## 1 Introduction
However, it is a great challenge to fabricate highly dispersed ultrafine metal compounds because the isolated ultra-small particles with high surface energy can susceptibly agglomerate into big particles in the synthesis process. Various strategies have been explored to minimize the agglomeration of isolated ultrafine particles; to this end, the most common approach is to restrict the further growth of nanocrystals by functional support and pore size confinement [32; 33; 34; 35]. For example, [PERSON] et al. designed highly dispersed TiO\({}_{2}\) nanoparticles anchored onto Co-nanoparticle-decorated carbon polyhedras derived from ZIF-67 (C-Co/TiO\({}_{2}\)) as a sulfur host. The as-prepared S@C-Co/TiO\({}_{2}\) cathode exhibited enhanced discharge capacity and long cycling capability due to the stronger chemisorption of C-Co/TiO\({}_{2}\) for polysulfides [35]. Recently, [PERSON] et al. successfully fabricated MoP-quantum-dot-modified N, P-carbon nanotubes (MPQ@NP-HCNT) as a multifunctional modification layer for the Li-S separator. As a result, MoP quantum dots had a large adsorption energy for S\({}_{8}\) and Li\({}_{2}\)S\({}_{n}\) and improved the kinetics of the subsequent Li\({}_{2}\)S reaction simultaneously [36]. Such impressive results can give some guidance to rationally design an ultra-small-size adsorber/catalyst for polysulfides within carbonaceous matrix composites. However, there are some drawbacks, especially in the preparation of precursors, such as expensive reagents, harsh reaction conditions, and synthetic complexity, which have become a major hindrance to their large-scale commercial applications. Consequently, designing a feasible synthesis strategy with advantages of a simple procedure and low cost for fabrication of new ultra-small-size metal compounds coupled with a carbon matrix is urgently demanded. Considering these issues, herein, we successfully fabricated highly dispersed ultrafine-Co-species-nanodot-interspersed g-C\({}_{3}\)N\({}_{4}\) nanosheets and graphene composites (Co@gC\({}_{3}\)N\({}_{4}\)/G) as separator coating materials using a facile and cost-effective method--pyrolysis of a mixed precursor consisting of g-C\({}_{3}\)N\({}_{4}\), CoCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O, and graphene oxide (GO) mixture. In this design, g-C\({}_{3}\)N\({}_{4}\) could partially decompose to realize the reduction of GO, while serving as a template to leave some pores during calcination treatment [37; 38]. Within the Co@g-C\({}_{3}\)N\({}_{4}\)/G products, the residual g-C\({}_{3}\)N\({}_{4}\), graphene, and ultrafine Co-species could form a unique heterostructure attributed to the intimate interface interactions between graphene and g-C\({}_{3}\)N\({}_{4}\), both with the 2D structure of nanosheets [39; 40]. Hence, graphene here acted as an ideal platform for electron transfer between the residual g-C\({}_{3}\)N\({}_{4}\) and the catalytic Co-species in the heterostructure. This heterostructural composite as a separator-modified layer can not only enable effective chemisorption toward polysulfides but also catalyze the conversion of intercepted polysulfides. As a result, the assembled Li-S cells using Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separators paired with a high-sulfur-content electrode (\(\sim\)70 wt\(\%\)) exhibit excellent rate capability and good cyclic stability. A high specific capacity of \(\sim\)860 mAh g\({}^{-1}\) at 2.0 C as well as a capacity-fading rate of only \(\sim\)0.035\(\%\) per cycle over 350 cycles at 0.5 C can be achieved. This bifunctional separator can even endow a Li-S cell at a low current density to exhibit excellent cycling capability, with a capacity retention rate of \(\sim\)88.4\(\%\) at 0.2 C over 250 cycles. Furthermore, a Li-S cell with a Co@g-C\({}_{3}\)N\({}_{4}\)/G-PP separator possesses a stable specific capacity of 785 mAh g\({}^{-1}\) at 0.2 C after 150 cycles and a superior capacity retention rate of \(\sim\)84.6\(\%\) with a high sulfur loading of \(\sim\)3.0 mg cm\({}^{-2}\).