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Journal of Materials Chemistry A Paper Hence, sulfur could be completely dissolved in SC-CO2 and be further transferred into the inner pores, voids and interlayer of carbon matrices to form a strong interaction between sulfur and carbon matrices. Beneting from this novel SC-CO2 synthetic strategy, we use AC as an example to fabricate AC@S composites in Li–S batteries. The results clearly demonstrate that the ob- tained AC@S cathode exhibits high specic capacity (905 mA h g1 at 0.1 A g1), prolonged cycling life (817 mA h g1 aer 100 cycles) and remarkable coulombic efficiency (>99%). This novel strategy provides new insight into the rational design and controllable synthesis of C@S cathodes for Li–S batteries. Conflicts of interest There are no conicts to declare. Acknowledgements The authors thank the nancial supports from National Natural Science Foundation of China (21403196, 51572240 and 51677170), Natural Science Foundation of Zhejiang Province (LY17E020010 and LY16E070004), Science and Technology Department of Zhejiang Province (2016C31012, 2016C33009 and 2017C01035) and Xinmiao Talents Program of Zhejiang Province (2016R403086). Notes and references 1 S. Urbonaite, T. Poux and P. Novak, Adv. Energy Mater., 2015, 5, 20. 2 A. Manthiram, S. H. Chung and C. Zu, Adv. Mater., 2015, 27, 1980–2006. 3 H.B.Yao,K.Yan,W.Y.Li,G.Y.Zheng,D.S.Kong,Z.W.Seh, V. K. Narasimhan, Z. Liang and Y. Cui, Energy Environ. Sci., 2014, 7, 3381–3390. 4 X. Tao, J. Wang, Z. Ying, Q. Cai, G. Zheng, Y. Gan, H. Huang, Y. Xia, C. Liang, W. Zhang and Y. Cui, Nano Lett., 2014, 14, 5288–5294. 5 F.Z.Zeng,A.B.Wang,W.K.Wang,Z.Q.JinandY.S.Yang,J. Mater. Chem. A, 2017, 5, 12879–12888. 6 J. Wang, Y. S. He and J. Yang, Adv. Mater., 2015, 27, 569–575. 7 Y. X. Yang, Z. H. Wang, G. D. Li, T. Z. Jiang, Y. J. Tong, X. Y. Yue, J. Zhang, Z. Mao, W. Sun and K. N. Sun, J. Mater. Chem. A, 2017, 5, 3140–3144. 8 R. Fang, S. Zhao, P. Hou, M. Cheng, S. Wang, H.-M. Cheng, C. Liu and F. Li, Adv. Mater., 2016, 28, 3374–3382. 9 R. Cao, J. Chen, K. S. Han, W. Xu, D. Mei, P. Bhattacharya, M. H. Engelhard, K. T. Mueller, J. Liu and J.-G. Zhang, Adv. Funct. Mater., 2016, 26, 3059–3066. 10 H. Hu, H. Cheng, Z. Liu, G. Li, Q. Zhu and Y. Yu, Nano Lett., 2015, 15, 5116–5123. 11 X. Fang, W. Weng, J. Ren and H. Peng, Adv. Mater., 2016, 28, 491–496. 12 X. L. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 2009, 8, 500– 506. 13 C. Oh, N. Yoon, J. Choi, Y. Choi, S. Ahn and J. K. Lee, J. Mater. Chem. A, 2017, 5, 5750–5760. This journal is © The Royal Society of Chemistry 2017 fresh electrode. Aer 100 cycles, although the surface of the AC@S electrode became smooth (Fig. 6e and f), the structural integrity of the cycled AC@S electrode was maintained well, suggesting the superior cycling stability of the AC@S electrode. In addition, on the micro scale, TEM and HRTEM images (Fig. 6c) showed the same morphology and lattice spacing as the fresh electrode. EDS mapping results (Fig. 6d, g and h) conrmed that the bright and strong S signal overlapped well with the C signal aer the long-term cycling, indicating that sulfur was still embedded uniformly in the cycled electrode. Moreover, the XPS results (Fig. 6i and j) further revealed that C–S–C (164.3 eV), C]S (165.8 eV) and C–SOx species (169.1 eV) existed in the cycled AC@S electrode (Fig. 6i). However, compared with the fresh AC@S sample (Fig. 2n), the intensity of the C–SOx species in the cycled AC@S electrode was dramati- cally enhanced. Additionally, apart from C–C (284.8 eV), C–S (285.6 eV) and C–O (286.8 eV), a peak at 288.8 eV in the cycled AC@S electrode (Fig. 6j) was assigned to the carboxyl group (O]C–OH).26 The enhanced intensity of the C–SOx species and the new peak related to the carboxyl group may be due to the decomposition of electrolyte and the side reaction during long- term cycling. However, as shown in Fig. 6i and j, the interaction between C and S was still retained well aer the long-term cycling, implying good affinity between sulfur and AC. Besides, the specic surface area of AC@S slightly increased to 25.46 m2 g1 aer 100 cycles (Fig. S10†). However, the specic surface area of AC@S was still much lower than that of pristine AC and AC-CO2. The increased specic surface area of the cycled AC@S sample may be attributed to the following. On the one hand, a small part of sulfur that is not well trapped by AC matrices will transform into polysuldes and further dissolve in the electrolyte, which may contribute to the specic surface area. On the other hand, Super-P with a large specic surface area was added to the electrode, and it cannot be completely removed from the cycled electrode when doing BET tests. The added Super-P will also increase the specic surface area of the cycled electrode, but this specic surface area change is very small in the cycled AC@S electrode. Therefore, it is easy to conclude that the SC-CO2 method is a facile and effective synthetic strategy for fabricating C@S cathodes with high cycling stability and strong polysulde trapping capability. 4. Conclusions In conclusion, a facile, low cost and environment friendly SC- CO2 synthetic strategy has been successfully developed to fabricate C/S composite cathodes in Li–S batteries. Taking the advantages of high inltrability, excellent diffusivity and supe- rior solvability, SC-CO2 plays multiple roles in achieving the highly efficient sulfur transfer and highly homogeneous sulfur distribution in carbon matrices. On the one hand, SC-CO2 serves as intercalator, penetrating into pores, voids and inter- layers of carbon matrices to expand and exfoliate the porous structure and tightly-stacked layered graphite structure. As a result, it can create extra space to store more sulfur in carbon matrices. On the other hand, SC-CO2 as a marvellous hydro- phobic solvent has the powerful solubility of nonpolar sulfur. J. Mater. Chem. A 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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