Construction of Co/Ni-Free P2-layered metal oxide cathode with high reversible oxygen redox for sodium ion batteries

To improve the energy and power density of P2-type layered cathode materials in sodium ion batteries, triggering oxygen-related activity is a promising strategy, which has attracted an amount of attention. However, stimulating lattice oxygen activity may inevitably yield adverse effects, such as detrimental structure distortion and irreversible voltage decay. Herein, a low-cost Co/Ni free layered P2-type Na0.67Mn0.6Cu0.3Mg0.1O2 cathode is designed to construct reversible anion reduction. Na0.67Mn0.6Cu0.3Mg0.1O2 exhibits an obvious anion reduction electrochemical behavior, confirming that oxygen occurred redox reactions and participated in the capacity compensation process. The density functional theory calculation proves that the presence of the covalency between Cu and O and the lager electron delocalization areas increase the anions redox activity. Furthermore, this compound provides an excellent cyclic stability and high capacity retention of 86 % at 0.2C rate after 100 cycles. In-situ Raman and ex-situ X-ray diffraction absorption spectroscopy reveal the reversible structure evolution and robust P2-type structure. The interaction between Cu and Mg plays a positive role in building the reversible anion reduction in terms of constructing electronic structure and regulating charge balance. Therefore, the Na0.67(Mn-Cu-Mg)O2 system provides an insight into designing a highly stable cathode material with anion reduction for sodium ion batteries.