Lithium-excess 3d-transition-metal layered oxides (Li1+xNiyCozMn1−x−y−zO2, >250 mAh g−1) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes.

Oxygen vacancies (OV) are native defects in transition metal (TM) oxides and their presence has a critical effect on the physicochemical properties of the oxide. Metal oxides are commonly used in lithium‐ion battery (LIB) cathodes and there is still a lack of understanding of the role of OVs in LIB research field. Here, we report on the behavior of OVs in a single‐crystal LIB cathode during the non‐equilibrium states of charge and discharge. We found that microcrack evolution in a single crystal occurs due to OV condensation in specific crystallographic orientations generated by the continuous migration of OVs and TM ions. Moreover, understanding the effects of the presence and diffusion of OVs in metal oxides enables the elucidation of most of the conventional mechanisms of capacity fading in LIBs and provides new insights for new electrochemical applications.

Oxygen vacancies (OV) are native defects in transition metal (TM) oxides and their presence has a critical effect on the physicochemical properties of the oxide. Metal oxides are commonly used in lithium‐ion battery (LIB) cathodes and there is still a lack of understanding of the role of OVs in LIB research field. Here, we report on the behavior of OVs in a single‐crystal LIB cathode during the non‐equilibrium states of charge and discharge. We found that microcrack evolution in a single crystal occurs due to OV condensation in specific crystallographic orientations generated by the continuous migration of OVs and TM ions. Moreover, understanding the effects of the presence and diffusion of OVs in metal oxides enables the elucidation of most of the conventional mechanisms of capacity fading in LIBs and provides new insights for new electrochemical applications.

Li‐ and Mn‐rich layered oxides (LMRs) have emerged as practically feasible cathode materials for high‐energy‐density Li‐ion batteries due to their extra anionic redox behavior and market competitiveness. However, sluggish kinetics regions (<3.5 V vs Li/Li+) associated with anionic redox chemistry engender LMRs with chemical irreversibility (first‐cycle irreversibility, poor rate properties, voltage fading), which limits their practical use. Herein, the structural origin of this chemical irreversibility is revealed through a comparative study involving Li1.15Mn0.51Co0.17Ni0.17O2 with relatively localized and delocalized excess‐Li in its lattice system. Operando fine‐interval X‐ray absorption spectroscopy is used to simultaneously observe the interplay between transition‐metal–oxygen (TM‐O) redox chemistry and TM migration behavior in real time. Density functional theory calculations show that excess‐Li localization in the LMR structure attenuates TM‐O covalency and stability, leading to overall chemical irreversibility. Hence, the delocalized excess‐Li system is proposed as an alternative design for practically feasible LMR cathodes with restrained TM migration and sustainable O‐redox chemistry.

We have explored the independent effect of mixed valence and oxygen defects on the catalytic origin by artificially engineering the stoichiometry of Li2RuO3.