In the manufacturing process of lithium batteries, battery electrode rolling is a critical link and plays a decisive role in battery performance. Among them, the two methods of primary rolling and secondary rolling have different effects on pole piece rebound and battery performance due to differences in operation.
Purpose and principle of electrode rolling press
The main purpose of pole piece rolling is to precisely control the pole piece thickness and porosity within the design range, enhance the pole piece peeling strength, and shorten the lithium ion transmission distance. During the rolling process, the pole piece is reduced in thickness and porosity under the strong pressure of the double rollers. The pole pieces of different material systems have different rolling performances. For example, the positive pole piece rebounds relatively less after rolling, while the negative pole piece rebounds more.
Effect of one-time rolling on rebound and battery performance
One-time rolling is relatively simple to operate, and the pole piece can reach the designed thickness and density through only one rolling. However, this method has obvious disadvantages. Under strong pressure, the active particles are very easy to break and pulverize. Taking the graphite negative pole piece as an example, after one rolling, the ID/IG value of the graphite in the pole piece is relatively high, indicating that the graphite particles are damaged to a large extent. Moreover, one rolling will also cause the pole piece pore size distribution to be uneven, and the pore size above the pole piece (far away from the copper foil) is significantly larger than that below the pole piece (close to the copper foil).
This uneven pore size distribution and damage to active particles have many adverse effects on battery performance. In terms of electrolyte wettability, the rupture of active particles and uneven pore size distribution hinder the smooth infiltration of electrolyte, increase the resistance to lithium ion transmission, and lead to a decrease in battery charge and discharge performance. From the perspective of resistance value, the inconsistent pore size above and below the pole piece will increase the internal resistance of the battery and reduce the charge and discharge efficiency. In terms of battery cycle life, the rupture and pulverization of active particles reduce the amount of active substances involved in the electrochemical reaction, resulting in a decrease in the battery cycle life. At the same time, in actual production, due to the high speed of a single roll press, it is difficult to accurately control the rebound characteristics of pole pieces of different materials, and the thickness, compaction density and quality of the pole pieces are often difficult to meet ideal requirements.
Effect of secondary rolling on rebound and battery performance
During the secondary rolling process, the pole piece is first rolled to a certain thickness, and then rolled for the second time, so that the pole piece finally reaches the designed thickness and density. Compared with the primary rolling process, the secondary rolling process has significant advantages. It can greatly reduce the cracking and pulverization of active particles and make the pore size distribution in the pole piece more uniform. Studies have shown that the pole piece that has been rolled twice has relatively less damage to its graphite particles, greatly improves the electrolyte wettability, and reduces the resistance of the pole piece.
From the perspective of pole piece rebound, since the pole piece has a certain amount of time to undergo elastic deformation after the first rolling and reaches a relatively stable state before the second rolling, the overall rebound degree is effectively controlled. In terms of battery performance, the uniform pore size distribution enables the electrolyte to better infiltrate the pole piece, reduce the resistance to lithium ion transmission, and improve the battery’s charge and discharge performance. At the same time, smaller active particle breakage and uniform structure are conducive to improving the battery cycle life. In addition, the secondary rolling can more accurately adjust the pole piece compaction density, reduce the pole piece rebound caused by unreasonable compaction density, and take into account the battery’s electrochemical performance and thickness requirements.
Comparative analysis and actual production suggestions
A comprehensive comparison between primary rolling and secondary rolling shows that secondary rolling has outstanding advantages in controlling the rebound of the electrode and improving battery performance. In actual production, for positive electrode sheets, given that the rebound of rolling is small, most domestic power battery companies can meet the demand by using primary rolling. However, for negative electrode sheets, secondary rolling is more suitable because of their large rebound.
When implementing the secondary rolling, some key points need to be noted. The time interval between the two rollings needs to be set reasonably. It is generally recommended to carry out the secondary rolling in a drying room 2 hours after the first rolling to ensure that the pole piece has sufficient time to complete the elastic deformation and stabilize. In addition, in order to further reduce the rebound of the pole piece, other processes can be combined, such as baking after rolling to accelerate the rebound of the pole piece and make it stabilize as soon as possible; hot rolling can be used to reduce the rebound of the pole piece while enhancing the battery performance.
Summary: During the rolling process of lithium battery pole pieces, the secondary rolling is significantly better than the primary rolling in dealing with pole piece rebound and improving battery performance. Battery manufacturers should reasonably select the rolling method based on the characteristics of pole piece materials and accurately optimize the process parameters to improve the quality and performance of battery products.
