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of the battery product. Among them, the manufacturing process of lithium-ion battery is relatively complex. The important production process includes three parts: the manufacturing of the front pole plate, the assembly of the middle cell, and the activation charge and discharge test of the rear cell. And the front pole piece manufacturing process. The result of the front-end technology of lithium ion battery is the preparation of the positive and negative pieces of lithium ion battery. The first procedure is stirring, that is, the positive and negative solid battery materials are mixed evenly, and the solvent is added, and the slurry is stirred by a vacuum mixer. The mixing of ingredients is the basis of the subsequent process of lithium battery, and the high-quality mixing is the basis of the subsequent coating and rolling process. Each step of the process contains a large number of process parameters, and the manufacturing process of lithium ion battery will directly or indirectly affect the safety performance and electrochemical performance of the battery. Among them, the positive and negative paste configuration control, coating quality control, drying, tablet and slice have the most obvious influence on the battery performance, and have a decisive influence on the final battery performance. Developing a set of mature technological process is the basis of producing high quality batteries, but it consumes a lot of manpower and material resources.
With the rapid development of computer technology, computer simulation provides convenience for the development of lithium ion battery manufacturing process. Using computer simulation technology can effectively study the battery manufacturing process, greatly shorten the development cycle, and is now more and more attention by everyone. Figure 2 is a lithium ion battery parameters - the microstructure characteristics - battery model parameters - performance relationship between computer simulation studies the overall train of thought, a piece of the manufacturing process chain simulation according to the parameters are deduced according to the craft of the pole piece structure parameters, and the pole piece structure characteristic parameters and the battery is directly related to the electrochemical model parameters, According to the battery model parameters, the relationship between the model parameters and the performance of the battery can be established by using a pseudo-two-dimensional electrochemical model. Finally, the relationship between process parameters, microstructure characteristics, battery model parameters and performance was established by computer simulation. Provide effective guidance for product design, process development and performance testing of lithium-ion batteries.
The preparation of slurry is the first process in the production of lithium-ion battery. The mixing process has more than 30% influence on the quality of the product in the whole production process of lithium-ion battery, and it is the most important link in the whole production process. Lithium-ion battery slurry is formed by electroactive substance, conductive agent and other solid particles dispersed in the binder solution. When coating, live substance, conductive agent and other solid components should be evenly dispersed in the solvent with tiny particles. The stirring process is the process of momentum transfer in the flow field, and the agitator is the device that inputs mechanical energy to the stirring medium by making it obtain the appropriate flow field. Therefore, choosing what kind of stirring method and agitator and how much energy to make the agitator supply what kind of flow field, and making the fluid appear appropriate circulating flow rate, turbulence degree or shear speed in the flow field, the purpose is to finally obtain the uniform product with high efficiency.
The primary problem of high viscosity liquid agitation is to solve the problem of fluid flow and circulation. In this case, the circulating flow of the agitator cannot be increased by increasing the stirring speed, because the discharge of the agitator is very small when the viscosity is high, and the high rotating speed will also form a groove in the high mucus, while the surrounding liquid is still dead zone. The important way is to make the blade push a wider range of liquid. Planetary stirring device is the use of a pair of planetary gear revolution and rotation, by a power source to drive the stirring shaft along the circumference of the barrel and the rotation of the stirring box, the other planetary shaft hollow structure, in which the high-speed dispersion disk power source is set, through mechanical transmission to make the high-speed dispersion disk also along the circumference of the revolution and rotation.
Battery slurry mixing process, to simulate the hydrodynamics method can be used in figure 3 is a fluid mechanics simulation mixing process instances, details see references "MixinganalysisofaNewtonianfluidina3Dplanetarypinmixer".
Coating process is one of the key processes in the production of lithium-ion battery electrodes. The quality of coating largely determines the capacity, voltage and other important parameters of lithium-ion battery. Computer simulation plays an increasingly important role in new product development and product manufacturing. Through the establishment of 2D or 3D model, the coating process is simulated, the coating law is mastered, the coating process can be visualized, and the process development time is shortened. The concrete coating simulation includes: the flow process of the internal flow field of the extrusion die head, the flow process of the slurry in the feeding system, the formation of the wet coating in the coating process, the optimization of the die head structure, the study of the coating mechanism, the improvement of the coating process and the determination of the coating window, etc. According to the theory of fluid mechanics, we can preliminarily determine the basic characteristics of the flow field, understand the phenomenon of the coating process, and the cause of the coating defect by calculating the force of the flow field and the characterization parameters of the flow field during the coating process. Finite element analysis of fluid mechanics can directly see the flow state of fluid and understand the flow process of coating more vividly. Fig. 4 is a fluid mechanics finite element simulation example of slit extrusion coating for lithium-ion batteries. For details, see Simulation Research on Initial Flow Field of Slit Coating for Lithium-ion Battery Slurry and Simulation Animation of Initial Flow Field of Squash Coating.
