Although metal compounds are inferior to carbon-based materials in terms of conductivity, specific surface area and pore structure; however, metal oxides often have a certain polarity and have a strong interaction with soluble intermediate products (polysulfide ions), which can limit the free movement of polysulfide ions to a certain extent and to a certain extent, and alleviate the shuttle effect caused by the migration of polysulfide ions, the loss of electrochemical substances and the corrosion damage of the metal lithium negative electrode; at the same time, certain metal compounds can also play a catalytic role in the electrochemical reaction process of the sulfur electrode, promote the efficient and orderly progress of the electrode reaction, and improve the electrochemical stability of the electrode and the battery; in addition, the density of general metal compounds is greater than that of carbon-based materials. Using this as a carrier material can also increase the tap density and compaction density of the positive electrode active material, which has important practical significance for increasing the energy density of the battery.Therefore, the rational use of metal compounds in sulfur electrodes may not only realize the full play of the electrochemical performance of active materials, but also promote the practical application of lithium-sulfur batteries.
At present, metal compounds that can be used as supports for sulfur-based composite materials can be divided into metal oxides, metal sulfides, metal nitrides, metal carbides, metal organic framework materials, and other metal compounds.
The early application of metal oxides in sulfur electrodes was mainly to compensate for the small specific surface area of the conductive agent. Utilize the higher specific surface area of nano metal oxide (such as Mg0.6Ni0.4O, La2O3, Mg0.8Cu0.2O, TiO2, Al2O3 and MgO, etc.) to improve the contact between sulfur and the conductive agent, to a certain extent inhibit the agglomeration of sulfur and other uneven distribution, and improve the utilization rate of sulfur. With the deepening of research, it has been discovered that metal oxides also have a certain adsorption and restriction effect on soluble lithium polysulfide, which helps to improve the cycle performance of sulfur electrodes. Therefore, metal oxides have gradually begun to be used as supports for sulfur-based composite materials and have shown relatively good advantages. By controlling the structure of the metal oxide, the effect of the metal oxide as a support material can be enhanced. Among them, some metal oxides with higher electronic conductivity can exhibit stronger interaction with Li2Sn, which is conducive to the realization of controlled uniform deposition and oxidation of Li2S. Therefore, the prepared sulfur-based composite shows better rate performance and cycle performance.
Recently, the Nazar research group has conducted an in-depth investigation of the influence of transition metal oxides with different intrinsic redox potentials (RP) on the electrochemical reaction of sulfur cathodes, as shown in Figure 1(a). The corresponding research results show that when the RP value of the metal oxide is in the range of 2.4~3.05V, S2 can be oxidized into thiosulfate (S2O32-) and polythionate under the action of the metal oxide. These sulfur-containing groups can be adsorbed on the surface of metal oxides and reduced to Li2S in the subsequent electrochemical reaction process, such typical metal oxides such as VO2 and MnO2. When the RP value is less than 1.5V, although the metal oxide does not react with Sn2-, it still has a strong adsorption capacity for Li2Sn, such as Co3O4 and Ti4O7. When the RP value is greater than 3.05V, Sn2- can not only be oxidized to S2O32-, but can further become non-electrochemically active SO42-, which will adversely affect the electrochemical reaction of sulfur. Such metal oxides are V2O5 (3.4V). In addition, the Cui research group discussed the mechanism of non-conductive metal oxides (such as MgO, Al2O3, CeO2, La2O3 and CaO) improving the performance of sulfur cathodes, and proposed that the performance of the cathode depends on the adsorption and diffusion of Li2Sn on the surface of the non-conductive metal oxides, as shown in Figure 1(b). Theoretical calculations show that Li2Sn (n=1, 8) exhibits single-molecule chemical adsorption on the surface of the metal oxide, and the diffusion process of Li2Sn on the surface of the metal oxide is predicted by the Li+ diffusion properties in the metal oxide lattice, and finally, some metal oxides that have excellent Li2Sn surface adsorption and diffusion capabilities are pointed out, such as MgO, La2O3 and CeO2 etc. After this type of metal oxide is compounded with elemental sulfur, a composite material with excellent cycle performance can be obtained. In addition, the microscopic morphology of the metal oxide can be flexibly controlled through a variety of preparation methods, which is more conducive to the loading of elemental sulfur. And with the help of the strong chemical adsorption of metal oxides on Li2Sn, the shuttle effect of soluble lithium polysulfide can be suppressed to the greatest extent, thereby greatly improving the electrochemical performance of the sulfur/metal oxide composite material.
Theoretical calculation results show that the adsorption energy of Li2Sn by metal sulfides is slightly lower than that of metal oxides. However, the electrocatalytic activity possessed by metal sulfides can promote the mutual conversion between long/short-chain lithium polysulfides, inhibit the dissolution of soluble lithium polysulfides, and can regulate the deposition of Li2S2/Li2S in order to achieve homogenization, thereby greatly improving the electrochemical performance of the lithium-sulfur battery.
With the gradual use of metal oxides in the preparation of sulfur-based active materials, metal sulfides, especially transition metal sulfides, have begun to be used as support materials alone or in combination with other supports to prepare sulfur-based cathode materials. The use of metal sulfide is not only conducive to the increase of the first week capacity of the composite material, but also can ensure the capacity retention after long-term cycling. This is due to the fact that metal sulfides can catalyze the mutual conversion between lithium polysulfides on the basis of adsorption of lithium polysulfides, and to speed up this process to a certain extent, so that the sulfur cathode can still maintain high capacity and high stability after a long period of charge and discharge cycles under high current density conditions.