How Does Low temperature Affect Lithium-Ion Batteries?

How Does Low temperature Affect Lithium-Ion Batteries?

lithium-ion batteries have been widely used because of their advantages of long life, large specific capacity and no memory effect.The low-temperature use of lithium-ion batteries has problems such as low capacity,serious attenuation, poor cycle rate performance, obvious lithium evolution phenomenon, and unbalanced lithium intercalation. However, with the continuous expansion of application fields, the constraints caused by the low low temperature performance of lithium-ion batteries are becoming more and more obvious. The operating temperature of traditional lithium-ion batteries is between -20 and +55 °C. However, in aerospace, military, electric vehicles and other fields, batteries are required to work normally at -40 °C. Therefore, it is of great significance to improve the low-temperature properties of lithium-ion batteries.

Reasons that restrict the low temperature performance of lithium-ion batteries

1. In a low temperature environment, the viscosity of the electrolyte increases, and even partially solidifies, resulting in a decrease in the conductivity of lithium-ion batteries.

2. The compatibility between the electrolyte and the negative electrode and separator in low temperature environment becomes poor.

3. The negative electrode of lithium-ion batteries in low temperature environment precipitates lithium seriously, and the precipitated metal lithium reacts with the electrolyte, and its product deposition leads to an increase in the thickness of the solid electrolyte interface (SEI).

4. The internal diffusion system of lithium-ion batteries in the active material under low temperature environment decreases, and the charge transfer impedance (Rct) increases significantly.

Discussion on factors affecting the low temperature performance of lithium-ion batteries

1. The electrolyte has the greatest influence on the low temperature performance of lithium-ion batteries, and the composition and physical and chemical properties of the electrolyte have an important impact on the low temperature performance of the battery. The problem faced by the battery cycling at low temperature is that the viscosity of the electrolyte will become larger, the ion conduction rate will be slowed down, and the electron migration speed of the external circuit will not be mismatched, so the battery will be seriously polarized and the charge and discharge capacity will be sharply reduced. Especially when charging at low temperatures, lithium ions can easily form lithium dendrites on the surface of the negative electrode, resulting in battery failure. The low temperature performance of the electrolyte is closely related to the size of the electrolyte's own conductivity, and the electrolyte has a large conductivity, the transport ions of the electrolyte are fast, and more capacity can be exerted at low temperatures. The more lithium salts in the electrolyte dissociate, the greater the number of migrations and the higher the conductivity. Therefore, high conductivity is a necessary condition to achieve good low-temperature performance of lithium-ion batteries. The conductivity of the electrolyte is related to the composition of the electrolyte, and reducing the viscosity of the solvent is one of the ways to improve the conductivity of the electrolyte. The good fluidity of solvent at low temperature is the guarantee of ion transport, and the solid electrolyte film formed by the electrolyte at the negative electrode at low temperature is also the key to affecting the conduction of lithium ions, and RSEI is the main impedance of lithium-ion batteries in low temperature environment. Low temperature characteristics of cathode materials in phosphate systems Due to its excellent volume stability and safety, LiFePO4 has become the main body of cathode materials for power batteries together with ternary materials. The poor low-temperature performance of lithium iron phosphate is mainly because its material itself is an insulator, low electronic conductivity, poor diffusion of lithium ions, poor conductivity at low temperatures, so that the internal resistance of the battery increases, the polarization is greatly affected, and the battery charge and discharge are blocked, so the low temperature performance is not ideal. Low temperature characteristics of lithium-ion battery anode materials Compared with the cathode material, the low temperature deterioration of the lithium-ion battery anode material is more serious, mainly as follows

Here are 3 reasons:

1  .When charging and discharging at low temperature and large rate, the polarization of the battery is serious, a large amount of metal lithium is deposited on the surface of the negative electrode, and the reaction products between metal lithium and electrolyte are generally not conductive;

2. From the perspective of thermodynamics, the electrolyte contains a large number of polar groups such as C-O, C-N, etc., which can react with the negative electrode material, and the SEI film formed is more susceptible to low temperature;

3. The carbon anode is difficult to insert lithium at low temperature, and there is asymmetry of charge and discharge. Research of cryogenic electrolytes The electrolyte plays the role of transferring Li+ in lithium-ion batteries, and its ionic conductivity and SEI film-forming properties have significant effects on the low temperature performance of the battery. To judge the advantages and disadvantages of low-temperature electrolytes, there are three main indicators: ion conductivity, electrochemical window and electrode reactivity. The level of these 3 indicators depends to a large extent on their constituent materials: solvents, electrolytes (lithium salts), additives. Therefore, the study of the low-temperature performance of each part of the electrolyte is of great significance to understand and improve the low-temperature performance of the battery. 2. Lithium salt is an important component of electrolyte. Lithium salt in the electrolyte can not only improve the ionic conductivity of the solution, but also reduce the diffusion distance of Li+ in the solution. In general, the greater the concentration of Li+ in a solution, the greater its ionic conductivity. However, the concentration of lithium ions in the electrolyte is not linearly related to the concentration of lithium salts, but is parabolic. This is because the concentration of lithium ions in the solvent depends on the dissociation and association of lithium salts in the solvent. Research of cryogenic electrolytes In addition to the battery composition itself, the process factors in actual operation will also have a great impact on the battery performance.

1. Preparation process In terms of capacity retention, the smaller the electrode load and the thinner the coating layer, the better its low-temperature performance.

 2. Charge and discharge state When the discharge depth is large, it will cause a large capacity loss and reduce the cycle life.

 3. Other factors The surface area, pore size, electrode density, wettability of the electrode and electrolyte and separator of the electrode all affect the low temperature performance of lithium-ion batteries. In addition, the influence of defects in materials and processes on the low temperature performance of the battery cannot be ignored.

 Therefore, in order to ensure the low-temperature performance of lithium-ion batteries, the following points need to be done:

  (1) Formation of a thin and dense SEI film;

  (2) Ensure that Li+ has a large diffusion coefficient in the active substance;

  (3) The electrolyte has high ionic conductivity at low temperatures. In addition, research can also open up another way to 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 capacity attenuation problem and cycle safety problem of batteries used at low temperatures.