Today, we will mainly introduce the three processes of baking, liquid injection and standing of the whole battery, first of all, we will introduce baking.
Of course, the main purpose of baking is to remove moisture from the bare cells. This moisture is mainly the moisture in the cathode material, the anode material and the separator. So why remove water, here are three main examples of influencing factors. Of course, as we will talk about later, if the moisture is not well controlled, it will also affect the electrolyte injected in the injection process, resulting in the first problem as shown in the figure above, the HF content in the electrolyte will increase. So at what level to control, let's look down.
Because there are many chemical systems, we cite a commercially mature material system here, lithium cobalt oxide + graphite anode. Water content tested using the Karl Fischer method. The overall water content needs to be controlled at 250 ppm. However, because there are many types of materials, when we use lithium iron phosphate and lithium titanate in the figure, the overall control level is about 400ppm because the BET of the material is too large and the water absorption is relatively strong. In fact, graphite itself is hydrophobic, but because it is an aqueous slurry and the cathoe electrode is an oil-based NMP system, the water content of the anode electrode is high. The picture in the bottom left corner of the image above is meant to illustrate that as the baking time increases, the water content can decrease, but it will reach a stable level. How to control the baking?
Here are three examples of factors that affect baking: temperature, vacuum and time. The temperature is relatively easy to understand, and increasing the temperature can help the volatilization of water, but because the heat resistance of the separator is poor, the temperature is not to be too high, and as you can see from the table below, the relationship between the vacuum degree and the boiling point of water. By increasing the vacuum, the boiling point can be effectively reduced, so as to achieve the effect of rapid baking and water removal, which is also a development direction of the current oven.
Next, let's look at the way of baking, which is heat transfer. There are also three influencing factors: heat conduction, heat radiation and heat convection. Our conventional oven baking time is relatively long, because under vacuum, only heat radiation conducts heat, and now the better baking is to use a contact oven.
In addition to the heat radiation, heat conduction can also be carried out through the contact hot plate, the heating speed is much faster, and the overall baking time is also greatly shortened.
We said that the baking is to remove moisture, and also talked about some control standards, have you ever thought about how to test the moisture? So let's start with the moisture test method.
At present, the Karl Fischer moisture test method is commonly used internationally for trace moisture. The basic mainstream is to use METTLER and Metrohm testers, so everyone's test results can be compared side-by-side. The simple thing is to heat the electrode to let the water run out, and then send it to the reaction pool through the carrier gas, the water reacts with the iodine element, and the moisture content is calculated by detecting the change of the iodine element.
This is a supplement, it can be seen that water and iodine are 1mol of the reaction amount, and iodine is produced from iodine ions through electrolysis, so in the end, the amount of iodine can be known through the record of electricity, and the amount of iodine is corresponding to water, so the relationship between electricity and water content is obtained.
After the battery is baked, it is cooled to room temperature and filled. The electrolyte on the side of Canrd has strict quality requirements, because as mentioned earlier, moisture has an impact on the performance of the battery. So basically the electrolyte needs to be configured in the glove box, and the standard of moisture is <20ppm. How much electrolyte needs to be injected into the injection, and how to determine the amount of injection, let's continue to see.
First, need to do some calculations to get the minimum amount of liquid you need to fill theoretically, then make some adjustments according to the customer's needs, and finally, you need to do some experience with some margins. So it is a value of calculation + experience, this value is related to materials, design, and application, and can only teach you one method. Taking pouch cells as an example, the electrolyte should be soaked in the bare cell, so as to ensure that the entire electrolyte is fully wetted. The process of liquid injection, because there are bare cells and electrolyte, must be completed in a low-humidity room, which is very important.
The last one is the standing process. Different from colleges and universities, the industry will use vacuuming, the main purpose of which is to extract the air in the aluminum-plastic film bag on the one hand, so that the electrolyte is in contact with the electrode, and on the other hand, to remove the air in the pores of the electrode, so that the electrolyte can be quickly infiltrated. At the same time, because the bare cells are in rolls, there is pressure between the layers, so it is also necessary to achieve full infiltration through high temperature and long time, generally in 36h. The coin cell in universities is usually a single unit with sufficient electrolyte and low compaction density, so the soaking time is usually only a few hours to complete.
We mentioned earlier that we need to control moisture, so what will be the impact if the moisture exceeds the standard? There are two main examples here, on the one hand, hydrogen will be generated, which consumes lithium ions, affecting efficiency and destroying the interface, and on the other hand, it will react with the components of SEI to continue to consume lithium ions, destroy SEI, and increase the internal resistance of the battery. As mentioned earlier, the increase of moisture will also lead to an increase in the HF content in the electrolyte, so what will the HF have on the battery?
There are three main effects of HF. On the one hand, the generated LiF is a component of SEI, and the internal resistance will increase, in addition, it will dissolve SEI, resulting in the stability and compactness of SEI being affected, and the cycle will deteriorate, and finally, for some cathode materials, it will cause metal ions to dissolve, resulting in deterioration of performance. Therefore, the message of today's course is to strictly control the moisture in the material and electrolyte, so as to get a battery with excellent performance.
