A wide variety of porous carbon is a good carrier for constructing sulfur-based active materials. The electrochemical performance of sulfur/porous carbon materials can be further improved by rationally regulating the structure, specific surface area, pore structure and distribution of porous carbon. However, porous carbon still has some shortcomings in terms of electronic conductivity and structural stability, which will affect the rate performance of the battery. Based on the consideration of electronic conductivity and structural stability of carbon materials, scientists have tried to use other carbon materials as the carrier of sulfur-based cathode materials, which can be divided into one-dimensional carbon[mainly carbon nanotubes and carbon nanofibers, as shown in Figure 1(b)], two-dimensional carbon[mainly graphene and its derivatives, as shown in Figure 1(c)] and three-dimensional carbon[figure 1 (d)].
One-dimensional carbon has a relatively large aspect ratio, which is conducive to the rapid movement of electrons. It also has superior physical properties such as high stability and mechanical strength, which can meet the basic requirements of sulfur carrier materials. In addition, the large aspect ratio of one-dimensional carbon makes it prone to entanglement, forming a good conductive network and even forming a self-supporting carrier. Among them, carbon nanotubes and carbon nanofibers are the most studied one-dimensional carbons.
Carbon nanotubes are typical sp2 hybrids, have excellent electronic conductivity, and also have an obvious hollow structure. Generally speaking, for carbon nanotubes with a small inner diameter, sulfur is preferentially distributed on the wall of the carbon nanotube; for carbon nanotubes with a larger inner diameter, sulfur can enter the inside of the nanotube. In view of the gap between carbon nanotubes and porous carbon in terms of specific surface area and pore structure, people often use the method of activation or composite with porous carbon to prepare porous carbon nanotubes with high specific surface area. In addition, surface functional group modification and heteroatom doping of carbon nanotubes can also improve the physical and chemical environment on the surface of carbon nanotubes, enhance the interaction with sulfur, and improve the performance of sulfur electrodes. CVD, template method, and suction filtration can be used to obtain carbon nanotube arrays and carbon nanotube films with higher mechanical strength and better flexibility, and the high performance of sulfur-based composites can be achieved by virtue of its good conductive network.
Carbon nanofibers also have a typical one-dimensional structure, but the degree of graphitization is slightly lower. Common carbon nanofibers have no internal cavity, which is not conducive to high sulfur loading. With the help of activation and other methods, holes can be made on the carbon nanofibers; carbon nanofibers with a hollow structure can be prepared by electrospinning, template method, etc., and these porous or hollow carbon nanofibers can greatly increase the sulfur loading and improve the electrode cycle stability. Carbon nanofibers can also be woven or wound to obtain a self-supporting structure, using the conductive network of carbon nanofibers and better flexibility to stabilize the sulfur cathode.
Carbon materials with one-dimensional structure have a certain gap compared with porous carbon in terms of specific surface area and pore structure. However, the structural characteristics of one-dimensional carbon can form a good conductive network, promote the transmission of electrons and ions, and can provide a certain space to load sulfur and even accommodate the volume change during the sulfur electrode reaction. However, one-dimensional carbon has a limited role in limiting polysulfide ions.
Graphene (g) with typical two-dimensional structure not only has ultra-high specific surface area, but also has excellent conductivity, mechanical properties and flexibility. It can be used as the carrier of sulfur based composites. At the same time, the surface of graphene, especially the edges and defects, has high activity, so it can be flexibly modified by functional groups and doped with heteroatoms, optimize the physical and chemical environment of graphene surface, improve the nucleation, growth and distribution of sulfur on its surface, increase the attachment and reaction sites of sulfur and polysulfide ions, enhance the adsorption of graphene on polysulfide ions, and improve the electrochemical stability of sulfur based composites. There are two common ways of loading sulfur on graphene. One is that graphene wraps sulfur particles to form a wrapping structure; The second is that sulfur accumulates on the surface and interlayer of graphene to form a sandwich structure.
Graphene Oxide (GO) rich in oxygen-containing functional groups has excellent dispersibility in water and can achieve controllable preparation of sulfur-based composite materials in aqueous solutions. In addition, the presence of oxygen-containing functional groups is conducive to the nucleation and growth of sulfur, and can enhance the binding effect of polysulfide ions with the help of stronger C-S bonds and O-S bonds, and improve the electrochemical performance of sulfur-based materials. However, the presence of a large number of oxygen-containing functional groups also seriously affects the conductivity of graphene oxide, which is not conducive to the rate performance of the material and the charge and discharge behavior under high current conditions. Surface modification and appropriate modification can be used to improve the rate performance of composite materials. Removal of some oxygen-containing functional groups by physical or chemical methods to obtain Reduced Graphene Oxide (rGO) can improve the poor conductivity of graphene oxide to a certain extent, but it will also cause a decrease in the specific surface area of the carrier material, which can be solved by the method of activating pores. Reduced graphene oxide can be directly compounded with sulfur to form a composite material, and can also be used as a coating material for sulfur/porous carbon composite materials to improve the electrochemical performance of sulfur-based materials under different conditions. In particular, it can improve the long-term cycle stability of the composite material under extreme temperature conditions (high or low temperature). At the same time, heteroatom doping has become an effective means to enhance the polar characteristics of the graphene surface and enhance the chemical adsorption of graphene to soluble polysulfide ions. Partial replacement of carbon atoms by non-carbon atoms (such as nitrogen, phosphorus, boron, and sulfur, etc.). Taking nitrogen as an example, it is not only beneficial to improve the conductivity of graphene and promote the transmission of Li+, but also can inhibit the shuttle of polysulfide ions by means of the strong interaction between nitrogen and lithium polysulfide, and improve the electrochemical stability of sulfur-based composite materials.