1 Principle and purpose of rolling process
The rolling process is to roll press a mixture of positive and negative electrode materials of batteries with active substances, conductive agents, binders, etc., to form a dense structure. The main purpose of roller pressing is to keep the surface of the pole piece smooth and flat, and to prevent potential battery short circuits caused by burrs piercing the separator on the pole piece surface. Compact the electrode material coated on the electrode current collector to reduce the volume of the electrode and improve the energy density of the battery. Make the contact between the active substance and conductive agent particles tighter, and improve the electronic conductivity. Enhance the bonding strength between the coating material and the current collector, reduce the occurrence of powder loss during the cycling process of battery electrodes, and improve the cycling life and safety performance of lithium batteries.
2 Roll pressing process
1. Material preparation: Coat the positive and negative electrode materials of the battery with a mixture of conductive agents, binders, etc. on aluminum foil or copper foil according to the process requirements.
2. Roller press commissioning: Place the positive and negative pole pieces of the battery in the roller press, and adjust the pressure, tension and speed of the roller to adapt to the process size and requirements of the battery.
3. Rolling process: The rolling machine starts running, and the rollers press the positive and negative electrode plates of the battery, causing the active material to undergo physical reactions with the mixture of conductive agents, binders, etc., forming a dense structure and achieving the required compaction thickness for the process.
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3 Factors affecting the effect of rolling process
1. Pressure and speed of the roller: The pressure and speed of the roller are important factors affecting the effectiveness of the rolling process. Excessive pressure on the roller can cause excessive deformation of the positive and negative electrode materials of the battery, affecting the battery's cycle life; Excessive speed of the roller can lead to insufficient compaction of the mixture of positive and negative electrode materials, active materials, conductive agents, binders, etc. in the battery, affecting the energy density of the battery.
2. Properties of positive and negative electrode materials: Different positive and negative electrode materials have different compaction densities, and appropriate rolling parameters need to be selected for processing.
3. Properties of mixtures of active substances with conductive agents, binders, etc.: The concentration and composition of the mixture of active substances with conductive agents, binders, etc. can affect the rolling thickness of the positive and negative electrode sheets of the battery.
4. Performance of roller press: The performance of roller press also affects the rolling process effect. For example, the accuracy, stability, and control system of a roller press can all affect the quality and efficiency of the roller press.
4 The influence of rolling process on battery performance
1. Energy density: During the rolling process, the roller wheel compresses the positive and negative electrode materials of the battery, reducing the thickness of the positive and negative electrode plates and thereby increasing the energy density of the battery.
2. Cycle life: During the rolling process, the roller wheel presses the positive and negative electrode materials of the battery, improving the adhesion of the positive and negative electrode powder, enhancing the structure and stability of the battery positive and negative electrode materials, reducing the internal resistance and capacity degradation of the battery, and thus improving the cycle life of the battery.
3. Safety: The rolling process can increase the contact area between the positive and negative electrode materials of the battery and the mixture of active substances, conductive agents, binders, etc., thereby improving the reaction rate and charge transfer efficiency of the battery and enhancing its safety.
4. Internal resistance: The rolling process can improve the conductive network of the electrode, reduce the internal resistance of the battery, and enhance the charging and discharging performance of the battery.
However, the rolling process also has some adverse effects. For example, rolling significantly reduces the porosity of the electrode, leading to a decrease in electrolyte infiltration efficiency, an increase in Li+transport resistance, and an increase in electrode polarization. In addition, the tensile fracture strength and Young's modulus of the pole piece increase after rolling, the elastic-plastic deformation weakens, and the brittleness increases. In the subsequent slitting and winding processes, stress concentration and large internal pressure are easily formed, leading to the performance degradation of the pole piece.
5 Common problems and solutions of rolling process
1. Uneven thickness of the polarizer: This is mainly due to the non parallelism between the roller axis and the axis of each roller. This problem can be solved by adjusting the parallelism of each roller axis.
2. Pitting on the surface of the polarizer: This is mainly caused by fatigue pitting corrosion on the surface of the rolling mill. This is related to the material of the rolling mill, uneven metallographic structure during heat treatment, poor fatigue strength of the rolling mill surface, and surface roughness of the rolling mill. This problem can be solved by improving the material and heat treatment process of the rolling mill, enhancing the fatigue strength of the rolling mill surface, and optimizing the surface roughness of the rolling mill.
3. Thickness rebound during pole rolling: This is mainly due to the large residual elastic deformation and high environmental humidity after pole rolling. The elastic deformation of the polarizer can be reduced by optimizing the rolling parameters, controlling the environmental humidity, and adopting a hot rolling process, thereby reducing thickness rebound.
4. Uneven polarizer plate shape: This is mainly due to uneven deformation during polarizer rolling, small and uneven tension before and after, or errors in polarizer coating thickness. This problem can be solved by optimizing the rolling process parameters, adjusting the tension before and after, and improving the consistency of the coating thickness of the polarizer.
6 The development trend of roller pressing technology
With the continuous development of lithium battery technology, the rolling process is also constantly innovating and improving. The future development trends include:
1. Equipment miniaturization and integration: In order to improve production efficiency, roller presses are developing towards miniaturization and integration. At present, the longest length of the roller body has reached 1600mm, and it may continue to increase in the future.
2. Improved level of intelligence: Due to their large production capacity, power and energy storage battery companies have higher requirements for the intelligence level of rolling equipment. For example, roller pressing equipment with high automation and accompanying detection methods will become increasingly popular.
3. Application of hot rolling process: In order to improve the continuity of the production line and reduce strip breakage, top lithium battery manufacturers tend to adopt hot rolling process. The hot rolling process can reduce the internal stress, rebound probability, and deformation resistance of the polarizer, and improve the quality of the polarizer.
4. Optimization of rolling parameters: Optimizing rolling parameters is the key to improving battery performance. In the future, we will continuously optimize the rolling parameters through experiments and simulations to achieve better battery performance.
The lithium battery rolling process is an important link in the production process of lithium-ion batteries. By reasonably controlling the rolling parameters and process conditions, lithium-ion batteries with excellent performance can be obtained. Meanwhile, with the continuous development of technology, the rolling process will also continue to innovate and improve to meet the needs of higher performance batteries.