It is well known that the four main parts of lithium-ion batteries are positive electrode material, negative electrode material, separator and electrolyte. However, in addition to the main four parts, the current collector used to store the positive and negative electrode materials is also an important part of the lithium battery. Today we will talk about the positive and negative current collector materials of lithium batteries.

For lithium-ion batteries, the commonly used positive current collector is aluminum foil, and the negative electrode current collector is copper foil. In order to ensure the stability of the current collector inside the battery, the purity of both is required to be above 98%. With the continuous development of lithium battery technology, whether it is a lithium battery for digital products or a battery for electric vehicles, we all hope that the energy density of the battery is as high as possible, and the weight of the battery is getting lighter and lighter, and the most important thing in the current collector is Reducing the thickness and weight of the current collector intuitively reduces the volume and weight of the battery.

1.​Requirements for the thickness of copper and aluminum foil for lithium batteries

With the rapid development of lithium batteries in recent years, the current collectors for lithium batteries have also developed rapidly. The positive aluminum foil has been reduced from 16um in previous years to 14um, and then to 12um. Now many battery manufacturers have mass-produced aluminum foil that uses 10um, and even uses 8um. As for the copper foil for the negative electrode, due to its good flexibility, the thickness of the copper foil has been reduced from the previous 12um to 10um, and then to 8um. At present, most battery manufacturers use 6um for mass production, and some manufacturers are developing 5um/4um are all possible. Since lithium batteries have high requirements for the purity of copper and aluminum foil used, the density of materials is basically the same level. As the thickness of the development decreases, the areal density also decreases accordingly, and the weight of the battery is naturally getting smaller and smaller, which is in line with our requirements for lithium batteries. need.

2.Surface roughness requirements of copper and aluminum foil for lithium batteries

For the current collector, in addition to the influence of its thickness and weight on the lithium battery, the surface properties of the current collector also have a great influence on the production and performance of the battery. Especially for the negative electrode current collector, due to the defects of the preparation technology, the copper foils on the market are mainly single-sided wool, double-sided wool, and double-sided roughened varieties. This asymmetry of the two-sided structure leads to asymmetric contact resistance of the coating on both sides of the negative electrode, which in turn makes the capacity of the negative electrode on both sides unable to be released uniformly; at the same time, the asymmetry of the two sides also leads to inconsistent bonding strength of the negative electrode coating, and yes, the charge-discharge cycle life of the negative electrode coating on both sides Serious imbalance, which in turn accelerates the decay of battery capacity.
In the same way, the positive aluminum foil is also developed to a double-sided symmetrical structure as much as possible, but at present, due to the influence of the aluminum foil preparation process, the single-sided smooth aluminum foil is mainly used. Since the aluminum foil is basically rolled from a thick aluminum ingot, the contact between the aluminum ingot and the roll needs to be controlled during the rolling process, so lubricants are generally added to the surface of the aluminum foil to protect the aluminum ingot and the roll. The lubricant on the surface has a certain influence on the battery pole piece. Therefore, for aluminum foil, the removal of lubricant from the surface is also a key factor.

3.Why use copper foil for negative electrode and aluminum foil for positive electrode?

First, copper and aluminum foil has good conductivity, soft texture and low price. We all know that the working principle of lithium batteries is an electrochemical device that converts chemical energy into electrical energy. In this process, we need a medium to transfer the electrical energy converted from chemical energy, and conductive materials are needed here. Among ordinary materials, metal materials are the most conductive materials, and among metal materials, copper foil and aluminum foil are cheap and good conductivity. At the same time, in the lithium battery, we mainly have two processing methods: winding and lamination.
Compared with winding, the pole pieces used to prepare the battery need to have a certain degree of flexibility to ensure that the pole pieces will not be brittle during winding. Among the metal materials, copper and aluminum foil is also a soft metal. . Finally, consider the cost of battery preparation. Relatively speaking, the price of copper and aluminum foil is relatively cheap, and there are abundant resources of copper and aluminum elements in the world.

Second, copper and aluminum foil is relatively stable in the air. Aluminum easily reacts with oxygen in the air, forming a dense oxide film on the surface of aluminum to prevent further reaction of aluminum, and this thin oxide film also has a certain protective effect on aluminum in the electrolyte. Copper itself is relatively stable in air and basically does not react in dry air.
The third is that the potential of the positive and negative electrodes of the lithium battery determines the use of aluminum foil for the positive electrode and copper foil for the negative electrode, not the other way around. The positive electrode potential is high, and the copper foil is easily oxidized at high potential, while the oxidation potential of aluminum is high, and the surface of the aluminum foil has a dense oxide film, which also has a good protective effect on the internal aluminum.