Superiority of native vacancies in activating anionic redox in P2-type Na2/3[Mn7/9Mg1/9□1/9]O2

Abstract

Anionic redox is another way of charge compensation to boost the energy density of the cathode materials. In addition to the conventional Li–O–Li configurations in the well-known layered Li-rich transition metal oxides, both the Na–O–Mg and Na–O-vacancy configurations have been found effective in triggering the oxygen redox for charge compensation. Therefore, it will be interesting to compare their superiority in activating the anionic redox reaction. Herein, P2-type Na 2/3 [Mn 7/9 Mg 1/9 □ 1/9 ]O 2 (□ for vacancy) is designed that contains both vacancies and magnesium ions in its transition metal (TM) layer. The co-existence of equal quantity of vacancies and Mg 2+ ions makes it possible to compare the superiority of these two configurations. Soft X-ray absorption spectroscopy and density functional theory calculations demonstrate that the vacancies are superior in facilitating the oxygen redox at low voltage. In addition, the asymmetry and flexibility of the vacancy-containing TMO 6 octahedrons ensure the high structural stability of the material. It delivers a reversible capacity of 212 mAh g −1 between 1.5 and 4.5 V and the reversibility of oxygen redox in 50 cycles between 2.1 V and 4.4 V. The capacity retention is good in 50 cycles between 2.1 V and 4.4 V. The strategy of introducing vacancies in the TMO 2 slab to stabilize the structure and to enhance the anionic redox will inspire new ideas in seeking and designing advanced cathode materials with superior performances. The existence of equal quantities of the Mg 2+ ions and vacancies in the TM layer of P2-type Na 2/3 [Mn 7/9 Mg 1/9 □ 1/9 ]O 2 provides an opportunity to compare their superiorities in triggering the oxygen redox and in stabilizing the structure. Our findings will inspire new ideas in designing new cathode materials with high energy densities and performance stability. • A P2-type Na 2/3 [Mn 7/9 Mg 1/9 □ 1/9 ]O 2 was prepared as a cathode material for the Na-ion batteries. • It contains equal quantities of the Mg 2+ ions and vacancies in the TM layer. • The vacancies are found superior to the Mg 2+ ions in charge compensation.