For lithium-ion batteries, aluminum foil is usually used as the positive collector and copper foil as the negative collector. In order to ensure the stability of the collector in the battery, the purity of both should be above 98%. With the continuous development of lithium battery technology, we all hope that the energy density of the battery will be as high as possible and the weight of the battery will be lighter and lighter, whether it is a lithium-ion battery used in digital products or an electric vehicle battery. The most important thing in the current collector is to reduce the thickness and weight of the current collector, so as to intuitively reduce the volume and weight of the battery. There are three reasons why the anode of lithium ion battery uses aluminum foil and the cathode uses copper foil:
First, copper aluminum foil has good conductivity, soft texture and low price.
We all know that the working principle of lithium-ion battery 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 here we need conductive materials. Among ordinary materials, metal materials have very good conductivity, while among metal materials, copper foil and aluminum foil are cheap and have good conductivity. At the same time, in lithium ion batteries, we have two important processing methods: winding and laminating. In terms of winding, the pole piece used to prepare the battery must have a certain flexibility to ensure that the pole piece will not break when winding. Among the metal materials, copper aluminum foil is also a soft metal. Finally, consider the cost of battery preparation. Relatively speaking, copper aluminum foil is relatively cheap, and the world is rich in copper and aluminum resources.
Second, copper aluminum foil is relatively stable in air.
Low temperature lithium iron phosphate battery 3.2V 20A
Low temperature lithium iron phosphate battery 3.2V 20A
-20 ℃ charging, - 40 ℃ 3C discharge capacity ≥ 70%
Charging temperature: - 20~45 ℃
-Discharge temperature: - 40~+55 ℃
-40 ℃ support discharge rate: 3C
-40 ℃ 3C discharge capacity retention rate ≥ 70%
Aluminum is easy to react with oxygen in the air, forming a dense oxide film on the aluminum surface layer to prevent further reaction of aluminum, and this thin oxide film can also protect aluminum in electrolyte. Copper is relatively stable in the air, and basically does not react in dry air.
Third, the positive and negative pole potentials of lithium ion batteries determine that the positive pole uses aluminum foil, and the negative pole uses copper foil, rather than the reverse.
With high positive potential, copper foil is easy to be oxidized at high potential, while aluminum has high oxidation potential, and aluminum foil has dense oxide film on the surface, which also has good protection for internal aluminum. Both of them are used as collectors because of their good conductivity, soft texture (which may also be conducive to bonding), and relatively cheap. At the same time, both surfaces can form a layer of oxide protective film.
1、 The size of the lattice octahedral void of metallic aluminum is similar to that of Li, and it is very 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 morphology are also damaged, so it cannot be used as the collector of the negative electrode of lithium ion batteries; However, during the charging and discharging process of the battery, Cu has only a small lithium insertion capacity, and maintains the stability of the structure and electrochemical performance, which can be used as the collector of the negative pole of the lithium ion battery; When Cu foil is at 3.75V, the polarization current begins to increase significantly, and rises linearly, and the oxidation intensifies, indicating that Cu begins to be unstable at this potential; The polarization current of aluminum foil is small and constant in the whole polarization potential range, and no obvious corrosion phenomenon is observed, maintaining the stability of electrochemical performance. Due to its small lithium intercalation capacity and electrochemical stability in the positive potential range of lithium ion batteries, Al is suitable for use as the positive collector of lithium ion batteries.
2、 The oxide layer on the copper/nickel surface is a semiconductor, and the electron is conductive. The oxide layer is too thick, and the impedance is large; The aluminum oxide layer on the aluminum surface is an insulator, and the oxide layer cannot conduct electricity, but because it is very thin, electronic conductivity is achieved through the tunnel effect. If the oxide layer is thick, the conductivity of the aluminum foil is poor, even insulation. Generally, the surface of the collecting fluid should be cleaned before use. On the one hand, the oil stain can be washed away, and at the same time, the thick oxide layer can be removed.
Low temperature high energy density 18650 3350mAh
Low temperature high energy density 18650 3350mAh
-40 ℃ 0.5C discharge capacity ≥ 60%
Charging temperature: 0~45 ℃
Discharge temperature: - 40~+55 ℃
Specific energy: 240Wh/kg
-40 ℃ discharge capacity retention rate: 0.5C discharge capacity ≥ 60%
3、 The positive potential is high, and the aluminum thin oxide layer is very dense, which can prevent the oxidation of collector fluid. However, the oxide layer of copper/nickel foil is looser. In order to prevent its oxidation, the potential is relatively low and better. At the same time, it is difficult for Li to form lithium intercalation alloy with copper/nickel at low potential. However, if the copper/nickel surface is oxidized largely, Li will react with copper/nickel oxide at slightly high potential. However, aluminum foil cannot be used as negative electrode, and LiAl alloying will occur at low potential.
4、 The collecting fluid shall be pure in composition. The impure composition of Al will cause the surface film to be not compact and pitting corrosion will occur, and even more, the destruction of the surface film will lead to the formation of LiAl alloy.
