## 3 Results and Discussion
### Post-Mortem Analysis  
After 700 cycles, both cells were disassembled in a glove box, and the electrodes were washed with EMC. The electrodes were observed visually at first, and no defect, delamination, or crack was found (Figure S2).  
The electrodes were further investigated in SEM by applying different magnifications (Figure 8). The images of lower magnification do not show any difference: Particles stack compactly, and particle surface is adhered by nano-sized carbon black. At higher magnification, some intergranular cracks on the particle surface of the pristine NMC622 are found, but none on the SiO\({}_{2}\)-coated one. On the pristine NMC622 material, the formation of intergranular cracks among the primary particles in the secondary particles was resulted from the anisotropic volume change due to Li-ionic (de)intercalation during charging and discharging, while the volume change was restricted by the SiO\({}_{2}\) layer. Moreover, the generated cracks expose more internal surface of the pristine NMC622 particles to electrolyte, which accelerate the side-reactions and consume capacity [6; 18]. SiO\({}_{2}\) as HF scavenger can reduce side-reactions and retain the stable layered structure. Therefore, no obvious cracks could be found on the surface. This is further proof for the above explanation of the higher capacity retention of SiO\({}_{2}\)-coated NMC622.  
The electrodes were placed in a special airtight argon-filled specimen holder and measured with XRD. In Figure 9, compared to an uncycled pristine NMC622 electrode, slight deviation (till 0.3\({}^{\circ}\)) of the reflexes position is detected. On the one hand, the shifting to the left reveals the higher c parameter, because more lithium ions could insert into the SiO\({}_{2}\)-coated NMC622 material than the  
Figure 8: SEM images for cycled Electrode: (**a,b**) Pristine (**c,d**) SiO\({}_{2}\)-coated NMC622.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **R\({}_{\text{s}}\) (\(\Omega\))** & **R\({}_{\text{sf}}\) (\(\Omega\))** & **R\({}_{\text{ct}}\) (\(\Omega\))** & **C\({}_{\text{f}}\)** & **C\({}_{\text{ct}}\)** & **C\({}_{\text{f}}\)** & **C\({}_{\text{t}}\)** & **U\({}_{\text{fl}}\)** & **W** & **(\(\Omega\)s-1/2)** & **X\({}^{2}\)** \\ \hline Pristine NMC622 & 6.279 & 6.875 & 149.400 & 5.45 e-4 & 6.24 e-3 & 7.484 e-9 & 0.093 & 1.41 \\ \hline SiO\({}_{2}\)-coated NMC622 & 12.500 & 15.720 & 107.200 & 5.30 e-5 & 6.42 e-3 & 1.160 e-7 & 6.387 & 5.43 \\ \hline \hline \end{tabular}
\end{table}
Table 3: EIS fitting results of the pristine and the SiO\({}_{2}\)-coated NMC622 after 700 cycles.  
pristine one, and the lithium slab is thicker. On the other hand, the measured sample is uneven electrode, which could lead to deviation of result. An extra reflex at 65\({}^{\circ}\) is from Al-current collector. The determination of full width of half maximum (FWHM) of main reflexes shown in Table S1 reveals that the SiO\({}_{2}\)-coated NMC622 electrode has sharper reflexes than the pristine one, which indicates a better crystallinity. The reason behind it could be homogenous crystal structure because of less-exposed surface. According to the Scherrer formula  
\[L=\frac{\text{K}\cdot\lambda}{\beta\text{-}\cos\theta}\]  
where \(\beta\) is the width of Bragg line in radians (here is FWHM), K is a shape factor 0.9, \(\lambda\) is the wavelength of the X-ray, \(\theta\) is the corresponding Bragg angle, reflexes with decreased width indicate larger mean crystallites [16], the calculated mean crystallite size is shown in Table 4.