Preparation technology and difficulties of lithium battery composite copper foil

Due to the decrease in thickness, copper foil is prone to defects such as pinholes, wrinkles, and indentations during production, resulting in uneven coating thickness and even leakage or penetration when applying active materials on its surface, thereby increasing battery internal resistance and reducing cycle life. At the same time, as the thickness decreases, its mechanical strength significantly decreases, leading to easy breakage during the negative electrode production process, which affects the stability and flatness of the negative electrode size.
At present, composite copper foils prepared by sputtering method using organic thin films as carriers have excellent plasticity of polymers and can reduce the overall weight of copper foils, greatly reducing the amount of copper raw materials used. Meanwhile, the insulating organic thin film intermediate carrier is beneficial for improving the safety of batteries and is a highly promising new type of lithium battery negative electrode current collector material in the current lithium battery industry.
This article reviews and prospects the preparation methods, technical challenges, research progress, and application prospects of polymer composite copper foil, providing reference for further theoretical research and industrial application of polymer composite copper foil.
Preparation method of polymer composite copper foil
The preparation methods of polymer composite copper foil can be roughly divided into three categories, and the specific preparation process is as follows: firstly, copper powder, conductive agent, and adhesive are uniformly mixed to obtain copper powder slurry, and then the slurry is coated on the polymer base film using gravure coating process. Through electrolytic copper foil process, a copper layer is thickened on the prepared base film, and finally composite copper foil is prepared. Due to the poor compatibility and dispersibility between copper powder and adhesive during the preparation of the slurry, copper powder is prone to agglomeration, resulting in significant differences in the resistance of the base film, and poor uniformity of the surface density of the prepared composite copper foil.
The second method is to use polyester film as the insulation layer, apply adhesive solution on the surface of the polyester film using a dry laminating machine, then dry the polyester film using a dryer, and then use a roller press to bond the electrolytic copper foil with the polyester film to produce a composite copper foil. The composite copper foil prepared by this process has defects such as poor peel resistance, which can lead to delamination or detachment during use.

The third is to use magnetron sputtering and electroplating process to produce composite copper foil. The specific preparation process is generally divided into two steps, as shown in Figure 1. The first process is to use magnetron sputtering deposition particle technology on a 4 µ m (PET/PI) polymer film. By using high-energy charged particles to bombard a copper target with a purity of 99.999%, copper atoms are given enough energy to splash out. A copper seed layer of 0.02-0.08 µ m is sputtered and deposited on the surface of the base film, giving the polymer surface metallic properties and obtaining a 4.5 µ m metalized base film. The second process uses electroplating technology to thicken the metalized film to 1 µ m on both sides, resulting in a new polymer composite copper foil with a thickness of 6.5 µ m. The composite copper foil prepared by this process has the characteristics of strong peel resistance, good ductility, and uniform thickness, and has been applied in domestic new energy storage equipment.
Compared with the above three processes, their advantages and disadvantages are also very obvious. The first two have lower equipment requirements and require less investment; The use of magnetron sputtering technology requires more stringent equipment and production conditions, and requires a larger investment. But in terms of product quality, the latter is superior, which is also the trend of the future development of the copper foil industry.

Technical difficulties and solutions for polymer composite copper foil
When using magnetron sputtering technology to prepare polymer composite copper foil, the high crystallinity, low polarity, and low surface energy of non-metallic polymer substrates can affect the adhesion between the coating and the substrate during sputtering. Additionally, polymer substrates are non-conductive insulators and cannot be electroplated for thickening. Therefore, surface activation treatment is required first to form a conductive metal film, followed by electroplating thickening process. So, the quality of the seed layer on the polymer surface ultimately determines the quality of the composite copper foil. Due to the thickness of the polymer matrix being only a few micrometers, magnetron sputtering deposition technology faces significant difficulties in terms of process and equipment.
2.1 Process aspect
(1) The copper seed layer has poor adhesion. Conventional magnetron sputtering deposition has low particle energy and cannot effectively activate the surface of polymer matrix, resulting in poor adhesion between copper seed layer and polymer matrix, as shown in Figure 2.

