Daikin Fluorochemicals high-performance materials for lithium-ion batteries, 90 years of professional research and development of fluororesins brings advanced solutions

Whether it is the mobile phones and computers that we cannot live without in our daily lives, or the increasingly popular camcorders and cameras, lithium batteries have become a vital part of these digital devices due to their small size, light weight, and large capacity. In recent years, as the use of lithium-ion batteries in automobiles (EV, HEV, PHEV, etc. = xEV), power storage, industrial machinery, airplanes, trains, etc. has continued to expand, the size of the batteries has also become larger.

From the first product launched into the market in 1991 to its wide application in many fields of social construction, science and technology, and people's livelihood, the development of lithium batteries in the past 30 years can be said to be mainly centered on the following three aspects: 1. Improving capacity and power density; 2. Reducing costs; 3. Maintaining and improving safety. It is worth mentioning that in the process of advanced development based on the above characteristics, "high-capacity batteries" have become a priority in the development of lithium-ion battery categories. After years of dedicated research and development, the superior performance of today's large-capacity batteries has been achieved.

The industry calls for a better solution: fluorine chemical materials in lithium-ion batteries

Fluorine compounds and fluorine resins are generally known for their high chemical stability. Many fluorine materials are also used in the manufacture of lithium-ion batteries, especially electrolyte binders. The reason why lithium-ion battery materials can produce high-performance batteries is that fluorine materials have made an indelible contribution.

The voltage of lithium-ion batteries can be as high as 4.6V. In order to stably maintain the performance of materials under such a high oxidation potential of the positive electrode, only fluorine compounds can do it. Moreover, the electrolyte or binder used in the positive electrode will be exposed to a strong oxidizing environment, so the material stability requirements are extremely high.

As an advanced company with 90 years of experience in fluorine chemistry research, Daikin Fluoro Chemicals has developed binders, electrolyte additives and solvents, CNT composite binder dispersions, sealing ring materials and other products for lithium-ion batteries, contributing to improving the performance and safety of lithium-ion batteries.

The manufacturing process of general lithium-ion batteries is divided into the "front process" before making electrodes and the "back process" of assembling battery cells and forming the battery shape. The electrode production in the front process adopts the coating method, and it is double-sided coating - the active material and conductive agent are dispersed in a solvent with a binder dissolved to make ink (slurry), and then the ink is applied to the metal foil collector, and after drying, it is rolled to form the electrode.

Among them, the positive electrode binder is generally made of PVdF material, and the solvent is the organic compound NMP (N-methyl-2-pyrrolidone). Because it is an organic solvent, improper disposal will cause environmental pollution. In addition, the organic solvent is a flammable solvent, and the factory building needs to adopt explosion-proof design specifications, which places extremely high requirements on factory safety construction and poses certain safety hazards.

In addition, the boiling point of NMP is about 204°C, and the drying temperature is 120-130°C, which is relatively high. Therefore, a large amount of energy is consumed in the drying process, resulting in a large amount of CO2 emissions.

In addition to the safety hazards caused by combustibles and the negative environmental impact caused by high energy consumption, NMP itself has a huge cost problem. Therefore, NMP cannot be consumed or discarded by incineration or other methods, and must be "recycled". NMP recycling equipment and recycling costs have also become a burden on manufacturing.

As for the negative electrode, water is usually used as a solvent, but the latent heat of vaporization of water is large, resulting in excessive energy consumption during the drying process. Considering the CO2 emissions and cost issues, there are also calls in the industry to use other methods.

Figure 1. Traditional electrode manufacturing process (wet process)

Lithium-ion battery dry process, Daikin Fluorochemicals provides innovative positive and negative electrode binder product solutions

So how to solve the problems arising from the manufacturing process of the positive and negative electrodes mentioned above?

One of the methods is to use a dry process. This method does not use a coating method to make electrodes, nor does it use liquids such as water and organic solvents. Therefore, there is no need to dry and recover the solvent. Only powders such as active materials, conductive agents, and binders are mixed to make a mixed powder for the electrode, and then a thin sheet is formed by some method to make the electrode.

The advantage of the dry process is that it does not require drying and recycling steps, but on the other hand, it becomes difficult to form thin electrode layers, which is relatively easy with the conventional wet process. It requires very high technical requirements to make the electrode material uniform and to make it extremely thin.

There are two types of dry processes currently being discussed in the industry. One method is to mix thermoplastic resin binder powder with active materials and conductive agents, heat to melt the binder resin, and use the molten binder to bond the active materials and conductive agents. Another method is to use PTFE (polytetrafluoroethylene) as a binder resin, apply appropriate shear force for mixing, and make the PTFE thin (fibrous) as a binder, and thin the electrode thickness by rolling and other methods.

For both dry process methods, there are binders suitable for the respective processing methods:

Thermoplastic resin binder

The current liquid lithium-ion battery, the positive electrode binder is almost all PVdF materials. This is because PVdF has electrochemical stability, even at very high oxidation potential will not decompose, it is used as a practical standard in lithium-ion batteries. Therefore, the dry process using thermoplastic resin as a binder is mainly developed and utilized PVdF material powder.

PTFE Binder

Another dry process is to fiberize PTFE under the action of shear force. These fine fibers are entangled with active materials and conductive agents to form blocks, which are then rolled into sheets to prepare electrodes. Of course, for the binder, it is not only expected to achieve fiberization through shear force, but also required that the binder has the necessary antioxidant properties on this basis.

Figure 2. Electrode manufacturing process (dry process using PTFE binder)

PTFE can not only be fibrous under shear force, but also has very high antioxidant properties. Therefore, under this method, PTFE is a material that is very likely to be used as a dry process electrode binder for the positive electrode.

However, the biggest problem with PTFE is that it will be reduced at the reduction potential of the negative electrode and may not be able to function as a binder for a long time. Therefore, it is not recommended to use it at the negative electrode. In this case, different binders need to be used for the positive and negative electrodes.

Daikin Fluorochemicals uses its fluororesin technology, especially PTFE design and synthesis technology, to promote the development of PTFE that can form membranes with less addition. As Daikin Fluorochemicals has emulsion polymerization technology and PVdF materials, it has the basic conditions for developing binders for both positive and negative electrodes, and can propose more advanced product proposals.

Based on the professional technology and market experience accumulated over many years in the field of fluorine chemistry, Daikin Fluoro Chemicals has always focused on the cutting-edge trends and needs of the industry, and played an important role in improving the performance and environmental protection of advanced lithium-ion batteries. In the future, it will continue to make greater contributions to social development through the development of fluorine materials.