 from thermal decomposition (post‐sintering XRD, XPS, and TEM). The optimized NCM811‐LATP composite cathode achieves a high initial capacity of 217.1 mAh g −1 at 0.1 C with 80% capacity retention over 248 cycles a 143% lifespan improvement compared to baseline systems, while maintaining charge‐transfer impedance below 97.8 Ω (EIS) and suppressing layered‐to‐spinel phase transitions (post‐cycling TEM). This work establishes a universal paradigm for oxygen‐interface co‐stabilization in ASSLIBs, balancing thermochemical stability with interfacial ion transport to unlock ultra‐stable cycling in high‐energy‐density solid‐state systems.