# Cu&Si Core-Shell Nanowire Thin Film as High-Performance Anode Materials for Lithium Ion Batteries
## 3 Results and Discussion
### Electrochemical Performance of Cu@Si CSNWFs
Figure 3: (**a**)Voltage profile of the Cu@Si CSNWFs at the 1 st, 2 nd, 5 th, and 10 th cycle at a current rate of 0.1 C between 0.01 and 1.5 V; (**b**) the specific discharge capacity and coulombic efficiency of the Cu@Si CSNWFs and pure Si films as a function of cycle number at a rate of 0.16 C between 0.01 and 1.5 V; (**c**) the specific discharge capacity of Cu@Si CSNWFs as a function of the cycle number and current rate; (**d**) cyclic voltammetry curves of the Cu–Si core–shell nanowires embedded Si thin films (CSNEFs) in the potential range of 0.01–1.2 V at a scan rate of 0.2 mV s\({}^{-1}\). The good rate performance can be understood by the fast and efficient electron transfer of the Cu nanowire core and by the decrease in diffusion length of the lithium ions, as schematically illustrated in Figure 4, in that the diffusion length of the lithium ion in pure Si films is much longer than in our Cu@Si CSNWF. Figure 3d exhibits CV curves for the Cu@Si CSNWF electrode in the potential range of 0.01-1.2 V at a scan rate of 0.2 mV s\({}^{-1}\). It is clearly seen that the CV curve of the first cycle is distinct from those of subsequent cycles, especially for the cathodic branch. In the first cathodic scan, the current increases gradually as the potential is decreased from 0.76 V to around 0.23 V, which is ascribed to the formation of a solid electrolyte interface (SEI) film [31,32]. Thereafter, it starts to increase rapidly, leading eventually to the formation of a broad peak between 0.05 V and 0.20 V and corresponding to the lithiation of Si. In the first anodic scan, a distinct peak at 0.31 V and a weak and broad peak at 0.48 V were observed, which could be assigned to the transformation between the Li-Si alloy to Si. Since the second cycle, the cathodic current was kept almost unchanged in the potential range of 0.72 to 0.23 V; the current is substantially lower than that in the first cycle. These data imply that the electrolyte decomposition during the second cycle is substantially suppressed. Instead, two new peaks located at 0.15 and 0.05 V appear, which could be assigned to the formation of the Li-Si alloy [33]. As of the third cycle, the cathodic and anodic current peaks settled rapidly and maintained a stable pattern, eventually resulting in the peaks superimposing on each other, suggesting a good cycling performance in the following cycles. In contrast, [PERSON] et al. reported that the current peak intensities increased gradually in the first few to ten cycles for the Si nanowire electrodes because the more active materials are activated to react with the lithium during cycling [7,34]. These results indicate that there is no gradual activation process of the active materials in our electrode; i.e., all active materials can be fully lithiated within two cycles, owing to the superior electrical conductivity of the Cu core in the core-shell structure and the void space for electrolyte impregnation, therefore reducing the diffusion length of the lithium ion. In order to clarify the mechanism of the enhanced electrochemical performance of the Cu@Si CSNWF in comparison with pure Si films, EIS, SEM, and STEM measurements were performed after the cycling test. Figure 5a,b exhibit the EIS spectra of the Cu@Si CSNWFs and pure Si films, respectively, after the cycling test in the state of full de-lithiation. As can be seen, for both electrodes the obtained Nyquist plots consisted of a high-frequency (HF) depressed semicircle and a low-frequency (LF) inclined line. It is accepted that in the fully delithiated state, the diameter of the HF-depressed semicircle is mainly ascribed to the intrinsic electronic resistance and the contact resistance (materia-material and material-current collector) while the LF inclined line is attributed to the lithium ion diffusion within the electrode [35,36].