Improved anionic redox reversibility of layered oxides by modulating transition metal–oxygen bonds for sodium ion batteries†

Abstract

Activating oxygen anionic redox within transition-metal layered oxides as cathode materials is a promising strategy to surpass the capacity constraints typically associated with conventional cationic redox processes in sodium-ion batteries. However, irreversible depletion of lattice oxygen during the charge process usually causes structural instability, resulting in the poor cycling capacity of layered oxides. Herein, a facile magnesium incorporation strategy for Na_0.72Li_0.24Mn_0.76O₂ (NLMO) cathode material was used to stabilize the structure by reducing the overall transition metal–oxygen (TM–O) bond lengths and expanding the sodium layer spacing. Theoretical calculations and electron paramagnetic resonance (EPR) results further confirm that the magnesium incorporation is beneficial for improving the overlap between Mn 3d and O 2p orbitals, thereby enhancing the stability of lattice oxygen. Modulating TM–O bonding covalency is crucial in enhancing the structure stability and electrochemical performance of layered oxide cathode materials for sodium-ion batteries.