Towards controlling the reversibility of anionic redox in transition metal oxides for high-energy Li-ion positive electrodes

Anionic redox in positive electrode materials in Li-ion batteries provides an additional redox couple besides conventional metal redox, which can be harvested to further boost the energy density of current Li-ion batteries. However, the requirement for the reversible anionic redox activity remains under debate, hindering the rational design of new materials with reversible anionic redox. In this work, we employed differential electrochemical mass spectrometry (DEMS) to monitor the release of oxygen and to quantify the reversibility of the anionic redox of Li2Ru0.75M0.25O3 (M = Ti, Cr, Mn, Fe, Ru, Sn, Pt, Ir) upon first charge. X-ray absorption spectroscopy, coupled with density functional theory (DFT) calculations, show that various substituents have a minimal effect on the nominal metal redox, yet more ionic substituents and reduced metal-oxygen covalency introduce irreversible oxygen redox, accompanied with easier distortion of the M-O octahedron and a smaller barrier for forming an oxygen dimer within the octahedron. Therefore, a strong metal-oxygen covalency is needed to enhance the reversible oxygen redox. We proposed an electron-phonon-coupled descriptor for the reversibility of oxygen redox, laying the foundation for high-throughput screening of novel materials that enable reversible anionic redox activity.