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110 S. Zeng et al. / Carbon 122 (2017) 106e113 Fig. 3. (a) SEM image of elemental sulfur. (b) SEM image of cp(S-TTCA)@rGO-80 and (c) the corresponding high-resolution SEM image of the selected area in (b). (d, e) TEM images of cp(S-TTCA)@rGO-80 at different magnifications. (f) High-resolution TEM image showing the lattices of crystalline nanoparticles on cp(S-TTCA)@rGO-80. (g) SEM image showing the selected area for EDS measurement; (h) the corresponding EDS spectrum and the summary of element contents. (i) Elemental mapping images of S, C, and N in cp(S-TTCA) @rGO-80. (A colour version of this figure can be viewed online.) cathode was further studied at a high rate of 1 C. As shown in Fig. 5, after 500 deep charge-discharge cycles, the specific capacity of cp(S-TTCA)@rGO-80 decreased from the initial value of 821 mAh g1 to 671 mAh g1, corresponding to 81.72% retention and a decay rate of only 0.0404% per cycle. Moreover, the coulombic efficiency was maintained at almost 100%. In contrast, a simple mixture of sulfur and TTCA supported on rGO (denoted as S&TTCA@rGO-80) showed a rapid decrease in capacity, retaining a capacity of ca. 500 mAh g1 after only 300 discharge-charge cycles. The much higher cycling stability of cp(S-TTCA)@rGO-80 is most likely due to the formation of highly crosslinked sulfur copolymers nano- particles on the rGO surface. To gain further insights into the structures of cp(S-TTCA)@rGO, we also carried out FTIR measurements (shown in Fig. 6a). The band at 792 cm1 is due to the stretching vibration of the CeS bond [41,53], and the one at 819 cm1 to the stretching vibration of SeS [53]. No apparent change was observed in the FTIR spectrum of cp(S-TTCA)@rGO-80 even after 500 discharge-charge cycles at 1 C, indicating that cp(S-TTCA) is structurally robust. Fig. 6b shows the typical Raman spectra of TTCA@rGO, and cp(S-TTCA)@rGO-80 before and after 500 discharge-charge cycles at 1 C, where one can find that the N]CeS deformation peak in TTCA remains at ca. 439 cml, indicating that the thiol groups were still chemically bonded with sulfur. That is, cp(S-TTCA) retained its highly cross- linked networks leading to effective confinement of lithium poly- sulfides during long-term discharge-charge cycles. To quantify the crosslinking of sulfur in cp(S-TTCA)@rGO, XPS measurements were conducted (Fig. 7). Calculations based on the integrated peak area of S2s in Fig. 7a suggested that the total S element in the as-synthesized cp(S-TTCA)@rGO-80 was ca. 81.0 wt.%PDF Image | Graphene-supported highly crosslinked organosulfur nanoparticles as cathode materials
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