Today we will introduce the process after rolling - slitting, welding, gluing, and winding.
After the large electrode is rolled, in order to obtain the required size, we also need to cut the large electrode into pole strips of a certain width. At present, the commonly used equipment for industrialization is semi-automatic slitting machine and automatic slitting machine.
In order to facilitate slitting and control the length of the electrode, if used the automatic slitting machine, it is also necessary to cut the electrode before slitting - cut continuous electrodes into large pieces based on their length dimensions, and then slitting,the slitting effect is like the picture in the middle above.
Automatic slitting machine, the whole process is driven by the transmission device, only need to roll the rolled electrode on the machine, adjust the slitting knife can be used for the integrated operation of slitting and winding, no need to cut pieces, very convenient; Some colleges and universities also make some pouch batteries, due to the small number, the lack of equipment may be directly cut with a blade, the process is very simple, but often the electrode slitting is prone to burrs, resulting in the battery in the later stage of the battery is prone to the problem of battery self-discharge.
At the same time, in the process of slitting, the following problems are also prone to occur.
For example, after slitting, the electrode deviates, resulting in some electrode cutting waste, which mainly occurs in the semi-automatic slitting process. Because the semi-automatic slitting is manually controlled into the piece, the electrode is not aligned with the baffle or the edge baffle is crooked, which may cause the electrode to deviate.
The edge of the electrode is not cut evenly, mainly because the knife is not fast, which will be easier to drop the material, so we must regularly check whether the slitting knife is sharp and whether there is rust.
Slitting wrinkles are mainly caused by the insufficient tension of the slitting process, so in the operation, the tension of slitting should be controlled. If it is a semi-automatic slitting machine, then the tension is controlled by a person - when entering the piece, the electrode should be gently pressed while entering the piece, so that the electrode is in a straight state
Here is a phenomenon: for the manual slitting equipment used in our laboratory, it is easy to appear in the "S" shape of the slitting length direction, that is, the edge of the slitting is not a straight line (this can be seen with a ruler card), which will lead to uneven winding, and the electrode will be damaged in serious cases. Teachers who use laboratory slitting equipment need to pay special attention to the following; Slitting with automatic slitting equipment will not cause this problem; Of course, the most important thing in the slitting process is to control the burrs, which may cause internal short circuits in the battery and even cause safety problems such as thermal runaway.
In view of the burr problem, here is a commonly used detection method, CCD detection, using microscope magnification to measure the burr state after the electrode is slitted, to see the position of the burr, whether it is parallel to the electrode or perpendicular to the electrode, and what is the length of the burr
Generally speaking, the length of the burr is controlled to be less than 1/2 of the separator, so that even if there is a burr, it is difficult to cause an internal short circuit due to the burr puncturing the separator.
After slitting here, the welding action will be done, generally the cathode aluminum foil is made of aluminum to nickel tabs, and the cathode copper foil is made of nickel tabs, which are welded together by an ultrasonic welding machine.
There are two problems to be avoided in this process, one is over-soldering and the other is virtual soldering. To put it simply, welding is to weld through, and the foil is welded through, resulting in insufficient welding strength, and in the later process, it is easy to fall off the foil together with the tabs; In addition, it is virtual welding, that is, the welding strength is not enough, resulting in no effective welding, the welding internal resistance is relatively large, and at the same time, the tab is also prone to fall off.
After the welding is completed, the gluing process will be carried out, but this tape is a special tape that needs to withstand the electrolyte soaking. When it comes to special adhesives, I actually want to tell you that the glue here is different from the general tape: first, there will be no other components in it that are soluble in the electrolyte, and second, this green glue can still be viscous after being soaked in the electrolyte.
There are several main purposes of gluing, one is that the position after welding needs to be glued, because the welding place is often relatively deep, and the surface is rough, in the subsequent shaping process, it may lead to puncture the separator, and produce a short circuit, in addition, the edge of the electrode material area will be pasted with green glue to prevent material dropping. Of course, sometimes this green glue is also used to control the paint area in the length direction to ensure that the overhang is sufficient during winding.
The picture on the left is the correct way to apply glue in the material area. If you look closely, you can see that the glue on the right does not cover all the coating area and the foil separation interface, which is not standardized.
This is a winding overall situation, and after winding it out, we call it a bare cell. In fact, it is to roll the electrode through the needle and roll it into a bare cell of a certain width.
The main attention here is to control the coverage area, generally the separator should completely cover the anode electrode, whether it is width or length, and the anode electrode should completely cover the cathode electrode. In this way, it can effectively ensure that all the lithium ions coming from the cathode electrode can be received by the anode electrode, so that there will be no lithium precipitation, which will eventually lead to potential safety hazards.
Before winding, we need to do some inspection of the electrode, generally there is an appearance like the following electrode, it will be judged as unqualified, such as the electrode is damaged, the anode electrode is dropped and other problems, these are safety risks, and the electrode needs to be picked out.
These are all relatively easy to encounter problems, or according to the previous principle - the separator should cover the anode electrode, and the anode electrode should cover the cathode electrode to ensure overhang. In the figure on the right, because the end of the anode electrode does not effectively fall on the position of the cathode electrode glue, there is no way to ensure the burr problem at the cutting edge of the anode electrode, so it is also unqualified.
Finally, we can take a look at the related processes of stacked.
The principle of stack is the same as that of the winding process, but the main difference is in the electrode. The electrode here does not need to be slitted, but cut into small pieces according to the size requirements, and then welded and glued in the same way, and then control the alignment to ensure the coverage of the separator and the anode electrode, and finally encapsulate If you need to stack several layers of electrodes, you can also do the stack first, and then weld the tabs, and the same kind of electrodes can share one tab; In the lower right corner of the picture, the electrodes are cut into small pieces one by one, and a continuous separator is used to laminate the pieces in a Z-shaped way.
