Charge Compensation Mechanism of Lithium-Excess Metal Oxides with Different Covalent and Ionic Characters Revealed by Operando Soft and Hard X-ray Absorption Spectroscopy

The charge/discharge capacity of current lithium-ion battery cathode materials is limited by the charge compensation of transition-metal redox during the charge/discharge processes. Recently, the use of oxide ion redox for charge compensation has been proposed to realize a higher charge/discharge capacity than that observed for transition-metal redox. Different stabilization mechanisms of the reversible oxide ion redox have been proposed. To clarify the mechanism, analysis of the electronic and local structures around oxygen is required. Because of the high-voltage region in which the oxide ion redox occurs, several reactions such as oxygen gas evolution and/or electrolyte oxidation are often included. Thus, operando measurements are required to directly prove this concept and generalize the understanding of the oxide ion redox. This study employs operando soft/hard X-ray absorption spectroscopy combined with X-ray diffraction spectroscopy for four lithium-excess electrode materials with different chemical bond natures. The experimental data together with online analysis of the generated on-charge gas reveal two extreme cases: significantly enhanced covalent or ionic characters in the metal–oxygen chemical bonds, which are necessary conditions to stabilize the oxidation of the oxide ions. This finding provides new insights with exciting possibilities for designing high energy density cathode materials based on anion redox.