# 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 3a exhibits the charge/discharge curves of the Cu@Si CSNWF electrode after 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. The potential window is 0.23-0.01 V during discharging and 0.23-0.55 V during charging, respectively. The voltage profiles exhibit a typical sloping and smooth curve without appearance of a distinct potential plateau. Figure 3b demonstrates the specific discharge capacity and  
Figure 2: (**a**) XRD pattern of the as-received Cu nanowire on a Cu foil substrate before and after heat treatment at 280 \({}^{\circ}\)C for 2 h in a reductive atmosphere of 95 v.% Ar + 5 v.% HZ, with and without Si deposition; (**b**) SEM image of the Cu nanowire (NW)-coated Cu foil substrate after heating treatment and the corresponding size distribution of the diameter of the Cu nanowire before (**b,d**) and after (**c,e**) Si deposition; (**f**) STEM image of a single Cu@Si core-shell nanowire and (**g**) its composition analysis by line scanning EDS along the straight line. charge efficiency of the Cu@Si CSNWFs electrode as a function of cycle number. The initial discharge capacity is 3193 mAh/g. After 500 cycles, the discharge capacity is still as high as 948 mAh/g, corresponding to an average capacity fading rate of only 0.2% per cycle. As for the charge efficiency, the initial efficiency is close to 87%, and then quickly increases and lies between 96% and 99.8% during subsequent cycling. These suggest a good cycling stability of the Cu@Si CSNWF electrodes. In contrast, the specific discharge capacity of 456 nm-thick pure Si films fades rapidly with cycling, which is typical for a thick Si film electrode [26]. The higher initial charge efficiency and better cycling performance of Cu@Si CSNWF electrodes than pure Si film could be attributed to the improved electric conductivity and special core-shell structure. The electrical conductivity of Cu\({}_{3}\)Si was about \(2\times 10^{4}\) S cm\({}^{-1}\), a little lower than the Cu nanowire but much higher than pure silicon of about 10-5 S cm\({}^{-1}\)[27]. Indeed, it has been reported that increasing the electronic conductivity of Si films could improve the initial coulombic efficiency significantly [28,29]. Moreover, the Cu\({}_{3}\)Si layer between the Si shell and Cu core functions as an adhesive layer, and could enhance the adhesion strength between the Cu nanowire core and Si shell and therefore improve the cycling stability of the Cu@Si CSNWF electrodes in spite of a little sacrifice in capacity due to the electrochemical inactivity of Cu\({}_{3}\)Si to Li [24]. [PERSON] et al. [30] also attributed the enhanced cycling stability of a Si-Cu-graphite composite to the formation of copper silicide in the interface between Cu and Si. Figure 3c exhibits the rate capability of the Cu@Si CSNWF electrode. The discharge capacity is about 2060 mAh/g at 0.2 C and exhibits a decreasing trend with the increase in current density. As the current density is increased to 0.5 C, 1.5 C, 3 C, and 6 C, the discharge capacity dropped down to \(\sim\)1900 mAh/g, \(\sim\)1600 mAh/g, \(\sim\)1200 mAh/g, and \(\sim\)700 mAh/g, respectively. When the current density was returned to 0.2 C, the discharge capacity was increased to 2025 mAh/g, almost the same value as that in the beginning, demonstrating a good rate capability of our Cu@Si CSNWF electrodes. In addition, the coulombic efficiency at different current densities is beyond 90%, with the exception of the first cycle at 0.2 C, 3 C, and 6 C rates, further confirming the good rate performance of the Cu@Si CSNWF electrodes.