German researchers have developed a dry-etch process for battery electrodes that eliminates the use of toxic solvents and boosts production throughput.
A dry electrode process has been key for production of battery cells by US car maker Tesla, which is building a battery plant in Berlin. Researchers at the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden have developed a similar process that will be key for boosting battery production for other European car makers.
Machines using the DRYtraec process do not require long drying tracks and so take up significantly less space than conventional battery electrode manufacturing systems and speed up production as well as reducing waste materials.
“The range of possible uses for the technology is not limited to a particular cell chemistry,” said Dr Benjamin Schumm, Group Manager for Chemical Coating Technologies at Fraunhofer IWS. “It could equally be used on lithium-ion cells as on lithium-sulfur or sodium-ion cells. We are even looking at solid-state batteries. These will be increasingly important in the future, but the materials cannot tolerate wet chemical processing.
The Federal Ministry for Economic Affairs and Energy (BMWi), which funded the research, predicts that Germany will need 655TWh of storage capacity by 2030, an increase of 20 percent compared to today which will require more efficient manufacturing. This demand has been reflected by Tesla in its push to build battery gigafactories around the world.
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A battery electrodes normally consists of a metal foil with a thin coating that contains the active components that store the energy.
“The conventional coating process uses a wet chemical method that applies what is known as slurry,” said Schumm. "The active material, conductive carbon and binders are dispersed in a solvent to make a kind of paste, which is initially applied to the metal foil to form a wet coating. Extremely large machines with very long drying tracks are needed to ensure that the solvent will evaporate afterward. With DRYtraec, we can design this process more efficiently,” he said.
The new coating process essentially uses similar raw materials as in the slurry process but works without solvents, instead using a special binder material.
Together, the materials form a dry mixture that is fed into the gap between two rollers rotating in opposite directions. The crucial detail is that one of the rollers must be turning faster than the other. This creates a shear force that ensures that the binder forms thread-like networks known as fibrils.
“Imagine it as a spider’s web that mechanically embeds the particles,” said Schumm. The pressure and motion form a fine film on the faster-rotating roller. This film is then transferred by a second roller onto a current collector foil. This allows both sides to be coated simultaneously without significant additional work. In the final step, the resulting coil is cut to the required size and the individual parts are stacked as appropriate in order to produce the finished battery cell.
Removing toxic solvents and long, energy-intensive drying machines from the process reduces the energy consumption of the battery electrode production process and requires only one-third of the equipment space of a conventional solution.
The first prototype DRYtraec systems were commissioned as part of the “DryProTex” funding project which demonstrated that it is possible to manufacture electrodes continuously regardless of the battery chemistry.
Discussions are currently under-way with several automobile and cell manufacturers to plan the construction of a number of pilot systems. Beyond manufacturing electrodes with DRYtraec, the researchers at Fraunhofer IWS are working on lithium ion films between 2 and 20 µm thick deposited on current collector foils using a new melt coating technology. These films form the basis for various anode concepts of future battery systems with high energy density.
It is also working on thin carbon coatings that are deposited on powders and flat substrates by liquid and gas phase processes (CVD). This reduces the interfacial resistances between active material and metallic current collector and enhances the electrically and ionically conductive encapsulation of the active materials
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