Metal nitrides, especially transition metal nitrides (called metallic nitrides) generally have the characteristics of high hardness, high melting point and high chemical stability, and have the appearance and conductivity of metals, which can be used in the semiconductor industry and energy storage devices. The earliest metal nitride used in lithium-sulfur batteries was mesoporous titanium nitride (TiN). TiN’s good electronic conductivity (46S/cm) gives the composite material better rate performance. In addition, mesoporous TiN has a certain restrictive effect on soluble lithium polysulfide, which promotes the sulfur/TiN composite to remain stable during long-term cycling. Subsequently, other transition metal nitrides, such as tungsten nitride, vanadium nitride, and molybdenum nitride, are also added to the ranks of sulfur-based positive electrode support materials. Studies have found that tungsten nitride has the strongest interaction with lithium polysulfide among these nitrides, and the sulfur-based active material prepared with this as a carrier exhibits the best electrochemical performance. At present, there are relatively few studies on metal nitrides in lithium-sulfur batteries, but the higher electrical conductivity of metal nitrides makes them very suitable as carrier materials. With the help of more advanced preparation technology to optimize the structure and morphology of metal nitrides, it is bound to make metal nitrides a place in the development of lithium-sulfur batteries.
Two-dimensional transition metal carbides (MXenes) have been developed in the fields of energy storage, adsorption, sensors, conductive fillers, etc. due to their high specific surface area, high conductivity, flexible and adjustable composition and controllable minimum nano-layer thickness. The research shows great application potential. At present, transition metal carbides used in lithium-sulfur batteries are mainly concentrated in titanium carbide series materials. Such materials can form a bond with lithium polysulfide, thereby restricting and constraining the free migration of soluble lithium polysulfide to a certain extent. The sulfur-based composite material constructed on this basis exhibits excellent discharge capacity and cycle stability. There are many types of transition metal carbides, but fewer types are used in lithium-sulfur batteries. In the future, we can investigate the actual effects of other transition metal carbides in lithium-sulfur batteries, and prepare sulfur/metal carbide composites with better performance through structural control.
The discovery and development of Metal-Organic Frameworks (MOFs) materials also provide new options for sulfur cathode carriers. This type of organic-inorganic hybrid materials formed by the self-assembly of organic ligands and metal ions or clusters through coordination bonds has many outstanding features. First of all, MOFs generally have a very high specific surface area (≥3000m2/g), which can provide enough reaction sites for the reaction of sulfur and lithium polysulfide; secondly, MOFs often have a certain pore structure, which can provide space for the load of sulfur and the accommodation of soluble lithium polysulfide; thirdly, the surface of MOFs material has abundant surface functional groups, which can not only be used as adsorption sites for lithium polysulfide, but also effectively restrain and limit lithium polysulfide; finally, the morphology of MOFs materials can be controlled, organic ligands and metal ions can be flexibly adjusted, and targeted design and preparation can be carried out according to actual needs. The addition of MOFs to sulfur cathode materials can improve the electrochemical performance of the composite to a certain extent. However, due to the inherent electronic insulation of the MOFs material, it is not conducive to the rate performance of the sulfur cathode and the cycle stability under high current density conditions. In view of the conductivity of MOFs materials, a method of directly carbonizing MOFs materials has been proposed to obtain a composite carrier material with high conductivity, high specific surface area, large pore volume and rich heteroatoms, and its application to the preparation of sulfur-based active materials shows good results. There are many types of MOFs materials, but relatively few are actually used in lithium-sulfur batteries. In view of the structural characteristics of MOFs materials themselves, in the future, we can try to apply more types of MOFs materials to lithium-sulfur batteries.
In addition to the metal compounds mentioned above, some composite metal compounds also play an important role in lithium-sulfur batteries, especially sulfur cathodes, such as NiFe2O4 with ferromagnetic properties and BaTiO3 with ferroelectric properties. The use of NiFe2O4 can provide sufficient bonding sites, effectively limit soluble lithium polysulfide, and significantly improve the cycle stability of sulfur-based composites. Relying on the excellent conductivity of carbon nanotubes, the composite material still exhibits a higher capacity and a better capacity retention rate at a higher current density. The simple introduction of BaTiO3 can produce instantaneous polarization during the sulfur discharge process, thereby forming an electric field inside the electrode, which limits the polar soluble lithium polysulfide to a certain extent, thereby improving the cycle stability, rate performance and cycle life of sulfur-based composites and even pure sulfur. However, the specific mechanism of action of the instantaneous polarization produced by ferroelectric materials on soluble lithium polysulfide is not very clear. With the deepening of research and further elucidation of the mechanism, composite metal compounds can also play their due role in improving sulfur cathodes.