Lithium-ion Full Cell Manufacturing Process Training--Separator section

Today, we will mainly introduce lithium-ion battery separators.

The first is the role of the separator, which is relatively simple, the separator needs characteristics such as electronic insulation, ion conduction, etc., so that lithium ions can be transmitted freely. So what kind of material can be used as a separator material?

The first is the insulation performance, which is easy to understand, if the separator is not electronic insulation, it will be short-circuited, and it will definitely not be used. The second is actually ion conduction, so the separator is required to be porous, and there are requirements for pore size and porosity, IIf the pore size is too large, the cathode?and anode?electrodes may be short-circuited due to powder loss or foreign bodies; If the pore size is too small, it is also bad for ion transport.

In terms of porosity, it also affects ion transport and the retention capacity of the electrolyte. If the pore size of the separator is too small and the retention capacity is insufficient, the internal resistance of the battery will be higher and the long-term cycling performance will be poor. For separators, as batteries require long-term use, the lifespan of power batteries may be more than 5 years. Therefore, it is necessary to ensure stable chemical and electrochemical performance throughout the lifecycle, without any side reactions.

The wettability of the electrolyte on the separator indicates whether the electrolyte can smoothly enter the pores of the separator, if the separator and the electrolyte are not wetted, the electrolyte cannot penetrate into the pores of the separator, and it cannot play the role of conducting ions. On the other hand, when studying electrolytes, the wettability of the electrolyte to the separator is also a very important indicator.

Mechanical performance, which should be easy to understand, means having a certain strength. It cannot be said that the separator is easily torn or damaged during the battery manufacturing process. In addition, this is mainly aimed at the safety performance of the battery, especially for power batteries, which often experience expansion, compression, and even puncture in practical applications. If the mechanical performance is too poor, it may not effectively protect the cathode?and anode?electrodes, leading to short circuits and safety issues.

Thermal stability mainly refers to the thermal shrinkage of the separator, that is, under high temperature conditions, the shrinkage of the separator?should be controlled within a certain range, otherwise the temperature rises, and the separator?shrinks, which leads to the inability to completely cover the cathode?and anode?electrodes.If there is a short circuit between the cathode?and anode?electrodes, safety problems such as self-discharge and even thermal runaway can occur.

What are the performance parameters of the general evaluation of the separator? Here's a tidy-up.

It is basically related to the requirements of the previous separator, and the general battery factory will carry out relevant incoming testing before the separator arrives to see whether the separator meets the requirements.

There are many types of separators, and the main commercial applications of lithium-ion batteries are PE and PP separators. Lithium-sulfur batteries are also a kind of lithium-ion batteries, and commercial separators can also be used, but many times when teachers in colleges and universities do research, in order to reduce other interference, they will deliberately select some materials to reduce interference. For example, if you are worried about the problem of lithium dendrites for lithium-sulfur batteries and lithium metal batteries, you may be able to use thicker PE and PP separators, and even glass fiber separators may be used.

In order to reduce the thermal shrinkage of the separator at high temperature, some research groups not only use ceramic separators to improve thermal stability, but also consider using some materials with higher melting points, such as PI and PET. aramid separator, etc., so that there will be no heat shrinkage at high temperatures.

At present, the relatively hot solid-state electrolyte studied by colleges and universities, in fact, from the perspective of the future, can also be regarded as a new type of separator, which not only plays the role of electrolyte, but also can be used as a separator to block the short circuit of the cathode?and anodee electrodes. Therefore, once the solid electrolyte is commercialized in the future, the current PE and PP separators will not be needed, of course,this time is still quite long.

At present, the commercial separator is made of PE and PP, and the obvious difference between the two can be seen from the SEM image of the separator.

?Dry PP separator is to peel off the crystalline surface of the film by stretching to obtain pores; The wet method is obtained by adding a second phase and then extracting and separating, so the pore size distribution is very uniform.

Comparing these two processes, the cost of dry PP is more advantageous than PE, so now due to the cancellation of subsidies for power batteries, many battery factories have begun to turn to PP separators. However, in the consumer category, it is basically the world of PE separator, because PE separator can be made relatively thin, the thinnest is currently 5um, and the thinnest PP is 12um, and it is not so stable. Of course, some price-sensitive products will also consider PP separators.

