Carbon materials are ideal carriers for preparing sulfur-based composite materials. This is mainly due to the relatively excellent electrical conductivity (electrons and ions) of carbon-based materials, which can alleviate the inherent insulation of elemental sulfur and lithium sulfide; carbon materials generally have a higher specific surface area, which can provide more reactive sites for the electrochemical reaction of sulfur and lithium sulfide, and promote the rapid and reversible conversion between sulfur and lithium sulfide; thirdly, the inherent pore structure of carbon materials can provide space for the storage of sulfur and reaction products (including intermediate product lithium polysulfide), to a certain extent, alleviate the volume effect and limit the free movement of polysulfide ions, which is conducive to the improvement of electrode and battery stability; carbon-based materials have a wide range of raw materials and simple preparation methods, which can realize large-scale controllable production and facilitate the commercialization of lithium-sulfur batteries. In addition, some non-metal elements such as boron, nitrogen, phosphorus and sulfur can be doped in situ on the carbon matrix, which flexibly adjusts the physical/chemical environment of the carbon surface, and increases the possibility of using carbon materials as a sulfur-based positive electrode carrier.
The porous carbon with rich pore structure, large specific surface area and excellent electronic conductivity, as shown in Figure 1, meets the basic requirements of sulfur cathode support materials. According to the pore size, porous carbon can be divided into microporous carbon (0.6~2.0nm), mesoporous carbon (2~50nm), macroporous carbon (>50nm) and multi-porous carbon (contains at least two of micropores, mesopores and macropores). Among them, the micropores in the microporous carbon have a strong restriction and adsorption effect on sulfur and polysulfide ions, and even limit the electrochemical reaction of sulfur within the micropores, avoiding the appearance of soluble long-chain polysulfide ions. It appears as a platform in the discharge process (corresponding to the “solid-solid” reaction mechanism), and the prepared composite material has excellent cycle stability. However, the sulfur content and discharge voltage platform in the sulfur-based composite material prepared with microporous carbon as the carrier are relatively low, which is not conducive to the improvement of battery energy density.
Mesoporous carbon rich in mesopores can not only meet the pore volume required for high sulfur loading, but also promote the transport of electrolyte and Li, but the electrochemical performance of sulfur-based composites prepared by using disordered mesoporous carbon as a carrier is not ideal. In 2009, Professor Nazar took the lead in proposing ordered mesoporous carbon CMK-3 as a sulfur carrier, achieving a discharge capacity of 1005mA·h/g in the first week, thus setting off a research boom in lithium-sulfur batteries. Subsequent in-depth research further found that the reasonable distribution of the pore diameter in the mesoporous carbon has an important influence on the electrochemical performance of the sulfur cathode. Among them, the mesopores with a smaller pore size have a stronger restriction on sulfur and polysulfide ions, and the mesopores with a larger pore size are conducive to the full infiltration of the electrolyte in the electrode. However, the limitation of sulfur and polysulfide ions in the practical application of mesoporous carbon is limited, and the carbon structure can be designed and optimized, or heteroatoms (such as nitrogen, phosphorus, boron, sulfur, etc.) can be used to improve the surface chemical environment of carbon materials and improve the adsorption and restriction of polysulfide ions by mesoporous carbon.
The open pore structure of macroporous carbon is very beneficial to the infiltration of electrolyte and the increase of sulfur loading. It is suitable for preparing high sulfur content cathodes, but the macropores cannot effectively inhibit the dissolution and diffusion of polysulfide ions, so there are few reports on single macroporous carbon as the supporting material of sulfur cathode. The hierarchical porous carbon can take into account the characteristics and advantages of different pore structures. It can not only use the strong limiting effect of micropores on soluble lithium polysulfide, but also use the adsorption effect of mesopores on Li+ transport and lithium polysulfide, and it can also use the contribution of macropores to sulfur load and electrolyte infiltration to achieve high load and strong adsorption, and significantly improve the electrochemical performance of sulfur positive electrode. In addition, the structure and pore distribution of the hierarchical porous carbon will affect the electrochemical performance of the sulfur cathode. At present, the hierarchical porous carbons that have been reported include microporous/mesoporous carbon, microporous/mesoporous/macroporous carbon, and single-layer or multi-layer porous hollow carbon spheres. Among them, the micropores are more distributed on the outside of the carbon to effectively limit the free movement and shuttle of polysulfide ions; mesopores and macropores are more concentrated inside the carbon, providing storage space for sulfur and lithium polysulfide; the porous hollow structure can better realize the high load of sulfur, slow down the volume effect and shuttle phenomenon in the electrode reaction process, and improve the rapid transport and migration of Li+.