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Journal of Materials Chemistry A Paper Fig. 2 (a–d) SEM images of AC, AC-CO2, AC@S and AC/S-155. (e–h) HRTEM images of AC, AC-CO2, AC@S and AC/S-155. (i–l) STEM image, EDS spectrum and EDS mappings of AC@S. (m and n) High-resolution C 1s and S 2p spectra of AC@S and AC/S-155 composites. View Article Online investigated. As shown in Fig. S1a,† MCMB the sample has a typically spherical morphology. Aer SC-CO2 treatment, MCMB-CO2 and MCMB@S samples were both exfoliated into small pieces of sheet-like particles (Fig. S1b and c†). Interest- ingly, there was no obvious morphological change in the MWCNTs sample (Fig. S2a–c†). MWCNTs-CO2 and MWCNTs@S samples still maintained the 1D tube-like structure. Since the microstructures of the three carbon matrices are quite different, this might be responsible for the signicant morphological changes. The MCMB sample is composed of planar and curved graphitic layers via weak van der Waals forces, therefore, SC- CO2 can directly intercalate into the intervals of layers in MCMB and exfoliate it into graphite sheets. The MWCNTs sample is dominated by strong C–C bonds that are not easily exfoliated or broken during the SC-CO2 process. The inner microstructure and chemical composition of AC, AC-CO2, AC@S and AC/S-155 samples were observed by high- resolution TEM (HRTEM), scanning TEM (STEM) and EDS tests. As shown in Fig. 2e–h, the interplanar distances of AC-CO2 and AC@S samples are both 0.39 nm, which is larger than that of AC (0.34 nm), indicating that well-dened expansion exists in the graphitic interlayers of AC. This phenomenon can also be detected in MCMB and MWCNTs samples as shown in Fig. S3a, b and S4a–c.† Aer incorporating sulfur into MCMB via the SC- CO2 process, the interlayer spacing between two lattice fringes expands from 0.34 to 0.39 nm. Such enlarged interlayer spacing in carbon matrices is benecial for increasing sulfur content, suppressing the polysulde shuttle, and facilitating electrolyte penetration. Furthermore, according to the STEM micrograph and EDS results (Fig. 2i–l and S3e–h†), the main elements in AC@S and MCMB@S samples are sulfur and carbon, matching well with the chemical composition of AC@S and MCMB@S. The EDS mapping results vividly display that the sulfur mapping and carbon mapping are well overlapped, indicating that sulfur is homogeneously embedded into carbon matrices. The high-resolution C 1s and S 2p spectra of AC@S and AC/S- 155 composites are exhibited in Fig. 2m and n. As depicted in Fig. 2m, the high-resolution C 1s spectra of AC@S and AC/S-155 samples can be assigned to three peaks with binding energies of 284.8 eV (C–C/C]C), 286.8 eV (C–O), and 285.6 eV (C]S).26,46 Additionally, according to the high-resolution S 2p spectra, the peaks located at 164.3 eV and 165.8 eV are attributed to C–S–C and C]S, respectively, whereas the peak at 169.1 eV is related to C–SOx species.46,47 It is worth noting that the percentage of C–S– S/C]S in AC@S is higher than that of AC/S-155 (Table S1†), suggesting the better interaction between C and S in the AC@S sample. Moreover, the intensity of C–SOx species in AC/S-155 is much higher than AC@S, indicating that sulfur is easily oxidized during the melt-diffusion process. In order to visually display the volume changes of various carbon matrices before/aer SC-CO2 treatment, the digital photographs of AC, MCMB and MWCNTs are presented in Fig. 3a, S1d and S2d,† respectively. Although all the samples have detectable volume expansion compared to their raw materials, the volumes of AC-CO2 and MCMB-CO2 samples are drastically expanded two times and three times, respectively. As shown in Fig. S5,† the apparent densities of AC (0.3 g cm3), MCMB (0.58 g cm3) and MWCNTs (0.1 g cm3) decreased sharply to 0.15 g cm3 (AC-CO2), 0.11 g cm3 (MCMB-CO2) and 0.07 g cm3 (MWCNT-CO2), respectively. Meanwhile, the apparent densities of AC@S, MCMB@S and MWCNTs@S increased to 0.28 g cm3, 0.19 g cm3 and 0.1 g cm3, J. Mater. Chem. A This journal is © The Royal Society of Chemistry 2017 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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