## 2 Results and Discussion
### Synthesis and Characterization of the Co@g-C\({}_{3}\)N\({}_{4}\)/G Heterostructure Composites
As the temperature increased to 550 \({}^{\circ}\)C, some crumpled, sheet-like structure was mainly displayed instead of the flower-like scrolled morphology. Specifically, numerous wrinkled, intersected nanosheets with a number of pores were observed in Figure 2b,e, which is mostly attributed to the partial decomposition of g-C\({}_{3}\)N\({}_{4}\) with increasing temperature, thus generating a number of pores during the carbonization process [38]. Figure 2c,f also present a typical large-size crumpled, sheet-like structure from a macro point of view after annealing at 600 \({}^{\circ}\)C. Therefore, it could be concluded that g-C\({}_{3}\)N\({}_{4}\) partially decomposes to realize the reduction of GO, meanwhile acting as a template to leave some pores in the calcination process. The microstructure of g-C\({}_{3}\)N\({}_{4}\)/G is similar to that of the Co@g-C\({}_{3}\)N\({}_{4}\)/G sample, as can be seen from the SEM images in Figure S3c,d. To further confirm the existence of elements in the prepared samples, the SEM mapping of Co@g-C\({}_{3}\)N\({}_{4}\)/G (550 \({}^{\circ}\)C) was performed, as shown in Figure 2g,h. In the SEM mapping of Co@g-C\({}_{3}\)N\({}_{4}\)/G, the elemental mapping images of a selective region of Co@g-C\({}_{3}\)N\({}_{4}\)/G exhibited a homogeneous dispersion of C, N, and Co, indicating these elements could be uniformly distributed on the surface of the sheet using this facile strategy. The fine microstructure of Co@g-C\({}_{3}\)N\({}_{4}\)/G after calcination at 550 \({}^{\circ}\)C was further investigated using TEM and high-resolution TEM (HRTEM) images (Figure 2i-k). A stacked layer-like structure with some wrinkles and no agglomeration of Co species was clearly observed (Figure 2i), mostly corresponding to the typical rGO matrix. Specifically, Figure 2j displays a number of macro-/mesopores distributed in the framework (marked in white circles), which is conducive to an improvement in electrolyte penetration and Li ion diffusion according to the previous reported literature [15]. From the high-resolution TEM image (Figure 2k), numerous ultra-small-size Co-based nanodots (red circles) uniformly dispersed in a carbon framework were observed. This phenomenon may be due to the dual-anchoring effect of g-C\({}_{3}\)N\({}_{4}\) and graphene oxide for Co\({}^{2+}\) ions in the preparation of a precursor, effectively preventing the aggregation of Co-species in the calcination process. These ultra-small Co-based species can significantly catalyze the polysulfide conversion due to their high surface energy and more active terminations [30; 36]. Fourier-transform infrared spectroscopy (FT-IR) was conducted to investigate the chemical structures of the as-prepared composites (Figure 3a). The broad characteristic peaks located at -3000-3500 cm\({}^{-1}\) are indexed to the stretching vibrations of N-H from the marginal amino groups and O-H from the free adsorbed hydroxyl species [39; 49]. Moreover, the adsorption peaks found at -1200-1750 cm\({}^{-1}\) for the g-C\({}_{3}\)N\({}_{4}\) sample can be  
Figure 2: FE-SEM images of prepared Co@g-C\({}_{3}\)N\({}_{4}\)/G samples at different calcination temperatures: (**a**,**d**) 500, (**b**,**e**) 550, and (**c**,**f**) 600 \({}^{\circ}\)C. (**g**,**h**) SEM mapping and EDS of Co@g-C\({}_{3}\)N\({}_{4}\)/G at 550 \({}^{\circ}\)C. (**j**) TEM and (**i**,**k**) high–resolution TEM (HRTEM) images of the Co@g-C\({}_{3}\)N\({}_{4}\)/G sample. ascribed to the stretching vibrations of N-containing heterocycles [50]. Notably, a sharp peak was observed at \(\sim\)808 cm\({}^{-1}\) in g-C\({}_{3}\)N\({}_{4}\), correlating with the typical tri-s-triazine ring vibration [39; 50]. In the spectrum of the G sample, the three peaks at \(\sim\)1632, 1565, and 1200 cm\({}^{-1}\) corresponded to the stretching vibrations of C=O, C=C, and C-O, respectively [51].