The anode and cathode materials are mixed just prior to being delivered to the coating machine. This mixing process takes time to ensure the homogeneity of the slurry.
Cathode: active material (eg NMC622), polymer binder (e.g. PVdF), solvent (e.g. NMP) and conductive additives (e.g. carbon) are batch mixed.
Anode: active material (eg graphite or graphite + silicon), conductive material (eg carbon black), and polymer binder (eg carboxymethyl cellulose, CMC)
N-Methyl-2-pyrrolidone (NMP): this is a toxic substance, widely used in the plastics industry as it is nonvolatile and able to dissolve a wide range of materials. NMP residual will be a Quality Control test downstream (Gas Chromatography-Mass Spectrometry can be used to test sample) as that will affect cell performance reactively.
Challenges
- Homogeneity of the mix
- Ensuring low ppm H2O – suppliers will be asked to guarantee a low ppm
- No breakup of the particles
- Particle and pore size distribution will be targeted with suppliers or via in process Milling
- Some developments concentrate on how to produce dual layers (to form a quasi-heterogeneous bi-layer) to aid electrolyte soaking. The calendaring process can achieve this to a degree.
- Moving from a batch mixing process to continuous mixing
- Ensuring no alien particulates are in the mix
- Magnetic filters often used to remove metal particles, this will only work down to a certain size
The mixing methods are divided into two groups [1]:
- Dry Mixing
- dry-ball mixer
- melt granulator
- extruder
- Wet Mixing
- high-shear mixer
- wet ball-miller
- ultrasound mixer
- magnetic stirrer
References
- Leire Fernandez-Diaz, Julen Castillo, Elene Sasieta-Barrutia, Maria Arnaiz, Marta Cabello, Xabier Judez, Alexandre Terry, Laida Otaegui, Maria C. Morant-Miñana, Aitor Villaverde, Mixing methods for solid state electrodes: Techniques, fundamentals, recent advances, and perspectives, Chemical Engineering Journal, Volume 464, 2023