What Is Aluminum Foil?
Most people think of Reynolds Wrap when they hear aluminum foil. In the battery world we mean something else entirely.
LOTTE Aluminium broke ground on their Elizabethtown plant back in 2022. I drove past the site last fall and the building was up, equipment going in. They picked that spot because Ford and SK are building the BlueOval battery complex five miles down the road. Makes sense. You want your foil supplier close when you are running 36,000 tons a year through a coating line.
The foil LOTTE will make is for cathodes. Every lithium-ion cell has two current collectors inside. Copper on the anode side. Aluminum on the cathode side. The active material-your NMC or your LFP or whatever chemistry you are running-gets coated onto these foils as a wet slurry. Once it dries you have an electrode.
People ask why not use the same metal for both. Copper would be simpler. One supply chain instead of two.
Here is the problem. Copper dissolves.
At the cathode you are sitting above 3.5 volts versus lithium. Copper cannot handle that. It oxidizes and goes into solution. Samsung SDI put out a technical bulletin a few years back showing copper starts corroding around 3.4 volts. You end up with copper ions floating around in your electrolyte and eventually plating out on the anode. That kills the cell.
Aluminum oxidizes too but the oxide layer it forms is different. Aluminum oxide is dense and stable. It actually protects the metal underneath. The layer is only a few nanometers thick so electrons tunnel right through. You get corrosion resistance and conductivity at the same time.
The foil itself is thin. We are talking 10 to 12 microns for most production cells. UACJ in Japan specs their rolls at plus or minus half a micron across the width. That is tight. For reference a sheet of copy paper is around 100 microns.
Thinner is better for energy density. Every gram of aluminum that is not active material is dead weight. Some cell makers have pushed down to 8 microns. The tradeoff is handling. Thin foil tears on the coating line. Your tension control has to be perfect. One wrinkle and that section of electrode is scrap.
"We see steady pressure from customers to go thinner every year," a process engineer at a Midwest coating operation told me last month. "The equipment can do it but your reject rate goes up. At some point you are losing more material to tears than you save in weight."
Purity matters too. Battery-grade aluminum needs to be above 98 percent. Impurities like iron or silicon create inclusions in the foil surface. Those inclusions become weak spots. The coating does not adhere properly or you get pinholes. Either way the cell fails QC.
Most of the world's battery foil still comes from Asia. UACJ and Toyo in Japan. LOTTE and Dongwon in Korea. Half a dozen big producers in China including Ding Sheng and Nanshan. The Kentucky plant is one of the first serious attempts at domestic production. There is talk of others following but nothing concrete yet.
For LFP cells a lot of manufacturers have moved to carbon-coated foil. Dunmore in Bristol, Pennsylvania does this. They put down a thin carbon primer before you coat the active material. It helps with adhesion since LFP does not stick to bare aluminum as well as NMC does. Also cuts contact resistance at the interface.
What else. Recycling. When cells reach end of life you can shred them and separate the aluminum from the black mass. The recovered metal goes back to a smelter. Not exactly closed loop since you lose some at each step but better than landfilling it.
I should mention the dyne value thing. Surface energy. The foil surface has to be clean enough that the electrode slurry wets it evenly. You measure this in dynes per centimeter. Most specs call for 32 or higher. If the foil sat in a warehouse too long and picked up contamination you will see the slurry bead up during coating. That is a bad day.
Anyway. That is aluminum foil in batteries. Nothing like what is in your kitchen drawer.
