Nitrogen-Doped Carbon Matrix to Optimize Cycling Stability of Lithium Ion Battery Anode from SiOx Materials

This study prepared silicon oxide anode materials with nitrogen-doped carbon matrices (SiOx/C–N) through silicon-containing polyester thermal carbonization. Melamine was introduced as a nitrogen source during the experiment. This nitrogen doping process resulted in a porous structure in the carbon matrices, a fact confirmed by scanning electron microscopy (SEM). Pyridinic and quaternary nitrogen, but mainly tertiary nitrogen, were generated, as shown via X-ray photoelectron spectroscopy (XPS). Electrochemical tests confirmed that, as anode materials for a lithium-ion battery, SiOx/C–N provided better cycle stability, improved rate capability, and lower Li+ diffusion resistance. The best performance showed an activated capacity at 493.5 mAh/g, preserved at 432.8 mAh/g after the 100th cycle, with 87.7% total Columbic efficiency. Those without nitrogen doping gave 1126.7 mAh/g, 249.0 mAh/g, and 22.1%, respectively. The most noteworthy point was that, after 100 cycles, anodes without nitrogen doping were pulverized into fine powders (SEM); meanwhile, in the case of anodes with nitrogen doping, powders of a larger size (0.5–1.0 µm) formed, with the accumulation of surrounding cavities. We suggest that the formation of more prominent powders may have resulted from the more substantial nitrogen-doped carbon matrices, which prevented the anode from further breaking down to a smaller size. The volume expansion stress decreased when the powders decreased to nanosize, which is why the nanosized silicon anode materials showed better cycling stability. When the anodes were cracked into powders with a determined diameter, the stress from volume expansion decreased to a level at which the powders could preserve their shape, and the breakage of the powders was stopped. Hence, the diameters of the final reserved powders are contingent on the strength of the matrix. As reported, nitrogen-doped carbon matrices are more robust than those not doped with nitrogen. Thus, in our research, anodes with nitrogen-doped carbon matrices presented more large-diameter powders, as SEM confirmed. Anodes with nitrogen doping will not be further broken at a larger diameter. At this point, the SEI film will not show continuous breakage and formation compared to the anode without doping. This was validated by the lower deposition content of the SEI-film-related elements (phosphorous and fluorine) in the cycled anodes with nitrogen doping. The anode without nitrogen doping presented higher content, meaning that the SEI films were broken many times during lithiation/delithiation (EDS mapping).