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After the electrode is coated and dried, the peeling strength of the living substance and the fluid collector foil is very low, and it is necessary to roll it to enhance the bonding strength of the living substance and foil, so as to prevent it from peeling off during the electrolyte immersion and battery use. At the same time, the electrode roll compression can compress the volume of the cell, improve the energy density of the cell, reduce the porosity between the living matter, conductive agent and binder inside the electrode, reduce the resistance of the battery and improve the battery performance.
First, roll press introduction
In order to improve the consistency of the density and thickness of the material on the surface of the battery electrode sheet, the positive and negative electrode sheet must be rolled after the coating process, which is called the rolling of the battery electrode sheet. At present, domestic and foreign lithium-ion battery manufacturers all use two-roll roll press to roll pole sheet, and the two-roll press is composed of two cast steel compaction rolls and a motor and a drive shaft. The roller diameter of the mainstream model is 500 mm, the length of the roll body is 500 mm ~ 700 mm, and the thickness consistency of the pole sheet pressed by the roll along the width direction is poor. In order to ensure the accuracy requirements of thickness consistency, the ratio of roll length to diameter is often small, resulting in a narrow roll width of the pole sheet.
FIG. 1 Schematic diagram of pole plate rolling production line
In the field of copper foil, aluminum foil and other foil manufacturing, the vast majority of foil is manufactured by a four-roll roller press, and the surface accuracy can reach a few microns at the same time, the roll width can reach more than 1 m. However, there is no precedent for the application of four-roll press in pole sheet roller pressing. (If so, please leave us a message to add)
FIG. 2 Left: Schematic diagram of roll system layout for two-roll press Right: schematic diagram of roll system layout for four-roll press
The complete rolling process is to finish the coated pole sheet, fixed in the unwinding mechanism, the pole sheet is correctly passed through the gap between the two rollers, and connected to the receiving coil system. After opening the roll mode, the motor drives the upper and lower rollers to rotate at the same time, and the pulling pole of the winding mechanism will steadily pass through the roll gap and finally be pressed to the required compaction density. When the roller press is not working, it needs to be coated with a thin oil layer to prevent rust, wipe the oil layer with anhydrous ethanol before use, and clean up the winding mechanism and automatic correction mechanism. Therefore, an excellent roller press must contain the following eight functional modules:
Figure 3: gas-liquid booster pump type pole mill
Roll pressure adjustment and rapid reaction function: The pressure adjustment between the two rollers of the rolling machine is a necessary condition to improve the compaction density of the active material of the battery pole sheet, due to the influence of the coating interval, single and double side crossing and other factors, the pressure adjustment between the two rollers must react quickly.
Roll gap adjustment and accurate reset function: The gap adjustment between the two rollers of the rolling machine is a necessary condition for obtaining the thickness of the battery pole. Due to the change of the coating mode and the need of the pole strip connection, the gap between two rolls needs to be accurately reset after rapid adjustment.
Tension adjustment before and after rolling and rapid response function: Adjusting the tension before and after the rolling of the battery can control the flatness of the battery plate. In the rolling process, the speed of the rolling line often changes instantaneously, and the rapid reaction of tension control is an important means to prevent the broken tape.
Roll stepless speed regulation and line speed synchronization function: The battery pole sheet rolling machine must be continuously variable during the start and stop process or according to the process needs, while ensuring that the line speed of the two rolls is consistent.
Electrode rolling temperature adjustment function: Adjusting the battery electrode rolling temperature can directly affect the deformation resistance and plastic deformation of the battery electrode during the rolling process.
Correction function of roll deformation: In the rolling process or temperature adjustment process of the battery pole sheet, the two rolls must have axial flexural deformation and radial swelling deformation, and the correction roll deformation directly affects the thickness uniformity and compaction density uniformity of the battery pole sheet.
Intelligent control function of the rolling process of the pole: with the continuous improvement of the rolling speed of the pole and the degree of automation, automatic loading and unloading, automatic tape connection, automatic pressure, automatic clearance adjustment, online monitoring, etc., require closed-loop control and even intelligent control.
