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

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 Na2/3[Mn7/9Mg1/9□1/9]O2 (□ 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 Mg2+ 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 TMO6 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 TMO2 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.