In low temperature environment, lithium-ion battery performance is not ideal. When commonly used lithium-ion batteries work at -10 ° C, their maximum charge and discharge capacity and terminal voltage will be significantly reduced compared with normal temperature [6], when the discharge temperature drops to -20 ° C, the available capacity will even be reduced to 1/3 at room temperature 25 ° C, when the discharge temperature is lower, some lithium batteries can not even charge and discharge activities, entering a "dead battery" state.
1, The characteristics of lithium-ion batteries at low temperatures
(1) Macroscopic
The characteristic changes of lithium-ion battery at low temperature are as follows: with the continuous decrease of temperature, the ohmic resistance and the polarization resistance increase in different degrees; The discharge voltage of lithium-ion battery is lower than that of normal temperature. When charging and discharging at low temperature, its operating voltage rises or falls faster than that at normal temperature, resulting in a significant decrease in its maximum usable capacity and power.
(2) Microscopically
The performance changes of lithium-ion batteries at low temperatures are mainly due to the influence of the following important factors. When the ambient temperature is lower than -20℃, the liquid electrolyte solidifies, its viscosity increases sharply, and its ionic conductivity decreases. Lithium ion diffusion in positive and negative electrode materials is slow; Lithium ion is difficult to desolvate, and its transmission in SEI film is slow, and the charge transfer impedance increases. The lithium dendrite problem is especially prominent at low temperature.
2, To solve the low temperature performance of lithium-ion batteries
Design a new electrolytic liquid system to meet the low temperature environment; Improve the positive and negative electrode structure to accelerate the transmission speed and shorten the transmission distance; Control positive and negative solid electrolyte interface to reduce impedance.
(1) electrolyte additives
In general, the use of functional additives is one of the most effective and economical ways to improve the low temperature performance of the battery and help form the ideal SEI film. At present, the main types of additives are isocyanate based additives, sulfur based additives, ionic liquid additives and inorganic lithium salt additives.
For example, dimethyl sulfite (DMS) sulfur based additives, with appropriate reducing activity, and because its reduction products and lithium ion binding is weaker than vinyl sulfate (DTD), alleviating the use of organic additives will increase the interface impedance, to build a more stable and better ionic conductivity of the negative electrode interface film. The sulfite esters represented by dimethyl sulfite (DMS) have high dielectric constant and wide operating temperature range.
(2) The solvent of the electrolyte
The traditional lithium-ion battery electrolyte is to dissolve 1 mol of lithium hexafluorophosphate (LiPF6) into a mixed solvent, such as EC, PC, VC, DMC, methyl ethyl carbonate (EMC) or diethyl carbonate (DEC), where the composition of the solvent, melting point, dielectric constant, viscosity and compatibility with lithium salt will seriously affect the operating temperature of the battery. At present, the commercial electrolyte is easy to solidify when applied to the low temperature environment of -20℃ and below, the low dielectric constant makes the lithium salt difficult to dissociate, and the viscosity is too high to make the battery internal resistance and low voltage platform. Lithium-ion batteries can have better low-temperature performance by optimizing the existing solvent ratio, such as by optimizing the electrolyte formulation (EC:PC:EMC=1:2:7) so that TiO2(B)/ graphene negative electrode has A capacity of ~240 mA h g-1 at -20℃ and 0.1 A g-1 current density. Or develop new low temperature electrolyte solvents. The poor performance of lithium-ion batteries at low temperatures is mainly related to the slow desolvation of Li+ during the process of Li+ embedding in the electrode material. Substances with low binding energy between Li+ and solvent molecules, such as 1, 3-dioxopentylene (DIOX), can be selected, and nanoscale lithium titanate is used as the electrode material to assemble the battery test to compensate for the reduced diffusion coefficient of the electrode material at ultra-low temperatures, so as to achieve better low-temperature performance.
(3) lithium salt
At present, the commercial LiPF6 ion has high conductivity, high moisture requirements in the environment, poor thermal stability, and bad gases such as HF in water reaction are easy to cause safety hazards. The solid electrolyte film produced by lithium difluoroxalate borate (LiODFB) is stable enough and has better low temperature performance and higher rate performance. This is because LiODFB has the advantages of both lithium dioxalate borate (LiBOB) and LiBF4.
3. Summary
The low temperature performance of lithium-ion batteries will be affected by many aspects such as electrode materials and electrolytes. Comprehensive improvement from multiple perspectives such as electrode materials and electrolyte can promote the application and development of lithium-ion batteries, and the application prospect of lithium batteries is good, but the technology needs to be developed and perfected in further research.
Post time: Jul-27-2023