In the production process of lithium-ion batteries, it is necessary to first mix the active material, conductive agent, and binder in different solvents, and then use a coater to apply the slurry to the surface of Al foil or Cu foil, and then use. The solvent in the slurry is removed at a high temperature, and after rolling, a porous structure electrode is finally formed. The microstructure of the electrode has an important influence on the electrochemical performance of the lithium ion battery. The porosity of the electrode and the tortuosity of the pore influence the Li+ on the electrode. Within the diffusion distance, the specific surface area of the active material affects the current density, so constructing a reliable and reliable lithium ion battery electrode model is of great significance for studying the influence of the electrode structure on the electrochemical performance of the lithium ion battery. In recent years, X-ray tomography technology The development of this method allows us to establish a 'real' electrode 3D model by reconstructing the lithium ion battery electrode. It can be said that the X-ray tomography technology bridges the gap between simulation and reality.
Compared to other methods, X-ray tomography has a large photon flux over a wide range of energy and can provide sub-micron resolution. It is ideal for scanning and reconstructing the positive electrode structure of lithium-ion batteries. Recently, Martin Ebner et al. of the Federal University of Technology in Zurich used X-ray tomography to study the electrode microstructure of NCM111 material. The analysis of different compaction pressures and conductive agent + binder content on the electrode porosity and electrochemical Effect of performance. The following figures a, b are NCM particles, SEM photographs of cross-sections of the electrodes, and c are photographs of electrode samples. Figure d is a picture of a sample scanned by X-rays. When X-rays pass through the sample Some of the rays are absorbed by heavy metals. The remaining X-rays are then converted to visible light by LuAG. The visible light image is then recorded by the CCD module. The following figure e shows the processed image. The light-colored part of the figure represents X. The area where there is more radiation absorption, that is, contains NCM particles with more heavy elements, and the position of dark color represents the area with less X-ray absorption, that is, the hole in the electrode. Gap, carbon black and binder etc. We can see the powerful strength of X-ray tomography in the following figures i and j. In the image we can clearly see the breakage of NCM particles in the electrode (this is often Due to the electrode being formed during the high-pressure process, this indicates that the packing density of NCM material greatly affects the compaction density. When the NCM particles cannot be rearranged, the NCM will absorb the pressure in the form of particle crushing.
Due to carbon black, the differences in the extent of X-ray absorption between the binder and the pores are very small, so it is difficult to distinguish them by X-ray absorptivity. In order to improve the accuracy of electrochemical simulation, Martin Ebner uses the distance transform method and the watershed algorithm These substances are distinguished. The following figures g and h are marked with color after differentiation. In order to verify the accuracy of the above algorithm, MartinEbner also divides the NCM particle size distribution results obtained by the algorithm and the NCM granularity obtained by the laser particle sizer. The distribution results were compared (as shown in the following figure a), and it can be seen that the two are in very good agreement, indicating that Martin Ebner's algorithm can accurately reflect the microstructure of the NCM electrode and is suitable for establishing 3D models for lithium-ion batteries. Electrochemical model for simulation.
Figure b below shows the particle size distribution and porosity distribution in the direction perpendicular to the current collector calculated by the above algorithm. From the figure, it can be seen that the small particles are more likely to be concentrated at the two boundaries of the electrode, and the large particles are in the grind. During the pressing process, it was squeezed into the middle position of the electrode. By comparing different electrodes, it can be found that all the electrodes will collect small particles at the interface of the current collector, but only the rolled electrodes will have small particles on the surface. phenomenon.
The following figure shows the electrode porosity data obtained by X-ray tomography (taking into account the impact of NCM particle crushing, carbon black and binder). Figure a shows the porosity and carbon black + PVDF quantity and compaction pressure. In the relationship, we see that the lower the number of carbon black+PVDF is, the lower the porosity is, and the lower the binder and conductive agent content is, the better the electrode is, but the pressure is high at high pressure. Next, on the contrary, the higher the carbon black and binder content, the lower the porosity, indicating that under high pressure, more conductive agent and binder fill the pores between the particles, reducing the porosity of the electrode Rate. At the same time, Martin Ebner also found that at a low conductive agent + binder content, the porosity distribution of the electrode becomes more inhomogeneous under low pressure, which may be inhomogeneous with the electrode during the homogenization, and the electrode is Rolled particles are rearranged.
The following figure shows the ratio of the electrode at constant current discharge (blue curve) and constant current-constant voltage discharge (purple curve) using 2% and 5% carbon black + PVDF electrodes after being rolled at pressures of 0 bar and 2000 bar respectively. In the performance curve, it can be seen that the NCM electrode with 5% of conductive agent + binder content can exert more capacity in constant current discharge, and the rate performance is better. 2% of NCM electrode with conductive agent + binder content In the constant current discharge to play a lower capacity, poor rate performance is poor, compared to the data we can also note that compaction density has almost no effect on the battery rate performance, indicating that the constant current discharge rate performance of NCM materials is mainly electronically conductive Impact, limited by ion diffusion.
Martin Ebner's work allowed us to use X-ray tomography to reconstruct the electrode structure, accurately analyze the effect of different binders, conductive agent content and different compaction densities on particle distribution and porosity inside the electrode. It is of great significance to establish a reliable and reliable 3D simulation model. MartinEbner's research also shows that the phenomenon of small particles gathering at the upper and lower interfaces and large particles at the center of the electrode appears on the NCM electrode. Electrochemical performance studies have shown that the NCM electrode magnification The performance is mainly affected by the electronic conductivity of the electrode and is less affected by the ionic conductivity.
