层状正极材料LiNi_(0.5)Co_(0.25)Mn_(0.25)O_2的结构及电化学行为

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

采用溶胶-凝胶方法制备了正极材料LiNi0.5Co0.25Mn0.25O2。XRD、XPS测试结果表明:LiNi0.5Co0.25Mn0.25O2中阳离子排列有序度较高,层状结构明显;Co、Mn分别以+3、+4价形式存在,Ni以+2、+3价形式存在,且Ni2+与Ni3+的含量之比约为1∶1。SEM测试结果表明:正极材料LiNi0.5Co0.25Mn0.25O2结晶粒径较均匀。充放电测试结果表明:与LiCoO2相比,尽管LiNi0.5Co0.25Mn0.25O2的放电电压平台较低,但放电容量较高;在恒流充电模式下,当充电截止电压由4.35V升高至4.75V时,首次放电容量由179mAh·g-1增至201mAh·g-1,50次循环后,容量保持率由74.95%增至78.48%;在先恒流再恒压的充电模式下,电池首次放电容量为212mAh·g-1,50次循环后,容量保持率提高到87.71%。循环伏安测试表明:在2.80～4.80V扫描范围内,该正极材料发生Ni2+/Ni3+,Co3+/Co4+两对电化学反应。EIS测试表明:随着充电截止电压的增大,该正极材料的传荷电阻变小。

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 Co^3＋, Mn^4＋, Ni^2＋ and Ni^3＋; and the atom ratio of Ni^2＋ to Ni^3＋ 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 50^th 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 50^th cycle. CV and EIS tests show that two redox couples of Ni^2＋/Ni^3＋ and Co^3＋/Co^4＋ 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.