The research on the decay mechanism of lithium-ion batteries is mainly focused on positive and negative materials. For example, many studies have shown that the loss of active materials, increase of internal resistance and other factors are the main factors causing the decline of lithium-ion batteries, while for binders in lithium. The role played by the decay of the ion battery is relatively small. In fact, although the proportion of the binder in the lithium-ion battery is very small (usually less than 5% of the active material), the binder has played a role. The key role. In lithium ion batteries, the role of the adhesive is to bind the active material particles and the conductive agent particles together to form a stable system. However, due to the presence of positive and negative electrodes during charge and discharge A certain volume change will destroy this stable structure. For example, the most common is the situation shown in the figure below. The layering phenomenon occurs between the adhesive/conductive agent and the active material particles, resulting in the active material The loss caused a decrease in the reversible capacity of the lithium-ion battery.
In order to analyze the role of adhesives in the decline of lithium-ion batteries, JM Foster at the University of Portsmouth, UK (come up with me, “Portsmouth” has a B-box feeling) By modeling, the effect of particle shape of the active material and the cyclic ratio on the adhesive properties of the adhesive was investigated. The results showed that the oval particles significantly increase the amount of the binder absorbed by the electrolyte after the expansion of the electrolyte in the upper and lower parts of the pellet. Strain, large charge-discharge rate (more than 1C) will also significantly increase the binder strain on the left and right sides of the active material particles, affecting the cycle performance of the battery.
JM Foster's model mainly consists of three hypotheses: 1) The electrode consists of spherical active material particles and elastic porous binder, and the pores of the binder are filled with the electrolyte; 2) The active material particles will participate in lithium insertion and delithiation. Volume expansion occurs; 3) The adhesive will swell when it contacts the electrolyte.
Based on the above assumptions, JM Foster used mathematical methods to model the motor (because the modeling process has a lot of mechanical knowledge, Xiao Bian is not a mechanical professional here, so it is not necessary to make a fuss. Interested friends can view the original) , we directly look at the results of the model.
In the actual electrode, there are tens of millions of active material particles and a large amount of binder. It is obviously unrealistic to directly solve the entire electrode. Therefore, JM Foster adopts a simplified method. JM Foster believes that in addition to electrode edges, Position, the internal force of the electrode is very uniform, so we can simplify the solution process of the entire electrode to solve the single active material particles and the adhesive around it, so that the model's solution process is greatly simplified.
The following figure a shows the stress distribution of the binder around the active material particles after the electrolyte is expanded. Figure C below shows the bonding of the P and E points of the active material particles after absorbing the electrolyte. From the figure, we can see that the strain at the point P near the surface of the electrode and the current collector increases as the absorption solution of the adhesive expands, and the strain at point E on the left and right sides of the particle increases. Due to the fluidity of the adhesive, the adhesive will push the adhesive from the top and bottom of the active material particles to both sides of the active material under the action of the strain.
The 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 adhesive stress distribution caused by the volume change of the active material is almost uniform, but careful study still found The adhesive strains on the left and right sides of the active material are still higher than the strains on the adhesives on the upper and lower ends of the active material. This indicates that the adhesive on the left and right sides of the active material particles is more likely to delaminate during the cycle. However, in practice, we need to note that since the positive electrode active material has a very small volume change during cycling (NMC 2-4%), the binder strain change caused by the volume expansion of the active material particles is actually much smaller than that due to PVDF. Volume expansion caused by the adhesive aspiration.
The previous analysis was for spherical particles, but in practice we used particles with many other shapes, so JM Foster analyzed the effect of different particle shapes on the strain of the adhesive. The figure below shows the different particle shapes. For the effect of the strain distribution after the adhesive is aspirated, the binder strain at the P-point of the elliptical particles is positive, and the binder strain at the E point is negative, as calculated. This is consistent with the previous analysis. It can also be seen from the figure below that the orientation 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, it will increase significantly. Adhesive strain.
The figure below shows the strain of the adhesive at different charge rates (Fig. a shows the strain of the adhesive in the positive electrode and Fig. b shows the strain of the adhesive in the negative electrode). The slowest charging rate used in the calculation needs 3100h charging is completed, and the fastest charging rate requires only 0.031h to complete the charging. It can be seen from the figure that the high charging rate significantly increases the strain of the adhesive at the position of the E point of the active material particle, resulting in the adhesive and activity. The problem of delamination of the material particles. In general, the rapid charging of more than 1C rate will be positive, and the adhesive of the negative electrode will be damaged, thereby affecting the life of the lithium ion battery.
JM Foster's work allows us to have a clear understanding of the strain distribution of the adhesive around the active material particles from the microscopic level, and the factors that affect the strain distribution of the adhesive—active material particle shape and charge/discharge rate, Conducted in-depth discussion, for the electrode material design and lithium-ion battery formulation design has a certain guiding significance.
