Why Does The Capacity Of Lithium Battery Become Lower in Winter?

Lithium-ion batteries have been widely used for their long life, large specific capacity, and no memory effect since they entered the market. Low-temperature use of lithium-ion batteries has problems such as low capacity, severe attenuation, poor cycle rate performance, obvious lithium precipitation, and unbalanced lithium deintercalation. However, with the continuous expansion of application fields, the constraints brought by the poor low-temperature performance of lithium-ion batteries have become more and more obvious.

According to reports, the discharge capacity of lithium-ion batteries at -20°C is only about 31.5% of that at room temperature. The operating temperature of traditional lithium-ion batteries is between -20 and +55°C. However, in aerospace, military industry, 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.

Factors Restricting the Low Temperature Performance of Lithium-ion Batteries

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

In a low temperature environment, the compatibility between the electrolyte, the negative electrode and the separator becomes poor.

Lithium is seriously precipitated from the negative electrode of the lithium-ion battery in a low-temperature environment, 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).

In a low temperature environment, the diffusion system of lithium-ion batteries in the active material decreases, and the charge transfer resistance (Rct) increases significantly.

Discussion on the Determinant Factors Affecting the Low Temperature Performance of Li-ion Batteries

Expert opinion 1: The electrolyte has the greatest impact 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 cycle at low temperature is: the viscosity of the electrolyte will increase, the ion conduction velocity will slow down, and the electron migration velocity of the external circuit will not match. Therefore, the battery will be severely polarized and the charge and discharge capacity will decrease sharply. Especially when charging at low temperature, 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 conductivity of the electrolyte itself. The electrolyte with high conductivity can transmit ions faster and can exert more capacity at low temperatures. The more dissociated lithium salts in the electrolyte, the more migration numbers and the higher the conductivity. The higher the conductivity, the faster the ion conduction rate, the smaller the polarization, and the better the performance of the battery at low temperature. Therefore, higher conductivity is a necessary condition for achieving 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 increase the conductivity of the electrolyte. Solvent The good fluidity of the solvent at low temperature is the guarantee of ion transport, and the solid electrolyte film formed by the electrolyte on the negative electrode at low temperature is also the key to the conduction of lithium ions, and 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 increased Li+ diffusion resistance at low temperatures, not the SEI film.

Low-temperature characteristics of cathode materials for lithium-ion batteries

Low temperature properties of layered cathode materials

The layered structure, which not only has the incomparable rate performance of the one-dimensional lithium ion diffusion channel, but also has the structural stability of the three-dimensional channel, is the earliest commercial lithium-ion battery cathode material. Its representative substances are LiCoO2, Li(Co1-xNix)O2 and Li(Ni, Co, Mn)O2 and so on.