The rolling of the battery pole piece is the friction between the roller and the battery pole piece. The battery pole piece is pulled into the rotating roller and the battery pole piece is compressed and deformed. The battery pole piece rolling is different from the steel piece. Rolling, rolling is a process in which the plate extends longitudinally and laterally. The density does not change during the rolling process, and the rolling of the battery pole pieces is a process of compaction of the battery material on the positive and negative electrodes, the purpose of which is to increase the positive electrode or The compaction density of the negative electrode material, the appropriate compaction density can increase the discharge capacity of the battery, reduce the internal resistance, reduce the polarization loss, prolong the cycle life of the battery, and improve the utilization of the lithium ion battery.
The battery pole rolling equipment is evolved from the steel rolling machinery. It is generally composed of a rack part, a transmission part and an electric control part. According to the mechanical structure and the roller pressure mode, three commonly used lithium ion battery pole roller presses are introduced in this paper. And its process characteristics: Manual screw-type pole piece mill, gas-liquid booster pump type pole piece mill, hydraulic servo-pressure pole piece mill.
1, manual spiral pressure pole piece mill
This kind of equipment is driven by the gear motor to rotate the high hardness roller, adopts the oblique block type roller gap adjusting device to mechanically adjust the gap of the roller, so that the pole piece is pressed and formed, and the pole piece density is increased. It is mainly used for rolling the monolithic battery pole piece. Rolling pressure is shown schematically in Fig. 1. This equipment is mainly used in laboratories. By setting the roll gap value, the roller is loaded on the pole piece. There is no additional pressurizing device. Therefore, the actual pressure is generally lower, and the roller pressure is relatively low. The pole piece compaction density is limited, and generally the maximum roller gap is limited by the mechanical device, there is a maximum value, generally can not roll too thick pole piece.
Fig.1 Manual Spiral Press Roll Adjustment
2, gas-liquid booster pump type pole piece mill
Gas-Liquid Intensifier Pump Pressurization The battery pole piece mill adopts wedge iron and screw to adjust the roll gap off-line. It can not adjust the roll gap and rolling force in real time online. The cost is relatively low, and the symmetrically coated battery pole piece can be rolled. , as shown in Figure 2.
Figure 2 pole piece mill real picture
The roll gap of this kind of rolling mill is adjusted by the variable thickness of the middle wedge. The principle of the gap adjustment: There are two slanted diagonal irons between the bearing seats at both ends of the roll. Usually one of the thinner ones is called Static oblique iron, moving another thicker called moving oblique iron, when the two oblique irons have relative displacement in the direction of the inclined plane, different thicknesses are combined, and then there are different roll gaps. As shown in Figure 3. Using a stepping motor to drive the movement of the inclined iron slider, the rotation of the stepping motor is converted into the adjustment of the distance between the rollers. Its structure is shown in Fig. 4. When the oblique iron is driven by a servo motor, it can be more intuitive. See the roll gap, so adjust the slant iron to the gap between the two ends of the roll is just zero, this position of the inclined iron is called the origin, and install a limit switch called the origin switch.
Figure 3 oblique wedge slotting schematic
Figure 4 Stepping motor mechanical structure of gap
Fig.5 Schematic diagram of a wedge-shaped battery pole piece rolling mill
Fig. 5 is a schematic diagram of force acting on a wedge-type battery pole piece rolling mill. The hydraulic cylinder pressure F acts on the bearing seats on both sides of the roller. When the pole piece is being rolled, the hydraulic cylinder pressure F is decomposed into the force acting on the wedge iron and its effect. The effective rolling force on the pole piece. The basic process of rolling is: Before setting the battery pole piece into the rolling mill, the pressure of the pressure cylinder of the rolling mill is zero, the pre-regulating roll gap value S0. After pressurizing with the gas-liquid booster pump, The roll chocks and wedges will be compressed and the center distance between the bearing housings will be shortened. Since the rolls will not come into contact, all the pressure will act on the wedges and the shortened distance is set to x0. The hydraulic cylinder preload force is F, then:
From this formula, it can be seen that the hydraulic cylinder pressure F, the pre-regulation roll gap S0, the incoming thickness H, etc. have an effect on the effective rolling force P and the roll thickness h of the pole piece. The elastic deformation curve A of the upper and lower roll systems, the battery pole piece The plastic deformation curve B and the bearing seat and the wedge iron elastic deformation curve C are drawn in the same figure. As shown in Fig. 6, the corresponding vertical and horizontal coordinates of the O point are the effective rolling force and the thickness of the pole piece rolling.
Fig. 6 Effective rolling force superimposed with elastoplastic curves with wedges - Roller thickness diagram
Process parameter adjustment points
However, in wedge rolling mills, the set cylinder pressure F does not completely act on the pole pieces, but is decomposed into two parts: the force acting on the wedges and the effective rolling force acting on the pole pieces. The component is different with the roller pressure parameter setting.
(1) When the hydraulic cylinder pressure F remains unchanged, the roll gap is set to different values. If the pre-roll gap S0 is relatively small, the bearing seat and the wedge iron are disengaged, and all the pressure acts on the pole piece. The pre-gap seam increases from small to small. Before a certain threshold, the roll thickness does not change, but this situation is not very stable. Beyond the critical value, the pre-roll gap S0 continues to increase, the effective rolling force acting on the pole piece decreases, and the pole piece thickness increases.
(2) When the pre-set roll gap S0 is relatively suitable and constant, if the set value of the pressure F of the hydraulic cylinder is less than a certain value, the bearing seat will be disengaged from the wedge iron when the roll rolls the pole piece, and the pressure is all Acting on the pole piece, as the cylinder pressure increases, the effective rolling force acting on the pole piece also increases, and the roller thickness decreases. However, after the cylinder pressure exceeds this value, the cylinder pressure continues to increase, and the increased pressure is basically consumed. Wedge iron, effective rolling force increase is not obvious.
