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.
Getting the perfect viscosity and homogeneity of the slurries for both electrodes is the most fundamental aspect to be considered for coating. Once that is established then the other parameters can be optimised further.
Coating is the second of the four electrode manufacturing steps: it takes the slurry from Mixing (cathode slurries typically use NMP as a solvent) and deposits it onto the foil, ahead of Drying and Calendering.
- Coating speed
- Coating Type
- Wet slurry: Slot-die/Comma
- Dry electrode
- Quality control
- measured parameters: thickness, coating weight/areal loading (mg/cm2, which sets areal capacity), and defect detection (pinholes, streaks, agglomerates) – see Quality
Coating Techniques
Electrode Coating Techniques: From Lab to Gigafactory
A look at the pro’s and con’s of:
- Doctor Blade
- Comma Coater
- Slot Die
For a directory of coating machine builders and slurry mixing suppliers, see Cell Manufacturing Equipment.
Slurry Viscosity & Solid Content
Slurry Viscosity & Solid Content Optimization
Viscosity in electrode manufacturing cannot be reduced to a single variable. It is the combined result of several physical mechanisms:
- particle packing
- binder distribution
- formation of conductive networks
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:
- Getting the width exactly right down to 0.1 mm -> Shim adjustments can solve this
- Avoiding/Controlling any “Fat Edges” i.e. thickness build ups at the edges -> Also shim adjustments
- 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.
- Coating width, straightness and mass-free-edge accuracy carry straight through to cell assembly tolerance stack-up – see Understanding and Eliminating Electrode Overlap Issues
- 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
References
- Marcel Schmitt, Slot die coating of lithium-ion battery electrodes, KIT Scientific Publishing
See also: Areal Loading