Stabilizing anionic redox in Mn-rich P2-type layered oxide material by Mg substitution

Abstract

Anionic oxygen redox reactions provide additional reversible capacity beyond 4 V in Mn-rich layered oxide cathodes; however, the irreversible anionic redox intensifies the structural deterioration, resulting in capacity fading. This study reports a stable anionic redox in Mn-rich P2-type Na0.67Mn0.8Fe0.1Ni0.1O2 by substituting Ni via Mg in the TM layer. The similarities in size and valence state of Mg2+ and Ni2+ confirm the homogenous substitution of Mg2+ into the TM layer, maintaining the layered structure. Mg2+ ion in the TM layer provides thermodynamic phase stability and facilitates the extra charge on the O atom that aggravates the oxidation of O, mitigating irreversible oxygen redox. As a result, Na0.67Mn0.8Fe0.1Mg0.1O2 displayed better reversibility in an expanded voltage range of 1.5–4.5 V and improved rate capability compared to Na0.67Mn0.8Fe0.1Ni0.1O2. Furthermore, Mg substitution reinforces the reversibility of the P2-O2 phase transition and improves Na+ diffusion kinetics. Hence, the profound study on Mg substitution in Mn-rich P2-type layered oxide is constructive to stabilize the anionic oxygen redox.