Deflated Carbon Nanospheres Encapsulating Tin Cores Decorated on Layered 3-D Carbon Structures for Low-Cost Sodium Ion Batteries

Novel deflated carbon nanospheres encapsulating metallic tin cores were successfully synthesized and decorated on 3-D layered carbon with wall-like structures. The composite was synthesized by facile in situ carbonization of walnut shell membranes and chemical vapor deposition (CVD) and can be used as negative electrodes for sodium ion batteries. The in situ reduction of SnO2 particles to metallic Sn partides could induce volumetric shrinkage of the particles due to the density difference. The encapsulation of Sn particles by carbon nanospheres under CVD conditions can form unique deflated Sn@C nanoparticles firmly attached on the surface of the 3-D carbon derived from a typical agricultural waste. Particularly interesting, all the carbon nanospheres are in deflated spherical shape, which could provide extra space for volume expansion during charging in Na insertion into the enclosed Sn. The as-prepared composite in the form of pieces of materials are ready electrodes that can be directly assembled into batteries without any further treatment, in contrast to the conventional multiple-step procedure in making electrodes. Preliminary results demonstrated the potential application of the as-prepared deflated Sn@C nanospheres on 3-D carbon in future sodium ion batteries. Impressive high Coulombic efficiencies were achieved at different currents in SIBs. It should be noted that we observed significant differences in the electrochemical performances of the as-prepared deflated Sn@C nanospheres on 3-D carbon in reversible Na and Li storage. Deflated carbon nanospheres would be broken during cycling in Na storage but were well preserved in Li storage. Those observations suggest the importance of the ionic size of Li and Na and their effects on the electrode materials during charge discharge cycling.