Beyond Superlattice Order: Full‐Composition Li Stoichiometry Screening Identifies Interfacial Spinel Passivation as the Decisive Factor for Durable Cobalt‐Free Li‐Rich Cathodes

Cobalt‐free Li‐rich layered oxides (LRMs) hold great promise for next‐generation high‐energy batteries, yet their practical deployment is severely hindered by irreversible oxygen redox and progressive interfacial degradation. Here, a full‐range Li stoichiometry screening across Li 1+0.65 x Ni 0.35 Mn 0.65 O 2+0.65 x (0 ≤ x ≤ 1) was conducted to decouple bulk superlattice evolution from surface chemistry. A structural crossover at x ≈ 0.5 with minimized Li 2 MnO 3 long‐range ordering is identified, yet optimal electrochemistry unexpectedly emerges at x = 0.375, governed by a triple synergistic mechanism: finely dispersed Li 2 MnO 3 nanodomains, moderate Li + /Ni 2+ antisite pillars, and—most critically— an in situ coherent spinel passivation interphase spontaneously formed during calcination. The optimized LRM0.375 delivers 227.1 mAh g −1 at 0.1C, 161.8 mAh g −1 at 5C, and an ultralow voltage fade of only 1.33 mV cycle −1 , with its graphite full cell retaining 78.48% capacity after 500 cycles. Operando XRD, RIXS, and DEMS collectively demonstrate that the spinel surface barrier, rather than bulk superlattice ordering alone, governs anionic redox reversibility and cycling durability. This work challenges the superlattice‐centered design paradigm and identifies interfacial spinel passivation as a key stability‐governing factor within a triple synergistic mechanism, providing targeted stoichiometric guidelines for designing durable anionic‐redox cathodes beyond conventional bulk‐structure‐focused strategies.