As we all know, the four main components of a lithium battery are the positive electrode material, the negative electrode material, the separator, and the electrolyte. However, in addition to these four important parts, the current collector used to store the positive and negative electrode materials is also a crucial component of a lithium battery. Today, we'll discuss the current collector materials for the positive and negative electrodes of lithium batteries.
1. Basic Information of Current Collector
For lithium batteries, the commonly used positive electrode current collector is aluminum foil, and the negative electrode current collector is copper foil. 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 for lithium batteries used in digital products or electric vehicles, we hope to maximize battery energy density and reduce battery weight. The most important aspect of current collectors is reducing their thickness and weight, which directly reduces the size and weight of the battery.
Thickness requirements for copper and aluminum foil used in lithium batteries
With the rapid development of lithium batteries in recent years, the development of current collectors for lithium batteries has also been rapid. The thickness of the positive electrode aluminum foil has decreased from 16µm a few years ago to 14µm, then to 12µm, and now many battery manufacturers are mass-producing 10µm aluminum foil, and even 8µm. For the negative electrode copper foil, due to its good flexibility, its thickness has decreased from 12µm to 10µm, then to 8µm, and currently a large number of battery manufacturers are mass-producing 6µm, with some manufacturers developing 5µm/4µm, which are also possible. Because lithium batteries have high purity requirements for the copper and aluminum foil used, and the material density is basically at the same level, as the thickness decreases, the areal density also decreases accordingly, and the battery weight naturally becomes smaller, meeting our needs for lithium batteries.
Surface roughness requirements for copper and aluminum foil used in lithium batteries
Regarding current collectors, besides their thickness and weight affecting lithium batteries, the surface properties of the current collector also significantly impact battery production and performance. This is especially true for negative electrode current collectors. Due to limitations in manufacturing technology, copper foil on the market is primarily available in single-sided roughened, double-sided roughened, or double-sided coarsened varieties. This asymmetry in the two-sided structure leads to asymmetrical contact resistance between the coatings on both sides of the negative electrode, resulting in uneven capacity release. Simultaneously, this asymmetry also causes inconsistent adhesion strength of the negative electrode coatings, leading to a severe imbalance in the charge-discharge cycle life of the two negative electrode coatings, thus accelerating battery capacity decay.
Similarly, positive electrode aluminum foil is also being developed towards a double-sided symmetrical structure, but currently, due to the limitations of aluminum foil manufacturing processes, single-sided smooth aluminum foil is still the most important. Since aluminum foil is basically rolled from aluminum ingots with relatively large thicknesses, the contact between the aluminum ingot and the rolls must be controlled during the rolling process. Therefore, a lubricant is usually added to the surface of the aluminum foil to protect the aluminum ingot and the rolls. The surface lubricant has a certain impact on the battery electrode, so the surface lubricant is also a key factor for aluminum foil.
Why is copper foil used for the negative electrode and aluminum foil for the positive electrode?
First, copper and aluminum foil have good conductivity, are soft, and are inexpensive. We all know that lithium batteries work by converting chemical energy into electrical energy—an electrochemical device. In this process, we need a medium to transfer the electrical energy converted from chemical energy, and this requires a conductive material. Among common materials, metals have the best conductivity, and among metals, copper and aluminum foil are inexpensive and highly conductive. Furthermore, lithium batteries primarily use two processing methods: winding and stacking. Regarding winding, the electrodes used to manufacture batteries need a certain degree of flexibility to prevent breakage during winding, and copper and aluminum foil are among the softest metals. Finally, considering battery manufacturing costs, copper and aluminum foil are relatively inexpensive, and copper and aluminum resources are abundant globally.
Secondly, copper and aluminum foil are relatively stable in air. Aluminum readily reacts with oxygen in the air, forming a dense oxide film on its surface, which prevents further reaction. This thin oxide film also provides some protection for the aluminum in the electrolyte. Copper itself is relatively stable in air and hardly reacts in dry air.
Third, the positive and negative electrode potentials of a lithium battery determine that aluminum foil is used for the positive electrode and copper foil for the negative electrode, not the other way around. The positive electrode has a high potential, and copper foil is easily oxidized at high potentials. Aluminum, on the other hand, has a high oxidation potential, and the surface of aluminum foil has a dense oxide film, which provides better protection for the internal aluminum.
