Rational Design of NMM811: A Cobalt‐Free Cathode Rivalling NMC811 in Performance and Stability

ABSTRACT The development of Co‐free, Ni‐rich layered oxide cathodes is crucial for cost‐effective, energy‐dense lithium‐ion batteries (LIBs). Yet, complete Co elimination remains challenging due to structural and electrochemical instability of high‐voltage cathodes. Using LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) as a reference, herein we demonstrate that replacement of 10 mol% Co with Mg is practically feasible. The Co‐free Li 1.1 Ni 0.8 Mn 0.1 Mg 0.1 O 2 (NMM811) exhibits markedly reduced antisite defect by increasing Ni–Li exchange energy barrier (∼305.1 meV vs ∼194.6 meV for NMC811), facilitated by partial oxidation of Ni 2+ and Li + migration toward NiO 6 octahedra, leading to expanded inter‐slab spacing and enhanced structural stability. Density functional theory reveals that Mg 2+ widens bandgap and strengthens Mg─O bonding, mitigating oxygen loss. The NMM811 delivers initial capacity of ∼197 mAh g −1 and energy density ∼620 Wh kg −1 . Importantly, the absence of H2→H3 phase transformation affords improved capacity of ∼82 mAh g −1 after 500 cycles at 1C (∼43 mAh g −1 in NMC811) with reduced voltage fading rate (∼0.39 mV cycle −1 ). Synchrotron‐based X‐ray absorption studies confirm robust Ni–O coordination prevents structural collapse, while differential scanning calorimetry indicates improved thermal stability (exothermic peak ∼197°C, 1607 Jg −1 ). Overall, the findings from this work establish NMM811 as a structurally resilient, thermally safe, Co‐free cathode for next‐generation LIBs.