Structural Characteristics and Electrochemical Behavior of Layered LiNi_(0.5)Co_(0.25)Mn_(0.25)O_2 as Cathode Materials of Lithium Ion Batteries

Cathode materials of LiNi0.5Co0.25Mn0.25O2 was synthesized by Sol-gel method in this study. XRD and XPS results reveal that the obtained LiNi0.5Co0.25Mn0.25O2 exhibits a typical layered structure composed of highly ordered Co3+, Mn4+, Ni2+ and Ni3+; and the atom ratio of Ni2+ to Ni3+ is 1∶1. A morphology of homogeneously distributed and well cyrstallized is observed by SEM. Compared with LiCoO2, the resulted materieals possess larger capacity and lower voltage plateau. It is also found that the initial discharge capacity and capacity retention of 50th cycle of LiNi0.5Co0.25Mn0.25O2 increase from 179 mAh·g-1 to 201 mAh·g-1, from 74.95% to 78.48%, respectivley, when the cutoff voltage is changed from 4.35 V to 4.75 V. Under the discharge-charge model of initial constant current and subsequent constant voltage, an initial discharge capacity of 212 mAh·g-1 can be achieved and the material keeps 87.71% of its initial capacity after 50th cycle. CV and EIS tests show that two redox couples of Ni2+/Ni3+ and Co3+/Co4+ are observed in the potential range of 2.80~4.80 V, and charge transfer resistance of the cathode decreases as the charge cutoff voltage increased.