## 3 Results and Discussion  
To systematically study the effect of particle size, we selected different sizes of SiO\({}_{2}\) particles. As shown in Figure 1, the morphology and size distributions of particles were characterized by TEM, SEM and DLS measurement. Figure 1a-d show the morphology of pristine SiO\({}_{2}\) particles with different sizes from 6 nm to 3 \(\upmu\)m. The TEM (Figure 1a,b) and SEM (Figure 1c,d) images clearly demonstrate all the silica particles have the spherical structures and uniform size. The size statistics by DLS further confirm that SiO\({}_{2}\) particles have a quite narrow size distribution (Figure 1e,h). The SiO\({}_{2}\) particles with different sizes were alloyed with molten Li metal under Ar atmosphere to form Li\({}_{x}\)Si/Li\({}_{2}\)O NPs, as schemed in Figure 2a. The powder color changes from white to black within ten minutes, indicating the formation of Li\({}_{x}\)Si alloy. SEM and TEM images (Figure 2b-e) demonstrate the morphology of as-prepared Li\({}_{x}\)Si/Li\({}_{2}\)O composites. After prelithiation, the surface of SiO\({}_{2}\) particles becomes rougher due to the coverage of oxide layer and volume expansion. Moreover, the images show that they have similar morphology despite being derived from variously sized SiO\({}_{2}\) particles. It is found that although the pristine SiO\({}_{2}\) particles varied from 6 nm to 3 \(\upmu\)m, the over-all sizes of as-lithiated samples are all around several micrometers, which can be clearly seen from the enlarged SEM images in the insets. This can be attributed to the smaller particles undergo more sever aggregation during SiO\({}_{2}\) lithiation. As a result, Nanoparticles trend to aggregate into microparticles, while micro- or submicro-ones prefer to retain their size in the lithiation process. Figure 3a shows the XRD patterns of prelithiated SiO\({}_{2}\) particles. A home-made sealed sample holder was used to avoid exposing the products to the air. The diffraction peaks of Li\({}_{2}\)O and Li\({}_{2}\)Si\({}_{5}\) are shown in the Figure 3a,  
Figure 1: (**a**,**b**) TEM images of SiO\({}_{2}\)-1 (6 nm) and SiO\({}_{2}\)-2 (20 nm); (**c**,**d**) SEM images of SiO\({}_{2}\)-3 (300 nm) and SiO\({}_{2}\)-4 (3 \(\upmu\)m); (**e**–**h**) The corresponding DLS analysis of the samples. demonstrating the formation of Li\({}_{2}\)O (PDF# 04-001-893) and Li\({}_{21}\)Si\({}_{5}\) (PDF# 00-018-747) phases. It has been reported that Li\({}_{21}\)Si\({}_{5}\) is the most thermally stable phase among the crystalline lithium silicates [21]. The XRD patterns of the four Li\({}_{x}\)Si/Li\({}_{2}\)O composites are similar, indicating that pristine particle size does not have a big effect on the formation of crystalline Li\({}_{21}\)Si\({}_{5}\). As calculated by the Scherrer equation, the crystalline sizes of the four composites are 28.2, 29.3, 31.9 and 37.5 nm, indicating that the SiO\({}_{2}\) microparticles produce relatively large Li\({}_{21}\)Si\({}_{5}\) domains. Figure 3: (**a**) XRD patterns of the four prelithiated composites; (**b**) The corresponding galvanostatic charge/discharge profiles of the Li\({}_{x}\)Si/Li\({}_{2}\)O samples at the first cycle. Figure 2: (**a**) Schematic of the thermal-alloying process of the SiO\({}_{2}\) with lithium metal; (**b–e**) SEM images of Li\({}_{x}\)Si/Li\({}_{2}\)O-1, Li\({}_{x}\)Si/Li\({}_{2}\)O-2, Li\({}_{x}\)Si/Li\({}_{2}\)O-3, and Li\({}_{x}\)Si/Li\({}_{2}\)O-4, respectively. To invest the electrochemical behavior of the prelithiated products, half cells were fabricated with Li metal as a counter electrode. Figure 3b shows the charging/discharging profiles of Li\({}_{x}\)Si/Li\({}_{2}\)O electrodes. The open-circuit voltage (OCV) of the half cells is around 0.35 V, which is significantly lower than \(\sim\)2.0 V of the SiO\({}_{2}\)/Li cell (SI, Figure S1). This proves that most SiO\({}_{2}\) particles were well lithiated.