Difficulties and solutions to industrialization of composite current collectors

1. Three advantages of composite copper and aluminum foil
2. Production process of composite copper and aluminum foil
3. Cost reduction and efficiency improvement of composite copper foil
4. Break the difficulties of industrialization of composite copper foil

Composite copper and aluminum foil, as a potential new lithium battery current collector material, has become one of the hottest hot spots in the industry due to its outstanding advantages such as high safety, long life, low cost, and strong compatibility.

According to forecasts, the market demand for power batteries equipped with PET composite copper and aluminum foil is expected to reach 294GWh in 2025. In China, there are some outstanding aluminum foil suppliers. You can read our article of top 5 aluminum foil manufacturers in China.

However, in the past two years, the industrialization process of composite copper and aluminum foil has generally been less than expected, which has also aroused the public’s concerns about the prospect of composite foil.

Three advantages of composite copper and aluminum foil

The composite copper and aluminum foil has a sandwich structure of “metal conductive layer-polymer support layer-metal conductive layer”, and the PET layer and flame-retardant structure can provide infinite resistance during short circuit in the battery to effectively avoid thermal runaway of the battery.

Traditional copper foil accounts for about 13% of the total weight of lithium batteries, and is a key material that affects the quality and energy density of batteries. And they have three advantages of weight reduction, cost reduction, and safety.

Compared with the 6μm copper foil, the copper thickness in the composite copper foil is reduced by 66.67%, and the aluminum thickness in the composite aluminum foil is reduced by 80% compared with the 10μm aluminum foil. After the saving of metal consumption is replaced by materials such as PET, the weight is lighter and the overall performance of the product is better while ensuring safety.

Moreover, the scale-up cost of PET copper foil is significantly lower than that of traditional copper foil. Taking 6μm copper foil as an example, the unit material cost is 3.73RMBandsquare meter. The copper foil thickness of 6.5 micron PET copper foil is 2 μm, and the total unit cost is 1.3 RMB and square meter, which is lower than the unit cost of copper foil, and the material cost advantage is obvious.

Production process of composite copper and aluminum foil

Composite copper and aluminum foil coating can be roughly divided into two steps:

1.  Evaporation or magnetron sputtering to form metal films with special properties.
2. Electroless plating to thicken the metal film layer.

Evaporation coating belongs to a kind of physical vapor deposition. It is a process in which the coating material is vaporized by a certain heating evaporation method under vacuum conditions, and the particles are deposited and condensed on the surface of the substrate to form a film.

Magnetron sputtering is a type of physical vapor deposition. After electrons collide with argon gas under the action of an electric field, high-energy argon atoms ionize and hit the surface of the target, causing the target to sputter, and the sputtered particles are deposited on the substrate. 

Electroless plating is a traditional electroplating process. After magnetron sputtering of substrates such as PET and PP, a thin metal layer is deposited on the surface of the substrate, and the sputtered metal layer-PET substrate-sputtered metal layer is electrochemically realized. Metal deposition is performed on both sides of the composite to increase the thickness of the metal layer and reduce the resistance.

The ultra-thin composite aluminum foil uses PVD evaporation on the surface of PET and PP and other substrates with a thickness of 4.5-6.0µm, and coats a layer of 1µm aluminum film on both sides to realize the metallization of the film surface and replace the traditional aluminum foil.

The ultra-thin composite copper foil is made of PVD magnetron sputtering on the surface of PET and PP and other substrates with a thickness of 3.0-4.5µm, and a copper film of 20-70nm is plated on both sides with a square resistance of about 0.5-2Ω. The surface of the film is metallized, and then the copper film is thickened to 1µm by electroless plating.

The difference between the composite copper foil and the electrolytic copper foil process is that the core process of the electrolytic copper foil is to deposit the copper sulfate electrolyte through the direct current electrodeposition in the cathode roll to make the original foil. The composite copper foil directly adopts the vacuum coating and electroless copper plating process to coat the copper film on the surface of the non-metallic material.

