Lithium-ion battery dry process report

Technology Sharing | A&S POWER | Sep 01, 2023

Recently, dry process has become a hot topic in the industry. This process refers to a method that does not use solvents or water in the production of electrodes for lithium-ion batteries. Here is a brief explanation of the reasons why this dry process is highly anticipated, as well as the technology and related environment required to introduce this process.

Lithium-ion battery manufacturing process and its problems


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

The positive electrode uses PVdF as the binder, and the solvent uses NMP (N-methyl-2-pyrrolidone) organic solvent. This organic solvent is a flammable solvent, so the factory buildings need to adopt explosion-proof design specifications. 

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

In addition to combustibles and environmental impact, the cost of NMP itself is also a huge issue. Therefore, NMP cannot be consumed or discarded through incineration, etc., and must be "recycled". NMP recycling equipment and recycling costs have also become a burden on manufacturing.

At the negative electrode, due to the large latent heat of vaporization of water, the energy consumption of drying is unexpectedly large. Considering the emission and cost of CO2, there are also calls in the industry to switch to other methods.


Dry processes as a solution


The disadvantages of using NMP as an organic solvent were introduced earlier, among which "removal of organic solvents" has attracted people's attention. As a method that does not use organic solvents, a request was made in the early stages of "Can a positive electrode slurry be formed in an aqueous system?" However, in the process of promoting the high performance of positive electrode active materials, the active materials themselves have become unsuitable for contact with water (the performance will decrease if exposed to moisture for a long time), and the process of using water-based slurry for positive electrodes has not formed a big trend until now. 

Therefore, one of the methods that comes to mind is the dry process. This method does not use coating methods to make electrodes, nor does it use liquids such as water and organic solvents. Therefore, drying and recycling of the solvent is not required. The electrode is made from a mixed powder of only active material, conductive agent and binder, which is then formed into flakes using a certain method.

The advantage of the dry process is that it does not require drying and recycling processes, but on the other hand, it becomes difficult to form thin electrode layers that were relatively easy before. Making the electrode material uniform and making it an extremely thin electrode requires very high technical requirements. The thickness of the material layer of the current electrode is about 50-100 μm (single side and excluding metal foil current collector). To form active materials into sheets of this thickness, conventional methods are not feasible. However, the aforementioned coating method (wet method) can be said to be a very good solution for making thin electrodes.

The dry processes discussed so far are roughly divided into two types. 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 active materials and conductive agents. Another method is to use PTFE (polytetrafluoroethylene) as a binder resin, apply appropriate shear force for kneading, and make the PTFE finer (fiberized) as a binder, and then roll it or other methods to make it slender. The thickness becomes thinner.