After preparation, the battery paste is coated on the collector metal foil and then dried. In the drying process, the coating always experiences a certain shrinkage when the solvent evaporates, and the solid materials close to each other in the wet coating, finally forming a porous dry electrode structure. In the process of coating shrinkage and solvent evaporation, additives such as binder and conductive agent are easy to migrate with solvent evaporation, and redistribute in porous electrode, resulting in uneven phenomenon. Fig. 5 is a simulation example of the solvent evaporation process for pole sheet drying. See "How to solve the problem of electrode surface enrichment in the drying process of PVDF adhesive" for details.
Fig. 5 Simulation of solvent evaporation process for pole plate drying
After the electrode sheet is compacted, the porosity of the coating is changed from the initial value ε C, 0 to & epsilon; C. The compaction process of lithium-ion battery electrodes also follows the exponential formula (1) in the field of powder metallurgy, which reveals the relationship between the coating density or porosity and the compaction load, as shown in Fig. 6.
Among them, & rho; C,0 is the initial value of coating density, ρ C is the density of the compacted coating. Ql is the line load used on the pole plate, which can be calculated from Equation (2) :
QL = FN/WC (2)
Fn is the rolling force applied to the electrode sheet and Wc is the width of the electrode sheet coating. ρ C, Max and & gamma; C can be obtained by fitting experimental data, respectively representing the maximum compaction density and compaction impedance of the coating under a certain process condition. The compaction density is converted into porosity, and the exponential formula (1) is converted into formula (3) :
According to the above compaction process model, the relationship between the compaction microstructure of the electrode sheet and the process parameters such as roller line load and the characteristics of the electrode sheet, such as surface density, active substance type and particle size distribution, was established. For a detailed description of the rolling process model, see "Basic Analysis of Rolling Process for Lithium Ion Battery Pieces" and "Compaction Process Model for Lithium Ion Battery Pieces: Investigation of the Influence of Active Substitute and Surface Densities on Porosities".
The relationship between the microstructure and electrochemical performance of lithium-ion battery electrodes can be established by electrochemical simulation. The electrochemical pseudo-2D (P2D) model of lithium-ion battery is established based on the porous electrode theory and the theory of concentrated solution, as shown in Fig. 7. The actual chemical reaction processes inside the battery are considered, including solid phase diffusion process, liquid phase diffusion and migration process, charge transfer process, and solid-liquid phase potential balance process. The Butler-Volmer equation was used to describe the electrochemical reaction on each electrode and the process of surface insertion and removal of lithium. Fick's second diffusion law was used to describe the diffusion process of lithium ions inside the particle. Several reaction process of partial differential equation and corresponding boundary conditions of the model, in a very short computing time obtains the external characteristic reaction battery charge and discharge curve, and can also get the reaction process of internal anode materials are solid phase concentration distribution and the solid phase electric potential distribution and solid phase and the liquid electrolyte concentration distribution details such as electric potential distribution, It is accurate, comprehensive and based on mechanism.
The basic structure of the lithium-ion battery is shown in Figure 7, which mainly includes the negative collector (copper), the negative electrode material, the separator, the positive electrode material and the positive collector (aluminum). The specific electrode structure parameters include: Ln is the thickness of the negative electrode, LS is the thickness of the diaphragm, Lp is the thickness of the positive electrode, L is the total thickness of the single layer, X is the transverse dimension between the positive and negative electrodes of the battery, and R is the radial dimension of the spherical coordinates of the electrode active particles. The porosity of the pole plate, the volume fraction of each component; Electronic conductivity, lithium ion diffusion coefficient, electrode reaction surface area, etc.
Finally, plate making the whole process of microstructure - battery performance simulation is summarized as figure 8, the characteristics of products from raw materials as input, slurry mixing model based on the mixing process model, the output characteristics of the thick liquid material parameters, then the slurry coating/drying process model parameters of input, output, dry electrode structure parameters, so on, According to the manufacturing process, computer simulation is carried out step by step, and the performance of the battery is predicted directly at last, so as to form a complete process chain of lithium ion battery electrodes and the simulation process of battery performance.
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