As the modern world continues to embrace electrification, lithium-ion batteries are becoming increasingly important. Building better batteries, more efficiently, would lower costs and help move us towards a battery-powered future. A recent development in this field is the application of continuous mixing of battery electrode slurries, which holds several advantages over the more common traditional method of batch mixing. If you can start with a better slurry, you can build a better battery.
Electrode slurry preparation
To understand how twin-screw extrusion improves the electrode slurry preparation process, it is important to know a little about battery chemistry (see the insert box) and the current predominant method in the battery manufacturing process, batch mixing. The batch method makes use of large planetary mixers, which look a bit like oversized countertop stand mixers that might be used for preparing cake batter. The slurry ingredients—a combination of active materials, solvents, binders, and additives—are combined into the large vessel and then stirred until homogeneity is achieved. This typically takes anywhere from four to eight hours. And during this time, the ability to monitor the potential resulting slurry is limited because the components are constantly undergoing the mixing process.
After the batch has been mixed, the slurries need to be coated onto a metal foil and dried before they can be incorporated into a battery. A slurry will be passed through a slot die and coated onto the electrode’s current collector foil and dried in long drying lines. The conventional coating process requires low-viscosity slurries with a content of at least 45% solvent in order to achieve the desired working consistency. (It is worth noting that the most common solvent used, NMP, is both toxic and expensive.) That solvent then needs to be evaporated away before the electrode coating—the slurry on metal foil—is usable in a battery. To achieve this evaporation, the electrode coating is passed through a drying line that is often ten to twelve meters long by a couple meters tall, thus necessitating a large workspace and lots of power. This drying process alone accounts for as much as 20% of the total energy required for cathode production.
Just as a good cake texture depends on getting the dry lumps of batter broken up and mixed, the quality of a battery depends heavily on the consistency of the electrode slurry. If the electrode slurry is poorly mixed, the conductive additive will not disperse well enough. Assembling a battery without properly dispersing the active material and the additives will concentrate the charge, which is undesirable because it causes non-uniform battery reactions; batteries need homogenous slurries for efficient charge transfer.
Extrusion manufacturing for batteries
Twin-screw extrusion is an appealing alternative to batch mixing. Twin-screw extrusion allows for continuous mixing, with a processing time on the order of minutes rather than hours. The twin-screw extrusion manufacturing process can be monitored in real time, which provides an element of control not available in batch mixing.
A twin-screw extruder is comprised of two Archimedes screws rotating within a cylindrical barrel. Material is fed into the screws via a port, and then that material is sheared between the screws and the barrel wall as the screw threads rotate and move the material along. The material can also be melted with heat or dispersed using solvents as it works its way down the barrel, before it is eventually compressed and pushed through a die to shape the extruded product. The screw speed, feed rate, and temperatures of various zones of the system can all be controlled.
It is this ability to control the constant processing that makes battery slurry production via extrusion manufacturing for batteries so appealing.
Batch vs continuous manufacturing
In electrode manufacturing, twin-screw extruders can be used for continuous cathode or anode slurry mixing. Although planetary mixers used in batch mode are the conventional manufacturing method, when one compares batch vs continuous manufacturing, it becomes evident that twin-screw extruders produce multiple advantages.
The simple process of a continuous feed of base materials provides immediate advantages in terms of consistency. With a steady input of measured ingredients, there is no variation from batch to batch, and reproducibility is greatly increased.
Since it only takes a few minutes for the materials to make their way all the way through the extruder, the residence time is significantly shorter than the multiple hours required for batch manufacturing. This reduces the chance for impurities to make their way into the system.
As the electrode slurry components process down the extruder barrel, the shear force acting on the material is higher than that of planetary mixers. This creates a more thorough dispersion, and large agglomerates that are sometimes present after batch mixing are ground down finer. This leads to another advantage, in that any filter screens further downstream become clogged less frequently and the need for screen changes is limited. The finer particle sizes and superior dispersion are readily apparent under a microscope, as shown in Figure 2.
The relatively small size of extruders means they take up far less space than planetary mixers and they are scalable. For R&D, there are even miniature extruders available that fit into safety workbenches. This makes for easier cleaning—and in fact, because the screws are “self-wiping,” there is also less chance for material to be left behind in the first place. On extruders, all contact parts can be changed out to ensure no cross contamination. With batch mixers, the equipment parts are so large that swapping out parts becomes impractical.
The biggest advantage of continuous vs batch manufacturing might be in energy savings. Because the shear forces of the twin screws are capable of compounding the material with less solvent or even no solvent in the mix, the solvent consumption is greatly reduced. This reduces the energy needed for drying, which as stated before can be as much as a fifth of the overall energy consumed in production. This can eliminate the space needed for a drying line, because that large equipment becomes effectively obsolete.
Twin-screw extrusion in lithium-ion battery manufacturing
Twin-screw extrusion can help optimize the manufacturing processes of lithium-ion batteries to make them safer, more powerful, longer lasting, and more cost-effective. Extruders have already been successfully used in research projects for innovative electrode manufacturing. With the ever-increasing demand for electrified vehicles and the batteries that power them, it seems likely that twin-screw extrusion will be a technology of key importance moving forward.
Additional resources:
- A video about Continuous vs. Batch manufacturing
- Watch a webinar presented by Thermo Fisher Scientific’s Annika Volp: Compound homogeneous electrode slurries fast and effectively
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