Since entering the market, lithium-ion batteries have been widely applied due to their advantages such as long lifespan, large specific capacity and no memory effect. Lithium-ion batteries have problems such as low capacity, severe degradation, poor cycle rate performance, obvious lithium plating, and unbalanced lithium intercalation and deintercalation when used at low temperatures. However, as the application fields continue to expand, the constraints brought by the poor low-temperature performance of lithium-ion batteries have become increasingly obvious.

It is reported that at -20℃, the discharge capacity of lithium-ion batteries is only about 31.5% of that at room temperature. The operating temperature range of traditional lithium-ion batteries is between -20 and +55℃. However, in fields such as aerospace, military industry, and electric vehicles, it is required that batteries can operate normally at -40℃. Therefore, improving the low-temperature properties of lithium-ion batteries is of great significance.

The factors restricting the low-temperature performance of lithium-ion batteries

In low-temperature environments, the viscosity of the electrolyte is high, and it may even partially solidify, resulting in a decrease in the conductivity of lithium-ion batteries.

The compatibility between the electrolyte and the negative electrode and separator deteriorates in a low-temperature environment.

In low-temperature environments, lithium is severely precipitated from the negative electrode of lithium-ion batteries, and the precipitated metallic lithium reacts with the electrolyte, resulting in the deposition of its products and an increase in the thickness of the solid electrolyte interface (SEI).

Under low-temperature conditions, the diffusion system within the active material of lithium-ion batteries decreases, and the charge transfer impedance (Rct) significantly increases.

Discussion on the Decisive Factors Affecting the Low-temperature Performance of Lithium-ion Batteries

Expert Opinion One: The electrolyte has a significant impact on the low-temperature performance of lithium-ion batteries. The composition and physicochemical properties of the electrolyte have an important influence on the low-temperature performance of the battery. The problems that batteries face when cycling at low temperatures are as follows: the viscosity of the electrolyte increases, the ion conduction speed slows down, causing a mismatch in the electron migration speed of the external circuit. As a result, severe polarization occurs in the battery, and the charging and discharging capacity drops sharply. Especially when charging at low temperatures, lithium ions can easily form lithium dendrites on the surface of the negative electrode, leading to battery failure.

The low-temperature performance of the electrolyte is closely related to the magnitude of its own electrical conductivity. A higher electrical conductivity enables the electrolyte to transport ions faster and can exert more capacity at low temperatures. The more lithium salts in the electrolyte dissociate, the greater the number of migrations, and the higher the electrical conductivity. The higher the electrical conductivity, the faster the ion conduction rate, the smaller the polarization it experiences, and the better the performance of the battery at low temperatures. Therefore, a relatively high electrical conductivity is a necessary condition for achieving good low-temperature performance of lithium-ion batteries.

The electrical conductivity of the electrolyte is related to its composition. Reducing the viscosity of the solvent is one of the ways to improve the electrical conductivity of the electrolyte. The good fluidity of the solvent at low temperatures is the guarantee for ion transport, and the solid electrolyte film formed by the electrolyte at the negative electrode at low temperatures is also a key factor affecting the conduction of lithium ions. Moreover, RSEI is the main impedance of lithium-ion batteries in low-temperature environments.

Expert 2: The main factor limiting the low-temperature performance of lithium-ion batteries is the sharply increasing Li+ diffusion impedance at low temperatures, rather than the SEI film.

The low-temperature characteristics of cathode materials for lithium-ion batteries

The low-temperature characteristics of layered structure cathode materials

The layered structure not only possesses the rate performance that one-dimensional lithium-ion diffusion channels cannot compare with, but also has the structural stability of three-dimensional channels. It is the earliest commercially available cathode material for lithium-ion batteries. Its representative substances include LiCoO2, Li(Co1-xNix)O2 and Li(Ni,Co,Mn)O2, etc.