Claus Luber and Steffen Straach of Fraunhofer FEP present a novel roll-to-roll process for producing lightweight metal-polymer current collectors that enhance both the energy density and safety of lithium-ion batteries.
Batteries are a key technology for the transformations of the 21st century. They are already widely used in numerous everyday products such as consumer electronics, communications technology and computers. In the future, other areas of application will gain importance, such as medical engineering (assistance robots in nursing care), defence and civil protection (drones for surveillance and defence), and space travel (satellites and space stations). High performance batteries are indispensable for electric mobility and, as stationary energy storage devices, make a significant contribution to the expansion of renewable energies and thus to the mobility and energy transition.
Commercial lithium-ion batteries (LIB) today have specific energies of 300 to 500Wh/kg. Nevertheless, a large part of global research efforts is aimed at further increasing this specific energy. One important approach is the development of high- performance electrode materials and the optimisation of cell structures to increase the proportion of active materials in the cell. However, improved passive components such as current collectors are equally important(1).
The reduction in the thickness of current collectors and thus the proportion of passive materials in the cell has become the focus of developments. Through continuous improvement of production processes, the thickness of current collector foils in current batteries has been reduced from 12μm to 8μm for aluminium foils and from 6μm to 4.5μm for copper foils.
This reduction in current collectors has reduced the passive mass within the cell by a few percentage points and increased the specific energy of the cell(2). However, due to their thinness, these metal foils are prone to wrinkling or tearing during the battery manufacturing process.
An alternative to these thin metal foils are plastic foils, which are coated with a thin layer of metal.
These metallised plastic films consist of a non-conductive core made of thin polymer film (e.g. PET), which is coated with copper or aluminium on both sides. This compound of a plastic film with metal layers serves as a current collector and replaces pure metal films.
Studies in battery cells have shown that the use of these current collectors can reduce the weight of passive materials, as much of the metal is replaced by lighter PET. In batteries with a temperature-stable separator, these current collectors can also provide protection against thermal runaway of the cell(3).
As part of the project funded by the German Federal Ministry of Education and Research (BMBF) ‘PolySafe – Increasing the safety of lithium-ion batteries through metal-polymer composite current collectors (FKZ 03XP0408)’, a process chain adapted to metal-polymer current collectors was investigated for the production of battery cells in various formats (round cell, pouch cell). The aim was to qualify the individual process steps to such an extent that the resulting safety advantage for different cell designs and cell chemistries could be demonstrated in an application-oriented manner.
In addition to integrating the manufactured metal-polymer current collectors (MPCC) into battery cells, the project partners wanted to adapt aluminium- and copper-polymer current collectors to the requirements of the respective cell design.
The challenge is to design the polymer substrates and the coating process in such a way that a total thickness comparable to current metal foils is achieved while ensuring optimum electrical conductivity of the metal layer. At the same time, the partners are pursuing the goal of bringing the production costs of the current collectors to a competitive level.
The project consortium consisted of six partners along the value chain of electricity collectors: Von Ardenne, Brückner Maschinenbau, the Fraunhofer institute for Electron Beam and Plasma Technology FEP, the Fraunhofer institute for Surface Engineering and Thin Films IST, the Battery Lab Factories (BLBs) of the Technical University of Braunschweig, and Varta Microbattery.
One of the biggest challenges for the use of MPCC in battery cells is the reliable and economical production of MPCC. The metallisation of plastic films is already an established technology in the packaging industry and in the manufacture of electronic components such as capacitors. However, only very thin metal layers in the range of tens to hundreds of nanometers are typically applied here by thermal evaporation or magnetron sputtering.
To be of interest for the battery sector, however, significantly thicker layers in the range of several hundred to thousands of nanometers must be applied with sufficiently high productivity. This poses a challenge for conventional coating processes. One way to overcome this is to sequentially deposit stacks of thin layers. Fraunhofer FEP’s technological approach consists of depositing such a thick layer in a single process step. However, depositing such a thick layer in a single step poses an enormous challenge in terms of cooling the plastic film during coating.
