Synergetic impact of dual substitution on anionic-Cationic activity of P2-type sodium manganese oxide

Abstract

P2-type sodium-deficient cathodes with the local Na-O-A configuration (A=Li, Mg, Zn, and vacancy) have attracted great interest due to their high capacity, stemming from the additional contribution of the oxygenredox chemistry. However, such materials suffer from irreversible oxygen redox, oxygen release, structural distortion, and capacity fading. Herein, we introduce a dual-doping Li/Cu strategy to improve the properties of P2-NaxMnO2 cathode material. The unique sodium-storage mechanism in P2-Na0.75[Li0.15Cu0.15Mn0.7]O2 is verified using operando X-ray diffraction (o-XRD), X-ray absorption, X-ray photoelectron spectroscopy, and magnetic susceptibility methods. O-XRD and 7Li solid-state nuclear magnetic resonance analysis reveal that the P2 phase is maintained within charge and discharge; however, at high voltage, a small portion of O-type stacking faults are emerged due to the migration of Li to octahedral Na sites. Furthermore, various spectroscopy and magnetometry techniques combined with density functional theory calculation demonstrate the stable activity of cationic Cu2+/Cu3+ and Mn3+/Mn4+ and anionic O2-/(O2)n- redox processes. This work highlights the efficacy of Li and Cu doping in P2-type layered materials and the contribution of oxygen redox to additional capacity at high voltages, suggesting the potential for future development of cathode materials for Na-ion batteries with oxygen redox.