The Role of Aluminum Foil as a Cathode Current Collector
In a lithium-ion cell the positive electrode coating is applied to a thin aluminium foil that carries the current out of the cell. This foil, usually called battery foil or cathode foil, must do more than conduct: it has to hold the active material firmly through thousands of charge and discharge cycles, resist oxidation at the high potentials of the cathode, and stay flat after the calendering and winding steps that follow coating.
Aluminium is selected because it forms a stable passive oxide layer at cathode potentials of 3 to 4.5 volts against lithium, so the foil does not corrode in the electrolyte the way copper would. It is also light, which directly raises the energy density of the finished cell, and it is an excellent electrical and thermal conductor, which limits resistive heating during fast charging.
Alloy Selection for Battery Foil
Battery foil is made from high-purity 1xxx alloys because conductivity and surface cleanliness matter more than strength.
Alloy 1060 is the most widely used cathode foil, with a nominal aluminium content of 99.6 percent, good conductivity and a consistent surface.
Alloy 1070 at 99.7 percent purity is chosen where the lowest possible resistance and the cleanest surface are required.
Alloy 1235 is used for thinner gauges and for foils where the rolling performance of a low-manganese alloy is an advantage.
Alloy 1145 offers the highest purity of the group and is reserved for demanding high-rate cells.
The alloy must be free of iron and silicon pickup from the casting process, because coarse intermetallic particles can initiate pinholes during rolling and can interfere with the adhesion of the cathode slurry.
Thickness, Width and Dimensional Tolerance
Thinner foil allows more active material in the same cell volume, so the trend is towards gauges of 8 to 12 micrometres for both electrodes. Foil thinner than about 6 micrometres becomes difficult to coat and handle without wrinkling.
| Parameter | Typical specification | Notes |
|---|---|---|
| Thickness range | 0.006 to 0.020 mm | 9, 12, 13 and 15 micrometres are the most common |
| Thickness tolerance | plus or minus 3 percent | measured across the full web width |
| Width | 200 to 1600 mm | slit to the coating line width |
| Core diameter | 76 mm or 152 mm | steel or aluminium core |
| Camber | as agreed, low value | controls tracking on the coating line |
Mechanical and Electrical Requirements
Tensile strength and elongation must be balanced: the foil has to survive web tension through the coating and slitting lines, yet still conform to the winding radius of the cell.
| Property | Typical value |
|---|---|
| Tensile strength | 190 to 260 MPa |
| Elongation at break | 3 percent or more |
| Electrical conductivity | 60 percent IACS or higher |
| Resistivity | not more than 0.0285 micro-ohm metre |
| Surface wetting tension | controlled to suit the slurry system |
| Moisture content | low, verified before packing |
Surface Quality and Coating Adhesion
Cathode slurry adhesion is decided by the state of the foil surface. Rolling oil left on the surface lowers surface energy, causes de-wetting of the slurry and leads to coating defects that show up later as capacity loss or as delamination during cycling. Production therefore ends with an online degreasing step that removes residual oil without scratching the surface, followed by inspection for pinholes, dents, streaks and edge burrs.
Key surface requirements include:
A consistent wetting tension so the slurry spreads evenly.
A low pinhole count, since a pinhole can allow electrolyte to reach the counter electrode.
Flat, burr-free edges after slitting, to prevent short circuits in the wound cell.
No visible roll marks or dark oxide patches.
Rolling Control, Inspection and Packaging
Battery foil is rolled on mills fitted with automatic thickness and flatness control, using blanks that are screened for hydrogen content, non-metallic inclusions and grain size before rolling. That upstream control is what makes a 0.012 mm foil hold a three percent thickness tolerance over a full web width. Finished rolls are inspected for thickness profile, mechanical properties, surface quality and moisture, then packed in moisture barrier bags with desiccant for shipment. Foil for cells is normally ordered to the dimensional and testing requirements of GB/T 3198 and to the mechanical and electrical values agreed in the purchase specification, with cell level performance assessed under the relevant cell standards such as IEC 62660-1 or GB 31241.
Frequently Asked Questions
Q: Why is aluminium foil used as the cathode current collector rather than copper?
Aluminium forms a stable passive oxide at cathode potentials of 3 to 4.5 volts against lithium, so it resists corrosion in the electrolyte, and its low density helps raise cell energy density.
Q: What is the most common thickness for battery foil?
Most cathode and anode foil is supplied between 8 and 15 micrometres, with 12 micrometres a very common gauge. Below about 6 micrometres handling during coating becomes difficult.
Q: Which alloy is normally specified?
Alloy 1060 and alloy 1070 account for most cathode foil, with 1235 used for thinner gauges and 1145 selected for the highest purity requirements.
Q: How does surface condition affect cell performance?
Residual rolling oil lowers surface energy, causing de-wetting and poor slurry adhesion. Clean, oil-free foil with the right wetting tension is essential for consistent coating and cycle life.
Q: Why is the pinhole count controlled so tightly?
A pinhole can let electrolyte contact the opposite electrode inside the cell, so pinhole count is limited by specification and checked during inspection.
Q: How is battery foil packed for export?
Rolls are wound on 76 mm or 152 mm cores, sealed in moisture barrier bags with desiccant, and shipped in cases that protect the roll edges from impact.
