Figure 2. (a) Voltage profile at first cycle, (b) Voltage profile at 80th cycle, (c) Cycling performance, and (d) Coulombic efficiency of GR100 and baseline samples with NMC622 full-cells.
The battery performance of 100GR and the baseline with NMC622 full-cells are shown in Figure 2 . The 100GR full cell exhibits an initial discharge capacity of 140.27 mAh.g-1, while the baseline only displays an initial value of 48.16 mAh.g-1 (Figure 2 a). After 80 cycles, the 100GR full cell delivered a specific capacity of 51.35 mAh.g-1 (Figure 2 b), which was reduced by 63.4% compared to the first cycle. Meanwhile, the baseline one displays a capacity of 32.50 mAh.g-1, which only decreased by 32.9% compared to the first cycle. These results could be explained by the porous and amorphous structure of SiO2/C facilitated a considerable amount of Li+ ions from the NMC cathode to intercalate in by shortening the Li+ ion diffusion pathway, resulting in the significant charging capacity. [10] However, the superior specific surface area of SiO2/C material reacted strongly with electrolyte to consume Li+ ions for SEI layer formation, which indicated “active lithium loss”. Hence the discharged capacity of the baseline is lower than the 100GR.
Furthermore, the cycling performance and Coulombic efficiency of the baseline were more stable than 100GR (Figure 2 c, d). Although the 100GR sample possesses a much higher discharged capacity than the baseline sample, capacity retention is lower (Figure 2 c). Minh et al. reported that the 100GR||NMC622 full-cell using the same electrolyte solution (1.2 LiPF6, EC: EMC 3:7 v/v) with N/P ratio of 1.24 delivered a discharged capacity of only 140.5 mAh.g-1 and remained 9.8 % after 30 cycles, respectively. [25] Moreover, the previous study reported that the well-designed graphite||NMC622 batteries retained about 50% initial capacity during the cell lifespan. [14] This rapid capacity fading of graphite anode full-cell may associate with active lithium loss due to the growth of the SEI layer or physical delamination of the graphite particles. [26] Aiping Wang et al. showed that the decomposition of ethylene carbonate (EC) during the cycling process would thicken the SEI layer and consequently decrease capacity retention. [14] In order to enhance the long-cycling performance of the baseline sample, the CM and EM methods were carried out to optimize the full-cell capacity.
Regarding the pre-lithiation SiO2/C using the direct contact method (CM) with lithium foil for a defined time, it can be expected that an adjustable amount of Li+ can be used for compensating active lithium loss during the first cycles for SiO2-based anode without promoting the Li dendrite formation. Figure 3 shows the battery performance in various contact times, including 10, 20, 30, and 40 minutes. The CM30 sample shows the highest capacity, while the baseline sample only displays the lowest capacity in the first and 80th cycles (Figure 3 a, b). The first reversible capacity of CM30 is 99.08 mAh.g-1, whích declines by 27.94% after 80 cycles, while these values for the baseline sample are 48.43 mAh.g-1 and 32.50 mAh.g-1, respectively. According to cycling performance and Coulombic efficiency (Figure 3 c, d), the CM30 sample continually witnesses the highest capacity during the testing time and maintains stable Coulombic efficiency of around 98% for 80 cycles. Furthermore, increasing the contact time enhances not only the discharged capacity but also improves the coulombic efficiency. Indeed, by increasing the contact time, the initial Coulombic efficiency increases from 84.42% (CM10 sample) to the highest of 93.23 % for the CM30 before declining by the time in CM40. Recently, similar results were demonstrated by Gerrit Michael Overhoff et al. with Si/C||NMC811 full-cell by varying the contact times. [21] As a result, the amount of Li+ ion was compensated to enhance battery performance by the CM method. Nevertheless, increasing the contacting time (over 30 minutes) leading to the deposition of lithium metal on the surface of the anode as well as the thickness of the SEI layer are the main reasons to explain the fast decrease of performance in the CM40 sample.