Research On Sulfur-containing Cathode Materials And Electrochemical Impedance For Lithium/sulfur Batteries

The lithium/sulfur (Li/S) battery is a promising next generation rechargeable battery due to its high theoretical specific energy. In addition, sulfur as a cathode active material is non-toxic, naturally abundant and environmentally benign. However, the practical realization of Li/S batteries has met huge challenges including low utilization of active materials and rapid capacity fading because of the insulating nature of sulfur and solid discharge products, the solubility of intermediary products and volume change during cycling.Aiming at these problems, we carried out some research works in terms of sulfur-carbon composite materials, preparation technique and electrochemical impedance. The main works and results are summarized as follows:(a) Vapor-grown carbon fiber (VGCF) is a carbon material with high electrical conductivity and high length/diameter ratio. We employed VGCF as the carbon matrix for Li/S batteries and VGCF was pretreated by KOH activation. It is found that activated VGCF (aVGCF) presents a porous structure, and sulfur is embedded into the pores of aVGCF network-like matrix after melt-diffusion process. The S-aVGCF composite cathode exhibits good electrochemical reversibility and active materials utilization because of the excellent conductivity, the fibrous and porous structure of aVGCF matrix.(b) We employed toluene as an extraction solvent for sulfur to optimize sulfur-actived carbon (S-AC) composite and to obtain a S-AC composite containing toluene (S-AC/T). The results indicate that after toluene extraction sulfur can be removed from the surface or macropores of AC. The existence of toluene is also proved. Although the S-AC/T cathode presents poorer electrochemical reversibility, the S-AC/T cathode exhibits excellent cycling stability as well and shows higher active material utilization. The existence of toluene promotes the reactions in upper potential plateau.(c) Electrode process of Li/S batteries was studied by using electrochemical impedance spectroscopy (EIS). An impedance model based on the analysis of EIS spectra as functions of temperature and depth of discharge is developed. Then, by monitoring the evolution of impedance during cycling process, capacity-fading mechanism of Li/S batteries was investigated. The results indicate that the signature of middle frequency semicircle is the charge-transfer resistance and its relative capacitance, and the semicircle at high frequency may be related to the interphase contact resistance and its relative capacitance. Moreover, impedance parameters vary with cycle number in different manners, and the charge-transfer resistance, which is strongly affected by the conductivity of sulfur cathode and the cathode surface properties, is an important factor contributing to the capacity fading of Li/S batteries.