The electrolyte is one of the key materials of lithium-ion power battery. It effects the battery performance, such as output power, internal resistance, cycle life, etc. The economical and effective method to improve the function of electrolyte is to add additives. In terms of function, the electrolyte additives can be sorted into film-forming additives, moisture inhibiting additives, conductivity enhancing additives, safety additives, low-temperature additives, high-voltage additives, etc. Base on the review of the research on the film-forming additives, low-temperature additives, high-voltage additives, and safety additives, it's pointed out that the key research direction of additives in the future is to develop multifunctional additives. 

    Lithium-ion batteries generally consist of a positive electrode, a negative electrode, a separator, an electrolyte, and a casing. As lithium-ion batteries' core material, the electrolyte usually composes of lithium salt and organic solvent. The current commercial lithium salt is mainly LiPF6. The organic solvent is usually carbonate solvent. Common ones are: ethylene carbonate (EC), carbonic acid Dimethyl ester (DMC), ethyl methyl carbonate (EMC), etc. Improving the functionality of the electrolyte through additives and thereby improving the performance of the battery is an important direction of current electrolyte research.

    Electrolyte additives can be divided into film-forming additives, high-voltage additives, low-temperature additives, safety additives, stabilizers, moisture inhibitors, etc., according to their mechanism of action.

     Film forming additives

     In the application field of new energy vehicles, batteries' long life and storage performance are very important points. As we all know, during the first charge and discharge of lithium-ion batteries, a layer called "solid electrolyte phase interface film" or SEI film is formed at the solid-liquid interface between the electrode material and the electrolyte. The dense and stable SEI film helps lithium. The ion power battery has a longer service life, good storage performance, and better adaptation to the environment. 

    The film-forming additives play an essential role in the formation of the SEI film. Film-forming additives are the ones that have been studied earlier and more frequently.

    VC is an organic film-forming additive with an excellent addition effect. It is a reducing additive that can improve the battery's cycle performance, especially the high-temperature cycle performance, and reduce the irreversible capacity. The main reason is that VC can polymerize on the graphite surface to form polyalkenes. Lithium carbonate membrane, thereby inhibiting the reduction of solvents and salt anions, has been widely used in commercial electrolytes.

    Resisting low temperature additives

    With the rapid increase in users of new energy vehicles, problems in the use process have gradually emerged. Among them, the reduction of cruising mileage in winter is the type of failure that consumers complain more. In order to improve the user experience, new energy vehicles have put forward higher requirements for low-temperature applications of lithium-ion batteries, such as hope that they can be charged and discharged at -20 ℃. In military equipment, low-rate discharge is required at -40 ℃; in the aerospace field, it is sometimes even required that batteries be charged and discharged at a low rate of -50 ℃.

    Commonly used low-temperature improvement additives include the following: organic sulfites, sulfones, vinylene carbonate (VC), and fluoroethylene carbonate (FEC). Among them, VC and FEC are also low-temperature additives that have been used more in recent years. Studies have shown that adding FEC to the electrolyte can better form a stable and low-impedance SEI film, reducing the battery's impedance at room temperature and low temperature, thereby improving the battery's low-temperature rate performance. FEC and VC can also improve the low-temperature performance and significantly improve the normal temperature cycle.