Introduction Lithium-sulfur batteries are one of the most promising secondary rechargeable batteries due to their high energy density, environmental friendliness, abundant reserves and low price. However, lithium-sulfur batteries have some problems such as poor electrical conductivity at the cathode, structural damage caused by volume expansion during charging and discharging, and shuttle effect caused by polysulfide dissolution, seriously restricting the development of lithium-sulfur batteries. One of the effective methods to solve the problems above is to prepare sulfur-carbon composite cathode materials by using carbon materials as sulfur carriers. Among them, porous carbon can increase the loading of monolithic sulfur, improve the electrical conductivity of the cathode material, inhibit the dissolution of soluble lithium polysulfide, and alleviate the volume expansion. It cannot inhibit the dissolution of soluble lithium polysulfide effectively for a long time due to the weak interaction between the no npolar and polar soluble lithium polysulfide of the carbon material. Pyridine nitrogen has the maximum binding energy with soluble lithium polysulfide (Li2Sx, 4≤x≤8), which has the most obvious inhibition effect on the shuttle effect. The nitrogen in C3N4 is as high as 55.1%, and it is mainly pyridine nitrogen. The preparation method is simple and low cost. Therefore, C3N4 modified bituminous mesoporous carbon composites were proposed as sulfur carriers for lithium-sulfur composite batteries. The morphology and structure of the composites, the effect of the composites on the adsorption-catalyzed conversion of soluble lithium polysulfide (Li2Sx, 4≤x≤8), and the electrochemical performance of lithium-sulfur batteries were investigated. Methods 5% mass fraction silane coupling agent (KH550) aqueous solution was used to surface treat SiO2 nanopowder(Jinan Zhiding Welding Materials Co., Ltd., China) and then porous carbon materials were prepared with treated SiO2 powder with asphalt powder. The porous carbon material was compounded with urea (Sinopharm Chemical Reagent Co., Ltd., China) and a sublimation sulfur powder (Shanghai McLean Biochemical Technology Co., Ltd., China)as amelted sulfur ina ratio of 40:60 to obtain the active material of cathode material. Al foil was used as acollector, and the electrode material was obtained via mixing the active material, carbon black and polyvinylidene fluoride (PVDF) in aratio of 8:1:1. The CR2025 coin-type cells were assembled in a glove box in Ar gas (<1 ppm of O2) with cathodes, separator, Li foil as an anode, and electrolyte. The phase composition and structure were characterized by X-ray diffraction. The surface composition of materials was determined by X-ray photoelectron spectroscopy. The surface morphology was characterized by scanning electron microscopy and transmission electron microscopy. The specific surface area was measured by specific surface area analysis based on the BET. The sulfur content was analyzed by thermo gravimetric analysis. The sample structure was determined by a model Sentera R200-L Raman spectroscope. The constant current charging and discharging tests were performed. The CV and EIS were determined by a model CHI660E electrochemical workstation. Results and discussion A porous carbon composite with C3N4 (i.e., AC/C3N4)has more reaction sites, greater adsorption, higher electrical conductivity and faster redox kinetics, compared with C3N4. This is mainly because AC/C3N4 has a large specific surface area and porosity, which is conducive to the capture of polysulfides. Also, this composite increases the contact area of the electrolyte, which is conducive to the penetration of electrolytes, accelerates the transfer of Li+ and electrons, thereby promoting the redox kinetics of polysulfides. Moreover, this composite provides more reaction sites for electrochemical reactions, enabling the redox reaction to proceed quickly and improving the utilization rate of sulfur. In the calcination process, urea pyrolysis forms C3N4 and adheres to the surface of porous carbon. AC/C3N4/S contains rich pyridine nitrogen, which can effectively anchor polysulfides. This composite has a great adsorption capacity for soluble lithium polysulfide (Li2Sx, 4≤x≤8) and can effectively inhibit the dissolution of lithium polysulfide (Li2Sx, 4≤x≤8), thus reducing the shuttle effect. The results of symmetric cell kinetic test and deposition kinetic test showed that AC/C3N4 has more catalytic active sites and a higher electronic conductivity, thus accelerating the catalytic conversion of polysulfides and promoting the nucleation and growth of Li2S. The first discharge capacities of the three samples, i.e., C3N4/S, AC/S, and AC/C3N4/S, are 705, 1 084 mA·h/g, and 1 248 mA·h/g, respectively, at a current density of 0.2 C. The voltage differences between the charging and discharging platforms of the three samples are 0.30, 0.21 V, and 0.19 V. Compared with C3N4/S, the polarization voltage of AC/C3N4/S is significantly reduced. The average discharge specific capacity of AC/C3N4/S is 986, 815, 736, 640, 520 mA·h/g and 859 mA·h/g at current densities of 0.2, 0.5, 1.0, 2.0, 4.0 C, and 0.2 C, indicating that AC/C3N4/S has abetter rate performance. This is attributed to the catalytic and adsorption effects of C3N4 on lithium polysulfide, which can accelerate the electrochemical reaction rate and inhibit the dissolution of soluble lithium polysulfide (Li2Sx, 4≤x≤8), resulting in an improved rate performance. The results of CV tests and EIS tests indicated that the difference between the oxidation and reduction peaks of AC/C3N4/S is smaller than that of AC/S. The charge transfer resistance of AC/C3N4/S is smaller than that of C3N4/S. This is because C3N4 has a catalytic effect on soluble lithium polysulfide (Li2Sx, 4≤x≤8), reducing the polarization, enhancing the electrochemical reaction kinetics, accelerating the charge transfer, and reducing the transfer impedance. Conclusions C3N4 was composited with asphalt porous carbon material to obtain a AC/C3N4 composite material, which was used as a sulfur carrier for lithium sulfur batteries. The AC/C3N4/S composite material hada large specific surface area and porosity, promoting the redox kinetics of polysulfides and providing a buffer space for the volume expansion of sulfur during charging and discharging processes. Meanwhile, polar C3N4 adhered to the surface and pores of porous carbon, effectively improving the material adsorption and catalysis of soluble lithium polysulfide (Li2Sx, 4≤x≤8), reducing shuttle effect and loss of active substances, improving sulfur utilization rate, and ensuring a long-term stability. The first discharge capacity of the AC/C3N4/S was 1 248 mA·h/g at a current density of 0.2 C, and the capacity still remained 862 mA·h/g after 100 cycling. The average discharge specific capacity of AC/C3N4/S was 986, 815, 736, 640, 520 mA·h/g and 859 mA·h/g at current densities of 0.2, 0.5, 1.0, 2.0, 4.0 C, and 0.2 C, indicating that AC/C3N4/S had abetter rate performance.