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.