The most comprehensive summary of common defects and solutions of battery electrode pieces
In the process of lithium-ion battery process development and quality control, battery electrode piece manufacturing belongs to the front-end process and occupies an important position in the whole process. The Japanese battery industry generally believes that 70% of the quality of the battery is related to the quality of the battery electrode. This is because the quality of the battery electrode not only affects the middle-stage assembly process of the battery, but also has a key impact on the back-end process, the electrochemical performance and safety performance of the battery. Therefore, how to minimize the defects of the battery electrode to improve the quality of the battery electrode is crucial to making high-quality batteries. Regarding the common defects and solutions of battery electrode, although there are many public account articles that have made relevant summaries, I personally think that the total categories of defects summarized in these articles are relatively limited. Based on the predecessors, this article summarizes more types of defects and corresponding solutions, hoping to help everyone have a more comprehensive understanding of the diversity and complexity of battery electrode defect problems. My personal level is limited. If there are any types of defects that are missed, you are also welcome to leave a message to supplement.
Point defects
1. Pinhole defects:
Phenomenon and causes: Pinhole defects caused by bubbles. The bubbles in the wet film migrate from the inner layer to the surface of the film and rupture on the surface of the film to form pinhole defects. After the electrode with bubbles is rolled by the roller press, the circular color abnormal area on the surface of the coating area is the bubble. The color inside the circular area is darker, indicating that the carbon layer here is thin and slightly concave, resulting in failure to roll; the bright edge of the circular area is because it is thicker here and has a higher compaction density. Bubbles mainly come from stirring, coating liquid transportation and coating process.
Solution: Strengthen the sealing and constant temperature of the slurry conveying system to remove bubbles that may be generated during slurry transportation; the slurry before coating needs to be degassed.
2. Foreign body shrinkage
Phenomenon and causes: During the coating process, when the coating substrate is contaminated by objects with lower surface tension (such as oil droplets, dust, etc.), the coating solution around the contaminant will flow in the direction with higher surface tension, forming shrinkage defects like craters or dimples). The mismatch of surface tension between materials is the main cause of shrinkage defects, but the viscosity, fluidity, drying wind speed and temperature of the slurry may change the surface tension and its action process, thereby inducing the formation of shrinkage defects.
Solution: Filtration of coating liquid to remove iron, environmental dust control, and surface cleaning of substrate.
3. Agglomerate particle protrusions:
Phenomenon: As shown in the figure below, a large area of particles protruding appeared on the surface of the pole piece. After rolling, the protruding area of the particles had a color difference (brighter than the surrounding color) due to the inconsistent compaction density with the surrounding area, and the appearance was abnormally similar to a pockmarked surface. When these places were magnified and observed, it was found that these were agglomerates of the conductive agent.
Cause: The slurry is not stirred evenly, and the conductive agent is not dispersed. This kind of defect will occur when agglomerates are formed.
Solution: Improve the slurry stirring process to eliminate it.
Linear defects
1. Scratches
Phenomenon: Linear thin areas or leaking foil lines parallel to the coating direction, as shown in the figure below
Reasons:
- 1) Foreign matter or large particles are stuck in the slit gap or coating gap;
- 2) The substrate quality is poor, causing foreign matter to block the coating gap between the coating roller and the back roller;
- 3) The die lip is damaged;
- 4) During the operation of the coating equipment, sometimes the electrode is scratched during the belt running process due to uneven tension or inappropriate oven air volume, which also causes scratches.
Solution: 1) Sieve before coating to remove large particles in the slurry; 2) Remove particles from the lip or coating gap and check the die lip; 3) For problems caused by scratches, it is necessary to stop the line and check one by one to find the scratch position, remove obstacles or adjust the oven air volume appropriately, and solve the scratch line problem caused by scratches while ensuring that the electrode is dry.
2. Regular vertical stripe defects:
Phenomenon: The “regular vertical stripe” (Ribbing) defect we usually refer to is parallel stripes that appear along the machine direction, and the entire coating width has this defect. It is like taking a comb or a rake to scratch along the machine direction on a rubber surface that has just left the coating head, and artificially scratching out the same appearance defect (in the figure below)
Cause: This phenomenon usually occurs when the speed is close to the upper limit of the coating window, and it is more obvious in thin coatings; the slurry viscosity is too high, it cannot be well leveled, and the distribution on the foil surface is uneven.
Solution: 1) Lower the slurry viscosity and improve the leveling of the slurry so that the slurry can be evenly spread after being transferred from the coating machine to the foil surface; 2) Reduce the coating speed to eliminate the “elasticity” of the slurry and level it in time; 3) Reduce the coating gap between the coating roller and the backing roller.
