What Are the Functions, Principles, and Influencing Factors of Rolling Press Machine?

Rolling refers to the process of compacting coated and dried lithium battery electrodes.

Rolling increases the energy density of lithium batteries and allows the binder to firmly adhere the electrode material to the current collector, preventing energy loss due to electrode material detachment during cycling. Before rolling, the coated electrodes must be dried to a certain degree; otherwise, the coating will detach from the current collector during rolling. The compaction amount must be controlled during rolling. Excessive compaction can affect the electrode material near the current collector, preventing proper lithium-ion intercalation and deintercalation, and causing the active material to adhere tightly together, making it easy for it to detach from the current collector. In severe cases, it can also increase the electrode's plasticity, making it unsuitable for winding and causing breakage.

Roll forming is one of the most critical processes in lithium-ion battery electrode manufacturing, and its precision significantly impacts battery performance.

ACEY-HRP100 hot roller press machine is mainly suitable for the electric rolling of battery materials in the laboratory, a small amount of precious metal materials such as gold and silver, and non-ferrous materials such as copper and aluminum at a certain temperature. The rolling thickness is adjustable and the operation is simple. It is especially suitable for thinning and increasing the density of lithium battery pole plates of clean energy materials .

The purposes of roll forming are as follows: Roll forming ensures a smooth and flat electrode surface, preventing short circuits caused by burrs piercing the separator and improving energy density. It also compacts the electrode material coated on the current collector, reducing electrode volume and further increasing energy density, cycle life, and safety.

The purpose of roll forming is to achieve a denser and more uniform bond between the active material and the foil. Roll forming must be performed after coating and electrode drying; otherwise, powder shedding and film peeling can easily occur.

Battery electrodes are copper (or aluminum) foils coated with electrical paste particles on both sides. The battery electrode strip undergoes coating and drying processes before roll forming. Before rolling, the electrical paste coating on the copper foil (or aluminum foil) is a semi-fluid, semi-solid granular medium composed of unconnected or weakly connected individual particles or clumps, exhibiting a certain degree of dispersibility and fluidity. The existence of voids between the electrical paste particles ensures that during rolling, the particles can undergo small displacement movements to fill these voids, allowing them to position themselves under compaction. Battery electrode rolling can be considered a continuous rolling process of semi-solid electrical paste particles in an unsealed state. The electrical paste particles adhere to the copper foil (or aluminum foil) and are continuously bitten into the roll gaps by friction, then compacted by rolling into battery electrodes with a certain density. The rolling principle is shown in Figure 2.

The rolling of battery electrodes differs significantly from the rolling of steel. During steel rolling, the workpiece initially undergoes elastic deformation under external force. When the external force increases to a certain limit, the workpiece begins to undergo plastic deformation. As the external force increases, the plastic deformation increases. The purpose of longitudinal rolling in steel rolling is to achieve elongation. During steel rolling, molecules extend longitudinally and widen laterally, resulting in a thinner rolled piece but no change in density. Battery electrodes are made by coating a compound slurry onto a substrate such as aluminum or copper foil. The rolling process compacts the electrical slurry particles on the electrode, increasing its density. A suitable compaction density can increase the battery's discharge capacity, reduce internal resistance, and extend its cycle life. The electrical slurry particles undergo displacement and deformation under pressure, and the electrode density changes with pressure according to a certain pattern, as shown in Figure 3.

In region I, as the contact pressure increases, the electrical slurry particles begin to undergo small-scale displacement, which gradually increases. At this point, the gaps between the electrical slurry particles are gradually filled, specifically manifested as a slow increase in the relative density of the electrode strip with increasing contact pressure.

In Region II, after the small-scale density increase in Region I, the electrical slurry particles continue to fill the gaps between them as the contact pressure increases. After rolling in Region II, the gaps between the particles are compressed and compacted. Specifically, the relative density of the electrode strip increases rapidly with increasing contact pressure, at a much higher rate than in Region I. Simultaneously, some deformation of the electrical slurry particles occurs in Region II.

In Region III, after the gaps between the electrical slurry particles in Region II are filled, the particles no longer shift. However, with increasing contact pressure, the electrical slurry particles begin to undergo large deformations. At this point, the relative density of the electrode strip no longer increases rapidly with increasing contact pressure; the electrode strip hardens, and therefore the change in relative density becomes a flat curve.

The changes in the electrical slurry particles on the battery electrode during the rolling process are highly complex. The increase in the relative density of the electrical slurry particles is mainly manifested in particle displacement. This displacement fills the gaps between particles, while a small portion of the particles deforms. Subsequently, due to the increased roller pressure, the electrical slurry particles undergo significant deformation after the gaps are filled, although some minor displacement also occurs during this stage.

The electrode quality problems caused by the battery electrode rolling equipment are mainly reflected in the unevenness of the electrode thickness after rolling. Inconsistent thickness leads to inconsistent compaction density of the battery electrode, which is a key factor affecting the uniformity of battery performance. Electrode thickness uniformity includes transverse thickness uniformity and longitudinal thickness uniformity, as shown in Figure 4. The causes of transverse and longitudinal thickness non-uniformity are different.

The main factors influencing the transverse thickness uniformity of the electrode sheet are the bending deformation of the rolls, the rigidity of the mill base, the elastic deformation of the main load-bearing components, the roll pressure, and the electrode sheet width. During mill operation, the roll pressure causes deformation of the rolls and mill base, ultimately manifesting as roll deflection, resulting in a transverse thickness uniformity where the electrode sheet is thicker in the middle and thinner at the edges. The main factors influencing the longitudinal thickness uniformity of the electrode sheet are the machining and installation accuracy of the rolls, bearings, and bearing housings. Machining errors in key components can cause periodic fluctuations in the roll pressure acting on the electrode sheet during roll rotation, resulting in uneven compaction thickness in the longitudinal direction.

Other factors affecting the quality of electrode sheet rolling include tension control devices, alignment devices, slicing devices, and dust removal devices. During the rolling process, the electrode sheet requires a certain tension. Insufficient tension can lead to wrinkles, while excessive tension can cause breakage. Dust removal devices ensure that surface defects caused by impurities do not appear on the electrode sheet surface during rolling. The correction device and the trimming device mainly affect the cutting dimensional accuracy of the electrode sheet.

Acey New Energy is a high-end equipment and complete line solution provider specializing in the field of new energy batteries. It is dedicated to providing full-cycle services from experimental development to large-scale production for global battery manufacturing enterprises, research institutions, and innovative energy organizations. Whether it is the sample trial production at the laboratory level or the process verification during the pilot stage.

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