Sulfur cathode structure design

Studies have shown that from the perspective of battery composition and structure, optimizing the structure of the sulfur cathode is also an effective way to improve the performance of sulfur cathodes and lithium-sulfur batteries, especially in the construction of sulfur cathodes with high sulfur content and high sulfur loading. Some typical sulfur cathode structures such as free-standing structure, sandwich structure and interlayer show advantages in stabilizing sulfur cathode and improving sulfur utilization.

The self-supporting structure means that there is no binder and current collector, and the sulfur-based composite is directly used as the cathode material of the lithium-sulfur battery. This can effectively reduce the proportion of inactive materials in the positive electrode, avoid the polarization effect of the binder on the electrode, and help increase the effective sulfur content and sulfur loading of the sulfur electrode. To obtain a self-supporting sulfur cathode with stable structure and performance, the key technology is to prepare a light-weight porous conductive carrier material with self-supporting structure or self-woven behavior; at the same time, the support material can maintain structural stability before and after loading sulfur and during the electrochemical process. The carrier materials generally used as self-supporting structures are carbon materials that have self-woven behavior or are easy to form films, such as carbon nanotubes, carbon fibers, graphene and their derivatives.

Generally, methods for preparing self-supporting structure carrier materials include electrospinning (suitable for preparing carbon fibers), chemical vapor deposition, and vacuum filtration (suitable for carbon nanotubes, graphene and their derivatives).

With the help of self-supporting carrier materials, sulfur can achieve a more uniform distribution and avoid agglomeration. In the actual charging and discharging process, the free movement of lithium polysulfide can be effectively suppressed. Under the premise of high sulfur content and high sulfur loading, the sulfur cathode can still achieve high utilization, low attenuation rate and long cycle life, showing electrochemical performance superior to that of the traditional structure sulfur cathode. It should be pointed out that the use of carbon fiber cloth as a self-supporting carrier needs to have a certain degree of mechanical strength and at the same time have a reasonable pore structure; the use of carbon film as a self-supporting structure must require the required raw materials to be lightweight and have good film-forming properties. The carbon film has porous characteristics to contain and limit sulfur and Li₂S_n, and a certain degree of flexibility and mechanical strength to withstand the volume expansion during the electrochemical process.

The sandwich structure flexibly distributes sulfur between two layers of materials (usually carbon materials). During the discharge process, the electronic conductivity of sulfur can be ensured, and the soluble lithium polysulfide can be fully confined to the positive electrode side with the help of structural characteristics. It avoids the free movement of lithium polysulfide and the inactivation of some active materials due to lack of electrical contact, alleviates the corrosion of lithium polysulfide on the metal lithium negative electrode, and improves the stability of the lithium negative electrode surface; during the charging process, lithium sulfide and lithium polysulfide can be fully converted into sulfur, which weakens the “shuttle effect” of lithium polysulfide and improves the coulombic efficiency of charging and discharging. The sandwich structure can suppress the volume effect of the sulfur cathode during repeated charging and discharging to a certain extent, and ensure the long-term cycling stability of the sulfur electrode. At the same time, the sandwich structure can also increase the sulfur loading of the sulfur electrode, which is of practical significance for constructing a high-energy lithium-sulfur battery.

After the sulfur-based composite material is reasonably designed, the dissolution and shuttle of Li₂S_n will be restricted to a certain extent. However, during the discharge process, Li₂S_n, driven by the concentration gradient and chemical potential, still inevitably diffuses to the negative electrode. For this reason, scientists proposed to preset an intermediate intercalation layer between the sulfur positive electrode and the separator to limit the uncontrollable migration of S_n^2-. This method is simple, direct and effective. The intermediate intercalation layer often has good contact with the positive electrode sheet, and has strong electronic and ion conductivity, as well as sufficient pore structure to adsorb S_n^2-. Because of this, the intermediate intercalation layer can even be used in pure sulfur electrodes and greatly improve the electrochemical performance of pure sulfur cathodes. Unlike the self-supporting structure, the raw materials of the intermediate layer are not limited to light carbon materials with good film-forming properties (such as carbon nanotubes, graphene and their derivatives, etc.). Conductive carbon black, porous carbon, conductive polymers and metal-based compounds can all be prepared into films by various methods and techniques and used as intermediate intercalation layers. A simple and cheap intermediate intercalation layer applied to lithium-sulfur batteries can effectively block the uncontrollable diffusion of S_n^2-. By optimizing the pore structure and surface functionalization of the material of the intermediate layer, the limiting effect of the intermediate layer on S_n^2- can be further improved. It is undeniable that the introduction of intermediate intercalation will increase the capacity and cycle stability of sulfur electrodes and lithium-sulfur batteries, but the intermediate layer as an inactive material will increase the volume and mass of the lithium-sulfur battery to a certain extent, resulting in a decrease in the volume/mass energy density of the battery system. Therefore, reducing the quality of the intermediate layer and improving the limiting effect of the intermediate layer on lithium polysulfide is the key to promoting the practical application of the intermediate layer in lithium-sulfur batteries. At present, the effect of the intermediate layer on lithium polysulfide is more concentrated on pure physical barrier and pore adsorption. In the future, the research of a multifunctional intermediate layer that combines physical barrier, pore adsorption and catalytic electrochemical reaction is of great significance to further improve the electrochemical performance of sulfur electrodes.