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consistent with that of elemental sulfur. This suggests that such a weight loss was due to the evaporation of sulfur nanocrystals in the nanocomposites [46]. From the weight loss, the sulfur contents were estimated to be 72.27 wt.%, 81.79 wt.%, and 90.86 wt.%. Thus, the three samples were referred to as cp(S-TTCA)@rGO-70, cp(S- TTCA)@rGO-80, and cp(S-TTCA)@rGO-90, respectively. DSC mea- surements were conducted to further ascertain the formation of crosslinking between sulfur and TTCA. As shown in Fig. 2b, pure sulfur sample exhibited four pronounced endothermic peaks in the heating thermogram, where the first one at ca. 118 C was ascribed to the solid phase transformation from orthorhombic to monoclinic forms, the second one at ca. 134 C was attributed to the trans- formation from solid to liquid state (melting), and the third one at about 200 C was due to the ring-opening of S8 to form radicals; whereas, the fourth peak centered at ca. 350 C was marked broad, starting from ca. 250 C to about 360 C, which is probably due to the combination of evaporation and boiling of liquid sulfur, coin- ciding with the sudden weight loss within this temperature range in the corresponding TGA curve for pure sulfur in Fig. 2a. For the three cp(S-TTCA)@rGO composites, the endothermic peaks at ca. 110 C, 134 C and 350 C were also well-resolved, whereas the peak at about 200 C completely disappeared, as highlighted by a green ellipse in Fig. 2b. This observation signifies that S8 ring has been opened to bond/anchor with thiol groups on TTCA, that is forma- tion of crosslinked polymers in these three cp(S-TTCA)@rGO com- posites. Note that the significant weight loss of TTCA is also at about 330 C due to decomposition (Fig. S3), therefore the weight loss and endothermal peak at around this temperature for the three cp(S- TTCA)@rGO composites depicted in Fig. 2b should be partially contributed by the decomposition of TTCA. From Fig. 2c, one can see that the XRD patterns of cp(S-TTCA) @rGO-80 resembled those of elemental sulfur, which indicates that sulfur remained crystalline in the composite. The bonding interaction between elemental sulfur and TTCA was evidenced in Raman measurements. Fig. 2d shows the typical Raman spectra of S, TTCA@rGO and cp(S-TTCA)@rGO-80. For TTCA@rGO, the vibra- tional band at ca. 446 cml might be attributed to the N]CeS deformation in TTCA [43]. After the mixture of TTCA and elemental sulfur was heated at 170 C to produce cp(S-TTCA)@rGO-80, this deformation peak red-shifted to 439 cml and concurrently a new peak appeared at ca. 473 cml due to the stretching of the SeS bonds. Again, these observations suggest that elemental sulfur (S8) reacted with the thiol groups on TTCA forming crosslinked poly- mers, as depicted in Fig. 1. The morphology of the cp(S-TTCA)@rGO composites was then studied by scanning electron microscopy (SEM) measurements. As depicted in Fig. 3a, granular domains with diameters ranging from tens of nanometers to hundreds of nanometers are observed with pure sulfur, whereas cp(S-TTCA)@rGO-80 exhibited a sheet-like crumpled structure with a rough surface (shown in Fig. 3b). From higher-magnification SEM studies of the selected area in Fig. 3b, one can see that nanoparticles up to 50 nm in width are densely packed on these sheet-like structures (shown in Fig. 3c). Typical TEM images of the cp(S-TTCA)@rGO-80 are shown in Fig. 3d and e, where one can find that the nanoparticles in Fig. 3c are actually comprised of smaller particles of less than 20 nm in diameter. In the corresponding high-resolution TEM image shown in Fig. 3f, one can see well-resolved lattice fringes with a d-spacing of 0.38 nm, consistent with the (222) planes of crystalline sulfur (PDF#08- 0247) [47], indicating the encapsulation of nanocrystalline sulfur in the cp(S-TTCA)@rGO composites. EDS measurements (shown in Fig. 3g) show that the cp(S-TTCA)@rGO-80 composite was comprised only of the C, O, N, and S elements, which were distributed homogeneously on the surface of the sample, as evi- denced in elemental mapping studies (shown in Fig. 3i). Calculations based on the integrated peak areas yielded a content of 15.45 wt.