Adding functional substances to the electrolyte has gradually become an important means to improve battery performance, especially in the lithium-sulfur battery system, the role of electrolyte additives is more important, and has become one of the main methods for people to improve the performance of lithium-sulfur batteries. This method is mainly based on the interaction between the additives and the positive and negative electrodes, which can form a stable SEI layer “in situ” on the lithium surface, improve the stability of the lithium/electrolyte interface, slow down the damage of the lithium structure and the irreversible loss of lithium polysulfide , and improve the efficiency and cycle life of lithium-sulfur batteries.
Some metal halides such as LiI, AlI3, MgI2, etc. can also be used as electrolyte additives. Among them, LiI is used as an additive in lithium-sulfur batteries. It was found that on the negative electrode side, a passivation layer containing iodine can be formed on the lithium surface, which reduces the possibility of side reactions of polysulfide ions on the lithium surface; on the positive electrode side, the presence of LiI will polymerize DME, forming a protective layer on the sulfur positive electrode. As an electrolyte additive, LiI can effectively stabilize the lithium negative electrode and the sulfur positive electrode, providing a guarantee for improving the performance of lithium-sulfur batteries, but its specific mechanism of action still needs to be studied in depth. For non-lithium metal halides, it is mainly based on that metal ions are deposited on the surface of lithium and even form a lithium alloy, thereby reducing the surface activity of lithium and slowing down the progress of dendrites and other side reactions. However, there are few studies on this type of additives in lithium-sulfur battery systems, and it remains to be studied whether they can continue to maintain battery stability in systems containing polysulfide ions.
In addition, some sulfides (such as phosphorus pentasulfide), rare earth compounds (such as nitric acid), etc. can also be used as electrolyte additives for lithium-sulfur batteries. The introduction of such substances will also make the lithium surface form a relatively stable passivation layer, which can effectively reduce the direct contact between lithium polysulfide and lithium, reduce the consumption of lithium polysulfide, and ensure the stability of lithium metal, thereby improve the capacity and cycle stability of lithium-sulfur batteries.
Not only inorganic salts can be used as additives, organic substances can also be used as additives, or even more effective additives. Drawing on the experience of organic film formers, some organic compounds such as methyl acetate, γ-butyrolactone and toluene can also be used as electrolyte additives for lithium-sulfur batteries. As a result, it was found that although the above-mentioned organic additives can increase the discharge capacity in the first week, methyl acetate and γ-butyrolactone have a negative impact on the battery cycle and cause rapid capacity degradation. After adding a certain amount of toluene, the cycle performance of the battery is significantly improved, which is mainly based on the addition of toluene that reduces the lithium/electrolyte interface impedance.
It is worth noting that the addition of methyl acetate can significantly improve the low-temperature electrochemical performance of lithium-sulfur batteries. By adding suitable substances to the electrolyte, the positive and negative electrodes of the lithium-sulfur battery can be protected, the properties of the electrode/electrolyte interface can be optimized, the irreversible consumption of active materials can be reduced, and the performance decay rate of the lithium-sulfur battery can be slowed down, the stability of the electrode and the battery can be improved, and a practical and efficient solution can be provided for the practical use of the lithium-sulfur battery.