The rapid development of the electric vehicle industry puts forward higher requirements for the quick charge capability of lithium ion batteries. At present, the design of the fast charge battery is more directed to the improvement of the electrode structure, for example, the positive and negative electrode materials with better magnification performance, Reduce the amount of electrode coating, reduce the compaction density and other methods, these methods have Li-ion battery fast charge performance has brought a great improvement.But for the moment to enhance Li + in the electrolyte migration coefficient is not much attention In the process of charge and discharge, the electric field between the positive and negative electrodes will promote the migration of the positive and negative ions between the positive and negative electrodes (Li + is positively charged and will migrate to the negative electrode under the action of an electric field. Anions such as PF6- The negative charge will migrate toward the positive electrode under the action of an electric field). Therefore, the conductivity of the electrolyte is composed of both anionic and cationic ions (so the high conductivity does not necessarily mean that Li + has a high migration efficiency), but in fact only Li + So for our purposes it makes sense that we define the number of Li + migrations as the ratio of the number of all ion transitions in the electrolyte to the number of Li + as shown in the following equation.
From the above equation, we can see that the closer to 1 the number of transitions of Li + is, the higher the proportion of Li + migrated in the electrolyte is, the higher the efficiency of the charge transfer between the positive and negative electrodes is. However, Of Li-ion battery electrolytes are LiPF6 electrolyte salts, so although they have very high conductivity (1-10 mS / cm), their Li + mobilities are all below 0.5, which means that most of the conductivity Are provided by the anion PF6-, so the significance of the conductivity of the electrolyte is also greatly reduced .This phenomenon is mainly due to the dissolution of Li + in the electrolyte will occur, resulting in the surface of a The layer solvates the shell, thus limiting the rate of Li + migration, while the anion is hardly solvated.
This poses a problem that too much anion accumulates on the surface of the positive electrode, creating a concentration gradient between the positive and negative electrodes, which causes a concentration polarization of the lithium ion battery resulting in a large overpotential, limiting The energy density and power density of Li-ion batteries have been improved.Researches show that when the migration number of Li + is increased to about 0.7, the rapid charge capacity of Li-ion battery can be significantly improved, so how to maintain the high conductivity σ High migration coefficient for improving the lithium-ion battery power density and energy density is of great significance.
To test the effect of Li + mobility on the performance of Li-ion batteries, Kyle M. Diederichsen of the University of California, Berkeley, USA, used a finite element model to simulate a porous graphite cathode, porous LiCoO 2The battery model of the positive electrode (as shown in the above figure a) was analyzed (in the experiment, the control group was a liquid electrolyte with a Li + mobility of t + of about 0.4 and a conductivity of about 10 mS / cm).
As can be seen from Figure b above, with the increase of the number of Li + migration, the overpotential (polarization) of the battery during charging is significantly reduced, while in the graphs c and d it can be seen that at higher charging rates (2- 6C), the battery with high Li + transfer number finally charged significantly higher than the low number of migrating cells, indicating that the high number of Li + migration can effectively improve the rapid charge capacity of lithium-ion batteries. Next, we come Take a look at the data in Figure E, that is, how much Li + is needed to achieve 75% and 85% SoC at different charge rates at a charge rate of 2C. The larger the number of Li + migrations, Then the correspondingly required electrolyte will have a lower conductivity, and if the transfer number of Li + reaches 0.8, the electrical conductivity requirement can be reduced by 50% compared to the liquid electrolyte.
Since the number of Li + migrations in the electrolyte is so important, why do most electrolyte manufacturers still use conductivity to characterize the electrolyte? This is because measurement of the migration number of electrolyte Li + is difficult compared to the established conductivity test It is difficult and difficult to measure the migration number of Li +, especially when most of the ions in the electrolyte are diffused. This is why most manufacturers do not use the Li + transport number to characterize the electrolyte important reason.
Of course, the measurement difficulty does not mean there is no way to measure the number of Li + migration is the most widely used method is Bruce and Vincent method, the principle of this method is first measured in lithium-ion battery just started current I O (At this time the electrolyte is still very uniform), after a period of stability, measure the steady current I SS , We assume that the net flux of anion between the positive and negative electrodes is 0 at this time. If we neglect some side reactions of the positive and negative electrodes and the electrolyte, the current ISS at this time is all conducted by Li +, so we have t + I SS / IOHowever, there are still some limitations in the method. First, the method is effective only in low concentration electrolytes, and it is also necessary to eliminate the side reactions that occur in the positive and negative electrolytes.
Recently, Balsara and Newman corrected the above method by adding Newman numbers to get more accurate Li + migration numbers, of course, there are still many other ways to test the migration of Li + in the electrolyte, but most of them are too complicated, so most of them Of researchers still assume that the electrolyte is a dilute solution, simplifying the measurement process.
In this article, we mainly discuss the relationship between the number of Li + migrations in electrolyte and the fast charge performance of Li-ion batteries. The research done by Kyle M. Diederichsen of Berkeley has given us a profound understanding of the importance of increasing the number of Li + migrations In the next article, we will continue to explore how to increase the number of Li + migrations in electrolytes.
