Recent Advances on Surface Modification of Li- and Mn-Rich Cathode Materials

With the rapid development of electric cars and energy storage power stations, there is an increasing demand for litluiun ion batteries with high energy density. Li- and Mn-rich (LMR) cathode materials with large specific capacity (>250 mAh.g(-1)) are supposed to accomplish lithium ion batteries with high energy density (>350 Wh.kg(-1)). The high capacity performance of LMR cathode materials are resulted from the lattice oxygen redox reaction induced by the electrochemical activation of the Li2MnO3 phase. However, the activation of the Li2MnO3 phase and oxygen redox reaction lead to lattice oxygen release and structure transformation, which cause some serious problems such as low initial columbic efficiency, poor rate capability, voltage and capacity degradation after subsequent cycles. The oxygen release and structure transformation always start from the surface, indicating that the surface stability is significant to LMR cathode materials. In this paper, surface modifications such as surface coating, surface doping and surface chemical treatment are reviewed and the mechanism of three surface modification methods for LMR cathode materials are discussed in further. Surface coating is one of the most widely surface modification methods, which can suppress the electrode/electrolyte side reaction and reduce the transition metal dissolution. The effect of surface coating on improving electrochemical performance of LMR cathode materials is always determined by the characteristic of coating layer materials including non-active coating layer, electrochemical active coating layer, Li+ conductive coating layer and electronic conductive coating layer. Surface doping has shown to be an effective method in suppressing oxygen release and structural transformation. Surface chemical treatment has resulted in reducing irreversible capacity loss by activating Li2MnO3 phase. On this basis, surface integrated strategies combined several surface modified methods are introduced and discussed in recent years. The surface intergrated strategies not only enhance the structural stability and suppress electrode/electrolyte surface-interface reaction, but also have an effective role on mitigating structure transformation and lattice oxygen release. Finally, we wish that our review would provide research directions for surface modified strategies of LMR cathode materials in future.