Metal-to-metal charge transfer for stabilizing high-voltage redox in lithium-rich layered oxide cathodes.

Abstract

Apart from conventional redox chemistries, exploring high-voltage anionic redox processes, such as pure oxygen or high-valent transition metal ion redox, poses challenges due to the instability of O nonbonding or O-dominant energy states. These states are associated with destructive behaviors in layered oxide cathodes, including local structural distortion, cationic disordering, and oxygen gas evolution. In this study, we suppress first-cycle voltage hysteresis and irreversible O₂ evolution in Li-rich oxide cathodes through covalency competition induced by the substitution of electropositive groups. We found that the nonequivalent electron distribution within an asymmetric M_A-O-M_B backbone (metal-to-metal charge transfer via oxygen ligands) increases electron density on electronegative transition metal ions, preventing them from reaching unstable oxidation states within an operating voltage range. This phenomenon is observed across diverse transition metal combinations, providing insights into controlling unnecessary oxygen redox activity. Our findings open new avenues for controlling intrinsic redox chemistry and enabling the rational design of high-energy density Li-rich oxide cathodes.