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Copper foil is the key material in the battery second only to the positive electrode, negative electrode and electrolyte, and has a great impact on the performance and cost indicators of the battery. In the development process of the lithium battery industry, the thickness of the negative electrode current collector material is gradually reduced from rolled copper foil to electrolytic copper foil. At present, the thickness of ultra-thin electrolytic copper foil is less than 4.5 microns, and it is more difficult to greatly reduce the thickness in the future. In order to further increase the density and reduce the cost, the composite copper foil current collector material has gradually become a hot spot of attention in the industry and investment circles. The production capacity under construction and planning is huge, and some of these issues are worthy of attention and discussion.
Composite copper foil refers to a new type of material formed by processing a copper conductive layer on one or both sides of organic film substrates such as PET/PP (the composite copper foil in this article refers to double-sided composite copper foil in particular). Compared with traditional copper foil, the advantages of composite copper foil as a negative electrode current collector material for lithium batteries are that the consumption of copper material is significantly reduced, the weight is significantly reduced, and the safety of the battery can be improved to a certain extent, in another word, it has low cost, high energy density and relative safety, and other advantages.
Since the polymer film substrate is a non-conductive insulator and cannot be directly electroplated, the current main production process of composite copper foil is to first use the PVD vacuum coating method on the substrate film for conductive pre-coating processing, and then use the traditional wet method. Thicken the electroplated copper layer (this process is referred to as the two-step method).
Two-step process
The first step is to pre-coat the conductive layer: Activate the surface of the 3-5 micron thick PET/PP and other polymer film substrates in a vacuum environment to improve the surface quality of the substrate and enhance the substrate and metal coating. Then use PVD process (physical vapor deposition process) to deposit a certain thickness (or a certain square resistance for short square resistance) metal conductive layer on the surface of the substrate (usually copper layer thickness range is 20nm-200nm, square resistance range is 0.2 ohm-2.5 ohm);
The second step to thicken the copper layer: In the atmospheric environment, transfer the pre-coated conductive layer film obtained in the first step to the continuous copper plating production line, and use the wet electroplating process to thicken the copper layer to 600-1000nm (0.6μm- 1 μm) to obtain the desired lithium battery composite copper foil current collector. (The corresponding film finishing and post-processing procedures are not within the scope of this article).Even if the performance indicators are excellent, the cost issue will definitely be the key factor for the large-scale industrial development of composite copper foil. In the comprehensive cost composition of composite copper foil, equipment investment, material and energy consumption are the main parts. Some technical details in the two-step method have a great influence on these parameters, which are worthy of attention and research.Because the two-step method involves two different sub-technical fields, vacuum coating and continuous wet electroplating, each of which is highly specialized, so the entire process of composite copper foil can be accurately judged in a short period of time and the cost of the best combination can be obtained. The control scheme is very difficult, and it may take a period of exploration and joint efforts of all parties. This paper attempts to start with two specific problems in the vacuum coating process, and analyzes the possible cost impact of different process routes on the composite copper foil and contribute on the development of this industry.
1、Selection of PET/PP base film
At present, the industry has not yet standardized the technical parameters of composite copper foil, and the choice of organic material base film is still unclear. There are many manufacturers who choose PET base film, but some manufacturers choose PP base film. Since PET and PP films are polar and non-polar polymers, the surface properties are quite different, and this difference has a significant impact on the bonding force between the substrate and the copper film, resulting in PVD equipment configuration and capacity indicators vary widely corresponding to different substrates.
At present, the hardware and process design especially for the PP base material can fully meet the requirements of the bonding force of the composite copper foil proposed by the manufacturer. Under normal circumstances, PVD coating equipment suitable for PP substrates can be compatible with PET substrates (which can be regarded as general-purpose equipment for substrates), but special coating equipment for PET substrates cannot be compatible with PP substrate coating. Due to the complicated activation and pretreatment of PP substrates, the cost of general-purpose equipment is significantly higher than that of special equipment for PET substrates; at the same time, the design processing speed when processing PP substrates is generally lower than the processing speed of PET substrates. Therefore, if the pre-coated conductive layer is calculated separately, regardless of the cost difference of the base material of PET/PP, the cost difference of the remaining parts may exceed 30%, which is worthy of attention. If users in the lithium battery industry uniformly clarify the types of substrates and the corresponding quantitative technical indicators as soon as possible, it will be able to speed up the finalization of related PVD coating equipment, reduce equipment investment costs, and greatly promote the development of composite copper foils.
