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The significance of rolling is to compact the electrode sheet and increase the energy density of the battery. After the electrode sheet is coated and dried in the previous process, the peel strength between the active material and the foil is very low, so it needs to be rolled to enhance the bonding strength between the active material and the foil to prevent it from being soaked in the electrolyte and during use. Appropriate compaction density can increase the discharge capacity of the battery, reduce internal resistance, reduce polarization loss, prolong the cycle life of the battery, and improve the utilization rate of the lithium-ion battery. The electrode sheet rolling is generally completed by a double-roller press, which is composed of two cast steel compacting rollers, a motor and a transmission shaft. After turning on the rolling mode, the motor drives the upper and lower rollers to rotate at the same time, and the winding mechanism pulls the electrode sheet to pass through the rolling gap steadily, and is finally pressed to the required compaction density.
The rolling process has a great influence on the performance of the cell. Several typical problems will appear in the rolling process: ① The thickness of the electrode sheet is inconsistent. The inconsistent thickness means that the active material density is inconsistent, and the transmission and conduction rates of lithium ions and electrons in the electrode sheet will be different. When the current density is different, it is very easy to cause the precipitation of lithium dendrites, which is detrimental to the performance of the battery cell. In addition, when the thickness of the electrode sheet is different, the contact resistance between the active material and the current collector is also different. The thicker the electrode sheet, the greater the internal resistance, and the more serious the polarization of the battery, which affects the capacity of the battery. ② Overvoltage occurs in part of the electrode sheet. Due to the excessive thickness of some parts during coating, overpressure may occur after rolling. The active material particles are broken at the position of overvoltage, and the contact between the active material particles is tight. During the charging and discharging process of the battery cell, the electronic conductivity is enhanced, but the ion movement channel is reduced or blocked, which is not conducive to the capacity development, during the discharge process, the polarization increases, the voltage decreases, and the capacity decreases. At the same time, overvoltage affects the wetting effect of the electrolyte, which also has a great impact on the performance of the battery.
From the perspective of technological process, the quality of rolling will directly affect the subsequent electrode sheet processing effect. The ideal state of the electrode sheet after rolling is that the surface of the electrode sheet is flat, the gloss is consistent, the blank part has no obvious waves, and the electrode sheet does not have a large degree of warping. However, in actual production, some problems will inevitably arise due to operational proficiency and equipment operation, which will affect the slitting of the electrode sheet in the subsequent process, resulting in inconsistent width of the slitting electrode sheet and burrs on the electrode sheet. The result of rolling will also affect the winding of the electrode sheet. Severe warping will cause a large gap between the electrode sheet and the diaphragm during the electrode sheet winding process. After hot pressing, some parts of the multi-layer diaphragm will be superimposed, which will become the stress concentration point and affect the performance of the cell.
To measure the advanced level of roller press technology, it mainly depends on the surface density and compaction density. The current leading equipment can make the specific capacity of the lithium iron phosphate cathode material reach 200 mAh/g, and the compaction density of the lithium iron phosphate cathode material reach 1.5 g/cm^3.