With the strong support of the state, the new energy vehicles developed rapidly in recent years and the market is growing rapidly. As the core component of new energy vehicles, lithium-ion battery has been developed rapidly. At the same time, the market for lithium-ion battery energy density, life expectancy, safety And other aspects continue to put forward new requirements, the state subsidy policy to adjust the battery cost also put forward higher requirements.Therefore, the battery factory must pay more attention to the quality and cost of the production process, and strive to improve product quality and consistency, reduce production costs. Lithium-ion battery pole piece manufacturing is the key process in the battery production process, including the preparation of the slurry, the pole piece coating and drying, the rolling compaction of the pole piece, and the pole piece cutting.At present, in the battery pole During the process of preparation, more and more on-line testing techniques are adopted to effectively identify the manufacturing defects of the products, eliminate the defective products, and timely feedback to the production line to automatically or manually adjust the production process and reduce the non-performing ratio.
The commonly used on-line inspection techniques in the manufacture of pole pieces include slurry characteristics testing, pole piece quality testing and dimensional inspection, for example: (1) the online viscometer is installed directly in the coating storage tank to detect the rheological properties of the slurry in real time , To measure the stability of the slurry; (2) using X-ray or β-ray direct measurement in the coating process to obtain the coating surface density, high accuracy, but the radiation, equipment, high prices and maintenance of trouble; (3) Laser on-line thickness measurement technology used to measure the thickness of the pole piece, the measurement accuracy of up to ± 1.0μm, but also real-time display of thickness and thickness measurement trends, for data traceability and analysis; (4) CCD visual inspection of the surface of the pole piece Defects, that is, linear array CCD scan of the measured object, image processing and analysis of real-time defect categories, to achieve non-destructive on-chip surface defect detection.
As a quality control tool, it is also essential to understand the relationship between defects and battery performance in order to determine the pass / fail criteria for semi-finished products.In this paper, a new method for the detection of surface defects of lithium ion battery pole pieces - Infrared thermography and a brief overview of the relationship between these different defects and electrochemical performance. D. Mohanty et al. Conducted an in-depth study and their work is expected to solve the following important scientific issues:
1) What are the possible defects in the electrode manufacturing process?
2) These defects on the lithium-ion battery charge-discharge cycle What is the impact?
3) The defect is how to change the Coulomb efficiency, rate performance and cycle life of lithium-ion battery?
4) Defective pole piece performance is impaired, is there a corresponding microstructure change?
1, pole piece surface defect detection technology
Infrared (IR) thermal imaging is also used to detect tiny defects on dry pole pieces that can damage the performance of lithium-ion batteries.On-line inspection, if electrode defects or contaminants are detected, they are marked on the pole piece , In the subsequent process will be removed, and feedback to the production line, the timely adjustment process to eliminate defects. Infrared is an electromagnetic wave, with radio waves and visible light of the same nature.Using some special electronic devices to the surface temperature The technique of distributing the image that is visible to the human eye and displaying the temperature distribution on the surface of the object in different colors is called infrared thermography and the electronic device is called an infrared camera.All objects above absolute zero (-273 ° C) Will emit infrared radiation.As shown in Figure 1, the infrared camera (IRCamera) using infrared detectors and optical imaging objective to accept the measured object infrared radiation energy distribution patterns and reflected to the infrared detector light-sensitive elements to obtain Infrared thermography, this thermal image and the object surface of the heat distribution field corresponds to the surface of the object when there is a defect, the area There will be temperature offset, therefore, this technology can also be used to detect defects on the surface of the object, especially suitable for some optical detection methods can not distinguish the defect.In lithium-ion battery dry pole online detection, the first pole after the flash Irradiation, the surface temperature changes, followed by the use of thermal imager to detect the surface temperature. Thermal image visualization and real-time image processing and analysis to detect surface defects in a timely manner to mark. D. Mohanty's research will be installed in the thermal imager The coating machine drying oven at the exit, the probe surface temperature distribution of the image.
Figure 1 thermal imaging probe pole surface appears schematic
Figure 2 (a) shows the temperature profile of the NMC positive plate coating surface detected by the thermal imager, which contains a very small defect that can not be resolved by the naked eye. The corresponding temperature distribution curve of the middle segment is shown in the interpolation diagram, Figure 2 (b) Image corresponding to the box there is a local temperature rise, corresponding to the surface of the pole piece defects. Figure 3 is the negative pole piece surface temperature distribution shows the presence of defects, which The temperature rise of the peak corresponds to bubbles or aggregates, the temperature drop in the area corresponding to the pinhole or out of material.
Figure 2 Positive pole piece surface thermal imaging temperature distribution, showing the pole piece surface defects
Figure 3 negative pole piece surface thermal imaging temperature distribution, showing the surface defects
Thus, the thermal imaging detection of temperature distribution is a very good means of detecting surface defects of the pole piece and can be used for the quality control of the pole piece manufacturing.
