Lithium-ion batteries have the advantages of high energy density, low self-discharge, high output voltage, long cycle life and no memory effect, occupying most markets in the field of consumer electronics represented by mobile phones, notebook computers, digital cameras, etc. At present, the application of lithium-ion batteries in power tools, electric bicycles and other fields has also shown a geometric progression.
With the rapid development of lithium-ion batteries in the field of electric vehicles and military industry, the low-temperature performance can not adapt to the special low-temperature weather or extreme environmental defects. The low-efficiency conditions, the effective discharge capacity and effective discharge energy of lithium-ion batteries will There is a significant drop, and it is almost impossible to charge in the environment below -10 °C, which seriously restricts the application of lithium-ion batteries.
Factors affecting the low temperature performance of lithium ion batteries
Lithium-ion battery is mainly composed of cathode material, anode material, separator and electrolyte. Lithium-ion battery in low temperature environment has the characteristics of lower discharge voltage platform, low discharge capacity, fast capacity attenuation and poor rate performance. The main factors of performance are as follows:
Positive electrode structure
The three-dimensional structure of the positive electrode material restricts the diffusion rate of lithium ions, and the effect is particularly obvious at low temperatures. The positive electrode materials for lithium ion batteries include commercial lithium iron phosphate, nickel cobalt manganese ternary materials, lithium manganate, lithium cobalt oxide, etc. Including high-voltage cathode materials in the development stage, such as lithium nickel manganese oxide, lithium iron manganese phosphate, lithium vanadium phosphate, etc. Different cathode materials have different three-dimensional structures. Currently, the cathode material used as the power battery for electric vehicles is mainly lithium iron phosphate. Nickel-cobalt-manganese ternary material and lithium manganate. Wu Wendi et al studied the discharge performance of lithium iron phosphate battery and nickel-cobalt-manganese ternary battery at -20 ° C. It was found that the discharge capacity of lithium iron phosphate battery at -20 ° C can only reach The room temperature capacity is 67.38%, and the nickel-cobalt-manganese ternary battery can reach 70.1%. Du Xiaoli et al found that the lithium manganese oxide battery can reach 83% of the normal temperature capacity at -20 °C.
High melting point solvent
Due to the presence of a high melting point solvent in the electrolyte mixed solvent, the viscosity of the lithium ion battery electrolyte increases in a low temperature environment. When the temperature is too low, the electrolyte solidification phenomenon occurs, resulting in a decrease in the transport rate of lithium ions in the electrolyte.
Lithium ion diffusion rate
The diffusion rate of lithium ion in graphite anode decreases under low temperature environment. The effect of graphite anode on the low temperature discharge performance of lithium ion battery is studied. The charge migration impedance of lithium ion battery increases under low temperature environment, which leads to lithium ion in graphite. The decrease of diffusion rate in the negative electrode is an important reason that affects the low temperature performance of lithium ion batteries.
SEI film
In the low temperature environment, the SEI film of the negative electrode of the lithium ion battery is thickened, and the impedance of the SEI film is increased, which leads to the decrease of the conduction rate of lithium ions in the SEI film. Finally, the lithium ion battery is charged and discharged in a low temperature environment to form a polarization to reduce the charge and discharge efficiency.
At present, many factors affect the low temperature performance of lithium ion batteries, such as the structure of the positive electrode, the migration rate of lithium ions in various parts of the battery, the thickness and chemical composition of the SEI film, and the choice of lithium salt and solvent in the electrolyte. Lithium-ion batteries are used in the field of electric vehicles, military industry and extreme environments. The development of lithium-ion batteries with excellent low-temperature performance is an urgent demand in the market.
