Achieving Reversible Anionic Redox via Homogeneous Transition Metal-Oxygen Coordination in Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are promising high-capacity cathode materials for next generation Li-ion batteries, but their practical application is hindered by voltage decay and capacity fading, which primarily originate from irreversible oxygen behaviors. Given that transition metal-oxygen (TM─O) bonding is crucial for stabilizing anionic redox, this study reveals the critical role of elemental composition in determining the homogeneity of the TM-O coordination environment within LLOs. This homogeneity directly influences the electrochemical behavior and structural stability of the material. Combining in situ X-ray diffraction (XRD) and density-functional theory (DFT) calculations on various model compounds, we demonstrate that while Co thermodynamically enhancing the Mn─O bonds, it forms highly covalent Co─O bonds that disrupt the uniformity of the TM─O bonding network. This inhomogeneity kinetically promotes irreversible ligand-to-metal charge transfer, exacerbates lattice strain along c-axis, and accelerates oxygen loss. In contrast, Ni promotes a homogeneous TM-O coordination environment, facilitating reversible charge compensation and accommodating lattice strain through gentle ab-plane expansion. Consequently, the Ni-rich cathodes achieve superior cycling stability and voltage retention. Our findings establish that a uniform TM─O bonding network is more crucial than the absolute bond strength for achieving reversible anionic redox, providing a new design principle for stable and high-energy cathode materials. © 2026 Wiley-VCH Gmb.