2、PVD vacuum copper plating and traditional wet electroplating copper thickening process parameters matching optimization
The first step of PVD vacuum copper plating (pre-coating conductive layer) is the key and necessary step in the two-step method. The PVD process has the advantages of environmental protection and cleanliness, small footprint and high degree of automation. Cost, energy and material consumption have a greater impact on the final product cost. When the PVD process is mass-produced, within a certain range of conditions, the energy and material consumption per unit area of the product is basically linearly proportional to the thickness (or square resistance) of the pre-coating. Since the depreciation cost per unit area is approximately proportional to the equipment capacity proportional relationship, and the equipment capacity is inversely proportional to the thickness of the pre-coating, so the depreciation cost per unit area and the thickness (or square resistance) of the pre-coating are also approximately linearly proportional. The technical characteristics of the PVD process determine that it can flexibly adjust and control the film thickness within a certain range, and it is very convenient to obtain the required optimal film square resistance (usually the parameter range selected in the pre-coating conductive layer is 20nm-200nm, and the square resistance is about 0.2 ohm-2.5 ohm), so the production process parameters of the first step can be selected and matched according to the optimal pre-coating square resistance data required in the second step of wet electroplating.
The cost of the thickening link of wet electroplating copper in the second step is usually related to the depreciation of related equipment (equipment capacity), material consumption, sewage treatment and labor site costs, etc., and is less sensitive to energy consumption (electricity charges). The square resistance of the pre-coated conductive layer needs to be at least lower than a certain value to achieve continuous wet electroplating thickening. It is generally believed that the pre-coating square resistance required by specially designed continuous wet electroplating equipment should be at least less than 2.5 ohms (or 2 ohms), relatively common continuous wet electroplating equipment requires lower square resistance of pre-coating, generally less than 700 milliohms (or less than 600 milliohms); usually within a certain range, the smaller the square resistance on the pre-coating conductive layer, the higher the production speed of continuous wet electroplating equipment (the investment of continuous wet electroplating equipment per unit capacity is correspondingly lower), resulting in depreciation and labor site costs per unit area of products in the wet electroplating process.
Based on the above two paragraphs, it can be seen that there must be a relatively optimal process parameter matching combination scheme in the two steps of vacuum plating and wet plating in the two-step method, which can achieve the goal of the lowest comprehensive cost of composite copper foil products (or the largest comprehensive production capacity per unit investment, etc.). The following is a further example to illustrate, classify and summarize and calculate the production capacity and cost parameters of the first step of PVD vacuum copper plating under several different conditions, for the reference of technology and researchers in the industry.
Capacity and cost analysis of a certain type of PET substrate PVD vacuum copper plating equipment
basic indicators
*）The length of the substrate film is greater than 30,000 meters, the thickness is greater than 3 μm, and the width is about 1.5 meters
*）The width of the finished composite copper foil is greater than 1.25 meters, and the coating speed is 5-20 meters per minute.
*）Double-sided coating, the thickness of copper layer on each side is 25-100nm (equivalent square resistance is about 0.5-2.5 ohms)
When equipment is depreciated over five years:
When equipment is depreciated over ten years:
(The annual design capacity is calculated based on the equivalent of 22 hours of coating time per day for 330 days in a year)
It can be seen from the above two tables that with the reduction of square resistance, the cost of PVD vacuum pre-coated conductive film gradually increases, but the relationship between the two is not linear. The price level of electricity is closely related. With reference to the above-mentioned various indicators, and further comprehensively considering the corresponding cost of the thickening link of wet electroplating in the second step, it is possible to obtain a matching parameter combination with the best overall cost.
In addition, with the advancement of technology, the PVD one-step method of forming a 1+1μm metal copper film directly on the surface of PET/PP and other polymer films with a thickness of 3.0-4.5 μm is directly used by PVD vacuum copper plating. Technical characteristics, after effectively reducing equipment costs and energy consumption indicators, are also an industry development direction worthy of attention.
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