Battery roll presses are critical equipment in the manufacturing process of lithium-ion batteries. They play a pivotal role in enhancing the energy density, safety, and lifespan of batteries.

Functions of Battery Roll Presses

Battery roll presses are primarily used to compress the positive and negative electrode materials, separators, and electrolytes into a tightly packed battery structure. This process serves several critical functions:

Enhancing Battery Energy Density:

Surface Smoothening: The roll pressing process ensures that the surfaces of the electrode sheets remain smooth and flat. This prevents any burrs on the surface from puncturing the separator, which could lead to battery short-circuiting.

Volume Reduction: By compressing the electrode materials coated on the current collector, the roll press reduces the volume of the electrode sheets, thereby increasing the energy density of the battery.

Improving Battery Conductivity:

Tight Material Contact :Under the high pressure applied by the roll cylinders, the battery materials are compressed together, forming tighter contact between the electrodes, separators, and electrolytes. This enhances the battery's conductivity and transmission performance, leading to improved output power.

Improving Battery Safety:

Uniform Material Density :After roll pressing, the density of the battery materials becomes more uniform, with smaller gaps between materials. This reduces air pockets within the battery, facilitating better heat dispersion and minimizing temperature fluctuations. Consequently, it reduces self-heating and self-discharge, enhancing battery safety.

Extending Battery Lifespan:

Material Distribution: Roll pressing ensures a more uniform distribution of materials within the battery, with tighter contacts between them. This reduces material aging and shedding, thereby extending the battery's lifespan.

Operating Principles of Battery Roll Presses

Battery roll presses operate through the application of pressure between two or more rollers, causing the material (such as battery electrode sheets) to undergo deformation, crushing, or mixing. Here's a detailed breakdown of the operating principles:

Roll Press Structure:

Battery roll presses typically consist of two cast steel compression rollers, along with motors and transmission shafts. The mainstream models have roller diameters of 500mm and roller lengths ranging from 500mm to 700mm.

Material Compaction Process:

Pre-Pressing Adjustment :Before the electrode sheet enters the roll press, the pressure in the hydraulic cylinder is set to zero, and a pre-adjusted roller gap (S0) is set.

Hydraulic Pressure Application: When the electrode sheet with slurry enters the roll press, the thickness of the sheet increases, causing a rolling force to act on it. The rollers and roller bearing seats undergo elastic deformation, and the force acting on the wedge decreases.

Effective Rolling Force: The force applied by the hydraulic cylinder (F) is divided into the force acting on the wedge (F1) and the effective rolling force (P) acting on the electrode sheet. The relationship between these forces and the roller gap, incoming material thickness, and roller pressure thickness can be mathematically modeled.

Elastic-Plastic Deformation:

The compaction process of lithium-ion battery electrode sheets follows the exponential formula in powder metallurgy, revealing the relationship between coating density or porosity and compaction load.

During roll pressing, the electrode sheet undergoes plastic deformation under increasing pressure, resulting in a change in coating density from its initial value.

Adhesion Enhancement:

Roll pressing increases the adhesion strength between the active material and the foil current collector, preventing peeling during electrolyte immersion and battery use.

Detailed Analysis of Roll Pressing Process

Roll Pressing Sequence:

Roll pressing follows the coating and drying processes. It is essential to perform roll pressing after drying to avoid powder shedding and coating detachment.

Material Characteristics Before Roll Pressing:

Before roll pressing, the slurry coating on the copper or aluminum foil is in a semi-fluid, semi-solid granular state, composed of unconnected or weakly connected individual particles or agglomerates.

Material Changes During Roll Pressing:

Displacement and Deformation :Under increasing pressure, the slurry particles undergo small-scale displacement and deformation, filling gaps between particles.

Density Increase: The relative density of the electrode strip increases gradually with increasing contact pressure, initially slowly and then rapidly as particles are packed more densely.

Hardening Phenomenon: After the gaps are filled, further pressure causes large deformation in the particles, resulting in a slower increase in density.

Factors Influencing Roll Pressing Quality

Thickness Uniformity:

Uneven thickness after roll pressing leads to inconsistent compaction density, which affects battery performance uniformity.

Factors such as tension control devices, alignment devices, slicing devices, and dust removal devices can impact thickness uniformity.

Tension Control:

Adequate tension is necessary during roll pressing to prevent wrinkles or tearing of the electrode sheet.

Dust Removal:

Dust removal devices ensure that impurities do not cause surface defects on the electrode sheet during roll pressing.

Alignment and Slicing:

Alignment and slicing devices affect the cutting accuracy of the electrode sheet.

Advanced Roll Pressing Technologies

Hydraulic Servo-Controlled Roll Presses:

These presses use hydraulic servo control systems to apply pressure and adjust roller gaps in real-time, ensuring precise control of the rolling force and gap.

They can achieve constant pressure and gap rolling, improving the quality of rolled electrode sheets, especially for alternating single and double-layer coated sheets.

Elastic Deformation Measurement:

The stiffness of the machine base is determined by pressing the rollers together without an electrode sheet present and measuring the relationship between rolling force and hydraulic cylinder position.

Conclusion

Battery roll presses are essential equipment in the production of lithium-ion batteries, enhancing energy density, conductivity, safety, and lifespan. Their operating principles involve the application of pressure between rollers to compact materials, resulting in plastic deformation and increased density. The quality of the rolled electrode sheets is influenced by factors such as thickness uniformity, tension control, dust removal, and alignment. Advanced technologies like hydraulic servo-controlled roll presses further improve the precision and quality of the roll pressing process. Understanding these principles and factors is crucial for optimizing battery manufacturing processes and achieving high-performance batteries.