As an important part of the battery system, the electrolyte has a direct impact on the performance of the electrode and the battery. Electrolytes generally used in battery systems need to have the characteristics of high conductivity, stable chemical properties, excellent electrode matching, easy access and low cost, less pollution, and suitable operating temperature window. The electrolyte in the lithium-sulfur battery is in contact with the sulfur positive electrode and the lithium negative electrode, and participates in the physical and chemical reactions on the surface of the positive and negative electrodes, which affects the structure and performance of the electrode. Later, the related research progress of electrolytes for lithium-sulfur batteries will be introduced from the aspects of lithium salts, solvents, additives and solid electrolytes.
Lithium salt is the core of the electrolyte system and can be dissociated in the solvent to provide Li+ to the electrolyte. Common lithium salts are shown in the table in Figure 1. LiPF6 and LiBF4 are the most commonly used lithium salts in secondary batteries, but their anions have potential side reactions with polysulfide ions in lithium-sulfur batteries. Therefore, these two types of lithium salts are rarely used in lithium-sulfur batteries. Generally only suitable for special material systems such as low-sulfur positive electrodes or sulfurized polyacrylonitrile. Although LiAsF6 has high ionic conductivity and good compatibility with lithium, its heavy metal elements cause great harm to the environment and human body, and it is not used much in lithium-sulfur batteries. Based on considerations such as battery system stability and compatibility with solvents, LiTf, LiTFSI, and LiFSI have become commonly used lithium salts in lithium-sulfur batteries.
The researchers first studied the single lithium salt system. Lithium perchlorate (LiClO4) is used as a lithium salt for lithium-sulfur batteries that show good electrochemical performance. This is mainly due to the fact that LiClO4 easily reacts with lithium, which can quickly form a dense SEI layer on the lithium surface, effectively shielding polysulfide ions in the system, preventing polysulfide ions from oxidation and loss on the lithium negative electrode side, and improving the cycle stability of lithium-sulfur batteries. For the electrolyte system using LiTFSI as the lithium salt, scientists put forward the concepts of “same ion effect” and “Solvent-in-salt”, and increase the viscosity of the electrolyte by increasing the concentration of lithium salt, so as to limit the free diffusion of polysulfide ions and improve the coulombic efficiency and stability of the system. Although as the concentration of lithium salt in the electrolyte increases, lithium dendrites are suppressed, and the performance of lithium-sulfur batteries is significantly improved, but too high a concentration of lithium salt will cause the electrolyte to become too viscous, which will easily cause the utilization rate of the cathode material to be limited, and the high price of the lithium salt itself will also increase the manufacturing cost of the lithium-sulfur battery. Therefore, high-concentration electrolytes face many challenges in the application of lithium-sulfur batteries.
Based on the study of a single lithium salt system, scientists have tried to use an electrolyte containing two lithium salts to apply to lithium-sulfur batteries. The use of LiTFSI/LiBOB, LiTFSI/LiODFB, and LiTFSI/LiFSI composite lithium salts has achieved relatively good results in lithium-sulfur batteries. A strong and stable passivation film can be formed on the surface of the lithium negative electrode to reduce the corrosion of lithium by soluble intermediate products and the loss of active materials, so that the lithium can maintain a flat and dense surface morphology after repeated charging and discharging, and inhibit the formation of lithium dendrites, effectively improving the capacity, coulomb efficiency and cycle stability of the lithium-sulfur battery.
In view of the effect of the double salt system in the lithium-sulfur battery, the tri-salt and multi-salt systems may exhibit advantages in regulating the chemical composition, ion distribution, electric field distribution, SEI (Solid Electrolyte Interphase) membrane composition and strength of the electrode surface in the lithium-sulfur battery system. In addition, some lithium salts that perform well in lithium-ion batteries, especially those that can generate a stable SEI film on the surface of the carbon negative electrode, may also play a positive role in lithium-sulfur batteries.