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The coating of the electrode is mainly composed of two parts of coating and drying. The coating mainly determines the coating width and the coating amount, and the microstructure of the electrode during the drying process has an important influence. The microstructure of the electrode is on the battery. The wettability, cohesiveness and diffusion kinetics of lithium ions have a significant effect. For example, during the drying process of the electrode, the solvent in the slurry gradually volatilizes with heating and enters the NMP recovery device. Since the NMP diffuses from the bottom to the surface during the drying process, and then volatilizes, the PVDF is in the electrode. The concentration gradient appears, the surface concentration of the electrode is high, and the surface concentration of the copper foil is low. Therefore, the drying speed has a crucial influence on the distribution of the binder such as PVDF in the electrode. Today, Xiaobian will discuss with you the formation characteristics of the onlooker structure during electrode drying.
StefanJaiser of the Karlsruhe University of Technology in Germany and his team used cryo-electron microscopy to study the changes in the microstructure of the graphite anode, the microstructure of the electrode and the concentration gradient of the binder at high speed. In the production of modern lithium-ion batteries, in order to reduce the production cost, the drying speed is often mentioned as fast as possible, and research shows that the high-speed drying will bond the electrode active material layer to the copper foil, and the binder distribution is uniform. The conductivity and conductivity of the electrodes have an effect, but we are still not clear about the mechanism, and the work of Stefan Jaiser just reveals the structural changes and influencing factors of the electrode during the drying process.
In the test, Stefan Jaiser used a graphite negative electrode as a sample, the binder was made of PVDF, and the slurry was coated on a copper foil by a small coater, and then the dry film was dried by using a dry air and a heating platform to simulate a real production environment. The slurry formulation and the electrode composition ratio after drying are shown in the table below.
During the drying process, according to the preset time, the pole piece being dried is directly put into the muddy liquid nitrogen for rapid cooling, the structure of the pole piece is preserved, and then the ion beam cutter is used at -160 ° C. The pole piece is cut to obtain a flat cross section. Finally, the morphology of the cross section of the pole piece is observed by low temperature electron microscopy. The schematic diagram of the process is shown in the following figure.
During the drying process, the active material layer shrinks due to the volatilization of the solvent NMP, and the shrinkage of the active material layer in different drying stages is as shown in the following figure. At the beginning, the solid content was 47.5%, but since the density of NMP was small, the volume occupied by the solvent was 71%, and after the electrode was completely dried, the electrode was close to each other due to the graphite particles, so the void ratio of the electrode was lowered. About 46.4% (this depends on the shape and size of the graphite particles).
Through the simulation of the factors affecting the evaporation, sedimentation and diffusion of the solvent during the drying process, at a lower drying speed, the sedimentation plays a decisive role, causing the particles to settle and gather away from the surface. At higher drying speeds, a rapid drop in the surface layer is caused, and the particles are aggregated to form a layer of particles on the surface of the electrode. If the rate at which the solvent evaporates is exactly equal to the rate of diffusion, no concentration gradient will occur inside the electrode. However, comparing the above figures, we can find that there is no obvious accumulation of graphite particles in the living bottom layer of the active material layer, and the graphite particles exhibit a uniform distribution throughout the active material layer. By analyzing the distribution of graphite particle size in the electrode of different drying degrees (as shown in the figure below), it can be seen from the data that as the drying, the size of the graphite particles gradually increases, but the graphite particles from the bottom layer to the surface The distribution of the layers has been relatively uniform, and there is no aggregation at the bottom or the surface, which indicates that the above model analysis does not match the actual production.
Since there is no concentration gradient in the distribution of graphite particles, is there any concentration gradient in other materials? Stefan Jaiser has studied the distribution of binder in the electrode layer in detail. Stefan Jaiser first divided the electrodes into four layers according to different distances from the surface. Four layers in each layer were used to analyze the content of F element (the marker of PVDF). The analysis results are shown in the figure below. From the figure, we can see that at the beginning, there is no concentration gradient of PVDF in the electrode, but with the volatilization of NMP, the concentration gradient of PVDF begins to appear, and the PVDF concentration of the electrode surface layer is higher than the concentration gradient of PVDF of the electrode bottom layer. And the concentration gradient gradually increases as the drying progresses. This can be explained by solvent diffusion, PVDF NMP solution diffuses from the bottom layer to the surface layer, then NMP volatilizes, and PVDF remains on the surface of the electrode.
What forces are driving the PVDF binder to migrate inside the electrode? We know that there are large and small pores between the graphite particles inside the electrode, and they are connected to each other due to capillary action. Exist, the solution in the macropores will migrate into the pores. At the beginning, this will push the NMP solvent in the bottom layer to migrate to the surface layer. When the amount of solvent is small to a certain extent, the solvent begins to accumulate in the smallest pore. This inhibits the migration of the binder to the electrode surface, so that the concentration gradient of PVDF is stopped to a certain extent. This also explains why the dried binder tends to appear at the junction of the particles and is also accompanied by a conductive agent because the carbon black material forms the smallest pore size, causing the final PVDF NMP solution to collect there.
The work of Stefan Jaiser reveals the changes in the microstructure of the electrode during the drying process. It is found that the graphite particles are evenly distributed throughout the drying process, and there is no delamination. However, the binder has a gradient distribution as the drying progresses, and the binder concentration on the electrode surface is higher than that in the bottom layer. According to research and analysis, this is mainly caused by the capillary action, and the NMP solution of PVDF migrates from the bottom layer to the surface layer. The work of StefanJaiser provides an important reference for guiding the production of lithium ion electrodes, which deserves our in-depth study.
Recently, the four ministries jointly issued the "Promotion of Action Plan for the Development of Automotive Power Battery Industry", proposing that by 2020, the power ratio of power lithium-ion battery cells will reach 300Wh/kg, the system specific capacity will reach 260Wh/kg, and the cost will drop to 1 yuan/Wh. a bit. Whether the specific energy index or the cost index of the single battery is very high, from the current technical level, there are great challenges. To achieve these indicators, we need to work together in materials, processes and structures. Electrode coating is a key step in lithium ion batteries. The quality of electrode coating directly affects the performance of lithium ion batteries. Therefore, detailed research on electrode coating is needed.