Anionic redox behaviors of layered Li-rich oxide cathodes

Lithium-rich and manganese-based oxides (LRMO) with anionic redox behavior are regarded as the cathode material for the next generation commercial lithium-ion batteries (LIBs) that are most likely to achieve the goal of long-distance cruise for electric vehicles (EVs), and the high-capacity, low-cost and environmentally friendly characteristics of LRMO have attracted a lot of research. This article reviewed the investigation of the structure and electrochemical behavior for LRMO in recent years, especially the reaction of anionic redox that is the origin of the high capacity of LRMO. The inseparability between the anionic redox and crystal configuration has been emphasized from the perspective of electron and energy band structure, and the generation of non-bonding O 2p states is a prerequisite for anionic redox. A global understanding of the reaction and structural evolution of LRMO has been drawn through outlining the reversible retention and irreversible transformation of anionic redox. The support for the reversibility of the reaction comes from the maintenance of the O 2p state by the structural changes during the deintercalation process of lithium, but the disadvantage is voltage hysteresis. However, the irreversible O loss and phase transition caused by the surface oxygen evolution are the beginning of the degradation for LRMO, and the resulting voltage and capacity decay shackle the application of LRMO cathode materials critically. This article suggests that the coexistence of reversibility and irreversibility for anionic redox also points out the way for the modification of LRMO. We look forward to the steady application of LRMO in the field of high energy density batteries by modifying unstable oxygen.