How to improve the conductivity of electrolyte at low temperatures?
Lithium-ion batteries still have many problems in practical applications. For example, the energy density is significantly reduced under low temperature conditions, and the cycle life is also affected accordingly, which also severely limits the scale of lithium-ion batteries.
Good low temperature performance is one of the important directions to broaden the range of use of lithium-ion batteries, and it is also one of the key issues that lithium-ion batteries must solve in new energy fields, military and aerospace applications.
As an important part of lithium ion battery, electrolyte not only determines the migration rate of Li + in the liquid phase, but also participates in the formation of SEI film, which plays a key role in the performance of SEI film.
At low temperature, the viscosity of the electrolyte increases, the electrical conductivity decreases, the impedance of the SEI film increases, and the compatibility with the positive and negative materials deteriorates, which greatly deteriorates the energy density and cycle performance of the battery.
At present, we can improve the low temperature performance of the electrolyte by adding an optimized solvent composition to the electrolyte.
The low temperature performance of the electrolyte is mainly determined by its low temperature eutectic. If the melting point is too high, the electrolyte tends to crystallize at low temperature, which seriously affects the conductivity of the electrolyte.
Ethylene carbonate (EC) is the main solvent component of the electrolyte, but its melting point is 36 °C. The solubility in the electrolyte decreases or even precipitates at low temperature, which has a great influence on the low temperature performance of the battery.
By adding low melting point and low viscosity components, reducing the solvent EC content, can effectively reduce the viscosity and eutectic point of the electrolyte at low temperature, and improve the conductivity of the electrolyte.
Kasprzyk et al. obtained an amorphous electrolyte by mixing EC and poly(ethylene glycol) dimethyl ether (PEG250). Only a glass transition temperature point appeared near -90 °C. The electrolyte greatly improves the performance of the electrolyte at low temperatures.
At -60 °C, its conductivity can still reach 0.014mS·cm-1, which provides a good solution for the use of lithium ion batteries at very low temperatures.
The chain carboxylic acid ester solvents have a lower melting point and viscosity, and their dielectric constants are moderate, which have a good influence on the low temperature performance of the electrolyte. Dong et al [using ethyl acetate (EA) as a cosolvent, 2 Mol·kg-1 of lithium bistrifluoromethanesulfonate (LiTFSI) as an electrolyte salt, the theoretical melting point of the electrolyte reaches -91 ° C, the boiling point reaches 81 ° C.
The results show that the ionic conductivity of the electrolyte reaches 0.2 mS·cm-1 even at the extreme low temperature of -70 °C, and the organic electrode (triphenylaniline PTPAN) is combined as the positive electrode and 1,4,5,8- Naphthalene anhydride (NTCDA) derived polyimide (PNTCDA) as a negative electrode, the battery still has 70% of normal temperature capacity at -70 °C.
Smart et al. have done a lot of research on the use of chain carboxylic acid esters as electrolyte cosolvents to improve the low temperature performance of batteries. Studies have shown that ethyl acetate (EA), ethyl propionate (EP), methyl acetate (MA), Methyl butyrate (EB) as an electrolyte co-solvent facilitates the improvement of the low-temperature conductivity of the electrolyte and greatly improves the low-temperature performance of the battery. They also optimize the electrolyte solvent combination to improve the overall performance of the battery, including EP, EB improves the cycle performance of the battery while improving the low temperature performance of the battery.