Manufacturing of battery electrodes, while relying upon common coating techniques, can be extremely challenging due to formulation limitations, raw material limitations, the need to limit process aids, their high solids content, high loading requirements, consistency of loadings to balance anode to cathode capacity ratios, limitations in compatible polymer binders which then define acceptable solvents, the need for high density or particle packing, as well as a necessity to create patterns both in the web direction and cross web direction. Common techniques often start with relatively crude tape casting, then progressing to: rotary screen, three roll reverse, comma coating, and slot die.
One of the great challenges of defining an electrode formulation involves how the physical properties of the active materials influence and impact the requirements of the non- active components such as: conductors, polymers / binders, rheology aids, solvents, mix processes, the need for particle milling or aggregate reduction, filtration, and potential impurities from equipment wear. One of the critical properties of the raw materials can be directly associated with particle surface area. For example, common battery materials such as LiCo2, LiMnO2, Sulfur, FeS2, etc. are typically less than one m2/gm. While carbons can range from 20 to 2000 m2/gm, other active material such as MnO2 may have a surface area of 30 and CFX maybe 250 or greater. Years of empirical work on electrode formulations has resulted in transfer functions which, when based off of a combination of BET surface area and DBP (DiButyl Phthalate) absorption, can define a starting point for polymer content, while the conductivity of the active materials can be used to define the amount and carbon preference.
For pattern or patch coating it’s important to measure the slurry rheology over a shear range that represents the coating process utilized. Since most battery coating formulations are shear sensitive or non-Newtonian, a single viscosity value can be very misleading. Additionally, and particularly for cross web mass free zones the type and speed (milliseconds) of mechanical, servo and or servo- mechanical processes, as well as fluid flow control, must be correlated to the web speed and length of the mass free zone. Lastly, it’s important to take into account the surface tension of the coating slurry and the substrate.