Lithium-ion battery paste is a multi-phase mixed fluid, which contains a solid phase of the active material, conductive agent, etc., also contains a liquid solvent and binder (dissolved in the solvent), the drying process will be lithium-ion battery The electrode structure has a significant effect. Marcus Müller et al. of Karlsruhe University of Technology in Germany found that the process of drying the graphite anode causes the PVDF to accumulate on the surface of the electrode. PVDF forms a concentration gradient inside the electrode, and with the drying Acceleration of the speed will lead to more PVDF concentration on the electrode surface, which will lead to a decrease in the content of PVDF binder at the graphite/copper interface, which will lead to poor adhesion of the active material and degraded battery performance. Stefan of Karlsruhe University of Technology Jaiser believes that the mechanism leading to uneven distribution of PVDF in the drying process is the capillary phenomenon caused by the porous structure of the electrode. With the evaporation of the surface solvent, capillary action will 'suck' the underlying solvent to the surface of the electrode, resulting in PVDF in the electrode. Inhomogeneous distribution.
According to research from Calhoun University of Technology in Germany, it is not difficult to find that a lower drying speed is conducive to forming a more uniform PVDF distribution and improves the adhesion of the electrode, but a too low drying rate results in a significant reduction in production efficiency. It is unrealistic to reduce the drying speed blindly in actual production. In order to resolve this contradiction, F. Font of the University of Technology of Catalonia, Spain, modeled the electrode drying process of a lithium ion battery and simulated it. During the drying process, the distribution of PVDF adhesives inside the electrode was optimized. The model was used to optimize the drying process. The drying rate was gradually reduced, and the internal electrode was kept relatively uniform with a reduced drying time. PVDF distribution.
F. Font believes that electrode drying can be divided into two processes: 1) The process of uniform electrode shrinkage. In this process, the NMP solvent is gradually evaporated and the thickness of the slurry film gradually decreases, but due to the stable properties of the electrode slurry. The active material particle distribution is still very uniform; 2) The second process is that the active material particles are completely in contact, but there are still more NMP solvents between the active material particles, and NMP in the particle pores during the subsequent drying process. Gradually evaporates, leaving pores between active material particles.
A one-dimensional model for the above-mentioned drying process F. Font is established, that is, the material diffusion proceeds only in the direction of the vertical electrodes, and the slurry is composed of a liquid phase and a solid phase (since the conductive agent content is small, the conductivity The agent is not calculated separately, but the conductive agent is taken into account in the binder). In addition, because the thermal conductivity of the slurry is very high, F. Font believes that there is no temperature gradient in the slurry film, which further simplifies the model.
Based on the analysis of the behavior of PVDF in the drying process, F.Font believes that PVDF is mainly affected by two kinds of forces in the drying process: 1) Due to the viscous drag caused by solvent evaporation, PVDF is dragged toward the surface of the electrode; 2 The diffusion effect caused by the concentration gradient, the PVDF is pushed back from the electrode surface with higher concentration to the electrode interior. Studies have shown that PVDF only crystallizes when the concentration reaches 77% (at 60°C), which shows that in the first step The PVDF will not reach the precipitation concentration during drying and shrinkage of the slurry membrane. PVDF will crystallize out in the second step of drying as the NMP evaporates from the inter-particle pores.
In the study of material transport in the drying process we need to use an important dimensionless parameter Pe (Pe = vl/D, where v is the characteristic speed, l is the feature length, D is the diffusion coefficient), Pe stands for The ratio of convection and diffusion transport in material transport. When Pe increases, the proportion of convection increases and the proportion of diffusion transport decreases. F. Font Divides the drying process into slow drying according to the value of Pe. <<1) 和快速烘干过程 (Pe>>1) .
During the slow drying process (Pe<<1) 中, 粘结剂浓度随时间的变化如下式所示, 由于烘干过程中物质的扩散输送速度要快于对流输送速度, 因此PVDF粘结剂在电极内部并不会形成浓度梯度, 从而形成PVDF粘结剂均匀分布的电极.
In the process of rapid drying (Pe>>1), convection diffusion becomes the dominant factor, so the concentration of PVDF adhesive at different locations within the slurry film can be expressed by the following formula. In the case of high-speed drying, in convection Under the effect of the adhesive will be brought to the surface of the electrode, resulting in a significant concentration gradient within the electrode.
Where A (t) is given by
The figure below shows the low speed drying (characteristic speed v=1.25x10-7m/s, Pe=0.1) and high-speed drying (Special speed v=1.25x10-5m/s, Pe = 10) In the two extreme cases, the distribution of the PVDF concentration inside the electrode in the Z direction at different time points can be seen in the case of low-speed drying (below a) at five times The calculated concentration curves are almost flat, indicating that the PVDF concentration inside the electrode changes very little in the Z direction. Let's take a look at the high-speed drying. We see the PVDF concentration on the electrode surface over time. Rapidly rising (upturn of curve end), and the concentration of PVDF adhesive at the interface between active material and Cu foil is very low, indicating that high-speed drying makes a very significant concentration gradient inside the electrode.
From the above analysis, it is not difficult to see that a lower drying speed is conducive to a more uniform distribution of PVDF, but in practical production, the production efficiency is also an important consideration for us. Therefore, we need to design a more suitable drying system. It can not only reduce the internal PVDF concentration gradient, but also improve the drying efficiency. F.Font compares the following three drying systems: The first one is the constant drying rate, and the second is the incremental drying rate. The three are decremental drying rates. From the calculation results in the following figure, the decreasing drying system can achieve a more uniform PVDF concentration distribution, while the incremental drying system will obtain the largest PVDF adhesive concentration gradient. It shows that in order to obtain a more uniform distribution of PVDF in actual production, we should use a drying system with decreasing speed.
The uneven distribution of PVDF adhesives caused by the drying process is a problem that has been plaguing us for many years. Although everyone knows that lowering the drying speed can improve the uniformity of the adhesive, this method is often used in practical over production of efficiency. Unable to adopt, F. Font's work allows us to see the hope of reconciling this contradiction. By adopting a speed-decreasing drying system, it is possible to ensure the uniform distribution of PVDF in the electrode and to effectively reduce the drying time. Win-win for quality and efficiency.