3. Horizontal lines:
Phenomenon: Waves or lines generated at fixed intervals perpendicular to the coating direction.
Causes: 1) Mechanical vibration; 2) Fluctuation of the tape speed; 3) Periodic fluctuations in the flow of the slurry feed
Solutions: 1) Confirm whether the frequency of mechanical disturbance is the same as the frequency of horizontal lines; 2) Replace the pump and coating equipment; 3) Add a vacuum box to the coating head.
4. Electrode cracks:
Phenomenon: The coating surface has criss-cross cracks.
Causes: Excessive and too fast drying. Because the pole pieces are heated unevenly in various parts when the machine is stopped, the heated edges dry too fast in some places and too slowly in some places during the drying process. The internal stress generated in the coating will crack if it is not completely released.
Improvement measures: The baking speed can be controlled by adjusting the oven temperature and the air intake size.
Edge defects
1. Thick edge:
Phenomenon: During the coating process, the edge is thick and the middle is thin (in the figure below)
Reasons: 1) When the die is extruded and sprayed, the viscoelastic slurry fluid will expand. Due to the additional stress of the die edge wall, the slurry expansion effect is more obvious at the edge, resulting in the thick edge phenomenon; 2) When the coating is drying, the drying speed is the same everywhere, but the solvent evaporates faster at the edge. Therefore, when the edge composition changes faster, if there is no surfactant or other additives in the slurry or the surface tension of the dispersed particle suspension is greater than the surface tension of the solvent, the slurry flows to the edge, eventually resulting in the thick edge phenomenon.
Improvement measures: 1) When the slurry flow rate is constant, reducing the slit size can increase the slurry outlet speed at the die head, thereby reducing the drag force ratio of the slurry, and then reducing the thickness of the thick edge coating. However, when the slit size is reduced, the pressure inside the die head is greater, which is more likely to cause the expansion of the die head outlet shape, resulting in uneven lateral thickness of the coating, which requires more precise coating equipment; 2) Reducing the coating gap can reduce the thickness and width of the thick edge coating to a limited extent; 3) Reduce the surface tension of the slurry, such as adding surfactants, adjusting viscosity, etc., to inhibit the slurry from flowing to the edge during drying; 4) Optimize the outlet shape of the slit gasket, change the direction and size of the slurry flow speed, reduce the stress state of the edge slurry, and weaken the slurry edge expansion effect.
2. Wrinkles on the edge of the pole piece during rolling
Phenomenon: the edge of the pole piece during rolling presents one or more smooth grooves that are small, longitudinally or obliquely locally raised.
Causes: 1) Rolling deviation, uneven rolling deformation, too low roller pressure, uneven thickness of the pole piece lead to uneven stress distribution and wrinkles; 2) The incoming material has a bad shape or transverse waves, and the tension is insufficient during winding; 3) The winding shaft is uneven, the sleeve is not round, etc., resulting in uneven curling tension.
Improvement measures: 1) Ensure that the direction of pole piece rolling is perpendicular to the axis of the roller; 2) Appropriately reduce the amount of pressure during rolling, increase the tension during winding, and make the deformation uniform; 3) Control the thickness and plate shape of the incoming pole piece to meet the rolling requirements; 4) Check the quality of the roller at any time, and deal with it immediately if the roller is not round.
3. Tailing phenomenon
Phenomenon: the tailing phenomenon is divided into watermark tailing and sawtooth tailing. There are no active substances and conductive agents in the watermark area, so the probability of uneven surface density is low, so the harm of watermark tailing is small. When the sawtooth tailing phenomenon occurs, the uneven surface density of the pole piece is serious and the harm is greater.
Reasons: 1) When the slurry viscosity is too low or the solid content is too low, the slurry will be solid-liquid stratified. Because the fluidity of the liquid is better than that of the solid, when the solid stops flowing, the liquid part or the part with low solid content will flow outward, forming a tailing phenomenon; 2) The difference in surface tension between the substrate and the slurry will also cause tailing. The wetting of the slurry on the substrate requires the slurry to have a low surface tension and a high surface energy of the substrate; otherwise, the coating will quickly dewet after coating, that is, the coating will retract from the coated area; 3) The slurry settles or the positive electrode slurry has a “jelly” phenomenon, and the tailing phenomenon will also occur; 4) The gap between the back roller and the coating roller is not parallel; 5) The back roller bounces too slowly
Improvement measures: 1) Prepare a slurry with appropriate solid content and viscosity; 2) The surface energy of the substrate should match the surface tension of the slurry, the surface energy of the substrate should be high, and the surface tension of the coating liquid should be low; 3) Adjust the gap between the back roller and the coating roller; 4) Increase the back roller bounce speed.
