Sulfur cathode is the core component of lithium-sulfur batteries, and it is also the key to restrict the electrochemical performance of batteries. In view of a series of problems such as the utilization rate, stability and shuttle effect of sulfur cathodes, current research focuses on building sulfur-based composite materials, exploring new sulfur-containing cathodes, and designing electrode structures.
Preparation of electrode materials.
The preparation of the sulfur cathode material also has a certain influence on the performance of the electrode. For different carrier materials “tailor-made”, choosing a suitable preparation method is also very important for realizing the efficient utilization of sulfur. As shown in Figure 1, taking the carbon support material as an example, the preparation method can be divided into a physical mixing method and a chemical reaction method. Among them, the physical mixing method can be divided into mechanical mixing method, heat treatment method (melt diffusion and gas phase diffusion method) and dissolution-recrystallization method; the chemical reaction law is mainly based on the reaction of sulfur-containing salt and acid to generate sulfur in situ. The mechanical mixing method is a simple and effective preparation method that can achieve rapid and more uniform mixing of sulfur and carrier materials. However, this method damages the morphology and tissue structure of the carrier material greatly, and the distribution of sulfur is mainly concentrated on the surface of the carrier. The heat treatment method mainly uses the melting or evaporation characteristics of sulfur at different temperatures to load the sulfur in the pore structure of the carrier material. The general treatment temperature is 150~300℃, and the heat treatment time ranges from 2 to 24 hours. Low-viscosity molten sulfur can enter the pore structure of the carrier, but at the same time it will also be enriched on the surface of the carrier material; gaseous sulfur can be easily poured into the pores of the material, which can avoid the accumulation of surface sulfur, but the sulfur content depends on the pore volume and pore size distribution of the material. The heat treatment method is a common method for preparing sulfur cathode materials, and is especially suitable for supporting sulfur on high specific surface area and large pore volume carrier materials. The dissolution-recrystallization method mainly utilizes the high solubility of sulfur in some solvents (especially non-polar organic solvents, such as carbon disulfide, carbon tetrachloride, etc.), disperses the carrier material in the sulfur-dissolved solvent, and volatilizes or removes the solvent, and the sulfur re-crystallizes from the dissolved state. The dissolution-recrystallization method is relatively simple to operate and is more suitable for self-supporting structures to load sulfur, but the size and size of crystalline sulfur are not easy to control, and the solvents generally used are more toxic. The chemical reaction method uses sulfur-containing salt (generally sodium sulfide, sodium thiosulfate, sodium polysulfide, etc.) to react with acid to generate elemental sulfur in an aqueous solution, which can achieve the control of sulfur particles and morphology, and achieve the goal of uniform combination of sulfur and carrier materials. The chemical reaction method is generally suitable for two-dimensional and three-dimensional carrier materials and carrier materials with functional groups on the surface to load sulfur, but the H2S gas produced by the chemical reaction will bring potential environmental and personal hazards.
Combining sulfur and a carrier to construct a cathode material is a commonly used method to improve the electrochemical performance and structural stability of sulfur cathodes. Sulfur-based composite materials can be divided into sulfur/carbon composite materials, sulfur/polymer composite materials, sulfur/metal compound composite materials, and metal/non-metal compound composite materials according to different carrier materials, which will be introduced in detail in the following article.