Roll cleaning and maintenance function: It is common for the surface of two rolls to stick powder during the rolling process of the battery pole sheet. Keeping the roll surface clean can reduce roll wear and improve the surface quality of the battery pole sheet. The rolling machine is also a necessary function for easy maintenance.
FIG. 4 Schematic diagram of manual helical pressure pole plate mill and adjusting roll gap
The laboratory usually uses the experimental manual helical pressure pole mill, which makes the roll load pressure on the pole plate by setting the roll gap value, and there is no additional pressure device. Therefore, the general actual pressure is relatively small, the compaction density of the roller electrode sheet is limited, and the general maximum roll gap is limited by the mechanical device, and generally cannot roll too thick pole sheet.
In addition, the pole rolling is divided into cold rolling and hot rolling two ways, at present, foreign countries have been widely used in the way of hot rolling for pole rolling, and the domestic is still more cold rolling, compared to cold rolling, hot rolling has the following advantages:
(1) Remove the water inside the pole plate; Reduce the internal stress of the pole sheet, because the release of the internal stress of the pole sheet is reduced during slitting or die cutting
(2) Reduce the material rebound of the pole sheet after rolling; Reducing the deformation resistance of the battery electrode is conducive to increasing the liquid absorption of the active substance.
(3) Because the material is in a molten state during the heating process: hot rolling can increase the adhesion between the active substance and the fluid, and reduce the occurrence of powder loss during the charge and discharge cycle of the battery, and improve the cycle life of the battery
In the rolling schedule: can be divided into one rolling and multiple rolling. The multi-rolling process is relatively complicated, but it can reduce the rebound of the pole sheet, and the pole sheet has good gloss and high thickness consistency.
Second, the purpose of rolling
The compaction density of the electrode plate has an important effect on the electrochemical performance of the battery. Within a certain range, with the increase of the compaction density, the distance between the active material particles decreases, the contact area increases, and the paths and Bridges conducive to ionic conduction increase, which is manifested in the reduction of the internal resistance of the battery in the macro aspect. However, if the compaction density of the electrode plate is too large, the contact degree between the active material particles is too close, the electronic conductivity increases. However, the reduction or blockage of lithium ion channels is not conducive to the play of capacity, and when discharging, the polarization increases, the voltage decreases, and the capacity decreases. When the compaction density is too small, the particle spacing is large, the lithium ion mobile channel is smooth, and the electrolyte has strong liquid absorption ability, which is conducive to the movement of lithium ions inside the battery, but because the contact degree between the particles is not close enough, it is not conducive to the conduction of electrons, and it is easy to lead to increased polarization during discharge.
The necessity of rolling: After coating and drying, the peeling strength of the living matter and the fluid collecting foil is very low, and it is necessary to roll it to enhance the bonding strength of the living matter and foil to prevent peeling during the immersion of the electrolyte and the use of the battery.
The purpose of rolling is as follows:
1) Ensure that the surface of the pole plate is smooth and flat, and prevent the burrs on the surface of the coating from penetrating the diaphragm and causing short circuit;
2) The electrode coating material is compacted to reduce the volume of the electrode sheet to improve the energy density of the battery;
3) Make the contact between the active substance and the conductive agent particles closer and improve the electronic conductivity;
4) Enhance the bonding strength of the coating material and the fluid collector, reduce the occurrence of powder dropping during the cycle of the battery pole, and improve the cycle life and safety performance of the battery.
Three, pole sheet rolling process and control
1. Rolling process
The process of battery plate rolling is that the friction generated between the roll and the battery plate is pulled into the rotating roll, and the battery plate is compressed and deformed. The rolling of battery electrode is different from the rolling of steel block. The process of rolling steel is a process of iron molecules extending longitudinally and spreading laterally, and its density does not change during the rolling process. The rolling of the battery electrode plate is a process of compacting the battery material on the positive and negative plate. When rolling the battery pole sheet, the rolling force should not be too large or too small, which should conform to the characteristics of the battery pole sheet material.
