After the electrode coating is completed, it needs to be rolled. The main purposes of electrode rolling are as follows:
Ensure the electrode surface is smooth and flat to prevent burrs on the coating surface from piercing the diaphragm and causing a short circuit;
The electrode coating material is compacted to reduce the volume of the electrode, thereby increasing the energy density of the battery;
This allows for closer contact between the active material and the conductive agent particles, thereby improving electronic conductivity.
This enhances the bonding strength between the coating material and the current collector, reduces powder shedding from the battery electrodes during cycling, and improves the battery's cycle life and safety performance.
During the production process, the higher the consistency of the rolled electrodes, the better. This is manifested in a smooth surface, consistent color, no dark spots, high consistency in transverse and longitudinal thickness, minimal thickness rebound, few wrinkles, and no cracks. Typically, the appearance of the electrodes can be judged visually, while the longitudinal and transverse thicknesses are measured with a micrometer or a thickness measuring device equipped on the roll press. Some companies also determine consistency by calculating the compaction density. So, what factors affect the quality of electrode roll forming?
I. The electrode itself
1. Electrode defects
Electrode defects mainly refer to defects caused by coating. Common coating defects include thick edges, material strips, foil leakage, dark spots, foreign matter, and poor areal density consistency. When rolling thick-edge electrodes, because the edge thickness is higher than the middle electrode thickness, the edge bears greater pressure, causing severe warping of the electrode, commonly known as a serpentine electrode. The uneven tension distribution of the serpentine electrode is detrimental to the slitting and winding processes, thus affecting the yield. Material strips and foil leakage depend on polarity and location; foil leakage of the negative electrode must be absolutely avoided, while foil leakage of the positive electrode must be controlled. Dark spots and foreign matter will become fully apparent after rolling. Electrodes with poor areal density consistency will have inconsistent porosity inside the porous electrode due to uneven stress after rolling, affecting the distribution of electrolyte and current distribution during charging and discharging, and significantly impacting the internal resistance and capacity consistency of the battery product.
2. Material properties
Besides the impact of electrode defects, another factor affecting the rolling quality is the material itself. Negative and positive electrode materials such as ternary, lithium cobalt oxide, graphite, and silicon carbide each have their own maximum compaction limits and mechanical properties. If high energy density is pursued forcibly, it may damage the raw materials with different particle size distributions, thereby affecting the compaction consistency of the electrode. Therefore, it is necessary to select an appropriate rolling force.
II. Rolling Conditions
Variations in rolling conditions also affect the quality of electrode rolling. These variations include changes in process parameters and equipment. Key parameters in the rolling process include rolling force, gap, tension control, and rolling speed. Rolling force and gap control the electrode thickness, microscopically reflecting the contact morphology between raw material particles, while tension and rolling speed affect the electrode's appearance. The equipment itself also significantly impacts the rolling effect; factors such as roll deformation, wear, roll aspect ratio, and equipment stability all influence the contact state between the rolls and the electrode, thus affecting the uniformity of electrode thickness.
Roll deformation refers to the deformation of the roll caused by roll pressure and roll torque, and it is a major factor affecting the thickness uniformity of the battery electrode along its width. During the rolling process, roll deformation leads to uneven distribution of unit roll pressure on the electrode surface. This uneven distribution of unit roll pressure results in inconsistent reduction of the electrode along its width, ultimately affecting the uniformity of the electrode thickness. Within a certain range, the greater the roll deformation, the worse the electrode thickness uniformity.
