Quantifying Effects of Ligand–Metal Bond Covalency on Oxygen-Redox Electrochemistry in Layered Oxide Cathodes

Oxygen-redox electrochemistry is attracting tremendous attention due to its enhanced energy density for layered oxide cathodes. However, quantified effects of ligand-metal bond covalency on the oxygen-redox behaviors are not fully understood, limiting a rational structure design for enhancing the oxygen redox reversibility. Here, using Li₂Ru_1-xMn_xO₃ (0 ≤ x ≤ 0.8) which includes both 3d- and 4d-based cations as model compounds, we provide a quantified relation between the ligand-metal bond covalency and oxygen-redox electrochemistry. Supported by theoretical calculations, we reveal a linear positive correlation between the transition metal (TM)-O bond covalency and the overlap area of TM nd and O 2p orbitals. Furthermore, based on the electrochemical tests on the Li₂Ru_1-xMn_xO₃ systems, we found that the enhanced TM-O bond covalency can increase the reversibility of oxygen-redox electrochemistry. Due to the strong Ru-O bond covalency, the thus designed Ru-doped Li-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode shows an enhanced initial coulombic efficiency, increased capacity retention, and suppressed voltage decay during cycling. This systematic study provides a rational structure design principle for the development of oxygen-redox-based layered oxide cathodes.