Casting

Casting, or coating, is the process in which a well-mixed viscous liquid slurry is converted into a solid electrode form. As most conventional casting of battery slurries results in a thin layer on a metal current collector substrate, this process is sometimes called coating. Casting is an important early step in the battery manufacturing process. Inhomogeneities in the slurry mixing process can result in structural defects in the electrode formation process. Process control is also important to preventing other types of defects from occurring in the finished electrode. Casting can be performed from lab-scale up to gigafactory-scale.

The casting process can be generalized to a few steps

1. Load slurry into casting vessel.
2. Distribute or disperse slurry in a controlled manner.

Casting is directly followed by drying in most battery manufacturing processes to remove the slurry solvent.

There are multiple methods of casting slurries into electrodes. The following are some methods which are commonly found in research labs and industry:

• Film applicator: a film applicator is a device used to evenly spread a liquid substance, such as paint, ink, or slurries, over a substrate. In the simplest form, an applicator with a defined slot thickness such as 50 or 100 μm is pushed by a bar which moves a deposit of slurry horizontally across the substrate. Applicators are usually metal bars that are manufactured to high tolerances to give consistent, repeatable results. Each bar will have a defined gap which gives a "theoretical wet film thickness" or, in other words, the thickness of the coating that should remain on the substrate after application. Even with high manufacturing tolerances, the actual wet film thickness can vary from 50% to 90% of the gap depending on the characteristics of the slurry. Applicators can take many forms, such as aluminum blocks with a single gap, a four-sided cylindrical bar with 4 different gap thicknesses, or a smoothly rotating bar with continuously varying gaps.
• Tape casting (also called doctor blading, knife coating, and shank shifting): tape casting is a method similar to a film applicator, but which originates from the field of ceramics processing. The slurry is loaded into a holding reservoir with a gap at the bottom. The doctor blade or knife is adjusted to define the gap between the reservoir wall and the substrate. To cast, either the substrate moves on a roller or the reservoir and doctor blade move along a fixed substrate.
• Comma head: comma head coating is similar to tape casting, but uses a comma-shaped bar to control the slurry instead of an angled blade like a doctor knife. This method is often used when the substrate is a foil which is running through several rollers as the comma head can be used to apply the coating as the foil wraps around a roller.
• Slot-die coating: slot-die coating is a coating technique for the application of solution, slurry, hot-melt, or extruded thin films onto typically flat substrates such as glass, metal, paper, fabric, plastic, or metal foils. In slot-die coating, the slurry is precisely fed using a syringe pump through a heated coating head which has a well-defined and adjustable gap. The high aspect ratio outlet and temperature of the slot-die coating head control the final delivery of the coating liquid onto the substrate. This results in the continuous production of a wide layer of coated material on the substrate, with adjustable width depending on the dimensions of the slot-die outlet. By closely controlling the rate of solution deposition and the relative speed of the substrate, slot-die coating affords thin material coatings with easily controllable thicknesses in the range of 10 nanometers to hundreds of micrometers after evaporation of the precursor solvent.^[1]

• Other methods include spray coating, screen-printing, and hot rolling, among others. Some casting methods are aimed at using dry processes to eliminate the use of solvent in battery manufacturing.

Electrode quality from casting is highly dependent on the process parameters, and each casting method has its own parameters. Among wet film coating methods, gaps and wet film thicknesses are some of the most important parameters to define the loading and thickness of the electrode. Since slurries are typically non-Newtonian fluids, the shear rate the slurry experiences as it goes through the gap can affect the resulting electrode thickness and uniformity. The temperature and humidity in the room of the casting device are important to control. For non-aqueous casting processes, the entire process can occur inside a dry room with controlled particulate density in the air.

Film applicators follow the ASTM standard D823.^[2]

Identification of defects is a key aspect of casting. Measuring wet film thickness is also important during the process to ensure that the gap is performing as desired.

For slot-die coating, the quality of the meniscus as the slurry exits the slot-die affects the uniformity of the electrode.

It is important to note that drying and calendering often proceed immediately after casting, so the quality control is not done until after these respective processes and the casting parameters are adjusted according to the results after all of these processes.

During casting, solvent if present will begin evaporating. Depending on whether the slurry is aqueous or non-aqueous and which solvents or additives it might contain, it can pose a varying degree of hazard to human and environmental health. These solvent vapors may need to be exhausted or captured and recycled or re-processed.