% for C, 1.02 wt.% for O, 4.67 wt.% for N and 78.85 wt.% for sulfur (shown in Fig. 3h). Consistent results were obtained in XPS measurements, where the contents of C, N and S were determined to be 18 wt.%, 7 wt.% and 75 wt.%, respectively (shown in Fig. S4). The performance of the cp(S-TTCA)@rGO composites as active cathode materials was then evaluated with a prototype battery in a coin-cell configuration, as depicted in Fig. 4a and Fig. S5. In cyclic voltammetric (CV) measurements (shown in Fig. 4b), two pro- nounced reduction peaks were resolved in the negative potential scan from þ3.0 V to þ1.5 V for the cp(S-TTCA)@rGO-80 cathode, where the first peak at about þ2.23 V was attributed to the con- version of polymerized sulfur in cp(S-TTCA) to high-ordered poly- sulfides (Li2Sn, 4 n 8) while the second peak at about þ1.93 V was ascribed to the further reduction of lithium polysulfides to lithium sulfides (Li2S). In the charging state, an anodic peak emerged at about þ2.51 V, which arose from the oxidation of Li2S to polysulfides [48]. It is worthy to note that no obvious change was observed between the 1st and 20th CV cycles of cp(S-TTCA)@rGO- 80, which signifies high electrochemical stability of the cp(S-TTCA) @rGO-80 cathode. To identify the effect of sulfur-rich nanoparticles on the electrical performance of cp(S-TTCA)@rGO-80, electro- chemical impedance spectroscopic (EIS) measurements was then conducted. From the EIS spectra in Fig. 4c, one can see that both the cp(S-TTCA)@rGO-80 sample and a simple mixture of sulfur and carbon black (S/C) showed a semicircle in the high-frequency re- gion and a radial oblique line in the low-frequency region. From the semicircles, the electron-transfer resistance (Rct) involved was estimated to be only 51.2 U for the cp(S-TTCA)@rGO-80 cathode, markedly lower than that (132.9 U) of the S/C electrode [49]. As for the linear segment in the low-frequency region, it was ascribed to the resistance of ion diffusion within the electrodes. Again the cp(S- TTCA)@rGO-80 showed a much lower resistance than the S/C electrode. These observations clearly demonstrate that copoly- merization of sulphur with TTCA to form crosslinked nanoparticles on rGO significantly facilitated the electron-transfer and mass- transfer dynamics. Notably, the cp(S-TTCA)/rGO-80 electrode showed an excellent rate performance (shown in Fig. 4d and Fig. S6). For instance, it displayed a highly reversible discharge-charge capacity of 1341 mAh g1 at 0.1 C (shown in Fig. S7) 1220 mAh g1 at 0.2 C, 1017mAhg1 at0.5C,861mAhg1 at1C,807mAhg1 at2C,and 645 mAh g1 at 5 C. These rate-capacity values are higher than those of sulfur-embedded benzoxazine polymers [50], covalent triazine frameworks [41], allyl-terminated copolymers, divinyl- benzene copolymers [51,52], and the copolymers of S and TTCA supported on carbon black (denoted as cp(S-TTCA)/CB-80 shown in Fig. S8). This was most likely due to the formation of small sulfur- rich polymer crystallites on the highly conductive rGO substrate (Fig. 1). The cycling performance of cp(S-TTCA)/rGO-80 was eval- uated at a higher rate current density of 0.5 C. As shown in Fig. 4e, although the specific capacity of cp(S-TTCA)/rGO-80 cathode diminished gradually with prolonging cycling, the discharge volt- ages remained almost invariant. Fig. 4f shows the cycling perfor- mance and coulombic efficiency of the series of cp(S-TTCA)/rGO cathodes. The initial capacity was determined to be 878 mAh g1 for cp(S-TTCA)/rGO-70, 1094 mAh g1 for cp(S-TTCA)/rGO-80 and 1026 mAhg1 for cp(S-TTCA)/rGO-90 at the same sulfur loading of 2 mg cm1). Yet, after 300 discharge-charge cycles, the specific capacity decreased to 673 mAh g1 for cp(S-TTCA)/rGO-70, 815 mAh g1 for cp(S-TTCA)/rGO-80 and 713 mAh g1 for cp(S- TTCA)/rGO-90, corresponding to a decay rate of 0.088%, 0.098% and 0.121% per cycle, respectively. From these results, one can see that cp(S-TTCA)/rGO-80 stood out as the best among the series. The long-term cycling stability of the cp(S-TTCA)@rGO-80 S. Zeng et al. / Carbon 122 (2017) 106e113 109PDF Image | Graphene-supported highly crosslinked organosulfur nanoparticles as cathode materials
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