Suddenly went to the frozen hands frozen feet in winter, the temperature fell to zero from time to time, in the northern Xiaobian, already down jacket plus cold weather not only makes us feel unbearable, in terms of lithium-ion battery is even more great The challenge is that the kinetic conditions of Li-ion batteries deteriorate under low temperature, the viscosity of the electrolyte increases, the polarization of the battery increases, leading to a sharp drop in the electrical properties.
In many factors that affect the low temperature performance of lithium-ion batteries, the electrolyte is a very crucial factor, the traditional lithium-ion battery electrolyte is mainly composed of carbonate-based solvents, solute salt is LiPF6, this electrolyte at room temperature conductivity High, and has good electrochemical performance, but the viscosity of the electrolyte rapidly increases at low temperature, which affects the kinetics of Li + diffusion between the positive and negative electrodes. Although the kinetic conditions can be improved by adjusting the solvent composition and adding some special additives However, at present, the lowest temperature of carbonate-based electrolyte is around -40 ℃, and the performance of lithium-ion battery will drop sharply at lower temperature.
Recently Marta Kasprzyk et al. Of Warsaw Polytechnic University developed an amorphous electrolyte that still achieves a high electrical conductivity of 0.014 mS / cm at -60 ° C, which greatly improves the lithium ion battery performance at extremely low temperatures Ability to work under.
In order to improve the low temperature performance of the electrolyte, Marta Kasprzyk uses a mixture of two solvents, one of which is our common ethylene carbonate EC and the other is polyethylene glycol dimethyl ether PEG250 (molecular weight of 250g / mol ). The melting point of EC is 36.5 ° C and the melting point of PEG 250 is -43 ° C. According to the knowledge of the phase diagram, the melting point of both substances will be lower than that of any single substance after mixing, so after mixing these two solvents Their melting point will certainly be lower than -43 ℃.
Marta Kasprzyk prepared electrolytes consisting of EC, PEG250 in varying proportions ranging from 100% EC to 100% PEG250 with one electrolyte at 5% intervals. The following is a differential thermal analysis of electrolytes with different EC ratios (DSG) curve can be seen from the curve can be seen in the EC when the proportion of less than 20%, we can observe that the electrolyte at about -40 ℃ there will be a melting point, when EC is added at a ratio of 25% -40% Between the electrolyte, the electrolyte will form an amorphous state, there is no crystallization point.When the EC ratio exceeds 40%, the electrolyte crystallization and melting point signal again.From the above analysis is not difficult to see that the most appropriate EC addition ratio Should be controlled between 25-40% in order to obtain amorphous electrolyte.
Below is the EC addition ratio of 25% of the electrolyte and pure PEG250 electrolyte, and commercial LB30 electrolyte DSG curve, we can see from the figure EC EC addition rate of 25% of the electrolyte only in the vicinity of -90 ℃ A glass transition temperature did not show a melting point signal, whereas a commercial LB30 electrolyte showed a melting point at -20 ° C and a pure PEG 250 electrolyte showed a melting point near -40 ° C. The EC addition was 25% Liquid is a kind of amorphous electrolyte that has no fixed crystallization point, and its performance at low temperature is obviously better than that of the other two electrolytes.
Through the above experiments, we will focus on the study of EC in the electrolyte ratio of about 40% of the electrolyte, the following table is different EC ratio and electrolyte salt ratio, the electrolyte glass transition temperature changes. It can be noticed two points. First, as the concentration of lithium salt increases, the glass transition temperature of the electrolyte increases. Secondly, the range of glass transition temperature of the LiPF6 electrolyte is obviously higher than that of the LiTDI electrolyte width.
The lower glass transition temperature means that the electrolyte has better performance at lower temperatures. The graph below shows the conductivity of 0.5 mol LiTDI electrolyte (EC: PEG 250 = 30: 70) as a function of temperature at -60 ° C, the conductivity of the electrolyte was 0.014 mS / cm.
The graph below shows the curve of the conductivity of 1 mol / L LiPF6 electrolyte as a function of temperature, and it can be seen that the conductivity of the electrolyte has also been significantly improved with the increase of the EC addition ratio.Although the commercial LB30 , The conductivity of LB30 was significantly better than that of the experimental group, but the LB30 crystallized at -30 ° C, so the LB30 electrolyte could not be used at -30 ° C. Therefore, although the amorphous EC / PEG250 The conductivity of the electrolyte is slightly lower, but still the best choice.
Marta Kasprzyk Through the use of EC and PEG250 mixed solvent obtained amorphous electrolyte, in the low temperature environment does not appear the problem of crystallization, only around -90 ℃ in the vicinity of a glass transition temperature point, this is not Crystalline electrolyte greatly improves the performance of the electrolyte at low temperature, its conductivity can still reach 0.014mS / cm at -60 ℃, which provides a good solution to the lithium ion battery under extremely low temperature s solution.
