Twin-screw extrusion provides an appealing alternative to production-scale batch mixers in the manufacture of battery electrode slurries. The continuous manufacturing process of a twin-screw extruder allows it to deliver an output equivalent to that of several batch mixers while taking up much less space. Beyond the advantages the extrusion process brings thanks to its continuous manufacturing abilities, the relatively small dimensions of lab-scale extruders — comparable in size to small lab dissolvers or centrifugal mixers — means they can be incorporated into much tighter spaces. One specific small space that could be of great use to electrode slurry research work is a glove box.
A glove box is a stable, sealed container used to manipulate materials where a separate atmosphere is desired. Picture a metal-framed glass box with invertible gloves mounted on one face: scientists can slide their hands into the gloves and reach into the interior of the glove box without ever opening the box or exposing its contents to outside air (or exposing the researcher to what’s in the box!). An attached antechamber that can be purged of air and then filled with an inert gas like argon allows for the transfer of items into or out of the main chamber. In particular, the glove box minimizes the introduction of humidity and oxygen into the box, while also preventing the escape of any into the surrounding room. Airborne matter can be controlled, as can the temperature, humidity, lighting, and climatic conditions of the box.
Lithium-ion battery slurry preparation
To understand why a twin-screw extruder inside a glove box has such great potential to help with battery slurry preparation and research, it helps to understand a little about lithium-ion battery chemistry and the battery manufacturing process. A lithium-ion battery’s charge transfer capabilities depend upon the quality of the electrode slurries. These slurries are a mixture of active conductive materials and various binders, additives, and solvent. For the anode, the key active ingredients are graphite and carbon black. While conductive to charge, these chemicals are not especially reactive or dangerous. But for the cathode, the major slurry component is a lithium mixed metal oxide (e.g., LiCoO2 or LiFePO4), which is one of the more readily reactive compounds known. This reacts with nitrogen, oxygen, and water — three significant components of our atmosphere. In fact, many lithium-ion cathode materials fall into the category of highly sensitive and reactive materials. Thus it is best if research and testing on these substances are done under a controlled atmosphere, such as within a glove box filled with argon.
While battery slurries are conventionally produced in large vessels called planetary batch mixers, the twin-screw extruder provides many advantages over the batch method. The traditional process involves mixing the various cathode slurry constituents for anywhere from four to eight hours, with a toxic solvent (NMP) making up as much as 45% of the mixture. Besides the risk of batch-to-batch variations, the labor-intensive process, and the need for production downtimes to collect & test the product and clean the equipment, a lot of physical space is required to hold the batch mixers. Extruder technology is scalable, so R&D can be performed on a small scale and then eventually expanded as needed. A twin-screw extruder can even perform a controlled mix of dry granules, far less solvent is needed than in batch processing.
Controlled Atmosphere Glove Box
An elegant solution to this challenge of performing small-scale research on air-sensitive electrode slurries is to combine a benchtop extruder with a discrete, controlled environment. Some extruders are small enough to fit into a glove box, which means that the extruder-inside-the-glove-box setup can process slurry under an inert atmosphere (e.g., argon). The small size of the overall setup also means that costs are reduced, because lower amounts of chemicals are used and less waste is generated.
Another potential benefit to this setup is that characterization of materials is possible inside the glove box. A typical rotational rheometer is small enough to fit into the glove box and be deployed alongside the twin-screw extruder. This allows for the testing of extruded slurry materials without the risk of contamination or adverse interactions with external atmosphere.
It is possible to produce and test a battery slurry in a glove box that takes up minimal space and uses fewer resources. Not only does the potential for lower solvent use make this a safer option for slurry manufacturing, the isolated chamber of the glove box lessens risk even further. The combination of a twin-screw extruder installed inside a glove box makes for a safe, convenient method of preparing sensitive battery slurries.
Additional resources:
- To see an example of a twin-screw extruder inside a glove box, watch this video: GLOVEBOX Video Energy 11 PROTECT integrated twin-screw extrusion glovebox for electrode slurry development. Also available on YouTube.
- Read this application note: Continuous twin-screw compounding of battery slurries in a confined space
- Explore Battery Manufacturing Enhanced with Extrusion and Rheology
Perfect and very informative topic,