Cyclic voltammograms were performed to study the reaction mechanism of SnO2@C and SnO2@C/Cu electrodes. The cyclic voltammetry (CV) curves of SnO2@C and SnO2@C/Cu electrodes at first 5 cycles between 0.05 and 3 V at the rate of 0.5 mV s-1 are shown in Fig. 5a and b. Fig. S6a shows the initial six CVs of bare SnO2 electrode for comparison. In the first cathodic scan, the sharp reduction peak at 0.75 V corresponds to the formation of solid electrolyte interface (SEI) layer and the reduction of SnO2 to metallic Sn, as described in equations (1) and (2). The other broad peak extending to 0.05 V is ascribed to the formation of a series of Li-Sn alloy. After forming stable SEI layer at the first cycle, the following reduced reactions corresponding to equations (1) and (2) provided peaks at around 1.1 V. This result is similar to the related reports . On the contrary, the oxidation peak at 0.62 V in the anodic scan indicates the highly reversible de-alloying of LixSn as pointed out by equation (3). The following peak at 1.25 V is ascribed to the partially reversible reaction in equation (2). Compared with the bare SnO2 electrode, both of CVs for SnO2@C and SnO2@C/Cu electrodes shown in Fig. 5a and b presented that the peaks extending from 1.0 V to 0.05 V are obvious broaden than that of SnO2 electrode, indicating the Li+ insertion into carbon shell. The related redox mechanism for other peaks presented in CVs is consistent with that of SnO2 electrode as above-mentioned. However, both of CVs for SnO2@C and SnO2@C/Cu electrodes display a different electrochemical behavior compared to the bare SnO2 electrode. The redox peaks of the following cycles are very close to the 1st one, which exhibits good reproducibility and similar shapes, revealing better electrochemical stability for SnO2@C and SnO2@C/Cu anodes. This phenomenon can relieve the aggregation and pulverization of SnO2 nanoparticles during the discharge/ charge process, suggesting only little capacity loss in the following cycles.