Effects of doped sulfur on electrochemical performance of carbon anode

An amorphous carbon doped with sulfur to be used as anode material for lithium ion batteries is prepared by heat-treatment of a mixture of polyacrylonitrile (PAN) and sulfuric acid. With X-ray photoelectron spectroscopy (XPS), elemental analysis, X-ray powder diffraction (XRD), scanning electron microscopy, and electrochemical capacity measurements effects of sulfur on properties of the carbon anode are investigated. The added sulfur favorably increases the charge capacity in close correlation with an increase of the size of graphite crystallites, L_c, the interlayer distance, d_{0 0 2}, and the number of micropores, and the doping of sulfur in the state of C-S-C. On the other hand, it also causes detrimental changes, for example the formation of amine-group nitrogen, sulfate/sulfonate deposits, and macropores. Nevertheless, as a result, both reversible and irreversible capacities are enhanced. This clearly shows, that added heteroatoms may have opposite influences, good and bad, on electrochemical performance of carbonaceous anode materials. Approaches towards suppressing the unfavorable ones in the application in lithium ion batteries are discussed.