Electrode Manufacturing - Battery Design

There are 4 steps in electrode manufacturing:

Mixing
Coating
Drying
Calendering

The electrode manufacturing process is the same for all of the cell chemistries and formats in terms of the basic steps.

Step 1 – Mixing

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

Step 2 – Coating

The anode and cathodes are coated separately in a continuous coating process. The cathode (metal oxide for a lithium ion cell) is coated onto an aluminium electrode. The polymer binder adheres anode and cathode coatings to the copper and aluminium electrodes respectively.

Challenges

Controlling thickness and thickness over time
- control across the width of the web can be more challenging than control along the length
One of the biggest challenges is getting the edges right:
1. Getting the width exactly right down to 0.1 mm –> Shim adjustments can solve this
2. Avoiding/Controlling any “Fat Edges” i.e. thickness build ups at the edges –> Also shim adjustments
3. Making sure the edges are always straight, and do not show waviness –> Slot die distance to the substrate, and slot die angle need to be controlled and adjusted correctly.
Foil surface oxidation homogeneity can add variance to the challenge
Controlling foil moisture levels
Besides thickness, tensiometer peel tests and even indentation resistance (hardness test) might be used to further qualify the coating integrity

Step 3 – Drying

Immediately after coating the electrodes are dried. This is done with convective air dryers on a continuous process. The solvents are recovered from this process.

Infrared technology is used as a booster on Anode lines.