Influence of Modification on the Charge Transfer Process and Strain of LiNi0.6Co0.2Mn0.2O2

Abstract

Charge transfer process and strain play a significant role in determining the rate performance and safety property of energy materials. As one of highly interesting materials for portal electronics and electric vehicles, LiNi0.6Co0.2Mn0.2O2 is limited by fade and safety. Some materials, including F and AlPO4, have been adopted to modify LiNi0.6Co0.2Mn0.2O2 to improve the performance. Influence of modification on the structure, surface elemental interaction, and electrochemical performance was investigated with various techniques, and the possible mechanisms were discussed. The changes of charge transfer process and strain were investigated using Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and non-destructive strain gauges at different charge states, respectively. Fluorine modified LiNi0.6Co0.2Mn0.2O2 with different fluorine contents has been synthesized successfully at low temperature using PTFE emulsion as the fluorine resource. In comparison with that of the pristine LiNi0.6Co0.2Mn0.2O2, the initial discharge capacities of LiNi0.6Co0.2Mn0.2O2-yFy (y=0.02, 0.05, 0.08) are slightly decreased at 0.1C, but the capacity retention ratios at higher current densities, higher potential and elevated temperatures are improved. Among the samples with different fluorine contents, LiNi0.6Co0.2Mn0.2O1.95F0.05 exhibits the best performance with lowest charge transfer impedance and strain during cycles. After 50 cycles at 1.0 C between 2.8 and 4.7 V, LiNi0.6Co0.2Mn0.2O1.95F0.05 improves the capacity retention ratio and the discharge capacity of the pristine sample from 80.0% to 90.4%, and from 156.7 to 176.3 mAh g-1, respectively. In order to further improve the high-temperature performance of LiNi0.6Co0.2Mn0.2O2, AlPO4 modification has been carried out using a simple route, and the discharge capacity retention ratio after cycles at 1C at 40 °C and 60 °C were respectively 86.20% and 85.44%, much higher than that of the corresponding values of the pristine sample. Fluorine and AlPO4 modification obviously decreases the strain as well as the residual stress of LiNi0.6Co0.2Mn0.2O2 during charge/discharge processes at evaluated temperatures. Cyclic voltammetry and electrochemical impedance spectroscopy results demonstrate that fluorine and AlPO4 modification significantly decreases the electrochemical polarization and charge transfer reaction impedance of LiNi0.6Co0.2Mn0.2O2 after cycles to enhance performance. The difference between F and AlPO4 modification was compared, and the changes of charge transfer process and strain were analyzed. Modification, a facile way to strengthen charge transfer, is a good alternative to improve the electrochemical performance and safety properties of electrode materials for high-performance batteries.