(3) The roller gap and cylinder pressure settings are unchanged, and the battery pole pieces of different thicknesses are rolled. When the incoming thickness decreases, the roller thickness also decreases, but the pressure loss on the wedge increases. The effective rolling force decreases, and the compaction density of the coating does not remain constant.
(4) At present, there is no unified method for adjusting the pressure of the roll gap and the hydraulic cylinder during the actual use of the pressurized type pole rolling mill of the gas-liquid booster pump. To set a relatively small roll gap, the hydraulic pressure of the hydraulic cylinder is smaller. ; Or set a larger roll gap, increase the pressure of the cylinder, can roll out the same thickness of the battery pole. In order to make the hydraulic cylinder pressure be used effectively, reduce the pressure loss caused by the system energy loss should The pressure on the wedge iron is reduced as much as possible, but in order to have a certain degree of richness, the cylinder pressure can be made slightly greater than the required rolling force. The required pre-gap can be calculated according to the following formula:
3, Hydraulic Servo Pressurized Plate Mill
AGC (Automatic Gauge Control) rolling mill is a pole piece mill with on-line automatic thickness adjustment technology. The most advanced at present is the full hydraulic pressure adjustment device. Hydraulic servo control pressurizing pole piece mill no longer uses wedge iron to adjust roll gap value The hydraulic cylinder pressure can fully act on the battery pole piece. In order to be able to control the pressure on the battery pole piece and the position of the hydraulic cylinder piston in real time, the pressure system uses the hydraulic servo control system of the valve control cylinder. This method is simple in structure and high in sensitivity. Can meet very strict thickness accuracy requirements, can achieve constant pressure, constant clearance rolling. The introduction of hydraulic servo control system with large transmission force and power enables the pole piece mill to realize online real-time adjustment of pressure and roll gap, rolling single When the double-layer alternately coated pole piece is used, the single-layer part can also get a better rolling effect, so that the quality of the rolling pole piece is greatly improved. During the rolling process, there is a rod cavity through the pressure reducing valve, the relief valve and the accumulator. The combination of the energy absorbers maintains a constant pressure. There are four cylinders between the upper and lower housings, and a constant pressure is maintained by a combination of a pressure relief valve and a relief valve to balance the upper roll system. the amount.
The rigidity of the base is determined by the roll pressing method. The specific process is as follows: There is no battery pole piece between the two rolls. Under the condition that the roll is idling, the upper roll is pressed down slowly so that the upper and lower rolls are in direct contact and pressed against each other. Afterwards, the hydraulic servo cylinder is controlled so that the upper roll continues to descend, so that the work stand of the rolling mill is elastically deformed. Then the upper roll is controlled to slowly rise, the two rolls are slowly separated, and the rolling force and the corresponding position of the hydraulic cylinder and the piston are measured. Relationship. The change in the relative position of the cylinder and the piston responds to the elastic deformation of the work stand.
Figure 7 Schematic diagram of a hydraulic servo-pressurized battery pole piece mill
Figure 7 shows the schematic diagram of the pressurizing mechanism of the hydraulic servo system for the pressurized battery pole piece rolling mill. The hydraulic pressure is all applied to the pole piece. The effective rolling force P is:
Among them, K is the stiffness of the entire frame, h is the thickness of the roller, and S0 is the pre-adjustment of the roller gap.
Hydraulic servo pressurizing pole piece rolling mill can control the pressure on the battery pole piece and the position of the hydraulic cylinder piston in real time. It has constant pressure and constant roll gap two rolling modes.
Constant roll seam rolling
Fig. 8 Curve of constant roll gap (100μm) rolling test
As shown in Fig. 8, when the roller is rolled from the battery part to the slurry-free process, the sudden roll-off of the battery pole causes the upper roller to drop suddenly and then quickly return to its original position. The elastic deformation is reduced and the rolling force is also reduced. When the roller is rolled from the baseband part of the battery pole piece to the slurry part, the upper roller will suddenly rise and then press down to the required position. The elastic deformation increases, and the rolling force increases accordingly. But overall, the displacement fluctuation is not great.
At present, there are even double closed-loop control systems. The inner ring position control loop (APC) is the core control link. The output is the actual position of the roll or the actual roll gap, that is, the constant roll gap rolling. The outer ring is the thickness control of the pole piece. Ring, real-time on-line detection of the thickness of the pole piece, the thickness feedback signal is used to correct the position setting of the roll gap, through hydraulic servo control, so that the roller quickly moves to achieve the purpose of quickly eliminating the thickness difference.
Constant pressure rolling
Fig. 9 Constant rolling force (400KN on one side) Rolling test curve
As shown in Fig. 9, when the roller is rolled from the battery portion to the no-slurry portion of the battery, there is a reduced fluctuation in the rolling force due to the sudden thinning of the battery pole piece, and the rapid recovery is achieved. At the set point, the upper roll also decreases accordingly. When the roll is rolled from the baseband portion of the battery pole piece to the slurry portion, the rolling force will have an increased fluctuation, and quickly return to the set value. The upper roll also rises accordingly. But overall, the pressure fluctuation is not great.
Due to the incomplete symmetry of the manufacturing and assembly of mechanical structures on both sides of the rolling mill, the transmission side is connected with the transmission shaft, and the position of the battery pole pieces between the roller systems cannot be guaranteed in the middle. There are certain differences in positional changes. Due to the particularity of the battery pole load How to overcome the gap between the pole pieces, reduce the pressure fluctuations and other issues have yet to be further solved.
