Transmission electron microscopy studies of structural degradation behavior of LiNi0.6Co0.2Mn0.2O2 cathode materials

The variations in the structural and chemical features of LiNi0.6Co0.2Mn0.2O2 (LMO2: L = Li, M = Ni0.6Co0.2Mn0.2) materials were investigated via transmission electron microscopy. The LMO2 materials were used as a cathode material in a Li-battery operated at cutoff voltages of 4.15 and 4.45 V, and subjected to as many as 2200 charge–discharge cycles. Charged and discharged materials of LMO2 had Li concentrations of 0 and ~16% at the surface region, respectively, while the c lattice constant varied by up to 2.5%. The spinel phase of Li(Ni0.6Co0.2Mn0.2)2O4 (LM2O4: L = Li, M = Ni0.6Co0.2Mn0.2) appeared reversibly during charging up to 1200 cycles at room temperature; however, the LM2O4 phase was irreversibly maintained when the battery was discharged up to 2200 charge–discharge cycles at the high temperature of 45 °C. In addition, the volume of the lattice cells of the layered LMO2 differed by ~6% from the equivalent of the spinel-type LM2O4 structures. On the other hand, two types of models were suggested for the generation of short-range-ordered (SRO)-type layered structures. In particular, the pseudo-SRO models are applicable to the surface region of the samples discharged, or the inner region of the sample discharged after being cycled like 2200 times at high temperature (45 °C).