As the world becomes more electrified and more energy storage options (i.e., new and improved batteries) are needed, the ability to reduce solvent use and reduce solvent waste during the battery manufacturing process will become increasingly important. Twin-screw extrusion can be a key to this development.
The current predominant means of battery electrode manufacturing involves the use of planetary batch mixers. Resembling countertop stand mixers you might find in a bakery, only much larger, planetary batch mixers are used to stir the lithium metal compounds, additives and binders that comprise the electrode slurry. The process of batch mixing requires the use of solvents in order to disperse the ingredients evenly and keep them under control while the mixing happens.
Although solvents are necessary for batch mixing, they do bring along a few negatives. For one thing, the most commonly used solvent, NMP (N-methyl-2-pyrrolidone) is an irritant and potentially toxic to humans. But the main issue for manufacturing is cost: A battery electrode slurry made in a batch mixer needs as much as 45% of the material to be solvent, which is not desired in the final product. The solvent needs to be evaporated away before the electrode slurries are actually usable. This requires the use of large drying lines that can be 5 to 10 meters long and 2 meters tall, as illustrated in figure 1. The energy used to remove and recycle the solvent can account for as much as 20% of the cost of cathode slurry production. If you could cut down on the use of a toxic substance and lower your production costs, why wouldn’t you?
Twin-screw extrusion for electrode slurry preparation
This is where twin-screw extrusion can really shine. Twin-screw compounding, when applied to batteries has the ability to mix electrode material with binders and additives while using significantly less solvent than batch mixers require, or even no solvent at all. A controlled dose of solvent can be fed into the extruder. This allows for a much more precise formulation, with reduced solvent waste. The mixed slurry, which has a thick paste-like consistency, is then extruded through a sheet die. This sheet of electrode material can be directly guided into a series of hard pressure rollers, known as a calender, to compress it. (See figure 2.)
The result is a thin ribbon of free-standing electrode material, pulled directly from the twin-screw extruder, as shown in figure 3.
A no-solvent process can be used in an electrode formulation where dry granules of cathode-active material are produced. In a dry granulation with a polymeric binder, a pre-blend of cathode materials is fed into the twin-screw extruder along with the binding component. Inside, the binder is dispersed and sheared to form a network surrounding the small particles of cathode-active material. This is only possible because of the mechanical shear utilized by the intermeshed parallel screws within the barrel of the extruder. The extruder is equipped with a discharge port that ejects self-formed granules without compaction. The extruded dry cathode formulate can be fed directly into a roller press and then calendered onto the current collector foil, with no drying step required.
Reduce solvent waste, save space, lower expenses
Producing electrodes without solvent, or with greatly reduced solvent levels, obviously saves on costs that would regularly be required for the drying step. The space saved by not needing all those batch mixers and drying lines can be significant too.
Applications of twin-screw extrusion in electrode manufacturing can reduce the solvent consumption in electrode paste and thus reduce energy consumption and other expenses. If these methods of low- or no-solvent electrode production catch on, twin-screw extrusion could become the talk of the industry.
References and Resources:
- Watch the Webinar: Twin Screw Extrusion in Battery Manufacturing and Research
- Wiegmann, Eike, Arno Kwade, and Wolfgang Haselrieder. “Solvent Reduced Extrusion‐Based Anode Production Process Integrating Granulate Coating, Drying, and Calendering.” Energy Technology (2022): 2200020.
- Astafyeva, Ksenia, et al. “High Energy Li‐Ion Electrodes Prepared via a Solventless Melt Process.” Batteries & Supercaps 3.4 (2020): 341-343.
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