## 3 Results  
### SiO2 Characterization  
Figure 2 shows the as-prepared SiO2 powder obtained from fly ash characterization by XRD (Figure 2a), FTIR (Figure 2b), SEM (Figure 2c), and XRF (Figure 2d). Based on the XRD pattern, the widening peak is detected at 15-35\({}^{\circ}\) of 2 theta; thus, it can be concluded that the samples exhibit amorphous properties. The impurity peaks (31\({}^{\circ}\) and 45\({}^{\circ}\)) of sodium chloride (NaCl) are detected in the \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Component** & **O** & **Si** & **Fe** & **Ca** & **Al** & **Na** & **Mg** & **K** & **S** & **Ti** & **Cl** & **P** & **Mn** & **Trace Metals** \\ \hline wt\% & 36.6 & 20.4 & 18.1 & 9.4 & 4.0 & 3.5 & 3.1 & 1.2 & 1.1 & 0.7 & 0.5 & 0.4 & 0.3 & 0.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: XRF analysis of coal combustion-derived fly ash from PT Semen Indonesia. Figure 1: Process flow diagram of SiO2 and SiO2/C composite synthesis. sample. The presence of impurities can be caused by an unfinished cleaning process or salt entrapment in silica matrices. In addition to the impurity peak, all peaks from all samples are well indexed to ICSD Card no. 061662. Based on the FTIR spectra, the absorption peaks at 3400, 1630, and 950 cm\({}^{-1}\) are attributed to the stretch vibration of -OH from siliconal (Si-OH) or H\({}_{2}\)O and Si-O from siloxane (Si-O-Si). The peak at 790 cm\({}^{-1}\) is attributed to symmetrical Si-O from Si-O-Si stretching vibration. Based on the XRD and FTIR result, SiO\({}_{2}\) is successfully obtained by caustic fusion and precipitation through following reactions [21]. \[\mathrm{SiO_{2(s)}+2 NaOH_{(aq)}\to Na_{2}SiO_{3(aq)}+H_{2}O_{(l)}} \tag{1}\]  
\[\mathrm{Na_{2}SiO_{3(aq)}+2 HCl_{(aq)}\to H_{2}SiO_{3(aq)}+2 NaCl_{(aq)}} \tag{2}\]  
\[\mathrm{H_{2}SiO_{3(aq)}+H_{2}O_{(aq)}\to Si(OH)_{4(s)}} \tag{3}\]  
\[\mathrm{Si(OH)_{4(s)}\to SiO_{2(s)}+2H_{2}O_{(g)}} \tag{4}\]  
SEM images (Figure 2c) confirm that there are two types of particles, i.e., primary and secondary. The secondary particle has a diameter of \(\sim\)7.5 \(\upmu\)m, which consists of clustering sub-micron-sized primary particles and small adherents on the surface. These adherents or aggregates enhanced the overall surface area of silica, which resulted in good ionic transfer during Li insertion. The XRF result shows that mostly silicon was obtained, while there are also small sodium impurities (6%), which is confirmed by the XRD result. The NaCl impurities is still detected due to the entrapment of NaCl during the silica precipitation process and exist even after multiple washing steps. Figure 2: Coal fly ash-derived SiO\({}_{2}\) characterization by (**a**) XRD, (**b**) FTIR, (**c**) SEM, and (**d**) XRF. The as-obtained SiO\({}_{2}\) powder was directly applied as anode in a NCA/SiO\({}_{2}\) battery. Figure 3 shows the electrochemical performance of the cell. The theoretical capacity of SiO\({}_{2}\) is adapted from a study by [PERSON] et al. (1965 mAh/g), while the theoretical capacity of NCA is estimated at 200 mAh/g. In the final cell assembly, the SiO\({}_{2}\) anode was selected as the limiting reactant; therefore, the specific capacity was referred to as the specific capacity of SiO\({}_{2}\). Figure 2a shows that the specific charge and discharge capacity of SiO\({}_{2}\) is approximately 1396 mAh/g and 1188 mAh/g, respectively. The discharge capacity was maintained for 10 cycles. Even though SiO\({}_{2}\) has a considerably lower specific capacity than its theoretical capacity, the capacity was stable at 1/10 C. At increasing rate, as shown in Figure 2b, SiO\({}_{2}\) exhibits an inferior rate performance and high level of irreversibility. This can be caused by the formation of electrochemically inactive lithiated SiO\({}_{2}\) at high rate (>0.5 C). Previous studies claimed that the reaction mechanism between Li-ion and SiO\({}_{2}\) was not been completely understood yet; however, most studies showed a significant irreversible capacity loss during initial cycling [16].