From other perspectives, the melting point of PE separator is about 130 °C, while the melting point of PP separator is about 160 °C, and the melting point of PP will be higher, but the puncture resistance of PP separator is significantly lower than that of PE separator, and the current separator is made of ceramic treatment, so on the whole, the quality and performance of PE separator are better than PP separator.

Therefore, we?have mentioned some of the advantages of PE and PP separators, what if they are combined?

This is Celgard's very successful product, PP/PE/PP three-layer composite separator. Sandwich structure – with a PE layer in the middle, two endothermic peaks are also clearly visible from the DSC.

This kind of separator is a good combination of the advantages of the two, a few years ago was the standard configuration of the power battery factory, although the price is very high, but still to buy, but due to the popularity of ceramic separator now, the safety characteristics of this separator have no obvious advantages.

The schematic diagram of the ceramic separator mentioned earlier is as shown above. By combining inorganic coatings such as aluminum oxide with organic separators, the thermal shrinkage of the separator is effectively reduced, the puncture strength of the separator is improved, and the safety performance of the battery is significantly enhanced. Ceramic separators?have other advantages, such as improving the wetting effect of the separator, allowing for rapid wetting, enhancing electrolyte retention performance, and prolonging service life.

In addition to the ceramic separator, the glued separator in the industry is also widely used, and here is an example, such as the deformation of the battery in the upper left corner. Due to the fact that the energy density of batteries, especially mobile phone batteries, is getting higher and higher, the expansion of materials is relatively large, and the application of materials such as silicon carbon also leads to the deformation of batteries. If a layer of polymer glue is applied to the surface of the separator, such as PVDF and PMMA, so that the cathode electrode and the separator, the separator and the anode electrode can be bonded together, and the whole battery becomes a whole piece, it can very effectively inhibit the expansion of the charging and discharging process. The following two pictures show that the battery is disassembled and the separator can be effectively bonded to the cathode and anode electrodes.

At?present, power batteries are also used in a large number of applications, because the cycle requirements of power batteries are very high, after long-term cycle expansion, the interface will become worse, if the glued separator is used, the stability of the interface can be maintained for a long time, and the increase of internal resistance can be reduced, so as to improve the life.

Briefly introduce some new separators, we?also mentioned cellulose separators?before, this is used in supercapacitors, of course, now there are also lithium battery applications, low cost, high porosity, good rate performance. We testing the DSC before, and the thermal stability is also good.

The following is some new technology, we mentioned the tortuosity before, that is, if the pores in the separator?are curved, the conduction path of lithium ions will be relatively long, so some people have thought of a way to improve the straight hole, and directly change the EMU to high-speed rail.

This is the PET separator, if you look at the DSC on the right, the melting point of the PET separator is about 250 degrees, and the PP is 160 degrees, so the thermal stability is greatly improved. However, the big problem of non-woven separator?is that the strength is too low, and the pores are too large, although the rate is good, but there are many pores, and they are very large, so the self-discharge is very large, so the structure on the right can be seen, this product will be combined with ceramics to fill the pores, so as to improve. Of course, we have also developed this kind of separator, but there is a big problem with non-woven separators, the thickness is difficult to make thin, and 16um is the limit. Because the strength of the separator?is not enough, it is easy to destroy during processing, and in addition, the cost is relatively high.

Finally, let's talk about the two development directions of the separator?- low cost and functionalization. Fortunately, in recent years, domestic separator?manufacturers have developed rapidly, after equipping Japan's advanced equipment, the quality of the separator?has been improved.?Functionalization is easy to understand, that is, it is developed according to the application, and if you want to have high energy density, the separator should be thin, reduce the thickness of the battery, and leave room for the filling of the active material, so now the thinnest is 5um. If you want a high magnification, then increase the porosity; If it is to be safe, it can be coated with ceramic and used in a three-layer composite. Separators with higher melting points can be used, such as PET, PI, aramid, etc.