Hierarchical porous nitrogen-doped carbon-supported Mo₂C particle composite materials (Mo₂C@PCN-X) were constructed by an anti-solvent method and a subsequent in-situ pyrolysis process with ammonium molybdate tetrahydrate and ammonium citrate as precursors, and sodium chloride as a template. The nitrogen-doped carbon pore structure and Mo₂C loading in Mo₂C@PCN-X were regulated via adjusting the addition ratio of ammonium molybdate tetrahydrate. The results show that the number of hollow cavities in Mo₂C@PCN-X increases with the increase in the amount of ammonium molybdate tetrahydrate added, providing abundant storage space for lithium polysulfides (Li₂S_n, n=4-8) and restricting the diffusion of LiPS through physical interactions. Furthermore, the Mo₂C particles exhibit significant adsorption capacity and catalytic activity toward LiPS, accelerating the conversion of soluble Li₂S_n to solid-state Li₂S₂/Li₂S, thereby effectively suppressing the shuttle effect. The S@Mo₂C@PCN-1.5 cathode delivers a high reversible discharge capacity of 1207 mA·h·g^-1 at 0.1 C and maintains 517 mA·h·g^-1 at 1.0 C. After 200 cycles, this cathode demonstrates a low per-cycle decay rate of merely 0.1%, exhibiting superior cycling stability compared to pristine S@PCN.

15 g of sodium chloride was dissolved in 50 mL of deionized water. Afterwards, 1.5 g of ammonium molybdate tetrahydrate [(NH₄)₆Mo₇O₂₄·4H₂O] and 4.786 g of ammonium citrate (C₆H₁₇O₇N₃) were sequentially added to the solution, followed by continuous stirring until complete dissolution. The mixed solution above was rapidly poured into 400 mL of industrial ethanol and vigorously stirred for 5 min. After precipitation, the NaCl@C₆H₁₇O₇N₃-1.5 precursor was obtained by filtration and drying. Also, NaCl@C₆H₁₇O₇N₃-0, NaCl@C₆H₁₇O₇N₃-1.0, and NaCl@C₆H₁₇O₇N₃-2.0 precursors were prepared via varying the addition amount of ammonium molybdate tetrahydrate (0, 1 g, and 2 g), respectively, based on the above procedure. The NaCl@C₆H₁₇O₇N₃-X (X=0, 1.0, 1.5, and 2.0) precursors were heated in a tube furnace to 450 ℃ at a rate of 5 ℃·min^-1 under an Ar-H₂ (10%, in volume fraction) mixed atmosphere for 2 h, and then continuously heated to 800 ℃ at the same rate and held for another 2 h. After cooling to room temperature, NaCl-Mo₂C@PCN-X powders were obtained. The powders were washed for three times with deionized water to remove the NaCl template (with template recovery), and dried at 70 ℃ to obtain Mo₂C@PCN-X composites and PCN support materials. Mo₂C@PCN-X was mixed with sulfur powder at a mass ratio of 3∶7, ground uniformly with a small amount of ethanol and dried. The mixture was heated in a tube furnace to 155 ℃ at a rate of 5 ℃·min^-1, held for 12 h, and then further heated to 200 ℃ and held for 20 min to remove the excess sulfur on the surface of Mo₂C@PCN-X. After natural cooling, the S@Mo₂C@PCN-X composite cathode material was prepared. In addition, S@PCN composite cathode material was also prepared as a control sample using PCN as a substrate according to the procedure above.

The analysis of micro-morphologies of PCN and Mo₂C@PCN-X composites shows that the number of elongated pore structures increases from Mo₂C@PCN-1.0 to Mo₂C@PCN-1.5, but decreases significantly from Mo₂C@PCN-1.5 to Mo₂C@PCN-2.0 as the addition amount of (NH₄)₆Mo₇O₂₄·4H₂O increases, possibly due to the collapse of excessive pore structures. Abundant pore structures are beneficial for the storage and transport of electrolytes. Compared with other samples, Mo₂C@PCN-1.5 exhibits a larger specific surface area and pore structure, which can provide more internal voids for sulfur loading and inhibit the dissolution of Li₂S_n. Meanwhile, the smaller pore diameters can enhance the adsorption capacity of carbon materials for polysulfides. Compared with pure carbon materials, the specific surface area of composites with added ammonium molybdate tetrahydrate increases significantly. This is likely because during the precursor preparation process, ammonium molybdate tetrahydrate crystallizes on the sodium chloride template after adding ethanol. In the subsequent carbonization process, ammonium molybdate tetrahydrate crystals extract carbon from the carbon material to form molybdenum carbide. In this process, the vacancies of extracted carbon form micropores, and unreacted molybdate leaves some micropores after washing. These micropores provide a larger specific surface area for the carbon material. However, the specific surface area decreases with the increase in the addition amount of ammonium molybdate tetrahydrate, possibly because the excessive addition causes larger crystal grains to form on the sodium chloride template. The larger grains extract too much carbon during the reaction, leading to the collapse of generated micropores. The adsorption capacity of the support materials for polysulfides is intuitively determined by visual experiments and UV-VIS absorption spectroscopy. After adsorption by Mo₂C@PCN-1.5, the solution color significantly fades, and the absorption peak intensities of Li₂S₆ and Li₂S₄ decrease significantly, indicating that the Mo₂C@PCN-1.5 support material can strongly adsorb polysulfides. The results of symmetrical battery and deposition experiments demonstrate that Mo₂C@PCN-1.5 with appropriate molybdenum carbide catalysis exhibits better polysulfide conversion kinetics. The electrochemical performance analysis of full batteries confirms that Mo₂C@PCN-1.5 has superior cycling and rate performance, as well as high sulfur loading capacity. As a cathode substrate, Mo₂C@PCN-1.5 is more conducive to promoting the retention of polysulfides and the utilization of active materials.

In this work, a Mo₂C@PCN-X composite material with a hierarchical pore structure was constructed by a green and facile anti-solvent strategy. The results showed that the introduction of the precursor (NH₄)₆Mo₇O₂₄·4H₂O had a regulatory effect on the crystallization behavior of the NaCl template, inducing the carbon component in the Mo₂C@PCN-X composite to form a hierarchical pore structure with additional cavities, and the cavity density was positively correlated to the molybdenum source concentration. When Mo₂C@PCN-X was used as a host for LSB batteries, the three-dimensional interconnected pores of the PCN matrix could promote electrolyte infiltration and adsorb soluble Li₂S_n (n=4-8) through physical action. Moreover, the Mo₂C particles loaded on its surface could enhance the chemical adsorption of Li₂S_n by the Mo₂C@PCN-X host material through d-p orbital hybridization, while reducing the energy barrier for Li₂S deposition/decomposition. However, Mo₂C could agglomerate as the loading amount increased, reducing the specific surface area of the material. Therefore, under the same conditions, the S@Mo₂C@PCN-1.5 positive electrode exhibited the optimum reversible discharge capacity and capacity retention rate during cycling. The crystallization regulation mechanism could provide an idea for the controllable preparation of porous carbon-based composites. Future research could focus on reducing the particle size of Mo₂C through strategies such as surface functional modification, so as to provide a theoretical reference for the design of positive electrode materials for high-energy-density LSBs.