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

First, the copper aluminum foil has good conductivity, soft texture and cheap price. As we all know, the working principle of lithium batteries is an electrochemical device that converts chemical energy into electrical energy, so in this process, we need a medium to transfer the electrical energy converted by chemical energy, here we need conductive materials. In ordinary materials, metal materials are the best materials for electrical conductivity, and in metal materials, the price is cheap and the conductivity is good: copper foil and aluminum foil. At the same time, in lithium batteries, we mainly have two processing methods: winding and laminating. Relative to the winding, the electrode sheet used for the preparation of the battery needs to have a certain softness to ensure that the electrode sheet does not occur in the winding brittleness and other problems, and the metal material, copper aluminum foil is also a soft metal. Finally, consider the cost of battery preparation, relatively speaking, the price of copper aluminum foil is relatively cheap, and the world's copper and aluminum resources are rich.

Second, the copper aluminum foil is also relatively stable in the air. Aluminum is easy to chemically react with oxygen in the air, generating a dense oxide film on the surface layer 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 the air and basically does not react in dry air.

Third, the positive and negative potentials of lithium batteries determine the positive electrode with aluminum foil, and the negative electrode with copper foil, rather than the other way around. The positive electrode potential is high, and the copper foil is easy to be oxidized at a high potential, while the oxidation potential of aluminum is high, and the surface layer of aluminum foil has a dense oxide film, which also has a good protective effect on the internal aluminum. Both are used as fluid collectors because they have good electrical conductivity, are relatively soft in texture (which may also be conducive to bonding), are relatively common and inexpensive, and both can form an oxide protective film on the surface.

The lattice octahedral void size of metal aluminum is similar to the size of Li, and it is easy to form metal interstitial compounds with Li, Li and Al not only form an alloy with the chemical formula LiAl, but also may form Li3Al2 or Li4Al3. Due to the high activity of the reaction between metal Al and Li, metal Al consumes a large amount of Li, and its structure and form are also destroyed, so it cannot be used as a fluid collector for the negative electrode of lithium-ion batteries. In the process of battery charging and discharging, Cu has only a small amount of lithium embedded capacity, and maintains the stability of its structure and electrochemical performance, and can be used as a fluid collector for the negative electrode of lithium-ion batteries. When Cu foil is at 3.75V, the polarization current begins to increase significantly, and increases linearly, and the oxidation intensifies, indicating that Cu becomes unstable at this potential. In the entire polarization potential range of aluminum foil, the polarization current is small and constant, no obvious corrosion phenomenon is observed, and the electrochemical performance is stable. Due to the small lithium insertion capacity in the positive potential range of lithium ion batteries, Al can maintain electrochemical stability, and is suitable for the positive collector fluid of lithium ion batteries.

Second, the copper/nickel surface oxide layer is a semiconductor, electron conduction, the oxide layer is too thick, the impedance is large; The alumina oxide layer on the surface of aluminum is an insulator, the oxide layer cannot conduct electricity, but because it is very thin, electronic conductance is achieved through the tunnel effect, if the oxide layer is thick, the conductivity level of aluminum foil is poor, and even insulation. General fluid collection before use is best to go through the surface cleaning, on the one hand to wash away the oil, while removing the thick oxide layer.

Third, the positive electrode potential is high, and the thin aluminum oxide layer is very dense, which can prevent the oxidation of the fluid collection. The copper/nickel foil oxide layer is relatively loose, in order to prevent its oxidation, the lower potential is better, and Li is difficult to form a lithium inlay alloy with Cu/ nickel at low potential, but if the copper/nickel surface is oxidized in large amounts, Li will react with copper/nickel oxide at slightly higher potential. Aluminum foil can not be used as a negative electrode, and LiAl alloying will occur at low potential. Fourth, the collector fluid requires pure composition. The impurity of the composition of Al will lead to the surface film is not dense and point corrosion occurs, and even more due to the destruction of the surface film leads to the formation of LiAl alloy. With the rapid development of lithium electricity in recent years, the development of fluid collector for lithium batteries is also rapid. Positive aluminum foil has been reduced from 16um in previous years to 14um, and then to 12um, and now many battery manufacturers have mass-produced 10um aluminum foil, and even 8um. The negative copper foil, due to its good flexibility of copper foil, its thickness is reduced from the previous 12um to 10um, and then to 8um, so far, a large number of battery manufacturers use 6um in mass production, and some manufacturers are developing 5um/4um is possible to use. Since the lithium battery has high purity requirements for the copper aluminum foil used, the density of the material is basically at the same level, and with the reduction of the development thickness, the surface density is also correspondingly reduced, and the weight of the battery is naturally getting smaller and smaller, which meets our needs for lithium batteries. The surface roughness requirements of copper aluminum foil for lithium battery: for the fluid collector, in addition to its thickness and weight have an impact on the lithium battery, the surface performance of the fluid collector also has a greater impact on the production and performance of the battery. In particular, 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 coarse-coated varieties. The asymmetric structure of the two sides leads to the asymmetric contact resistance of the coating on both sides of the negative electrode, so that the negative capacity of both sides can not be released evenly. At the same time, the asymmetry of both sides also causes the bond strength of the negative coating to be inconsistent, and the charge-discharge cycle life of the negative coating on both sides is seriously unbalanced, thus accelerating the attenuation of the battery capacity.