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Journal of Materials Chemistry A Paper clean and non-toxic solvent. However, most of the routine methods (such as the dissolution method and melt-diffusion method) oen use highly toxic CS2 as the solvent, which is not environmentally benign, and will even cause serious health issues for humans. (2) Compared to chemical techniques (chemical deposition and electrochemical deposition), the SC- CO2 method does not involve complex chemical reactions and expensive chemical reagents, and the nal products need not be further puried. (3) The SC-CO2 method can guarantee the uniform sulfur distribution and high utilization of carbon matrices. In contrast, the pore-channel structure of the carbon matrix is easily blocked during the melt-diffusion process, meanwhile the sulfur precursor solution could not penetrate well into carbon matrices during the chemical synthesis process. Thus, it is difficult to take the advantages of carbon matrices with high specic surface area and abundant porous structures. (4) The reaction temperature of the SC-CO2 method is only 32 C, which is much lower than that of the vapor-phase inltration method (500 C) and the melt-diffusion method (155 C); therefore, the SC-CO2 method can save more energy and cut down the cost. (5) The electrochemical performances of the carbon–sulfur composites prepared via the SC-CO2 method surpasses those of the samples derived from other methods. Therefore, the reversible specic capacity, long-term cycling stability and coulombic efficiency have been greatly enhanced with the assistance of the SC-CO2 method. The structure stability and polysulde adsorption capability of AC@S and AC/S-155 cathodes have been studied. As shown in Fig. 5a, AC@S and AC/S-155 samples were directly soaked in the electrolyte. Aer soaking for 12 h at 80 C, the white occulent precipitate was detected on the surfaces of both the AC@S and AC/S-155 samples. The color of the mixed solution also had a visible change, in which the colorless transparent solution became light yellow. However, compared to the AC@S sample, the AC/S-155 sample had more occulent precipitate. The color of the AC@S sample was lighter, compared to the AC/S-155 sample. These results indicate that sulfur is tightly enveloped in the AC via the SC-CO2 method, compared to the routine melt- diffusion method. Moreover, the cycled cells of the AC@S and AC/S-155 samples were disassembled to verify their polysulde trapping capabilities. Fig. 5b displays that the cycled cathodes with absorbed electrolyte were directly soaked into the DOL/ DME (v/v 1⁄4 1 : 1) mixed solution. It is clear that the solution of AC@S cathode remained light yellow, suggesting that poly- suldes are strictly conned within AC matrices. In contrast, a yellow solution of AC/S-155 sample was obtained aer soak- ing, implying that the AC/S-155 sample (melt-diffusion method) could not effectively prevent polysuldes from continuously dissolving into the electrolyte during the long-term cycling. Additionally, as illustrated in Fig. 5c, only trace amounts of yellow polysuldes could be observed on the surface of the separator from the AC@S cell. In contrast, the yellow area and color depth on the separator of the AC/S-155 cell were much larger and deeper, compared to the AC@S cell. These observa- tions clearly reveal that the AC@S cathode has superior struc- ture stability during the long-term cycling. View Article Online J. Mater. Chem. A This journal is © The Royal Society of Chemistry 2017 Table 1 The comparison of different synthesis methods for carbon–sulfur composites in Li–S batteries Capacity (mA h Method Physical techniques Sulfur source (solvent) S(—) Temperature (C) RT g1)/efficiency (%) 643 Sulfur distribution Bad Poor Cost Low Low Toxicity Ref. Chemical techniques Non-toxic Highly toxic 55 Ball-milling Dissolution S(—) S(CS2) S(CS2) S(CS2) S(—) S(CS2) S(CS2) S(CS2) S(SC-CO2) Na2S + S(H2O) C6H5NH2 + S(H2O) H2S/(Na2S + S + H2O) RT RT RT RT 500 155 155 155 32 60 15 RT 615/96 630/$100 541/$98 466/$100 $700/96 792/92.8 797.9/96 670/98.7 817/$100 826/97 633.1/98 756/$90 Non-toxic 32 50 Highly toxic 51 22 52 Low toxic 36 Highly toxic 35 53 13 Vapor-phase inltration Melt-diffusion Good Good Moderate Moderate SC-CO2 Chemical deposition Good Poor Low High This work Low toxic 54 Electrochemical deposition Poor High Published on 24 November 2017. Downloaded by University of Texas Libraries on 08/12/2017 20:16:36. 37PDF Image | Supercritical CO2 Mediated Incorporation of Sulfur into Carbon Matrix
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