Briefly describe the role of binders in the decay of lithium-ion batteries

Most studies on the mechanism of lithium-ion battery decay are concentrated on positive and negative materials. For example, many studies have shown that active material loss, increased internal resistance and other factors are the main factors causing the decline of lithium-ion batteries, while the binder is in lithium. There is still little research on the role played by ion battery decay. In fact, although the binder is small in lithium-ion batteries (usually less than 5% of the active), the binder plays a crucial role. In a lithium ion battery, the role of the binder is to bond the active material particles and the conductive agent particles together to form a stable system. However, in the process of charge and discharge, there are certain volume changes in the positive and negative electrodes, which will destroy this stable structure. For example, the most common case is the case shown in the following figure, adhesive/conductive agent and active material particles. Stratification occurs between them, resulting in loss of active material, causing a decrease in the reversible capacity of the lithium ion battery.

In order to analyze the role of the adhesive in the decline of lithium-ion batteries, the University of Portsmouth (come, everyone read with me: "Portsmouth", there is no B-box feeling) JMFoster passed the model method was established to study the effect of particle shape and cycle rate of the active material on the bond characteristics of the adhesive. Studies have shown that the oval particles can significantly increase the strain on the upper and lower parts of the particles after the adhesive absorbs the expansion of the electrolyte. The large charge-discharge rate (over 1C) also significantly increases the strain of the binder on the left and right sides of the active material particles, affecting the cycle performance of the battery.

JMFoster's model mainly consists of three hypotheses: 1) the electrode consists of spherical active material particles and elastic porous adhesive, the micropores of the adhesive are filled with electrolyte; 2) the active material particles will occur during the process of lithium insertion and delithiation. Volume expansion; 3) Adhesive expansion occurs after the adhesive contacts the electrolyte.

According to the above assumptions, JMFoster uses mathematical methods to model the motor (since the modeling process has been designed with a large amount of mechanical knowledge, I’m not a mechanical professional here, and there is no way to go to the door, interested friends can view the original text), Let us look directly at the results of the model.

In the actual electrode, there are tens of millions of active material particles and a large amount of adhesive. It is obviously unrealistic to solve the entire electrode directly. Therefore, JMFoster adopts a simplified method. JMFoster thinks that except for the position of the electrode edge, the force inside the electrode is very uniform, so we can simplify the solution process of the whole electrode to solve the single active material particles and the adhesive around it, which greatly simplifies the solution process of the model.

Figure a below shows the stress distribution of the binder around the active material particles after the absorption of the electrolyte, and Figure c below shows the adhesion of the P and E points of the active material particles after the electrolyte is absorbed. From the graph, we can see that the strain at the P point near the electrode surface and the current collector is increased after the expansion of the binder absorption solution, and the strain at the E point on the left and right sides of the particle. Decreasing, due to the fluidity of the binder, the binder is pushed from the top and bottom of the active material particles to both sides of the active material under the action of strain.

Figure b below shows the strain distribution of the surrounding adhesive during the volume change of the active material particles. It can be noted from the figure that the binder stress distribution caused by the volume change of the active material is almost uniform, but careful study still finds The strain of the adhesive on the left and right sides of the active material is still higher than the strain of the adhesive on the upper and lower ends of the active material, which indicates that the adhesive on the left and right sides of the active material particles is more likely to stratify during the circulation. However, in fact, we need to note that since the volume change of the positive active material during the cycle is very small (NMC is 2-4%), the change in binder strain caused by the volume expansion of the active material particles is actually much smaller than that due to PVDF, the volume expansion caused by the absorption of the adhesive.

The previous analysis was for spherical particles, and in practice the particles we used had many other shapes, so JMFoster analyzed the effects of different particle shapes on the strain of the adhesive. The figure below shows the effect of different particle shapes on the strain distribution after the adhesive is absorbed. From the calculation results, the adhesive strain of the elliptical particles at the P point is positive, and the viscosity at the E point. The knot strain is negative, which is consistent with the previous analysis. At the same time, it can be seen from the following figure that the alignment direction of the elliptical particles also affects the strain of the adhesive. When the longer side of the ellipse is parallel to the surface of the electrode, the strain of the adhesive is significantly increased.

The figure below shows the strain of the adhesive at different charge rates (Figure a is the strain of the adhesive in the positive electrode, Figure b is the strain of the adhesive in the negative electrode), and the slowest charge rate used in the calculation is required. Charging is completed at 3100h, and the fastest charging rate only needs 0.031h to complete the charging. It can be seen from the figure that the high charging rate will significantly increase the strain of the adhesive at the position of the active material particle E, resulting in adhesive and activity. The problem of stratification of matter particles in general, fast charging exceeding 1C rate will cause damage to the positive and negative adhesives, thereby affecting the life of the lithium ion battery.

The work of JMFoster allows us to have a clear understanding of the strain distribution of the adhesive around the active material particles at the microscopic level, and the factors affecting the strain distribution of the adhesive - the shape of the active material particles and the charge and discharge rate In-depth discussion has certain guiding significance for the design of electrode materials and lithium-ion battery formulation design.

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