The results of the high magnification microscope showed that the color of cathode changed from black to golden gradually with SOC increasing shown in Figure 1. This phenomenon probably resulted from the formation of LiC6 and lithium metal produced by insertion of lithium ion into cathode carbon layer2. From the SEM images, it can be seen that the cathode were getting powdery with an increase in SOC. When SOC was 0%, the XRD result of cathode showed four diffraction peaks, which were at 2θ=25.23 with d=3.52, 2θ=26.40 with d=3.38, 2θ=42.36 with d=2.13 and 2θ=44.50 with d=2.03 respectively. Compared with the XRD standard spectra, they can be ascribed to plane (101), (102), (100) and (101) of LiC6 and LiC12. The diffraction peak intensity of LiC12 which is produced at the beginning of charge reduced2, while the diffraction peak intensity of LiC6 increased along with the lithium ions intercalating into the graphite. From the DSC results, it can be seen that there were two exothermic peaks for cathode. The first exothermic peak which occurred at ~112 ℃ had no change with SOC increasing for the decomposition of electrolyte and SEI (solid electrolyte interface). The other one occurred at ~280 ℃, and its output heat increased with an increase in SOCs.
From the SEM results of anode, it can be seen that an identical shape appeared below SOC=100%, whereas an obvious aggregation could be found in the SOC=120% sample for the lithium ion exchange reaction2. From the XRD results, it could be seen that all diffraction peak angles of anode are getting broader and shifting to lower diffraction angle with SOC increasing. This phenomenon suggests that an internal stress is induced by lithium extraction3. When SOC was 0%, the DSC results of anode showed that there was one endothermic peak at ~169 ℃ and two exothermic peaks. The two exothermic peaks were at ~289 ℃ and ~347 ℃.The endothermic peak could be ascribed to the melting of LiPF64. The onset temperature of the two exothermic peaks which are caused by the reactions between oxygen with electrolyte shifted to lower position with an increase in SOC. These reactions become more intense due to the increasing amount of releasing oxygen of anode.
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