The positive current collector commonly used in lithium-ion battery is aluminum foil and the negative current collector is copper foil. The function of a current collector is primarily to conduct electricity. It's similar to our wire. It's made of copper or aluminum. This is because the conductor can be divided into silver → copper → gold → aluminum → tungsten → nickel → iron according to the conductivity coefficient. Silver is the best bet, but precious metals are rare, and the world is rich in copper and aluminum, which are relatively cheap.

At the same time, in the lithium battery, we mainly have winding and lamination two processing methods. Compared with winding, it is necessary to make the electrode sheet of the battery with a certain degree of flexibility in order to ensure that the electrode sheet does not brittle fracture when winding. On the other hand, copper foil in the air is also relatively stable. Aluminum reacts easily with oxygen in the air, forming a dense oxide film on the surface of the aluminum that prevents further reaction. This thin oxide film also protects the aluminum in the electrolyte. Copper in the air itself is relatively stable, in the dry air basic does not react. The oxide layer on the surface of copper/nickel belongs to semiconductor, and the oxide layer on the surface of aluminum belongs to insulator, and the oxide layer on the surface of aluminum can not conduct electricity, but because it is very thin, the electronic conductivity is realized through the tunnel effect, even insulation. In general, it is best to use the collector after surface cleaning, on the one hand to wash away oil, at the same time can remove thick oxide layer.

The positive and negative electrode potentials of lithium batteries determine that aluminum foil is used for the positive electrode and copper foil is used for the negative electrode instead of the other way around. The positive electrode potential is high, the oxide layer of copper/nickel foil is loose, it is easy to be oxidized at high potential, but the oxidation potential of aluminum is high.

Another is that aluminium is consumed by alloying with lithium at low potential (such as Lithium 0V or graphite 0.2 v) . The gap size of the lattice Octahedron of metallic aluminum is similar to that of Li, so it is easy to form gap compounds with Li. Li and Al Not only form alloys with chemical formula Li, but also form Li3Al2 or Li4al3. Because of the high activity of the reaction between Al and Li, AL consumes a lot of Li, and its structure and morphology are destroyed, so it can not be used as a current collector of lithium-ion battery, it has a small capacity of Lithium intercalation, and maintains the stability of structure and electrochemical properties. It can be used as a current collector of lithium-ion battery anode, but if the surface of cupric nickel is heavily oxidized, Li will react with cupric nickel oxide at slightly higher potential, at 3.75 v, the polarization current of Cu foil begins to increase significantly, and the polarization current increases linearly, and the oxidation intensifies, which indicates that Cu becomes unstable at this potential, while the polarization current of aluminum foil is small and constant in the whole polarization potential range, no obvious corrosion was observed, which kept the stability of electrochemical properties. Because Al has a small lithium intercalation capacity in the lithium-ion battery potential range, it can maintain electrochemical stability and is suitable as a positive current collector for lithium-ion battery. If a high-potential anode (lithium titanate 1.5 v) is used, the current collector can also be aluminum foil

1. In order to ensure the internal stability of the current collector in the battery, the purity of copper and aluminum foil for lithium battery is above 98% . The impurity of AL will cause the surface film not compact and pitting corrosion, even more because of the surface film damage lead to the formation of LiAl alloy.

2. The thickness of copper-aluminum foil for lithium battery is required with the development of lithium battery in recent years. Positive aluminum foil from the previous years 16um to 14um, then to 12um, now many battery manufacturers have mass production of aluminum foil using 10um, even use 8um. The negative copper foil, because of its good flexibility, the thickness of copper foil from 12um before reduced to 10um, then to 8um, until now there are a large number of battery manufacturers mass production with 6um, and some manufacturers are developing 5um/4um are possible to use. Because of the high purity of copper and aluminum foil used in lithium batteries, the density of the materials is basically at the same level. As the development thickness decreases, the surface density also correspondingly decreases, and the weight of the batteries naturally becomes smaller and smaller, meet our needs for lithium-ion batteries.

3. In addition to the thickness and weight of the current collector, the surface properties of the current collector have a great influence on the production and performance of the lithium battery. In particular, negative current collector, due to the defects of the preparation technology, the copper foil on the market to single-sided wool, double-sided wool, double-sided roughened varieties. The asymmetry of the two-side structure leads to the asymmetry of the contact resistance of the coating on both sides of the negative electrode, which leads to the uneven discharge of the capacity of the negative electrode on both sides, the charge-discharge cycle life of the two-side negative electrode coating is seriously out of balance, which accelerates the capacity decline of the battery. 

Source: The web

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