Effect of composite structure on capacity instability of SnO2-Coated multiwalled carbon nanotube composite anode

In this study, a multiwalled carbon nanotube (MWCNT) composite anode with SnO2 coated on the inner and outer surfaces of the surface-treated MWCNT was fabricated using a simple wet synthesis method. According to the X-ray diffraction and transmission electron microscope diffraction patterns, the SnO2 particles on the inner and outer surfaces of the MWCNT were a mixture of crystalline and amorphous phases. The peak separation of X-ray photoelectron spectra showed that 93.17 wt % Sn4+ and 6.83 wt % Sn2+ coexist, indicating that the Sn compound in the synthesized composites was composed of SnO2 and SnO. The cycle voltage profile showed a typical SnO2 oxidation/reduction reaction and reversible Sn-Li alloying and de-alloying reactions. Although the Sn compounds have a high Li-charging capacity, the capacity of the prepared composite anode decreased rapidly after several cycles. In addition, a repeated decreasing/increasing behavior of the capacity was observed after 20 cycles of charge/discharge. This rapid decrease and repeated capacity increase/decrease behaviors are attributed to the formation of coarse Sn particles through the re-agglomeration on the surface of MWCNT after grinding because of their charging and discharging. These results suggest that it is very important to avoid the presence of Sn compounds on the surface of MWCNTs when preparing a composite anode of Sn compounds and MWCNT as a power source for wearable devices.