In lithium-sulfur batteries, in addition to being used as separators and binder materials, polymers can also be used as carriers for sulfur composite materials. Such as common polyacrylonitrile, polythiophene and its derivatives, polyaniline and polypyrrole, as shown in Figure 1. These polymers form a soft chain structure, which can alleviate the volume effect in the electrochemical process to a certain extent; at the same time, the heteroatoms or functional groups in the polymer skeleton structure or branched chain structure have a certain restrictive effect on lithium polysulfide, and improve the electrochemical stability of the sulfur positive electrode during the cycle.
In 2002, Wang et al. took the lead in applying polyacrylonitrile to construct sulfur-based cathode materials. The elemental sulfur and polyacrylonitrile are heat-treated at a certain temperature (280°C and above) to obtain a composite material, in which the cyano group in the skeleton forms a ring, and the main chain is a polyacetylene-like structure. In the actual discharge process, the sulfur/polyacrylonitrile composite exhibits an inclined plateau of 2.1~1.8V, and exhibits excellent cycle stability in carbonate electrolytes. Scientists subsequently carried out the influence of heat treatment temperature on the properties of composite materials, and explored the distribution of sulfur in composite materials, the bonding situation and the reaction mechanism during charging and discharging. Some scholars believe that during the high-temperature heat treatment process, sulfur only bonds with the carbon in the polyacrylonitrile skeleton, not with nitrogen; in the actual charging and discharging process, the conjugated framework of polyacrylonitrile has a certain capacity contribution, which can explain the phenomenon that the sulfur/polyacrylonitrile composite material exceeds the theoretical capacity during the discharge process. However, there are still controversies about the specific structure of sulfur/polyacrylonitrile and the changes in the substance and structure during actual charging and discharging. However, it is undeniable that the sulfur/polyacrylonitrile composite exhibits extremely high capacity and excellent cycle stability in carbonate-based electrolytes.
Polythiophene and its derivatives (such as polyethylene dioxythiophene), as typical conductive polymers, are also used in the preparation of sulfur-based cathodes. In actual use, polythiophene and its derivatives are mostly used as coating materials to coat the surface of sulfur particles or sulfur/carbon particles to form a typical core-shell structure. With the help of the stability and good ionic and electronic conductivity of polythiophene and its derivatives, it promotes the smooth progress of sulfur oxidation/reduction reactions, and eases the dissolution and shuttle of lithium polysulfide to a certain extent. Scientists used polythiophene and its derivatives to prepare a series of sulfur-based composite materials, which realized the slow decay and high capacity of sulfur-based cathodes during long-term cycling.
Polyaniline has a certain degree of conductivity due to its p-electron conjugated structure in its molecular chain, and it can also be used to prepare sulfur-based composites. Common application methods are also used as coating materials to prepare core-shell structure or coating structure composite materials. Utilize the structural characteristics of polyaniline and better electronic conductivity to improve the electrochemical stability of sulfur-based materials. At the same time, based on the reaction of sulfur and polyacrylonitrile, polyaniline can also undergo a vulcanization reaction with sulfur to obtain a three-dimensional sulfur/polyaniline composite material. And use the S-S bond in the chain or between the chains to fix sulfur and lithium polysulfide to improve the electrochemical performance of the composite material under different current density conditions.
Polypyrrole is a heterocyclic conjugate conductive polymer that has been studied and used more frequently. Usually pyrrole monomer which is liquid at room temperature is used as raw material, synthesized by electrochemical oxidation polymerization or chemical polymerization method, especially easy to form film by electrochemical polymerization method (applicable to acidic aqueous solution and various organic electrolytes). Polypyrrole has good stability in the air and exhibits good stability in general solvents. The conductivity and structural stability of polypyrrole are closely related to the polymerization conditions. In addition, the introduction of certain ions into the polypyrrole by chemical doping can make the polypyrrole have a certain ionic conductivity. In the process of preparing sulfur/polypyrrole composite materials, polypyrrole is more in the form of a coating layer and constitutes sulfur/polypyrrole materials with different morphologies. The introduction of polypyrrole improves the electronic conductivity of the positive electrode material and reduces the charge transfer resistance during the electrochemical reaction. At the same time, it can also avoid the agglomeration of sulfur particles and improve the capacity and cycle stability of the composite material. In addition, by further doping and modifying polypyrrole, the polymer can be turned into a sulfur-based material carrier with both electronic and ionic conductivity, and the performance of the lithium-sulfur battery can be significantly improved.
In addition to the above-mentioned polymers with certain electronic conductivity, other polymers such as polyvinylpyrrolidone (PVP), polyvinyl alcohol (PEG), Nafion and polydopamine (PDA) are also used in the preparation of sulfur cathodes. The functional groups possessed by such polymers appropriately modify the surface of the sulfur particles, improve the micro-environment of the sulfur surface, and have a positive influence during the electrochemical reaction of sulfur, which promotes the improvement of the performance of the composite material. However, there may be a certain gap between the flexibility of the polymer itself and the mechanical strength required by the sulfur cathode. This problem can be solved by compounding with the carrier material. It is undeniable that polymers, especially conductive polymers, have achieved gratifying results in the construction of high-performance sulfur-based cathode materials. In the future, the interaction between lithium polysulfide and polymer molecules will be enhanced, the microscopic morphology of the material will be optimized, and the position of sulfur/polymer composites in sulfur-based composites will be effectively improved.