(2) Copper film has a high pinhole rate. The conventional magnetron sputtering deposition area is relatively small, and the controllability, density, and uniformity of the copper seed layer structure prepared by depositing on the surface of a wide polymer substrate are poor, which greatly increases the probability of pore formation during the electroplating thickening process, as shown in Figure 3.

(3) Wrinkles and perforations in the substrate. As shown in Figure 4, when copper metal is magnetron sputtered onto the surface of a polymer substrate, due to the high energy of the sputtered copper atoms, the substrate temperature will significantly increase, causing local wrinkling phenomenon; At the same time, copper metal deposits in a high-temperature molten state may melt through polymers, causing perforation problems and subsequently leading to strip breakage and other issues in the continuous production process of subsequent rolls. When preparing composite copper films using magnetron sputtering, in order to improve the adhesion between the copper seed layer and the base film and solve the above-mentioned problems, it is usually necessary to preheat the base film to control the substrate temperature at 40-50 ℃.

(4) The compatibility issue between copper seed layer and electroplating process. The density and roughness of the copper seed layer can affect the quality of the final composite copper foil due to the end discharge effect.
2.2 Equipment aspect
(1) Tension control issue. In order to achieve continuous deposition of copper films on the surface of polymer substrates, vacuum roll to roll continuous deposition is required. However, due to the thickness of only a few microns, polymers have low tensile strength and are prone to stretching and deformation during the production of wide width materials, resulting in problems such as wrinkling and tape breakage.
(2) The uniformity of coating is low. During magnetron sputtering, if the angle or size of the magnetic field injection is not properly controlled, it may cause uneven adhesion of the seed layer, resulting in significant differences in the final product surface density.
(3) The utilization rate of target material is low. Target material, as one of the key substrates for polymer composite copper foil, consumes a huge amount during magnetron sputtering, with a utilization rate of about 30%. How to improve the utilization rate of target materials is currently a technical barrier that restricts the further development of this industry.
(4) The deposition rate of magnetron coating is low. Although conventional magnetron sputtering equipment has a fast rate for metal deposition, the high energy of the metal during the sputtering process causes a significant increase in the temperature of the base film, leading to high-temperature shrinkage and deformation of the polymer. Therefore, heat dissipation treatment is required. How to keep the substrate at a lower temperature while ensuring rapid deposition is currently an unresolved problem.
(5) The compatibility issue between sputtering deposited seed layer and electroplating thickening equipment. Conventional magnetron sputtering deposition requires breaking the vacuum in the magnetron coating chamber, which requires a large amount of vacuum pumping time, resulting in low sputtering deposition efficiency and difficulty in matching with electroplating thickening equipment.

2.3 Solution
At present, in response to the technical difficulties faced by the preparation of polymer composite copper foil using magnetron sputtering technology in terms of process and equipment, domestic research institutions represented by Lanzhou Institute of Physical and Chemical Research, in conjunction with local enterprises, have focused on the design and manufacturing of magnetron sputtering deposition equipment, magnetron sputtering deposition technology, and preparation process matching. Feasibility solutions have been proposed based on the technical difficulties mentioned above, including:
In terms of equipment, by adopting a multi cavity isolation design and optimizing the configuration of multiple sputtering targets, rapid deposition can be achieved while ensuring good density and uniformity of the seed layer. At the same time, the application of multi motor constant speed and constant tension film winding transmission and control technology enables efficient and continuous production.
In terms of technology, high-energy non-metallic atom bombardment, etching, and cleaning techniques are used to inject high-energy copper atoms into polymer substrates, forming an interfacial interpenetrating network structure, effectively avoiding poor bonding strength, detachment, or peeling between the two, and achieving the preparation of high-quality composite copper films.