Lithium ion batteries are lightweight, high density power sources for many different devices, like mobile phones, laptops, electric cars and even aircraft. Polymer separators, similar to regular battery separators, are used in Lithium ion batteries to separate the anode and cathode while also enabling the movement of ions through the cell. These multilayered separators are usually composed of one or more polyethylene layers and at least one polypropylene layer. N-Methyl-2-pyrrolidone (NMP) is a chemical base required in the production of lithium batteries. The steam produced during production is toxic and rapidly flammable. The use of NMP is associated with high costs. Recycling reduces the costs as well as the harmfulness.
Monitor the manufacturing process of separators
Advantages of lithium batteries are a high energy density, a high cell voltage as well as a wide temperature range. Lithium is used as active material in the negative electrode. Because of its standard potential of around -3.05 V and the resulting high cell voltage, lithium is the best possible negative electrode material for batteries.
However, the production of lithium batteries is associated with many dangers.
For the production of lithium batteries, iron rods are fed into an NMP and PVDF solution in order to bind lithium ions to the rod. NMP is used as a solvent to dissolve PVDF, so PVDF can coat the positive rod. At the end the iron rods are put into a drying oven, where the NMP evaporates.
NMP is volatile from a temperature of 203°C and is very harmful to the environment and health. In addition, NMP is rapidly inflammable. Therefore, the steam is diverted and collected in a special container, names recovery tower. Water is added to this steam. In the container, the hot NMP steam mixes with the cold water, the concentration of NMP in the air decreases and the concentration in the water rises. The NMP changes its aggregate state and becomes liquid and is no longer highly inflammable. The water gets lukewarm. It is important to keep in mind that the temperature of the water should not increase, otherwise you would have hot steam with a high NMP concentration again.
In-line refractometers by Schmidt + Haensch are the perfect tool to monitor the manufacturing process of those separators and to maintain highest quality standards.