## 2 Results and Discussion
### Synthesis and Characterization of the Co@g-C\({}_{3}\)N\({}_{4}\)/G Heterostructure Composites
This result may be attributed to the heavy aggregation of metallic Co nanoparticles with the rise in the calcination temperature, thereby leading to the formation of large-size particles, displaying a distinct diffraction peak in the XRD pattern. Therefore, we could deduce that a minor feature of Co-species in the final products at a relatively low temperature was achieved, meanwhile giving rise to strong chemical adsorption/catalyzation conversion toward polysulfides due to enhanced surface energy and abundant active sites [29; 31]. Figure 1: (**a,b**) XRD spectra, (**c**) Raman spectra, and (**d**) N\({}_{2}\) adsorption–desorption curve of as-prepared samples (the corresponding pore size distribution curves are inserted in (**d**)). The * in Figure 1a is a diffraction peak of CoO in the CoO@G sample. The Raman spectra of the three samples at different temperatures were also investigated, as displayed in Figure 1c. In each Raman spectrum, two apparent peaks could be seen at 1350 cm\({}^{-1}\) and 1590 cm\({}^{-1}\), corresponding to the D-band (sp\({}^{3}\)-hybridized carbon) and G-band (sp\({}^{2}\)-hybridized carbon) [10; 25]. Generally, the intensity ratio of the D band to the G band (I\({}_{\rm D}\)/I\({}_{\rm G}\)) is used to evaluate the graphitization degree of carbon-based materials. The I\({}_{\rm D}\)/I\({}_{\rm G}\) value significantly decreases with increasing calcination temperature, suggesting that a relatively high temperature could promote the graphitization of graphene. The porous properties of the as-prepared samples were investigated using the N\({}_{2}\) adsorption-desorption test at 77 K. The shape of the isotherms for the g-C\({}_{3}\)N\({}_{4}\) and Co@g-C\({}_{3}\)N\({}_{4}\)/G samples were similar (Figure 1d and Figure S2b), that is, a certain amount of adsorption volume at a low relative pressure (_P/P\({}_{0}\)_ = 0.01-0.20), a H3 hysteresis loop at a medium relative pressure (_P/P\({}_{0}\)_ = 0.50-0.80), and an obvious uptake tendency at a high relative pressure (_P/P\({}_{0}\)_ = 0.90-1.0). These observations imply that the two prepared samples possessed a hierarchical porous structure, with the coexistence of micropores, mesopores, and some accumulated macropores [16; 42], which were further identified from their pore size distribution curves. In addition, it was observed that Co@g-C\({}_{3}\)N\({}_{4}\)/G exhibited a tiny adsorption capacity at a low relative pressure (_P/P\({}_{0}\)_ = 0.01-0.20) compared to the g-C\({}_{3}\)N\({}_{4}\) sample, indicating a relatively low specific surface area. This remarkable difference in these two samples may be due to the unique heterostructure of the Co@g-C\({}_{3}\)N\({}_{4}\)/G sample. The specific surface areas of g-C\({}_{3}\)N\({}_{4}\)/G and Co@g-C\({}_{3}\)N\({}_{4}\)/G were 124.4 and 48.73 m\({}^{2}\) g\({}^{-1}\). In addition, the pore size of Co@g-C\({}_{3}\)N\({}_{4}\)/G was predominately situated at meso-macropores regions, as can be seen from its pore size distribution curve (inserted in Figure 1d), which is conducive to promoting electrolyte penetration and Li ion transport in the cycling process [10]. The micromorphology of the corresponding samples was characterized using SEM and TEM, as shown in Figure 2. The SEM images of g-C\({}_{3}\)N\({}_{4}\) and the precursor are displayed in Figure S3 (see Supporting Information). The g-C\({}_{3}\)N\({}_{4}\) sample exhibited a flower-like scrolled structure with numerous crumpling and scrolling nanosheets (Figure S3a). As for the dried precursor, some flower-like scrolled g-C\({}_{3}\)N\({}_{4}\) uniformly dispersed on the surface of wrinkled bulk graphene oxide nanosheets was clearly observed (Figure S3b). After calcination treatment at 500 \({}^{\circ}\)C, there was some flower-like scrolled g-C\({}_{3}\)N\({}_{4}\) (marked in red circle) and large-size sheet-like substance (a typical feature of rGO marked in the white curve in Figure 2a,d), suggesting the coexistence of g-C\({}_{3}\)N\({}_{4}\) and graphene.