Problems and their origins of Ni-rich layered oxide cathode materials

Ni-rich layered oxides, LiNixCoyMnzO2 (NCM) and LiNixCoyAlzO2 (NCA) with x ​+ ​y ​+ ​z ​= ​1 and x ​≥ ​0.8, are regarded to be the best choice for the cathode material of high energy Li-ion batteries due to their combined advantages in capacity, working potential and manufacture cost. However, their application in practical Li-ion batteries is hindered by two essential problems of (1) performance degradation and (2) safety hazard over the whole life of battery. Performance degradation behaves as declines in battery's capacity and working voltage as well as the battery's swelling and impedance growth; Safety hazard arises from thermal runaway under abuse conditions such as overcharging, overheating, and electric shorting. It appears that nearly all problems can be ultimately attributed to the loss of oxygen, especially caused by the oxidation of lattice oxygen in H3 phase where the capacities are contributed by both of the Ni and O redox couples. In this review, the problems and their origins of Ni-rich layered oxides are overviewed, and the solutions attempted to mitigate these problems are outlined.