About Lithium-ion Battery Positive and Negative Electrode Materials
For lithium-ion batteries, aluminum foil is commonly used as the positive current collector, and copper foil is commonly used as the negative current collector. 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 lithium batteries for digital products or batteries for electric vehicles, the energy density of batteries is required to be as high as possible, and the weight of batteries is getting lighter and lighter. As for the current collector, the most important thing is to reduce the thickness and weight of the current collector, and intuitively reduce the volume and weight of the battery.
Reason Analysis of Aluminum Foil for Lithium Battery Positive Electrode and Copper Foil for Negative Electrode
There are three reasons why aluminum foil is used for the positive electrode of lithium-ion batteries, and copper foil is used for the negative electrode:
1. Copper and aluminum foil has good conductivity, soft texture and cheap price.
The working principle of a lithium battery is an electrochemical device that converts chemical energy into electrical energy. Then, in this process, a medium is needed to transmit the electrical energy converted from chemical energy, and a conductive material is needed here. Among ordinary materials, metal materials are the most conductive materials. While among metal materials, copper foil and aluminum foil are cheap and have good conductivity.
At the same time, the processing of lithium batteries mainly has two methods: winding and lamination. For winding, the pole piece used to prepare the battery needs to have a certain degree of flexibility, so as to ensure that the pole piece will not be brittle and broken during winding. Among metal materials, copper and aluminum foil are also soft metals.
Finally, consider the cost of battery preparation. Relatively speaking, the price of copper and aluminum foil is relatively cheap, because the resources of copper and aluminum elements are abundant in the world.
2. Copper and aluminum foil is relatively stable in the air.
Aluminum is easy to chemically react with oxygen in the air, forming a dense oxide film on the surface of aluminum to prevent further reaction of aluminum. And this very 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.
3. The potential of the positive and negative electrodes of the lithium battery determines the aluminum foil for the positive electrode and the copper foil for the negative electrode.
The positive potential is high, and the copper foil is easily oxidized at high potential.
The oxidation potential of aluminum is high, and there is a dense oxide film on the surface of aluminum foil, which also has a good protective effect on the aluminum inside.
Both are used as current collectors because they have good electrical conductivity and relatively soft texture (maybe this will also be conducive to bonding). In addition, it is relatively common and cheap. And at the same time, a layer of oxide protective film can be formed on the surface of both.
Material requirements for positive and negative electrodes of lithium batteries
Below we will introduce in detail the requirements for materials for the positive and negative electrodes of lithium batteries from a chemical point of view:
The size of the lattice octahedral voids of metal 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 may also 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. Besides, its structure and morphology are also destroyed, so it cannot be used as a current collector for the negative electrode of lithium-ion batteries.
However, Cu has only a small lithium intercalation capacity during the charging and discharging process of the battery. What’s more, it maintains the stability of structure and electrochemical performance, so it can be used as a current collector for the negative electrode of lithium-ion batteries. When the Cu foil is at 3.75V, the polarization current begins to increase significantly, and it rises linearly, and the oxidation intensifies, indicating that Cu becomes unstable at this potential.
On the other hand, the polarization current of the aluminum foil is small and constant in the whole polarization potential range. Moreover, no obvious corrosion phenomenon is observed, and the electrochemical performance is kept stable. Because in the positive electrode potential range of lithium ion batteries, Al has a small lithium intercalation capacity and can maintain electrochemical stability, so it is suitable as a positive electrode current collector for lithium ion batteries.
The oxide layer on the surface of copper/nickel is a semiconductor, and the electrons are conductive. If the oxide layer is too thick, the impedance will be relatively large.
The aluminum oxide layer on the surface of aluminum is an insulator, which cannot conduct electricity. But because it is very thin, the electronic conduction is realized through the tunnel effect. If the oxide layer is thicker, the conductivity of the aluminum foil is extremely poor, or even insulated. Generally, it is better to clean the surface of the current collector before use. On the one hand, the oil stains can be washed away. And at the same time, it can remove the thick oxide layer.
The positive potential is high, and the thin aluminum oxide layer is very dense, which can prevent the oxidation of the current collector.
While the copper/nickel foil oxide layer is looser, in order to prevent its oxidation, it is better to have a lower potential. At the same time, it is difficult for Li to form a lithium intercalation alloy with Cu/Ni at low potentia. But if the copper/nickel surface is oxidized in large quantities, Li will intercalate lithium with copper oxide/nickel at a slightly higher potential.
However, aluminum foil cannot be used as a negative electrode, and LiAl alloying will occur at low potential.
04 The current collector requires pure components.
The impurity of the Al composition will lead to the incompact surface film and pitting corrosion, and the damage of the surface film will lead to the formation of LiAl alloy. With the rapid development of lithium telecommunications in recent years, the current collectors of 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 and used 10um aluminum foil, and even used 8um.
As for the copper foil used for the negative electrode, due to its good flexibility, its thickness is reduced from 12um to 10um, and then to 8um. So far, most battery manufacturers use 6um for mass production, and some manufacturers are developing 5um/4um.
Since lithium batteries have high requirements on the purity of the copper and aluminum foil used, the density of the material is basically at the same level. As the thickness of the development decreases, the surface density also decreases accordingly, and the weight of the battery naturally becomes smaller and smaller, which meets our demand for dry lithium batteries.
Surface roughness requirements of copper and aluminum foil for lithium batteries
For current collectors, in addition to its thickness and weight, the surface properties of the current collector also have a greater impact on the production and performance of the battery. Especially for the negative electrode collector, due to the defects of the preparation technology, the copper foils on the market are mainly single-sided, double-sided, and double-sided roughened.
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 leads to the inability to release the capacity of the negative electrode on both sides evenly. At the same time, the asymmetry on both sides also leads to inconsistent bonding strength of the negative electrode coating, which makes the charge-discharge cycle life of the negative electrode coating on both sides seriously unbalanced, thereby accelerating the decay of battery capacity.