For lithium-ion battery current collectors, do you only know that aluminum foil is used for the positive electrode and copper foil for the negative electrode?
So why were these two materials chosen? Why not other materials? Why are different materials used for the current collectors of the positive and negative electrodes? Can the two materials be used in reverse order? Are there other more suitable materials to replace them?
1. Aluminum foil and copper foil have good electrical conductivity.
This doesn't require much explanation; lithium batteries work by converting chemical energy into electrical energy. The current collector's role is to collect and output the current generated by the active material or to input the electrode current to the active material, which requires a conductive material. Among many conductive materials, metals are a good choice. (Therefore, this point is based on the metallic properties of copper and aluminum foil.)
2. Aluminum foil and copper foil are soft and have good ductility.
Lithium-ion batteries come in various types, including hard-case, soft-pack, and cylindrical. While a small number utilize a stacking process, most lithium-ion batteries employ a winding process. This winding process requires the current collector to have a certain degree of flexibility (otherwise, how could it be rolled up like paper?). Of course, to improve the energy density of lithium-ion batteries, the current collector thickness needs to be as thin as possible (generally controlled at around 10μm). At such a thin thickness, rolling up copper or aluminum foil is quite a "roll."
3. Aluminum foil and copper foil have good ductility.
There's a reason why this point is discussed separately, rather than combined with the previous one. It also involves a common lithium-ion battery manufacturing process—rolling. This process compacts the active material, increasing its density (otherwise the cell's volumetric energy density would be too low), reducing internal resistance, and improving electrochemical performance. Rolling requires the current collector to have good ductility. When the active material and current collector are rolled together, the coating area of the active material tends to increase. If the current collector doesn't change accordingly, the active material is highly likely to separate from it. Secondly, the good ductility of copper and aluminum foil also prevents them from breaking due to the high rolling pressure.
4. Inexpensive
This point will not be explained further.
5. It has good stability in air.
Four points have been listed above, but many might ask, "Iron also meets these conditions, so why not use iron foil?" Here are two points (feel free to add more): First, iron's conductivity isn't particularly good (lithium batteries require relatively high conductivity from fluids; otherwise, excessive heat can cause a series of safety issues); second, iron's chemical properties are relatively unstable (aluminum is more chemically reactive than iron, but a dense aluminum oxide film forms on its surface), making iron prone to rust. The resulting rust is detrimental to battery stability and safety. Conversely, aluminum foil and copper foil have good stability in air.
6. Why is aluminum foil used for the positive electrode and copper foil for the negative electrode, and vice versa?
That's a great question!
Copper has a high positive electrode potential, and it is easily oxidized at high potentials, so it is not suitable as a positive electrode current collector. Aluminum has a high oxidation potential, and the dense aluminum oxide film formed on the aluminum surface can further protect the inner aluminum layer. Of course, the passivation layer formed on the aluminum foil surface is not a good conductor of electrons, but this passivation layer is very thin and electron conduction can be achieved through the tunneling effect.
So, can aluminum be used as a negative electrode current collector? Lithium and aluminum easily form lithium-aluminum alloys (LiAl, Li3Al2, Li4Al3, etc., because the octahedral structure of aluminum lattice has a similar void size to lithium, making it easy to form intermetallic compounds), so it is not suitable. Conversely, copper does not easily form copper-lithium alloys with lithium.
