Rechargeable Li-ion batteries with higher energy density are in urgent demand to address the global challenge of energy storage. In comparison with anode materials, the relatively low capacity of cathode oxides, which exhibit classical cationic redox activity, has become one of the major bottlenecks to reach higher energy density. Recently, anionic activity, such as oxygen redox reaction, has been discovered in the electrochemical processes, providing extra reversible capacity for certain transition-metal oxides. Consequently, a more complete understanding and precise controlling on anionic electrochemical activity in these high-capacity oxides have become a flourishing, yet challenging subject.

Compared to TM cations, oxygen anion electrochemical activity is more challenging to experimentally prove and quantify for the following reasons: First, oxygen participates in the electrochemical activity at highly charged states when the partially delithiated oxides are typically very sensitive to high energy electron beam or X-ray source exposure. Second, peroxo-like species and O-holes resulted from oxygen oxidation are extremely reactive and unstable in air and carbonate based liquid electrolytes. Third, M(nd)−O(np) metal−ligand hybridization and rehybridization make it intricate to quantify and differentiate cation and anion contributions to electronic structure changes as well as their contributions to the extra capacity. Based on the above considerations, a multimodal characterization approach must be established for oxygen anion electrochemical activity.

Here, we select Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ as the model compound and systematically study its structure response to the oxygen redox activity by combing neutron pair distribution function, X-ray absorption spectrum, resonant inelastic X-ray scattering and DFT calculation. An obviously increased short O-O pairs accompanying oxygen redox reactions is for the first time detected, and which should be ascribed to the shrink of the interlayer O-O distance. Also, theoretical analysis indicates that the selective decrease of O-O distance is originated from the different oxygen chemical coordination environment. Further, a coupling relationship between the oxygen redox and transitions metals migration is experimentally demonstrated, which reveals the structural origin of the stable lattice oxygen redox reactions. Based on these understandings in our work, an optimization guidance for anionic acitvity is unambiguously presented: designing a robust layered structure with high-covalency TMs while constructing a flexible local structure with high-ionicity TMs to achieve the reversible high energy density.