Research on Low Temperature Electrolyte

The electrolyte plays the role of transferring Li+ in the lithium-ion battery, and its ion conductivity and SEI film-forming performance have a significant impact on the low-temperature performance of the battery. There are three main indicators for judging the quality of low-temperature electrolytes: ion conductivity, electrochemical window, and electrode reaction activity. The level of these three indicators depends to a large extent on its constituent materials: solvent, electrolyte (lithium salt), and additives. Therefore, the research on the low-temperature performance of each part of the electrolyte is of great significance for understanding and improving the low-temperature performance of the battery. Compared with chain carbonates, the low temperature characteristics of EC-based electrolytes have a compact structure, greater force, and higher melting point and viscosity. However, the large polarity brought about by the ring structure makes it often have a large dielectric constant. EC solvent's large dielectric constant, high ionic conductivity, and excellent film-forming properties can effectively prevent the co-insertion of solvent molecules, making it indispensable. Therefore, most commonly used low-temperature electrolyte systems are based on EC, and then mixed Small molecule solvent with low melting point. Lithium salt is an important component of the electrolyte. Lithium salt in the electrolyte can not only increase the ionic conductivity of the solution, but also reduce the diffusion distance of Li+ in the solution. Generally speaking, the greater the concentration of Li+ in the solution, the greater the ionic conductivity. However, the concentration of lithium ions in the electrolyte is not linearly related to the concentration of lithium salt, but is parabolic. This is because the concentration of lithium ions in the solvent depends on the dissociation of the lithium salt in the solvent and the strength of the association. In addition to the battery composition itself, the research of low-temperature electrolyte will also have a great impact on battery performance due to process factors in actual operation. (1) Preparation process. Yaqub et al. studied the influence of electrode load and coating thickness on the low-temperature performance of LiNi0.6Co0.2Mn0.2O2/Graphite batteries and found that in terms of capacity retention, the smaller the electrode load, the thinner the coating layer, the better the low-temperature performance. . (2) Charge and discharge state. Petzl et al. studied the influence of low-temperature charge and discharge conditions on the cycle life of the battery, and found that when the depth of discharge is greater, it will cause greater capacity loss and reduce the cycle life. (3) Other factors. The electrode surface area, pore size, electrode density, wettability of the electrode and electrolyte, and separator, etc., all affect the low-temperature performance of lithium-ion batteries. In addition, the impact of material and process defects on the low-temperature performance of the battery cannot be ignored. In summary, in order to ensure the low-temperature performance of lithium-ion batteries, the following points need to be done: (1) form a thin and dense SEI film; (2) ensure that Li+ has a large diffusion coefficient in the active material; (3) the electrolyte is in the It has high ionic conductivity at low temperature. In addition, the research can also blaze a new trail and focus on another type of lithium-ion battery-all-solid-state lithium-ion battery. Compared with conventional lithium-ion batteries, all-solid-state lithium-ion batteries, especially all-solid-state thin-film lithium-ion batteries, are expected to completely solve the problem of capacity decay and cycle safety when the battery is used at low temperatures.