FIG. 5 Relation between electrode density and rolling force
Region I is the first stage: in this stage, when the rolling force starts to gradually increase, the density of the electrode sheet increases rapidly, because in this stage, the electrode material particles are displaced and the pore structure is filled, and the first stage is generally called the sliding stage. This stage is the one in which the density of the polar plate increases the fastest among the three stages.
Region II is the second stage: Because the pore structure of the electrode material in the first stage is filled, the density of the electrode coating material has reached a fixed value, and a certain compressive deformation resistance appears during the electrode rolling in the second stage. Compared with the first stage, although the rolling force continues to increase, the density increase rate of the pole plate has decreased. From a microscopic point of view, this is because the displacement of the inner pole coating material particles at this stage has been very small, but the large displacement movement of the coating material particles has not yet begun.
The III region is divided into the third stage: when the rolling force exceeds a certain size, the density of the battery electrode sheet begins to increase with the increase of the rolling force, and then the increase rate gradually decreases. This is because when the rolling force exceeds a certain value, the displacement of the electrode coating material particles gradually begins, and the density of the electrode sheet begins to increase. When the rolling force increases to a certain value, the deformation of the electrode coating material is more severe, resulting in work hardening. If the rolling force continues to increase at this time, the further deformation of the electrode coating material is difficult. Therefore, as the rolling force continues to increase, the density of the pole sheet does not increase much, and the increase rate also decreases.
Two, roll control
The basic mechanism of battery electrode rolling: battery electrode rolling belongs to powder rolling, and its purpose is to improve the compaction density and uniformity of the active substance of the battery electrode, improve the adhesion of the active substance, and improve the surface roughness. The rolling process follows the law of constant weight.
Vertical compaction and longitudinal extension: in the rolling process, the pressure of the two rolls on the battery pole is actually the resultant force of vertical pressure and horizontal pressure, and its size depends on the compression amount of the active material of the pole sheet and the Angle of roll bite. Under the premise of a certain amount of active material compression, the vertical pressure and horizontal pressure depend on the bite Angle of the two rolls. If the bite Angle is large, the horizontal pressure is large, and the bite Angle is small, the vertical pressure is large. The compaction density depends on the vertical pressure and the longitudinal elongation depends on the horizontal pressure.
Polar plate compaction density uniformity and surface roughness: Assuming that the coating thickness of the electrode sheet is uniform, the uniformity of the compaction density of the battery electrode sheet depends on the parallelism of the contact bus between the two rolls, and the influencing factors are mainly the coaxiality of the roll, the cylindricity of the roll body, the bearing accuracy, the rigidity stability of the equipment, the adjustment of the gap at both ends of the roll, and the roughness of the rolling surface of the electrode sheet depends on the particle size of the active substance and the roughness of the roll surface.
Fluid collection extension and active material particle slip: Aluminum foil or copper foil fluid collection is difficult to extend when rolled on large roll rolling equipment, but the active material bound on the fluid collection will slip under the push of horizontal pressure, and then drive the battery pole sheet fluid collection extension, and the elongation affects the flatness and conductivity of the pole sheet.
The local extension compression and internal stress of the battery electrode plate are uneven: there is an error in the thickness of the battery electrode plate coating, and there is also an error in the parallelism of the two rollers' contact bus bars. Therefore, the local compaction density of the active material on the battery electrode is not uniform, and the coexistence of local extension and peripheral compression causes the internal stress of the battery electrode plate is not uniform, and then affects the flatness of the battery electrode plate.
Compaction density, elongation and roll neck: the bite Angle of the two rolls directly affects the compaction density and elongation of the active material of the pole sheet, and the diameter of the roll body directly determines the bite Angle. If the roll diameter is large, the biting Angle is small, and if the roll diameter is small, the biting Angle is large.
