Why Copper Foil for Anode and Aluminum Foil for Cathode in Lithium-Ion Batteries?

For lithium-ion batteries, the commonly used cathode electrode current collector is aluminum foil, and the anode electrode current collector is copper foil. In order to ensure the stability of the current collector inside the battery, both require a purity of over 98%

For lithium-ion batteries, the commonly used cathode electrode current collector is aluminum foil, and the anode electrode current collector is copper foil. In order to ensure the stability of the current collector inside the battery, both require a purity of over 98%. With the continuous development of lithium battery technology, whether it is lithium batteries used in digital products or electric vehicle batteries, we hope that the energy density of batteries is as high as possible, and the weight of batteries is becoming lighter and lighter. In terms of current collectors, the most important thing is to reduce the thickness and weight of current collectors, intuitively reducing the volume and weight of batteries. There are three reasons why aluminum foil is used for the cathode electrode and copper foil is used for the anode electrode of lithium-ion batteries:

One is that copper 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, which requires conductive materials. Among ordinary materials, metal materials have the best conductivity, while copper foil and aluminum foil are cheaper and have good conductivity among metal materials. Meanwhile, in lithium batteries, we mainly have two processing methods: winding and stacking. Compared to winding, the electrode sheets used for battery preparation need to have a certain degree of flexibility to ensure that the electrode sheets do not break during winding, and copper aluminum foil is also a relatively soft metal material. Finally, considering the cost of battery preparation, copper aluminum foil is relatively cheap, and there are abundant resources of copper and aluminum elements in the world.

Secondly, copper aluminum foil is relatively stable in air. Aluminum is prone to chemical reactions with oxygen in the air, forming a dense oxide film on the surface of the aluminum to prevent further reactions. This thin oxide film also has a certain protective effect on aluminum in the electrolyte. Copper itself is relatively stable in air and does not react significantly in dry air. The third is that the cathode and anode electrode potentials of lithium batteries determine the use of aluminum foil for the cathode electrode and copper foil for the anode electrode, rather than the other way around. The cathode electrode potential is high, and copper foil is easily oxidized at high potential, while aluminum has a high oxidation potential and a dense oxide film on the surface of the aluminum foil, which also provides good protection for the internal aluminum. Both are used as current collectors because they have good conductivity, soft texture (which may also be beneficial for bonding), are relatively common and inexpensive, and both can form a layer of oxide protective film on their surfaces. 1. The size of the octahedral voids in the lattice of metallic aluminum is similar to that of Li, making it easy to form metal interstitial compounds with Li. Li and Al not only form alloys with the chemical formula LiAl, but may also form Li3Al2 or Li4Al3. Due to the high reactivity of the reaction between metal Al and Li, metal Al consumes a large amount of Li and its own structure and morphology are also destroyed, so it cannot be used as a current collector for the anode electrode of lithium-ion batteries; During the battery charging and discharging process, Cu has only a small lithium insertion capacity and maintains stable structure and electrochemical performance, making it a current collector for the anode electrode of lithium-ion batteries; At 3.75V, the polarization current of Cu foil begins to significantly increase and shows a linear rise, indicating that oxidation intensifies and Cu becomes unstable at this potential; And aluminum foil covers the entire polarization potential range.

1. The size of the octahedral voids in the lattice of metallic aluminum is similar to that of Li, making it easy to form metal interstitial compounds with Li. Li and Al not only form alloys with the chemical formula LiAl, but may also form Li3Al2 or Li4Al3. Due to the high reactivity of the reaction between metal Al and Li, metal Al consumes a large amount of Li and its own structure and morphology are also destroyed, so it cannot be used as a current collector for the anode electrode of lithium-ion batteries; During the battery charging and discharging process, Cu has only a small lithium insertion capacity and maintains stable structure and electrochemical performance, making it a current collector for the anode electrode of lithium-ion batteries; At 3.75V, the polarization current of Cu foil begins to significantly increase and shows a linear rise, indicating that oxidation intensifies and Cu becomes unstable at this potential; However, aluminum foil has a small and constant polarization current throughout the entire polarization potential range, and no significant corrosion phenomenon was observed, maintaining stable electrochemical performance. Due to the cathode electrode potential range of lithium-ion batteries, Al has a small lithium insertion capacity and can maintain electrochemical stability, making it suitable as a cathode electrode current collector for lithium-ion batteries.

2. The copper/nickel surface oxide layer belongs to semiconductors, with electron conduction. The oxide layer is too thick and has a high impedance; The aluminum oxide layer on the surface of aluminum is an insulator, and the oxide layer cannot conduct electricity. However, due to its thinness, electronic conductivity is achieved through tunneling effect. If the oxide layer is thick, the conductivity level of aluminum foil may vary, and even insulation may occur. Generally, it is best to clean the surface of the current collector before use, which not only removes oil stains but also removes thick oxide layers.  

3. The cathode electrode potential is high, and the aluminum thin oxide layer is very dense, which can prevent the oxidation of the current collector. The oxide layer of copper/nickel foil is looser, and to prevent oxidation, it is better to have a lower potential. At the same time, Li is difficult to form lithium intercalation alloys with Cu/nickel at low potentials. However, if the surface of copper/nickel is heavily oxidized, Li will undergo lithium intercalation reaction with copper/nickel oxide at slightly higher potentials. Aluminum foil cannot be used as a anode electrode as LiAl alloying occurs at low potentials.

4. The current collector requires pure composition. The impure composition of Al will lead to the densification of the surface facial mask and pitting corrosion, and even more, the destruction of the surface facial mask will lead to the formation of LiAl alloy. The thickness requirement for copper aluminum foil used in lithium batteries has been met with the rapid development of lithium batteries in recent years, and the development of current collectors for lithium batteries has also been rapid. The cathode electrode aluminum foil has been reduced from 16um in previous years to 14um, and then to 12um. Nowadays, many battery manufacturers have mass-produced and used 10um aluminum foil, and even 8um. The copper foil used for the anode electrode, due to its good flexibility, has reduced its thickness from 12um to 10um, and then to 8um. Currently, a large number of battery manufacturers use 6um for mass production, and some manufacturers are developing 5um/4um which may be used. Due to the high purity requirements of copper aluminum foil used in lithium batteries, the density of the material is basically at the same level. As the development thickness decreases, the surface density also decreases accordingly, and the weight of the battery naturally becomes smaller and smaller, which meets our needs for lithium batteries. Surface roughness requirements for copper aluminum foil used in lithium batteries: For the current collector, in addition to its thickness and weight, the surface properties of the current collector also have a significant impact on the production and performance of the battery. Especially for anode electrode current collectors, due to defects in preparation technology, copper foils on the market are mainly of single-sided, double-sided, and double-sided roughened varieties. This asymmetric two-sided structure leads to asymmetric contact resistance between the anode electrode coatings on both sides, resulting in uneven release of anode electrode capacity on both sides; At the same time, the asymmetry between the two sides also leads to inconsistent bonding strength of the anode electrode coating, resulting in a serious imbalance in the charge and discharge cycle life of the anode electrode coating on both sides, thereby accelerating the decline of battery capacity.