Solvents used in lithium-sulfur battery electrolytes are often divided into three categories: the first category is ester solvents, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), etc.; the second category is ether solvents, mainly including chain ethers such as ethylene glycol dimethyl ether (DME) and triglyme (TEGDME), and cyclic ethers such as 1,3-dioxolane (DOL), tetrahydrofuran (THF) and their homologues, etc.; the third category is other organic solvents, such as toluene (TOL), dimethyl sulfide (DMSO), etc. The specific physical properties are shown in Table 1.
Although ester solvents can easily form a denser and stable SEI film on the lithium surface, polysulfide ions are prone to side reactions with ester solvents, especially for high-sulfur battery systems, resulting in extremely low sulfur utilization. Therefore, ester solvents are only suitable for a few sulfur positive electrode systems such as low sulfur content positive electrodes and sulfurized polyacrylonitrile. Cyclic ether solvents have limited dissolution of polysulfide ions, but they are good for polymerization to form a protective layer on the surface of lithium metal. If the amount is too large in the electrolyte, the final product of the sulfur reduction reaction will not be Li2S, resulting in a partial loss of the sulfur electrode capacity; chain ether can effectively dissolve lithium polysulfide, but excessive addition can easily cause the polysulfide ions to dissolve excessively, which is not conducive to inhibiting the shuttle effect of polysulfide ions, but also aggravates the corrosion of polysulfide ions on the lithium negative electrode, and reduces the coulombic efficiency and cycle stability of the battery system. Therefore, the electrolyte for lithium-sulfur batteries is often composed of two or more solvents, and the composition and ratio of the solvents are also important factors that affect the performance of lithium-sulfur batteries.
Some sulfur-containing solvents (such as dimethyl disulfide, DMDS) can also be used as a solvent for lithium-sulfur battery electrolyte. The addition of sulfur-containing solvent has a promoting effect on the multi-step reaction of the sulfur cathode, and significantly improves the performance of the sulfur cathode, but it cannot prevent the corrosion damage of lithium by polysulfide ions, and cannot stabilize both the sulfur positive electrode and the lithium negative electrode at the same time, and the improvement of the long-term cycle stability of the lithium-sulfur battery is limited. Relevant studies have shown that LiF plays a very important role in stabilizing the surface properties of metallic lithium and improving the electrochemical performance of lithium-sulfur batteries. Therefore, some fluorine-containing ester and ether solvents such as fluoroethylene carbonate (FEC), 1,1,2,2-tetrafluoroethyl ether (ETFE) And 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) have begun to be used in lithium-sulfur batteries and have shown better results. The use of fluorine-containing solvents can form a strong LiF-rich SEI film on the surface of lithium metal, effectively improving the stability of the lithium metal/electrolyte interface, ensuring that lithium is protected from the continuous corrosion damage of polysulfide ions, and reducing the consumption of polysulfide ions on the negative electrode side, and is beneficial to enhance the stability of the lithium-sulfur battery during the cycle. Studies have also shown that some fluorine-containing solvents such as TTE can not only form a protective layer on the lithium negative electrode, but also can form a protective layer with a certain effect on the sulfur positive electrode. The introduction of fluorine-containing solvents can promote the formation of a more stable SEI film on the surface of metallic lithium, which is an effective means to improve and improve the structure and electrochemical stability of lithium-sulfur batteries from the perspective of solvents. Conventional ether solvents have relatively narrow potential windows, low melting points and boiling points, and the safety and stability of the electrolytes they make up need to be improved urgently. Ionic liquids have many advantages over ether solvents and have gradually become a hot spot in electrolyte research, especially room temperature or near room temperature ionic liquids. According to their different cations, ionic liquids can be divided into imidazoles, pyrrolidines, piperidines, pyridines, quaternary ammonium salts, quaternary phosphorus salts and other types of ionic liquids.
Imidazole-based ionic liquids are the first to be used in lithium-ion batteries, but their higher reduction potential (~1.0Vvs.Li/Li+) has certain difficulties in matching carbon and metal lithium anodes. But the use of suitable anions such as TFS factory, FS factory or long-chain alkyl substitution method can appropriately solve the problem of poor matching. The imidazole ionic liquid is effective in stabilizing the morphology of the negative electrode and improving the performance of the battery.
Pyrrolidine-based ionic liquids have higher lithium ion migration number and conductivity than imidazole-based ionic liquids, and have a significant effect on inhibiting lithium dendrites. In lithium-sulfur batteries, the addition of pyrrolidine ionic liquids improves the cycle stability of lithium on the basis of improving the compatibility of the electrolyte/metal lithium and increasing the ionic conductivity of the electrolyte. It provides a reference for the application of pyrrolidine ionic liquids in lithium-sulfur batteries.
Piperidine-based ionic liquids and pyrrolidine-based ionic liquids have a very similar structure and good electrochemical stability, but their high viscosity and poor film-forming properties limit their application in electrolytes. However, the high viscosity of the electrolyte is conducive to limiting the diffusion of polysulfide ions and reducing the probability of side reactions between lithium and polysulfide ions, which is beneficial to the improvement of the performance of lithium-sulfur batteries. Therefore, the high viscosity of the piperidine ionic liquid can be used to stabilize the sulfur positive electrode and the lithium negative electrode in a lithium-sulfur battery. The high viscosity of piperidine ionic liquids effectively inhibits the shuttle of polysulfide ions in the electrolyte; even without adding LiNO3, a relatively stable SEI film can be formed on the surface of lithium metal, and the battery can maintain a higher capacity and better stability. Pyridine ionic liquids also have good thermal and electrochemical stability. The addition of pyridine ionic liquids makes the surface of metal lithium form a dense SEI film. Although the thicker SEI film will increase the impedance of the battery, it can shield polysulfide ions, prevent its corrosion of metal lithium, and inhibit the “shuttle effect.” By controlling the added amount of pyridine ionic liquid, the viscosity of the electrolyte can be adjusted, so that the lithium-sulfur battery can exert higher electrochemical performance. In addition, when the anions in the pyridine ionic liquid match the carbon matrix and sulfur loading in the sulfur cathode, the performance of the battery can be improved, especially the ionic liquid containing the FS plant has a significant effect on improving the performance of the lithium-sulfur battery.
Quaternary ammonium salts, quaternary phosphate salts, guanidines, and stream plasma liquids generally have higher melting points and decomposition temperatures, and they have good safety. But its conductivity is low, and it needs to functionally replace the alkyl group on the cation or mix it with other solvents and functional additives. These types of ionic liquids are currently only used in systems that use metal oxides and lithium-containing transition metal compounds as positive electrodes, but there are few reports on their applications in lithium-sulfur battery systems. In view of the protective effect of this type of ionic liquid on the negative electrode, there may be some applications in the lithium-sulfur battery system in the future.