Polar sheet rolling thickness rebound and rolling speed and ambient humidity: slow rolling speed will reduce the amount of elastic deformation of the active material of the polar sheet, that is, the amount of thickness rebound after rolling will be smaller. However, the fact is that when the rolling speed is increased to a certain value, the thickness rebound of the pole sheet after rolling becomes smaller, which is caused by environmental humidity. The water absorption of the active substance not only affects the surface alkalinity of the active substance, but also affects the thickness rebound.
Uneven internal stress and tension control: the process of rolling is the process of compression deformation and extension deformation. In this process, the inlet tension affects the internal stress distribution of the pole sheet, and the outlet tension affects the flatness of the pole sheet.
Deformation resistance of hot rolling and pole sheet: Generally speaking, material deformation resistance will become smaller with the increase of temperature, and the amount of plastic deformation will also increase. Hot rolling of the pole sheet also helps to reduce the surface wear of the roll. However, there is no obvious effect on the comparison of cold and hot rolling of polar sheet, which shows the complexity of influencing factors.
Fourth, the problems in the rolling process and solutions
Figure 6: gas-liquid booster pump type pole mill
Uneven sheet thickness: there are many factors that cause uneven sheet rolling thickness, such as uneven sheet coating thickness, roll coaxiality error, roll cylindricity error, roll contact bus non-parallel, roll roll axial deflection, poor rigidity stability of rolling equipment, and so on. The transverse thickness is not uniform, and the thickness of the left and right pole plates is often inconsistent during the rolling process. When the thickness of the left and right pole sheet is inconsistent, the influence of the pole sheet coating process should be eliminated first. When the thickness of the left and right pole sheet is consistent in the test, the roll pressure should be adjusted to ensure that the left and right compaction density of the pole sheet is consistent after rolling. During the rolling process, the pole plate should be tested regularly to prevent the pressure change during the rolling process. The longitudinal thickness is not uniform, and sometimes after the pole piece is rolled, the thickness of the test pole piece meets the requirements, but the thickness increases during slitting. This is the rebound phenomenon of the pole sheet, the pole sheet rebound is that there is more water inside the pole sheet, but the speed is too fast when rolling. The problem of pole plate rebound can be solved by using a hot roll process and controlling the roll speed.
Sickle bending of pole sheet: This situation is mainly caused by the two rolls contact bus is not parallel or the thickness of the pole sheet coating on both sides is not the same. Because the edge thickness is several microns or ten microns larger than the middle part, when the roll pressure acts on the pole sheet, the region with large edge thickness bears more rolling force, resulting in inconsistent transverse density of the pole sheet roll compaction, resulting in serious warping after the pole sheet roll compression, and adversely affecting the subsequent splaying process. To control warping, the key is to control the coating quality of the pole sheet. The warping of the pole sheet can be effectively reduced by controlling the surface tension, pump pressure, strip speed, roll pressure and other parameters. Of course, under the condition of meeting the design requirements.
The wave edge of the pole sheet: this situation is mainly caused by the relatively large expansion rate in the rolling process of the pole sheet. The causes are small diameter of the roll body, small tension before rolling, large compression of the thickness of the pole sheet, and convex on both sides of the pole sheet coating. When the pole sheet is in the process of rolling, the living material is squeezed between each other, and a certain pressure is applied to the copper foil and aluminum foil, which will produce a certain extension. During rolling, the part that is not coated with living material does not extend, and the pole piece with living material produces extension under the action of roll pressure. The extension does not form wavy folds on the edge of the foil strip in appearance, and the parallel wave traces are perpendicular to the direction of the foil strip movement.
Dark stripe on the surface of the pole: this situation is mainly caused by the existence of vibration on the surface of the roll, the cylindricity error of the roll body, and the front tension is small and uneven.
Curling of the pole sheet: This situation is caused by excessive elongation of the pole sheet. The main solutions are to increase the diameter of the roller body, reduce the compression amount of the pole sheet and adjust the tension before and after the pole sheet.
