## 3 Results and Discussion
### Electrochemical Performance of Silicon-Carbon Nanocomposite  
The electrochemical properties of the electrodes were measured by the EIS and CV measurements. Figure 7a shows the EIS of the electrodes after different cycles. The impedance spectrum consists of a semicircle, which is related to the charge-transfer impedance at the electrode-electrolyte interface in the high-frequency region, and a straight line, which implies the lithium diffusion impedance in the low frequency region. As the number of charge/discharge cycles increases, the diameter of the semicircle in the high-frequency region tends to increase. This is mainly due to the volume expansion of Si nanocomposite during the charge/discharge progress, which leads to the destruction of SEI film and the exposure of the Si nanocomposite surface. The formation of new SEI film consumes the electrolyte and thus increases the impedance. However, there is no significant difference among the three curves after 50 cycles. This indicates that the increase of interface impedance is limited during the cycles, and the SEI film formed is relatively stable [41], proving that the composite structure can efficiently alleviate the volumetric changes of Si nanocomposite. Figure 7b shows the initial five CV curves of the composite. In the cathodic scan of the first cycle curve (discharge processes), a cathodic peak located at 0.1 V (vs. Li/Li\({}^{+}\)) is related to the Li\({}_{x}\)Si (amorphous) alloy formation from Si (crystalline) and a phase transition from Li\({}_{x}\)Si (amorphous) to Li\({}_{15}\)Si\({}_{4}\) (crystalline); a broad reductive peak at about 0.25-0.9 V (vs. Li/Li\({}^{+}\)) corresponds to the decomposition reaction of electrolyte and the SEI film formation on the surface of the composite, which disappeared from the second cycle. Correspondingly, during the anodic scan (charge processes), the two anodic peaks at 0.36 and 0.52 V (vs. Li/Li\({}^{+}\)) are ascribed to the dealloying reactions from  
Figure 6: XRD pattern (**a**) and Raman pattern (**b**) of the silicon-carbon nanocomposite prepared by DC magnetoelectric plasma under optimal conditions. Li\({}_{x}\)Si (amorphous) or Li\({}_{15}\)Si\({}_{4}\) (crystalline) to Si (amorphous). From the second cycle, a cathodic peak at about 0.2 V and the two anodic peak (0.36 and 0.52 V) are characteristic of the phase transition between Li\({}_{15}\)Si\({}_{4}\) (crystalline) and Si (amorphous) [42; 43; 44]. The increase in peak current is due to the continuous reaction of Si (crystalline) in the inner of the nanoparticles with Li during the repeated lithiation/delithiation [45; 46]. The conclusion seems reasonable that the Si nanocomposite is the main reactant with Li and the main provider of Li storage capacity in the silicon-carbon nanocomposite. Figure 8a displays the charge/discharge profiles of the silicon-carbon nanocomposite for 1 st, 2 nd, and 50 th cycle, respectively. The galvanostatic cycling was carried out at a current density of 150 mA g\({}^{-1}\). The initial discharge and charge capacities of the silicon-carbon nanocomposite are 1469 and 1193 mAh g\({}^{-1}\), respectively, and the coulombic efficiency of the first cycle is 81.2%. The initial irreversible capacity is mainly due to the SEI film formation on the composite, and the irreversible process disappeared during the subsequent cycles. For the second cycle, the discharge and charge capacities of the electrode are 1217 and 1106 mAh g\({}^{-1}\), respectively, and the coulombic efficiency increases to 90.8%. After the second cycle, the coulombic efficiency rapidly reaches 99% and remains stable for 50 cycles. After 50 cycles, the discharge-specific capacity of the composite electrode is 1007 mAh g\({}^{-1}\) with a high capacity retention of 68.6 % as shown in Figure 8b, and the good cycling stability attributed to the silicon-carbon nanocomposite structure effectively alleviates the volumetric expansion during the cycles.