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Journal of Materials Chemistry A Paper interface between carbon matrices and SC-CO2 is much smaller than that of a carbon–liquid (e.g. CS2) interface or a carbon– solid (e.g. sulfur powder) interface, since SC-CO2 has low interfacial tension and high diffusivity. The excellent wetting interfaces allow better penetration of sulfur into the pores of carbon matrices, resulting in homogeneous sulfur distribution and high sulfur content. More interestingly, SC-CO2 as an intercalator can expand and exfoliate tightly-stacked, layered carbon materials during the abrupt pressure release process.45 This ability of SC-CO2 to synchronously tune the layer structure or porous structure of carbon matrices will offer plenty of space to store small sulfur allotropes (S2–4) in carbon matrices. Active carbon (AC) (amorphous carbon), MCMB (graphitic carbon) and MWCNT (1D carbon) are three typical carbon materials, which are selected in this work as examples to study the universality of the SC-CO2 method for synthesizing C/S cathodes in Li–S batteries. It is highly expected that SC-CO2 technology may synchronously realize the highly efficient sulfur transfer and precise microstructure regulation of S/C composites. 2. Experimental 2.1 Materials preparation All reagents were used as purchased without any further puri- cation. In a typical procedure, 0.5 g carbon (active carbon (AC), mesocarbon microbeads (MCMB), multi-walled carbon nano- tubes (MWCNTs)) and 0.6 g sulfur were transferred into a 100 mL stainless-steel milling jar. Subsequently, CO2 (99.9%) was pumped into the milling jar until the pressure reached 8.5 MPa. A planetary ball mill (Nanjing, QM-1SP2) at 350 rpm was used in the milling process, and the ambient temperature was strictly kept at 32 C for 12 h. Aer milling, CO2 gas was immediately released, and the as-prepared C@S composites were denoted as AC@S, MCMB@S and MWCNTs@S, respec- tively. Meanwhile, C–CO2 samples were synthesized under the same conditions without adding sulfur, and were denoted as AC-CO2, MCMB-CO2 and MWCNTs-CO2. In order to make a comparison, the control samples were prepared by the routine melt-diffusion method as well. In detail, sulfur and AC were mixed in a mass ratio of 1.2 to 1, and then the mixture was heated in a vacuum oven at 155 C for 12 h to obtain the nal products, denoted as AC/S-155. 2.2 Materials characterization X-ray diffraction (XRD) patterns were obtained using a Rigaku Ultima IV X-ray diffractometer with Cu Ka radiation (l 1⁄4 0.15418 nm). The Raman spectra were recorded using a DXR Raman microscope (Thermo Fisher Scientic) with He–Ne 532 nm laser excitation in the range of 200–2000 cm1. The surface area was determined by the Brunauer–Emmett–Teller (BET) method based on nitrogen adsorption–desorption tests using an ASAP 2020 (Micromeritics Instruments). The pore size distributions were calculated by the Barrett–Joyner–Halenda method. Thermogravimetric analysis was performed on a SDT Q600 (TA Instruments) under Ar owing atmosphere with a heating rate of 10 C min1 from room temperature to 500 C. To address the aforementioned issues in Li–S batteries, extensive efforts have been devoted to incorporating sulfur into various carbon matrices (e.g. meso/microporous carbon,11–13 hollow carbon spheres,14–17 graphene/graphene oxide18–22 and carbon nanotubes23–29) to construct sulfur/carbon (S/C) composite cathodes. These carbon matrices can simulta- neously act as an elaborate conducting framework to contain sulfur and facilitate electron transport, and as a strong adsorbing agent to suppress the soluble polysulde shuttle, resulting in the signicantly enhanced electrochemical perfor- mance of Li–S batteries.2,11 Although the incorporation of sulfur and carbon matrices can ameliorate the intrinsic inferior characteristics of the sulfur cathode, a suitable sulfur encap- sulation technique is more important for realizing the high sulfur utilization, high cycling stability and rate capability in Li– S batteries. To date, various synthetic strategies have been developed for the incorporation of secondary materials into the carbon matrix, which can be classied as direct incorporation by the assembly process,30 or post-incorporation.31 The advan- tage of direct incorporation is that the secondary materials can be uniformly embedded into the carbon matrix; however, compared with direct incorporation, post-incorporation is more convenient to operate and the range of application is wider. The synthetic methods for C/S materials mainly involve the latter, which can be generally divided into three major categories including mechanical mixing,32 heat treatment33–36 and wet- chemistry synthesis.37–39 Conventional methods oen involve complex manufacturing processes, high toxic solvents and high energy consumption, and it is very difficult to guarantee the precise sulfur content, uniform sulfur distribution and good affinity between sulfur and carbon. Specically, sulfur can hardly reach the inner pores and voids of carbon matrices via the aforementioned methods, leading the poor dispersion of sulfur and low effective utilization of carbon matrices. There- fore, the development of a more facile, efficient and green strategy is urgently needed for the synthesis of high- performance sulfur cathode in Li–S batteries. Supercritical uids (SCFs) are unique solvents with both “gas-like” and “liquid-like” physicochemical properties that offer huge opportunities to manipulate the reaction environ- ment in terms of density, diffusivity, viscosity and surface tension via controlling pressure and temperature.40,41 Particu- larly, due to the relatively accessible critical parameters (Tc 1⁄4 31.1 C, Pc 1⁄4 7.38 MPa), non-toxicity and non-inammability, supercritical carbon dioxide (SC-CO2) is the most widely used in many elds such as materials processing, materials drying and separation.42–44 However, to date, the application of SC-CO2 to synthesize S/C composites has rarely been reported. In this work, we attempt to develop a novel SC-CO2 method for the synthesis of a S/C composite with high sulfur content, uniform sulfur distribution and strong sulfur affinity, as shown in Fig. 1. Compared with routine methods, the exploitation of SC-CO2 has several unparalleled merits. First of all, SC-CO2 is a promising hydrophobic solvent that has the comparable dis- solving ability of nonpolar sulfur to that of the highly toxic carbon disulde (CS2), guaranteeing that sulfur can be effec- tively dissolved at the molecular level. The surface tension of the J. Mater. Chem. A This journal is © The Royal Society of Chemistry 2017 View Article Online 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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