Understanding the degradation mechanisms of layered lithium nickel-cobalt-manganese oxides as a cathode material for lithium ion batteries

Abstract

LiNixCoyMnzO2 (NCM, 0 < x,y,z < 1, x+y+z=1) has become one of the promising cathode materials for next generation lithium ion batteries due to its high capacity and the cost effectiveness compared to LiCoO2. However, the high voltage operation of NCM ( > 4.3 V) that is required for a higher capacity inevitably accompanies more rapid capacity fading over cycles. Here, the degradation mechanisms of LiNi0.5Co0.2Mn0.3O2 are investigated during cycling at various charge cut-off voltage conditions. The surface crystal structure of the LiNi0.5Co0.2Mn0.3O2 suffers from an irreversible transformation, and the type of a surface structure varies depending on the cut-off voltage condition. The surface of the pristine rhombohedral phase transforms into a mixture of spinel and rock salt phases which contributes to a gradual increase in charge transfer resistance. Furthermore, the formation of the rock salt phase is more dominant with a higher voltage operation (~ 4.8 V), which is due to the highly oxidative environment that triggers the oxygen loss from the surface of the material. The presence of the ionic insulating rock salt phase may cause a sluggish kinetics, thus, deteriorate the capacity retention. It implies that the prevention of the surface structural degradation can provide a way to the high capacity and stable cycle life of LiNi0.5Co0.2Mn0.3O2 for high voltage operation.