Hierarchical Garnet‐Type Interfacial Engineering Enables Reversible Oxygen Redox in Li‐Rich Layered Cathodes

ABSTRACT Li‐rich layered oxides (LROs) are promising high‐capacity cathodes for next‐generation Li‐ion batteries, yet suffer from irreversible oxygen release and interfacial degradation, leading to voltage decay, capacity fading, and gas evolution. Herein, we report an i n situ constructed garnet‐type Li 5 La 3 Nb 2 O 12 (LLNO) fast‐ion conductor coating on LRO particles via a sol–gel process using C 4 H 4 NNbO 9 ·nH 2 O as the Nb precursor. During pyrolysis, released NH 3 acts as a mild reducing agent, promoting surface reconstruction accompanied by near‐surface Nb 5+ doping, formation of a spinel transition layer, and a gradient oxygen‐vacancy distribution. The resulting hierarchical architecture integrates: (i) interfacial protection and chemical stability from the LLNO coating, (ii) structural coherence and multidimensional Li + transport from the spinel layer, and (iii) reversible oxygen‐anion redox stabilization enabled by Nb 5+ –O bonding and oxygen vacancies. Benefiting from these cooperative effects, the optimized cathode delivers 281.7 mAh g −1 at 0.1 C and maintains 97.6% capacity retention after 200 cycles at 1 C, outperforming the pristine counterpart. This work demonstrates a multifunctional interfacial‐engineering strategy that unifies coating, doping, and surface reconstruction, providing a generalizable pathway toward durable, high‐energy LROs.