2, common surface defects
(A, b) Bump / agglomerate; (c, d) Dropout / pinhole; (e, f) Metal foreign body; (g, h) Uneven coating
Figure 4 is a common defect on the surface of a lithium ion battery pole piece, the left is the optical image, the right is the image captured by the thermal imager, of which:
(a, b) bump / agglomerates which can occur if the slurry is not uniformly agitated or the coating feed rate is unstable The agglomeration of the binder and the carbon black conducting agent results in a low active ingredient content , Pole piece light weight.
(c, d) Dropouts / pinholes that are uncoated, usually by bubbles in the slurry, reduce the amount of active material and expose the current collector to the electrolyte, reducing Electrochemical capacity.
(e, f) metal foreign matter, slurry or equipment, metal foreign objects introduced into the environment, the metal foreign body harmful to lithium batteries huge size of the metal particles directly pierce the diaphragm, resulting in short circuit between the positive and negative, which is Physical short circuit. In addition, when the metal foreign body mixed into the positive electrode, after charging the positive potential increases, the metal dissolves, spread through the electrolyte, and then precipitated on the negative electrode, and ultimately pierce the diaphragm, the formation of short circuit, which is chemical dissolution short circuit. The most common metal foreign bodies in the factory are Fe, Cu, Zn, Al, Sn, SUS and so on.
(g, h) Inhomogeneous coating, such as insufficient stirring of the slurry, streaks easily when the fineness of the particles is large, leading to non-uniform coating, which will affect the consistency of the battery capacity, even the appearance of completely uncoated stripes , Have an impact on capacity and security.
3, the pole piece surface defects on the battery performance
3.1, the battery capacity and the efficiency of coulomb effect
Figure 5 shows the effect of agglomerates and pinholes on cell capacity and Coulomb efficiency, but agglomerates actually improve cell capacity but decrease Coulombic efficiency. Pinholes reduce cell capacity and Coulombic efficiency, and the decrease in Coulombic efficiency at high magnification Big.
Figure 5 Effect of positive aggregates and pinholes on battery capacity and coulombic efficiency
Figure 6 is a graph of the effect of the non-uniform coating and the metallic contaminants Co and Al on cell capacity and coulombic efficiency. The non-uniform coating reduces the battery unit mass capacity by 10% -20%, but the overall battery capacity drops by 60% Indicating a significant decrease in the mass of living matter in the plate.The metal Co foreign material reduces the capacity and the coulombic efficiency even with no capacity at 2C and 5C high magnification which may be due to the metal Co alloy formation in the electrochemical reaction impedes the Lithium and lithium intercalation, may also be metal particles clogging the pores of the diaphragm caused by micro-short circuit.
Figure 6 Positive non-uniform coating, as well as metallic foreign bodies Co and Al on cell capacity and Coulomb efficiency
Positive pole defect Summary:
Aggregates in the positive pole piece coating reduce the coulombic efficiency of the battery.
The pinhole of the positive electrode coating reduces the coulombic efficiency, resulting in poor rate capability, especially at high current densities.
Non-uniform coatings show poor rate performance.
Metal particle contaminants can cause micro-shorts, which can drastically reduce battery capacity.
Figure 7 is the negative electrode foil stripe leakage rate on the battery capacity and Coulomb efficiency, the negative electrode leakage foil significantly reduce the battery capacity, but the g capacity is not significantly reduced, the impact on the efficiency of Coulomb is not significant.
Figure 7 negative foil leakage stripe on the battery capacity and Coulomb efficiency
3.2, the battery cycle rate performance
Figure 8 is the pole piece surface defects on the battery magnification cycle results, the impact of the results are summarized as follows:
Aggregate: 2C, the capacity of the defect-free plate cell was maintained at 70% for 200 cycles and that of the defective cell was 12%. The capacity retention rate was 50% for the defect-free plate cell and 50% for the defective cell during 5C cycle.
Pinholes: The capacity decay is obvious, but there is no decay of agglomerate defects, and the capacity retention rates of 2C and 5C at 200 cycles are 47% and 40% respectively.
Metal foreign matter: metal Co foreign body several times the capacity is almost zero, metal foil Al foil 5C cycle capacity attenuation significantly.
Leakage Foil Stripes: With the same leaking foil area, the battery capacity of multiple stripes of small size decay more rapidly than a large size stripe (47% capacity retention at 200 cycles at 5C cycle) (200 cycles at 5C cycle Capacity maintenance rate of 7%) This shows that the greater the number of fringes on the battery cycle greater impact.
Figure 8 pole piece surface defects on the battery cycle rate
