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

Abstract

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.

Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7

“…O K-edge X-ray absorption spectra ( Fig. 2b inset) show the emergence of a new absorption peak at 531.5 eV after the charge process, which corresponds to the excitation of an O 1 s core electron to an O 2p hole, as commonly observed for other oxygen-redox electrodes [35][36][37][38] . To monitor the detailed valence partial density of states (pDOS) of oxygen, resonant inelastic Xray scattering (RIXS) spectra were measured using an incident photon of 531.5 eV.…”

High capacity manganese layered-perovskite cathode for fluoride ion batteries involving cationic and anionic redox reaction

“…On the other hand, further fluorination beyond x ~ 2 resulted in the emergence of a new O K-edge peak at around 530.8 eV, and this peak increases its intensity with x, while it reversibly disappears upon fluorine deintercalation. A similar peak has been observed in peroxides such as Li2O2 46 and also in lithium-rich cathodes containing oxidized oxygen species (e.g., O n-, O2 -, O2 2-) 10,12,45,47 . These observations strongly suggest that in our oxyfluoride cathode, oxidized oxygen species are involved in charge compensation during the charge/discharge process for 2 < x < 4.…”

Elucidating Influence of Mg‐ and Cu‐Doping on Electrochemical Properties of O3‐Nax[Fe,Mn]O2 for Na‐Ion Batteries

“…Oxide ions are likely to contribute to the redox reactions partly as oxidation of Fe 3+ is accompanied by hole formation in the oxide ions and charge transfer through the hybridized O 2p‐Fe 3d orbitals. Therefore, by oxygen K‐edge X‐ray absorption spectroscopy for O3‐Na 5/6 FeMn and O3‐FeMnMg during charge and discharge, we investigated contribution for anion redox of oxide ions [ 26–30 ] into the redox reactions.…”