The rolling process, performed on lithium-ion battery electrodes after coating and drying, is a crucial step in the manufacturing process. Simply put, rolling is a process that uses the pressure of a high-tonnage rolling mill to compact the loose, porous active material coating on the current collector (aluminum or copper foil) into a dense thin film.
This step is indispensable primarily for the following core purposes:
1. Increasing Energy Density
This is the most direct purpose. After coating, the active material on the electrode surface (such as lithium cobalt oxide, lithium iron phosphate, graphite, etc.) is in a loose, granular aggregate state, with numerous voids between the particles.
Effect: Through rolling compaction, the voids between particles are reduced, increasing the mass of active material that can be accommodated per unit volume, thereby directly increasing the volumetric energy density of the cell. More energy can be packed into the limited cell space.
2. Reducing Internal Resistance and Improving Rate Performance
The internal resistance of a battery directly affects its charging and discharging speed and heat generation.
Contact Resistance: Before rolling, the contact points between active material particles and between the active material and the current collector are merely physical, resulting in a long electron transport path and high resistance. After rolling, the particles are flattened and rearranged, forming surface contact or tighter point contact, constructing a stable conductive network. Simultaneously, the bonding force between the active material layer and the current collector is significantly enhanced. This greatly reduces the battery's ohmic internal resistance, allowing electrons to move rapidly, thus supporting high-current charge and discharge (high-rate performance) and reducing heat generation during charge and discharge.
3. Improved Cycle Life
If the electrode is not rolled or is insufficiently compacted, it will face stability issues during battery charge and discharge cycles.
Suppressing Delamination: Rolling enhances the peel strength between the coating and the current collector. If the coating is loose, it is prone to detaching from the foil or pulverizing during repeated charge and discharge cycles (lithium-ion insertion and extraction causing volume expansion and contraction of the active material), leading to rapid capacity decay.
Structural Stability: A dense electrode structure better constrains the volume changes of the active material during cycling, maintaining the integrity of the electrode structure and thus extending battery life.
ACEY-HRP100 roll pressing 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 .
4. Controlling Thickness Uniformity and Subsequent Processes
Thickness Uniformity: The winding or stacking process places extremely stringent requirements on the thickness tolerance of the electrode sheets. If the electrode sheets are not rolled, the coating thickness fluctuates greatly and the surface is uneven, making misalignment likely during subsequent winding. During stacking, uneven thickness may puncture the separator, causing a short circuit risk.
Thickness Reduction: The thickness of the rolled electrode sheets is typically only 60%-80% of the original coating thickness. This saves space for subsequent winding/stacking processes, which is beneficial for cell miniaturization.
5. Optimizing Porosity
Rolling is not about pressing as densely as possible; rather, it's about controlling the porosity within a reasonable range.
Key Balance: A suitable porosity (typically 25%-35% for lithium-ion anodes and 20%-30% for cathodes) ensures both sufficient electrolyte storage space, guaranteeing smooth lithium-ion transport channels between the solid and liquid phases (ionic conductivity), and sufficient electronic conductivity. Roll forming is a crucial means of achieving this balance between electronic and ionic conductivity.
In conclusion: Without the roll forming process, the electrode will have a "sponge-like" structure. Such electrodes not only have low energy density, but also suffer from poor electronic conductivity and weak bonding with the current collector, making them prone to problems such as high polarization, severe heat generation, lithium plating (negative electrode), and powder shedding during charging and discharging. This results in batteries failing to meet commercial cycle life and safety requirements.
Additional note: While roll forming is critical, the compaction density needs to be precisely controlled based on different material systems (e.g., lithium iron phosphate is sensitive to compaction, while ternary materials require a balance of toughness). Over-compression leads to excessively low porosity, making electrolyte wetting difficult and hindering lithium-ion transport, which is detrimental to battery performance and can even cause electrode brittleness and breakage in severe cases. Therefore, the roll forming process is often a core aspect of quality control.