Figure 3. (a) Voltage profile at first cycle, (b) Voltage profile at 80th cycle, (c) Cycling performance, and (d) Coulombic efficiency of baseline, CM10, CM20, CM30, and CM40 samples with NMC622 full-cells.
FT-IR Spectroscopy was further performed to evaluate the formation of the SEI layer on the anode surface by analyzing the vibrational state of chemical bonds. Figure 4 exhibits similar qualitative spectra but differentiates quantitatively in peak intensity, indicating the heterogenous formation of the SEI layer. In general, there are three main frequency regions observed, and each of these regions is associated with typical group frequencies: 4000 – 3000 cm-1 assigned for the O – H stretching, 3000 – 1750 cm-1 for the C – H stretching, and 1700 – 800 cm-1 for the C = O stretching regions, as earlier reported by Guorong V. Zhuang et al. [27] The wavenumber region of 2800 – 2000 cm-1 might represent the CO2 molecular absorbed by LiOH or Li2O, which happens during sample reservation and testing process (Figure 4 ). In addition, the SEI layer was reported to be composed of Li2CO3 and lithium alkyl carbonate (ROCO2Li) compounds due to the decomposition of ethyl carbonate (EC) electrolyte. [28] Thereby, IR fingerprints of these samples proved the present of Li2CO3 on the electrode surface by the broad band of 920 – 800 cm-1 and the double peak at 1520 – 1380 cm-1 wavelength. It can be observed that the peak intensity of Li2CO3 also increases with the longer contact duration, which is consistent with the explanation of the thick SEI layer. Additionally, three wavelength regions including 1820 – 1740 cm-1, 1100 – 1000 cm-1, and 780 – 600 cm-1 represent organic carbonate from ROCO2Li compounds was observed and also increased with the contact time. [28]