# Scalable Fabrication of Si-Graphene Composite as Anode for Li-ion Batteries
## 3 Results and Discussions
The intensities of the D and G peaks in the Si-graphene composite are similar, indicating the graphene possesses many defects that could provide extra diffusion paths for Li ions. In addition, the significantly reduced 2D band in Si-graphene composite indicate that the multilayer graphene structure is gone, which further confirms the homogeneous distribution of the Si nanoparticles and the graphene nanosheets. A coin cell with a lithium electrode was used to measure the charge/discharge capacities of the Si-graphene composite electrode. For comparison, a coin cell composed of lithium electrode and Si-based electrode was fabricated and evaluated. The specific charge and discharge capacities were calculated based on the total weight of the Si-graphene composite used to fabricate the half cell. Figure 5a shows the specific charge and discharge capacities of Si-graphene and Si anodes at a current rate of 0.05C. As expected, the Si-based coin cell shows a much higher specific capacity than the Si-graphene composite coin cell for the initial cycle. This is due to the intrinsic high capacity of the Si nanoparticles. Figure 5b shows the specific charge and discharge capacities of Si-graphene anode at different current rates of 0.05C, 0.1C, and 0.5C, respectively, with a voltage ranging from 0.05 to 0.7 V. The initial specific discharge capacity and charge capacity of the formation cycle were 1393 mAh g\({}^{-1}\) and 1180 mAh g\({}^{-1}\) at 0.05C, respectively. Therefore, the coulombic efficiency (CE = charge capacity/discharge capacity) was calculated to be 84.7%. After the formation cycle, the Si-graphene coin cell yielded discharge capacities of 1307 mAh g\({}^{-1}\) at 0.1C and 433 mAh g\({}^{-1}\) at 0.5C, and charge capacities of 1313 mAh g\({}^{-1}\) at 0.1C and 513 mAh g\({}^{-1}\) at 0.5C. Therefore, the coulombic efficiencies were calculated to be 100.5% at 0.1C, and 118.5% at 0.5C. It is noticed that the capacity decays slightly faster when the cell was tested at a lower current rate. This might be due to the smaller volume expansion of the Si nanoparticles at relatively higher current rates, at which the transportation rate of the Lithium ions is relatively slower, resulting in a more stable SEI layer that prevents the irreversible loss of capacity. The energy density of the Si-graphene active material at 0.1C was calculated to be 522.8 Wh kg\({}^{-1}\) using Equation (1). \[\text{Energy density}=\text{C}*\text{V} \tag{1}\]  
Figure 4: (**a**) XRD patterns of Si nanoparticle and Si-graphene composite; (**b**) Raman spectra of Si, graphene and Si-graphene composite. where C is the specific discharge capacity (1307 mAh g\({}^{-1}\) at 0.1C), V is the voltage at 50% state of charge (0.4 V in this case). Figure 5c shows the cycle performance of the Si-graphene electrode and the Si electrode at a current rate of 0.1C. The Si-graphene electrode showed an initial discharge capacity of 1307 mAh g\({}^{-1}\) at a current rate of 0.1C. At the 25 th cycle, the electrode retained a discharge capacity of 1270 mAh g\({}^{-1}\), with an excellent capacity retention of 97%. At the 50 th cycle, the electrode still retained high capacity retention of 89%. As a comparison, the pure Si anode possessed a higher initial discharge capacity (over 3000 mAh g\({}^{-1}\) at 0.1C), while the decay of the capacity retention was much faster, because Si nanoparticles experienced dramatic volume change during the charge/discharge process which ultimately led to limited cycle life. The enhanced cycling stability of the Si-graphene electrode was mainly attributed to the graphene structure that accommodates the volume expansion of the Si nanoparticles. However, the capacity fading became faster beyond 50 cycles. At the 100 th cycle, the electrode delivered a specific discharge capacity of 810 mAh g\({}^{-1}\) with 62% retention. Although the specific capacity decreases at long cycles, it is noticed that the columbic efficiencies obtained were all over 90%, indicating a stable structure of the solid electrolyte interface.