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Paper Journal of Materials Chemistry A Fig. 1 Schematic diagram of the synthetic process of AC@S composites via SC-CO2 technology. View Article Online The morphology and microstructure were observed using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30) with an energy dispersive spectroscopy (EDS) detector. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Analytical spectrometer with an Al Ka monochromatic X-ray source. The peak positions were calibrated based on the C 1s peak at 284.8 eV. 2.3 Electrochemical measurements The cathode slurry was comprised of 10 wt% Super-P, 10 wt% polyvinylidene uoride (PVDF) binder and 80 wt% active materials. The resultant viscous slurry was coated onto Al foil via a glass rod. The working electrodes were dried in a vacuum oven at 60 C overnight. The areal mass loading of sulfur on each electrode was approximately 2.1 mg cm2. Lithium foil and Celgard 2300 membrane were used as the anode and separator, respectively. The electrolyte was 1 M LiCF3SO3 and 0.2 M LiNO3 in the co-solvent mixture of 1,2-dimethoxyethane and 1,3-diox- olane (v/v 1⁄4 1 : 1). The volume of electrolyte in the coin cells was approximately 80 mL (electrolyte/sulfur ratio 1⁄4 15 mL mg1). Cyclic voltammogram (CV) measurements were recorded on a CHI650B electrochemical workstation (Chenhua, Shanghai, China) at a scanning rate of 0.1 mV s1 in the voltage range of 1.8–2.6 V. The galvanostatic charge–discharge proles were recorded on a battery testing system (Shenzhen Neware Tech- nology Co. Ltd.) at various current densities in the voltage window of 1.8–2.6 V. All electrochemical tests were performed based on CR2025 button cells in ambient environment. 3. Results and discussion Fig. 2a and b vividly illustrates the morphology and structure of AC before/aer the SC-CO2 process. The pristine AC sample is composed of irregular particles with size of ca. 5 mm (Fig. 2a). Aer SC-CO2 treatment, the particle size of AC-CO2 sample gradually decreased; some small cracked particles are observed in Fig. 2b. Additionally, when sulfur was incorporated into AC matrices via the SC-CO2 process, lots of ne particles were ob- tained in the AC@S sample, and the particle size was smaller than that of AC-CO2. The reason for this phenomenon could be explained as follows. During the SC-CO2 process, sulfur pene- trates into the pores of the AC matrices with the assistance of the SC-CO2 uid. When exposed, there was an abrupt decrease in pressure, SC-CO2 uid was immediately transformed into CO2 gas, resulting in a large pressure difference between the inner pores and the ambient environment. Synchronously, sulfur remained in the pores and in intervals of AC matrices, causing huge mechanical stress. As a result, the AC@S sample was crushed into small pieces, compared to the AC-CO2 sample. However, the AC/S-155 sample prepared by the routine melt- diffusion method had no distinct change in morphology or particle size (Fig. 2d). Furthermore, the effects of the SC-CO2 process on MCMB and MWCNTs carbon matrices were also This journal is © The Royal Society of Chemistry 2017 J. Mater. Chem. A Published on 24 November 2017. Downloaded by University of Texas Libraries on 08/12/2017 20:16:36.PDF Image | Supercritical CO2 Mediated Incorporation of Sulfur into Carbon Matrix
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