## 3 Results and Discussion
### Electrochemical Measurement  
Figure 4 shows the cyclic voltammetry of the pristine and SiO\({}_{2}\)-coated NMC622 electrodes in the measured voltage range of 3.0-4.3 V vs. Li/Li+ with a scan rate of 0.05 mV/s. Only one oxidation peak and one reduction peak are observed. This implies that SiO\({}_{2}\) coating is electrochemically inactive in the testing voltage range. The first oxidation peak of both sample is found at a higher potential (approximately 0.1 V) compared to the following peaks, which indicates the irreversible reactions during the initial cycle. The oxidation peak of the SiO\({}_{2}\)-coated NMC622 electrode shows a slighter less shift (from 3.85 to 3.78 V) compared to that of the pristine NMC622 electrode (from 3.99 to 3.84 V) after the first cycle. Figure 5 compares the rate capability of the pristine and the SiO\({}_{2}\)-coated NMC622 samples with discharge rates of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C (the specific capacity is shown in Figure S1). The NMC622 material initially had outstanding rate performance, compared to 0.1 C, at 2 C and maintained 84% of its initial capacity with excellent reproducibility. The SiO\({}_{2}\) coating layer did not deteriorate the rate capability of the NMC622 material, as the amorphous SiO\({}_{2}\) coating layer probably  
Figure 4: CV curves of (**a**) pristine and (**b**) SiO\({}_{2}\)-coated NMC622 electrodes. Figure 3: TEM images of (**a**) SiO\({}_{2}\)-coated NMC622 particle edge and (**b**) elemental mapping by EDX. allowed the pass of Li-ions because of its lower dense network structure, which did not significantly affect the ionic conductivity. The cycling performance of the pristine and SiO\({}_{2}\)-coated NMC622 electrodes is shown in Figure 6. Long-term cycling measurements were set in the voltage range of 3.0-4.3 V with 0.05 C (1 C = 160 mA/g) for formation. After five cycles at 0.05 C, a long-term cycling test was performed under 0.5 C for CCCV-charging (CV till 0.05 C) and 2.0 C for CC-discharging. As expected, both types of materials were able to be cycled for a long term. An obviously improved capacity retention of above 80% after 700 cycles was observed for SiO\({}_{2}\)-coated NMC622 material, compared to pristine NMC622 which was only 74%. For the pristine NMC622 material, its capacity decreased sharply within the first 200 cycles, which implies that the most material degradation happens there. However, for the SiO\({}_{2}\)-coated NMC622 material, the decreasing rate of capacity retention was constant. Figure 6b,c illustrates charge-discharge curves of the initial, 1 st, 100 th, 300 th, and 700 th cycle. Both samples show similar voltage profiles without any additional plateaus. After the first 100 cycles, the discharge potential plateau of the coated one did not decrease obviously, while the pristine one had an obvious decrease of approximately 0.3 V (the decreasing is shown by blue arrows). Furthermore, the CV (constant voltage) part percentage of the pristine NMC622 material increased from 1.3 to 12.4% after 700 cycles, while that of NMC622 with SiO\({}_{2}\) coating from 1.1 to 6.8%. This implies that the pristine NMC622 has higher polarization than the coated one. SiO\({}_{2}\) can react with HF in the electrolyte, namely preventing a series of HF-involved side-reactions, which leads to dissolution of TM [6]. Therefore, it is assumed that SiO\({}_{2}\) coating can effectively prevent NMC622 material surface degradation. After 700 cycles, the EIS measurement was done, which was tested after discharging (the open circuit voltage was approximately 3.7 V) in a frequency range of 500 kHz-100 mHz. The Nyquist plots of both pristine and SiO\({}_{2}\)-coated NMC622 cells after 700 cycles are shown in Figure 7, and the EIS at initial cycle is shown in Figure S2 as reference. The patterns of both samples are similar and consist of two semicircles.