Enhanced rate performance and high potential as well as decreased strain of LiNi0.6Co0.2Mn0.2O2 by facile fluorine modification

Abstract

Fluorine is interesting for improving the electrochemical performance of current electrode materials, but the preparation process faces challenges because of the difficult homogeneous mixing of powders. A facile novel strategy of surface fluorine modification using polytetrafluoroethylene (PTFE) emulsion at low temperature, where the powders are easy to be homogeneously mixed and evenly covered by fluorine polymer, has been proposed to enhance the electrochemical performance of LiNi0.6Co0.2Mn0.2O2 material. Though the initial discharge capacities of LiNi0.6Co0.2Mn0.2O2-yFy (y = 0.02, 0.05, 0.08) are slightly decreased at 0.1C, the capacity retention ratios at higher current densities, higher potential and elevated temperatures are improved compared to that of the pristine one. Among the samples with different fluorine contents, LiNi0.6Co0.2Mn0.2O1.95F0.05 exhibits the best performance, and the average discharge capacity fade rate in one cycle and the residual stress after 50 cycles at 1.0 C, as well as the discharge capacity after 10 cycles at 4.0 C are respectively about 0.03%, 10.3 MPa, 129.5 mAh g−1, while the corresponding values of the pristine samples are around 0.12%, 29.4 MPa, and 99.7 mAh g−1, respectively. 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. The electrode reaction process and fluorine modification mechanisms have been discussed using various techniques. This method is easily achievable and may be used to modify other powder materials via a simple solid-solution mixing and an in-situ surface modification.