So why is the composite copper foil not all magnetron sputtering? The advantages of magnetron sputtering vacuum coating are good stability, good uniformity, dense film layer and good bonding force. However, magnetron sputtering has high requirements on the purity of metal materials, and the processing process requires special gases such as high-purity argon, and the processing cost per unit area is higher than that of electroplating.

In addition, magnetron sputtering has a single coating thickness of nanometers. To achieve micrometer copper thickness, multiple sputtering is required, and the relative efficiency is lower than that of electroplating.

There is another question: Why is the composite copper foil not directly electroless plated? That is because PET and other polymer materials have high crystallinity, low polarity and low surface energy, which will affect the adhesion between the coating and the substrate, and most polymer materials are non-conductive insulators, so chemical plating cannot be performed directly.

It is necessary to perform surface treatment and activation on the polymer material first, so that a conductive metal film is deposited on the surface, and then chemical plating is performed. The magnetron sputtering vacuum coating technology can metallize the surface of non-metallic materials such as PET, realize the conductivity of the material, and ensure the density and bonding force of the film layer.

Cost reduction and efficiency improvement of composite copper foil

From the perspective of production, the raw material cost of traditional copper foil accounts for about 83%, while the raw material cost of composite copper foil accounts for about 31%, and the cost of equipment is as high as 50% due to the impact of the initial stage of industrialization. There is more room for effect to reduce costs.

Equipment is an important factor to promote the development of the composite copper foil industry. The main cost reduction path is to dilute the unit fixed cost by improving production efficiency and yield. The production cost of composite copper foil can be disassembled into fixed cost and variable cost. The fixed cost is mainly the depreciation of equipment and workshop, which currently accounts for 50%.

With the scale effect, the unit production cost will be greatly reduced. Direct material costs are difficult to reduce by optimizing production. Direct labor and other manufacturing costs can be reduced by improving production efficiency and yield, but the reduction is relatively limited.

With the advancement of technology, PVD magnetron sputtering is used to complete double-sided copper coating on the surface of 3.0-4.5μm PET and PP polymer film. The industrialization of composite copper foil has landed.

Break the difficulties of industrialization of composite copper foil

Taking composite copper foil as an example, there are three major difficulties in conventional coating technology, which hinder its industrialization process:

Foil perforation: In the process of sputtering the copper seed layer, the high-temperature metal melt splashes and melts through the foil to form perforations. Secondly, because conventional magnetron sputtering is generally atomic deposition, the density of the copper seed layer is poor, which also increases the follow-up. The occurrence rate of pinholes in the plating thickening link.

Poor adhesion of copper film: The particle density of conventional magnetron sputtering technology is low, which cannot effectively activate the surface of the PET and PP polymer matrix, resulting in poor adhesion between the copper film and the polymer matrix, increasing battery safety risks and may cause battery explosion. Here is an article of lithium ion battery explosion for your reference.

Capacity bottleneck: Limited by the rhythm of conventional magnetron sputtering technology, the deposition speed of copper metal is 20-30nmandmin, and the deposition time of thick copper foil is long, which makes it difficult to achieve roll-to-roll production and hinders high-efficiency delivery.

In the face of the above technical constraints, some companies have carried out targeted process iterations. First, high-energy and low-density argon ions are used for bombardment, etching and cleaning, and then copper metal is injected into the flexible organic film substrate to form an interpenetrating network structure at the interface, which overcomes the bottleneck of the bonding strength of the copper foil interface.

Therefore, the long-term integrity of the copper foil interface is maintained, and the copper layer is prevented from falling off due to factors such as battery collision or the volume change of the anode material during charging and discharging. 

On the other hand, the periodic etching of argon ions superimposes the deposition of high-ionization metal copper atoms to jointly densify the copper foil and reduce the risk of perforation of the copper film.

The argon ion etching process can also roughen the surface of the conductive current collector to make it fully contact with the active material, improve the conductivity, and speed up the rhythm of magnetron sputtering. With the winding equipment that completes PET and PP double-sided coating at one time, the production efficiency is greatly improved.

It is worth noting that the constant tension characteristics and intelligent control environment of the film winding transmission system also effectively avoid the stretching and deformation of the copper film and ensure the product yield.

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