## 3 Results and Discussion
Even though the Si is progressively amorphized, the curve area gets increased at the extended cycling 20 th cycle represent more Si is activated in the alloying/dealloying reactions. TEM images of the Si/C nanocomposite electrode are shown in Figure 4a,b. The selected area electron diffraction (SAED) patterns show the Si (111) and (220) planes. From high-resolution TEM image, the lattice inter-planar distance (d-spacing) was calculated for Si, and the d-spacing was \(\sim\)0.32 nm, which is corresponding to the Si (111) plane. It has been observed that the nanocomposite was comprised of nanocrystalline Si, amorphous Si and amorphous carbon. The nanocrystalline Si was distributed on the amorphous silicon and carbon matrix. The amorphous Si phase could also be formed due to amorphization of Si (supported by background broadening in the X-ray diffraction results) for the longer mechanical milling. Amorphous carbon phase could be produced in the form of interface between Si powders and coating on the Si powders from the heat treatment of the citric acid (carbonization). In addition, carbon was expected to diffuse into the Si crystals through surface and boundary diffusion. Such an impregnation of the carbon is believed to be effective to both improve the milling efficiency (refinement of Si crystal size) and act as a mechanically strong (highly elastic) buffering matrix against the Si volume change. However, as shown in Figure 4c, the HR (High resolution)-TEM image of the Si/C nanocomposite electrode at the 100 th cycle, a complete amorphization of the nanocomposite powder was observed, which could be due to the stress generated by the Si during the lithiation and delithiation due to extended cycling. Figure 4: _Cont._  
Figure 5 shows the electrochemical impedance spectroscopy (EIS) and equivalent circuit of the Si/C composite. EIS measurements were taken for 1 st and 20 th cycle in the frequency ranging from 100 kHz to 10 mHz and an AC signal of 10 mV in amplitude as the perturbation. As shown in Figure 5, two semicircles were observed in between the high-frequency region to medium frequency region, which indicates the resistance caused by SEI film formation (R\({}_{\rm{sei}}\)), and charge transfer resistance (R\({}_{\rm{ct}}\)) between the electrode to electrolyte interface and the angled line in the low-frequency range corresponds to the ion-diffusion process within the anodes [50]. The equivalent circuit diagram (Figure 5) was used for fitting the EIS data, where R\({}_{\rm{s}}\) is the bulk resistance raised from electrode, electrolyte, and separator; R\({}_{\rm{sei}}\) is the resistance of the SEI film and its relevant capacitance is denoted as CPE1; R\({}_{\rm{ct}}\) is charge transfer resistance and its relevant capacitance is denoted as CPE2; and Zw is the Warburg impedance [50]. As per the fitting results, the R\({}_{\rm{sei}}\), and R\({}_{\rm{ct}}\) of the Si/C nanocomposite electrode showed about 6 and 40 \(\Omega\) for the initial cycle, and 5 and 17 \(\Omega\) for the 20 th cycle, respectively. Therefore, it is obvious that the inner resistance of the electrode is small in the case of extended cycles. This result suggest that the electrode possesses a great conductive network owing to the carbon phase. Consequently, stable SEI film formation and efficient charge transport between active Si and carbon matrix during the electrochemical reaction lead to the lower internal resistance. The tight attach of Si nanoparticles in the carbon matrix is the key factor that ensured the good structural integrity and fast electronic/ionic transport during the lithiation/delithiation process, finally leading to high reversible capacity retention and good cycling stability of the Si/C nanocomposite electrode. Figure 4: (**a**) Selected area electron diffraction (SAED) pattern; (**b**) HR (High resolution)-TEM image of Si/C nanocomposite electrode; and (**c**) HR-TEM image of the Si/C nanocomposite electrode after 100 th cycle.