Lithium ion power batteries have been widely used in the field of electric vehicles due to their high voltage, high specific energy, and good cycling performance.

1 Test

Taking a lithium-ion power battery with a rated capacity of 70Ah for electric vehicles as an example, a room temperature (20 ℃) capacity test and a low temperature (-20 ℃) capacity test were conducted to compare the impact of temperature on the capacity of lithium-ion batteries. The lithium-ion battery has a charging cut-off voltage of 36V and a discharge cut-off voltage of 2.0V. Then, discharge capacity tests are conducted in environments of 20 ℃ and -20 ℃, and the test is terminated when the discharge cut-off voltage is 2.0V.

2 Main factors affecting low-temperature capacity

The low-temperature performance of lithium-ion batteries is mainly affected by the types of electrolytes, positive and negative electrode materials, etc. The low-temperature performance of lithium-ion batteries also varies depending on the type of electrolyte and positive and negative electrode materials. Under low temperature conditions, the solidification of some solvents in the battery electrolyte results in difficulty in ion migration and a decrease in conductivity; The transfer resistance of lithium ions in electrode materials increases; The lithium diffusion and charge transfer between the electrode and electrolyte interface are slower, and the wettability of the electrolyte to the separator and the penetration of lithium ions to the separator become worse.

2.1 The influence of electrolyte

The influence of electrolyte is an important component of lithium-ion batteries. Ionic transfer between the positive and negative electrodes inside the battery is generally carried out using a mixture of non-aqueous organic solvents dissolved in lithium salts. Organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) are currently widely used in lithium-ion battery electrolytes. The impact of electrolytes on low-temperature performance is mainly reflected in their effects on conductivity and solid-electrolyte interface (SEI) film properties.

1) Conductivity.

Conductivity is an important parameter for measuring the performance of electrolytes, and a higher conductivity is a necessary condition for achieving good low-temperature performance of lithium-ion batteries. From the perspective of organic solvents, the main factors affecting conductivity are the dielectric constant and viscosity of the solvent. The larger the dielectric constant of the solvent, the weaker the electrostatic force between lithium ions and anions, and the easier it is for lithium salts to decompose and increase the number of free ions.

The viscosity of solvents mainly affects the mobility of free ions. The higher the viscosity, the smaller the mobility, and the lower the conductivity. Conversely, the opposite is true. As the temperature decreases, the dielectric constant of the solvent will decrease the interaction force between lithium ions and anions, and the difficulty of lithium salt decomposition will be enhanced. As the temperature decreases, the viscosity of the electrolyte increases, and the migration rate of ions decreases. These will reduce the conductivity of the battery and affect the capacity of lithium-ion power batteries.

2) SEI membrane.

The composition of the electrolyte not only determines the ionic conductivity of the electrolyte, but also affects the formation of SEI (solid electrolyte phase) films. The performance of SEI membranes has a significant impact on the irreversible capacity, low-temperature performance, cycling performance, and safety performance of batteries. An excellent SEI film should have an organic solvent insolubility that allows lithium ions to freely embed or detach from the electrode, while solvent molecules cannot penetrate, thereby preventing solvent molecules from damaging the electrode and improving its cycling life.

Research has found that the SEI film resistance is much greater than the electrolyte resistance, and as the temperature decreases, the SEI film resistance increases, corresponding to the rapid deterioration of battery performance. An appropriate amount of film forming additives can be added to the electrolyte of lithium-ion batteries to reduce the SEI film resistance, improve the performance of the SEI film, and thus improve the low-temperature performance of the battery.

2.2 Effect of Particle Size of Electrode Materials

Under low temperature conditions, the decrease in battery discharge voltage indicates an increase in the polarization of the inner and outer layers of positive and negative electrode particles, i.e. an increase in the transmission impedance of lithium ion positive and negative electrode solid particles, leading to the premature reaching of the discharge termination voltage during the discharge process and a corresponding decrease in discharge capacity.

Research has found that under low temperature conditions, fully charged graphite electrodes can relatively easily release embedded lithium ions below -20 ℃. However, at the same temperature, embedding lithium ions in fully discharged graphite electrodes encounters serious obstacles. By reducing the particle size of the electrode material, the low-temperature performance of the battery will be significantly improved.

2.3 Optimization and improvement of electrolyte and electrode materials

We conducted in-depth analysis and research on the formulas and processes of the electrolyte and electrode materials for the above-mentioned 70Ah low-temperature capacity unqualified products. The product electrolyte was adjusted from the ternary electrolyte solvent composed of EC, DMC, and DEC to a quaternary electrolyte solvent composed of EC, PC, DMC, and DEC.

The types of lithium salts and additives remained unchanged, but the proportion was adjusted to improve the manufacturing process of the electrode materials, The electrolyte has undergone dozens of proportional adjustments and experiments to make the particles of the electrode material smaller.

The improved battery has a discharge starting voltage of 3.293V and a discharge cut-off voltage of 2.0V at 20 ℃. The discharge capacity of 74.6Ah is 106.6% of the rated capacity, which meets the standard requirements. The discharge capacity at 20 ℃ is between 100% and 110% of the rated capacity; Under the condition of -20 ℃, the discharge starting voltage is 3.189V, and the discharge cut-off voltage is 2.0V. The discharge capacity is 56.1Ah, which is 80.1% of the rated capacity. The discharge capacity at -20 ℃ that meets the standard requirements is not less than 70% of the rated capacity (49.0Ah)

3 Conclusion

Under low-temperature conditions, the discharge performance of lithium-ion power batteries deteriorates. The discharge voltage decreases, and the discharge capacity significantly decreases. Due to the low-temperature characteristics of the aforementioned batteries, they pose a significant obstacle to the popularization and development of electric vehicles in low-temperature areas. Therefore, a significant improvement in the low-temperature performance of batteries is conducive to the development of electric vehicles and also promotes the application process of lithium-ion batteries in military, aerospace and other fields.