Pole strip breakage: this situation is mainly caused by uneven tension instability, lack of tension rapid response mechanism, and serious bulges at the edge of the pole strip coating. For example, in the coating process, if there are uneven texture phenomena such as small particles on the surface of the pole sheet, the small particles will be squeezed in the direction of the foil belt under the pressure of two rollers during roller pressing, and the soft particles can be ground into powder and then fall off, while the hard particles will squeeze the foil strip, causing the foil strip to break holes or even break. During the coating process, if the surface density of the pole plate is different, one piece will overroll and the other will underroll during the rolling process. In the process of pole strip walking, under the same tension control, part of the living matter will fall off or even break the foil in the place of insufficient roll pressure. Controlling the winding tension and preventing large particles from falling on the surface of the electrode can effectively reduce the fracture of the electrode.
The tension on both sides of the pole plate is different: this situation is mainly caused by the axis of the roll and the axis of the rod is not parallel, and the parallelism of the roller line can be adjusted to solve the problem.
Roll surface pitting: this situation is the fatigue pitting of the roll surface, mainly caused by uneven roll material and thermal treatment metallographic structure, poor fatigue strength of the roll surface, and also related to roll surface roughness.
Pole sheet rolling thickness rebound: this situation is mainly caused by large residual elastic deformation after pole sheet rolling and large ambient humidity. You can try hot rolling, slow rolling, high-speed rolling, reduce the relative humidity of the environment and other measures.
Uneven shape of the pole plate: this situation is mainly due to the uneven amount of rolling deformation of the pole plate, small and uneven tension before and after, or the thickness error of the pole plate coating.
In addition, there are some operational errors, such as scraping material when measuring the thickness of the pole plate, and human errors such as the problem point is not marked in time, which can be solved by strengthening training and raising awareness.
Five, the impact of roller press process on the battery
1. The influence of roller pressure on the processing state of the pole sheet
The ideal condition of the pole sheet after rolling is that the surface of the pole sheet is flat, the gloss is consistent under the light, the white part has no obvious wave, and the pole sheet has no large degree of warping. However, in the actual production operation proficiency, equipment operation, etc., will cause some problems. The most direct effect is to affect the pole slice, the width of the pole slice is inconsistent, and the pole slice appears burr; The result of rolling affects the winding of the electrode sheet, and serious warping will cause a large gap between the electrode sheet and the diaphragm during the winding process. After hot pressing, some multi-layer diaphragms will be superimposed, which will become the stress concentration point and affect the performance of the cell.
2. The impact of roller pressure on lithium batteries
Impact on the specific energy and specific power of the battery: according to Faraday's Law, the amount of electricity passed by the battery electrode is proportional to the mass of the active substance. Electrode rolling directly affects the compaction density of the active material and the specific energy of the battery.
Influence on battery energy density and power density: The same compaction density of the active material directly affects the energy density and power density of the battery.
Impact on battery cycle life: electrode rolling directly affects the adhesion of the active substance on the battery collector fluid, which directly affects the separation and shedding of the active substance in the battery charging and discharging process. This in turn affects the cycle life of the battery.
The effect on the internal resistance of the battery: the compaction density and the degree of shedding of the active substance on the pole greatly affect the ohmic internal resistance and electrochemical internal resistance of the battery, which directly affects the various performance of the battery.
Impact on the safety of the battery: the compaction density uniformity of the active substance on the electrode, the surface roughness caused by the rolling of the battery electrode, etc., will directly affect the negative lithium, positive copper, and sharp Angle discharge of the battery. The result is a safety accident.
6. Summary
There are many influencing factors in the production process of lithium-ion battery, after solving the process problems that may occur in each process, it will directly reduce the waste of production materials, improve the quality and efficiency of subsequent assembly, liquid injection, packaging and other processes, improve the quality and consistency of the final product, reduce production costs, and then make lithium-ion battery products have a stronger market competitiveness.
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