Sulfur/non-metal compound composite material
In addition to metal compounds, some non-metal compounds can also be used as supports for sulfur-based composite materials, such as silicon oxide, silicate, and graphite phase carbon nitride. The surface of silicon oxide can be flexibly modified with different functional groups, so as to use functional functional groups to limit the free movement of intermediate products during the reaction to a certain extent, alleviate the attenuation of sulfur cathode capacity, and improve the stability of the electrode and battery system. The composition and structural characteristics of silicate itself are conducive to strong adsorption of soluble polysulfide ions and a strong interaction with lithium polysulfide, which effectively alleviates the adverse effects of lithium polysulfide on sulfur electrodes and lithium-sulfur batteries, and realizes the improvement of sulfur utilization and cycle stability. Graphite carbon nitride (g-C3N4) is a kind of carbon nitride, which is a typical two-dimensional layered structure with good visible light response and stability. It is widely used in photocatalytic hydrogen production, water oxidation, organic degradation, photosynthesis and carbon dioxide reduction and other catalytic fields. It is a non-metallic photocatalytic material with important research value. g-C3N4 is rich in nitrogen atoms, among which pyridine nitrogen has a strong effect on polysulfides and can be used to construct sulfur-based cathodes. Studies have confirmed that the nitrogen atoms of g-C3N4 can form a bond with lithium in polysulfides, thereby limiting the uncontrollable migration of polysulfides. Similarly, g-C3N4 can significantly enhance the redox reaction of polysulfide ions. In addition, the inherent electronic conductivity and ionic conductivity of g-C3N4 are poor, and its electrochemical performance is limited under the condition of increasing the high current density of the sulfur cathode; the pore structure of g-C3N4 is less than that of conventional carbon materials, which is not conducive to increasing the sulfur loading in sulfur-based composite materials; the low density of g-C3N4 will adversely affect the tap density and compaction density of the prepared sulfur cathode material.
Novel sulfur-containing cathode material
In addition, some sulfur-containing compounds and even organic compounds can be used as cathode materials for lithium-sulfur batteries. Among the more common ones are lithium polysulfide, lithium sulfide, lithium polythiophosphorus, and sulfur-containing organic compounds.
Lithium polysulfide (Li2Sn, n=4~8) is an intermediate product in the process of sulfur charging and discharging. In this way, it can be directly used as the active material of the sulfur electrode (usually liquid, used in conjunction with porous carbon cloth), can realize normal charge and discharge, and show good cycle stability.
Li2S is the final discharge product of S and can also be used as a positive electrode material. Because it contains lithium, other negative electrodes (such as silicon, tin and metal oxides) can be used on the negative side to replace metal lithium, and the theoretical specific capacity is 1166mA·h/g. When lithium sulfide is used as a positive electrode active material, it is first charged and converted into elemental sulfur, and then the same electrochemical reaction as a conventional sulfur electrode occurs. Similarly, Li2S itself has low ion and electronic conductivity, which makes it face the same dilemma as sulfur cathodes. At the same time, Li2S is extremely sensitive to water and oxygen, and its synthesis conditions are harsh. In recent years, in response to a series of Li2S problems, scientists have carried out related research and made certain progress. At present, the main improvement methods include preparing nano-Li2S particles, constructing composite materials of Li2S and carbon, and optimizing the structure and composition of Li2S from the perspective of synthesis. At present, the safety performance of the battery has gradually become the most concerned point in the actual production and use of the battery. Li2S instead of elemental sulfur can circumvent the use of lithium metal anodes, and significantly improve the safety of lithium-sulfur batteries. However, its own insulation properties, relatively complex preparation process and required relatively harsh experimental conditions limit the popularization and application of Li2S in lithium-sulfur batteries.
The discovery of lithium polythiophosphate has enriched the choice of positive electrodes for lithium-sulfur batteries. The formation of lithium polythiophosphate is based on the reaction of S atoms with PS4-3 to form S-S bonds. During the charging and discharging process, the S-S bond in the material is broken and re-formed, thus completing the electrochemical reaction. This kind of sulfur-containing inorganic compound has high conductivity (5×10-5S/cm, 25℃) at room temperature, and can release 1272mA·h/g capacity at 0.1C. It exhibits higher capacity and better cycle stability at 60°C. At the same time, lithium polythiophosphate can also be used as a positive electrode material for all-solid-state lithium-sulfur batteries, and exhibits better electrochemical performance and higher safety performance.
In addition, some sulfur-containing organic compounds can also be used as cathode materials for lithium-sulfur batteries.
As early as 2002, Yang Yusheng and others of the Chemical Defense Research Institute creatively proposed a series of organic sulfur-containing materials that can be used as lithium-sulfur primary batteries and secondary batteries, such as polythiocarbyne, polythiopolyphenylene acetylene, polythiopolyphenylene, polythiopolyaniline, polythiopolypyrrole, polythiopolythiophene, etc. In 2008, the design idea of sulfur-containing organic compounds with “main chain conduction and side chain energy storage” was proposed, which verified the actual effect of polythiocarbyne as a lithium-sulfur secondary battery. The material shows good electrochemical performance in ester electrolyte. At the same time, some polyphosphazenes commonly used as flame retardants for lithium-ion batteries generate other sulfur-containing polyphosphazene derivatives through appropriate reactions, and can be used in lithium-sulfur batteries, significantly inhibiting the “shuttle effect” of soluble lithium polysulfide. The use of sulfur-containing organic compounds as a positive electrode can significantly improve the cycle stability of the positive electrode, but the actual discharge capacity of the material is relatively low. In the future, with people’s in-depth research on high-capacity sulfur-containing organic materials, they will surely get corresponding applications in lithium-sulfur batteries.