The morphologies and sizes of SnO2, SnO2@C and SnO2@C/Cu were characterized by SEM measurements. As shown in Fig. 2a and b, the bare SnO2 products are uniform pomegranate-core-like nano-spheres with diameters of approximately 90–110 nm. It is also observed that the pomegranate-core-like SnO2 nano-spheres are composed of many smaller nanoparticles with a diameter of _8 nm. Meanwhile, Fig. 2c and d present the panoramic SEM images for the SnO2@C composite, respectively. With the carbon coating, the entire SnO2@C composite are composed of smooth ball-like NPs with the similar diameters in comparison of bare SnO2. As further shown in the SEM images of Fig. 2e and f, the detailed morphologies and structure features of SnO2@C/Cu are depicted. These SEM images clearly display that adding Cu particles does not change the general shape of SnO2@C NPs but results in the difference in the surface morphology of the hybrid products. It is observed that massive Cu nanoparticles are attached on the surface and gap of SnO2@C nano-spheres. The EDS result in Fig. S2 also supported the existence of Cu nanoparticles. These uniform Cu nanoparticles are beneficial to the fast diffusion and migration of electronics in the metal conductor especially at low temperature, thereby improving their electrochemical perfor- mance of electrodes. In order to further confirm the presence of Cu, element mapping was used to observe the distribution of each element. As presented in Fig. 3, the color points are due to the presence of the element. It can be seen that C, O, Sn and Cu in the sample are homogeneously distributed in the composite. Similarly, the Fig. S3 also indicated this phenomenon