As part of the project, Fraunhofer FEP developed a new process for depositing metal coatings using a roll-to- roll process. In this process, the metal layers are applied to the polymer films by electron beam evaporation. The influence of parameters of web travel, substrate pretreatment, and metal evaporation was investigated, as well as their optimal settings for meeting technological and economic requirements. It is essential to minimise the heat load during the deposition process by using a special cooling method. A gas cooling drum from Von Ardenne, which was tested and optimised in the PolySafe project, was used.
The process was developed in a roll-to-roll pilot coater (the novoFlex® 600) at Fraunhofer FEP. The pilot coater has various technology modules arranged around two cooling drums: a boat evaporator for evaporating aluminium, several sputtering stations with planar and rotatable magnetrons, and an electron beam evaporator. In addition, there are options for in-line measurement of layer resistances and optical spectra of the deposited layers. The pilot coater can process films up to a width of 650mm and, depending on the process, can be operated at web speeds from 1m/min to 600m/min.
For the work presented here, the electron beam evaporator of the novoFlex® 600 was used. In this process, an electron beam is directed into a crucible containing the material to be evaporated in order to melt the material and convert it into the vapour phase. The metal vapour then condenses on the substrate foil, which is guided over a cooling roller located above it.
For the developed process, an electron beam with an acceleration voltage of Ub = 50kV and a beam power in the range P = 35–50kW was used. Higher beam power generally results in a higher coating rate. A high coating rate always causes a high heat input into the film during coating. This heat input results mainly from the heat of condensation of the coating material, which is an inevitable consequence of the coating rate. The heat radiation from the crucible and crucible ingot also contributes to the heat input, albeit to a much lesser extent. This contribution can be significantly reduced with specially developed shields without disrupting the evaporation process.
The necessary cooling of the substrate, a basic requirement for high-rate coating of thermally sensitive, thin polymer substrates, was achieved by a gas cooling drum provided by project partner Von Ardenne.
This creates a fully homogeneous gas cushion between the cooling drum surface and the back of the film to be coated via a special gas supply line. This gas cushion increases the heat transfer several times over compared to a standard cooling drum.
All of these measures combined enable the coating of comparatively thin polymer films with thick metal layers in a single pass with high productivity.
As a result, the project was able to demonstrate coppercoated films with a double-sided metal layer thickness of approximately 1μm per side on PET film substrates with thicknesses ranging from 6μm to 12μm. Measurements of the layer resistance showed resistance values of Rsq = 22mOhm for 1μm thick metal layers. This corresponds to a specific resistance of the deposited layer of ρ = 2.1μΩcm.
The coatings were applied on a coating width of 500mm. A dynamic coating rate of DDR = 7.5μm m/min was achieved. The coating and winding quality was not impaired by any disruptive creases or other defects, which enabled these materials to be further processed into battery test cells, using a roll-to-roll process.
For aluminium, coated films with a double-sided metal layer thickness of approximately 1.4μm per side on PET film substrates with thicknesses ranging from 6μm to 12μm were demonstrated.
Measurements of the layer resistance showed resistance values of Rsq = 34m for these layers. This corresponds to a specific resistance of the deposited layer of ρ = 4μΩcm. Here too, coatings were carried out on a coating width of 500mm, with the dynamic coating rate DDR increased to up to 25μm m/min. Here, too, good coatings and winding quality was achieved without any disruptive wrinkles or defects, which also enabled these materials to be further processed into battery test cells using the roll-to-roll process.
Electrochemical characterisation was carried out by Fraunhofer FEP’s partner TU Braunschweig, which manufactured and characterised multilayer pouch cells from the materials.
References:
1 Z. Zhang, Y. Song, B. Zhang, L. Wang, X. He, Adv. Energy Mater. 2023 , 13 (36).
2 Y. Ye, L.-Y. Chou, Y. Liu, H. Wang, H. K. Lee, W. Huang, J. Wan, K. Liu, G. Zhou, Y. Yang, A. Yang, X. Xiao, X. Gao, D. T. Boyle, H. Chen, W. Zhang, S. C. Kim, Y. Cui, Nat. Energy2020 , 5 (10), 786–793.
3 M. T. Pham, J. J. Darst, W. Q. Walker, T. M. Heenan, D. Patel, F. Iacoviello, A. Rack, M. P. Olbinado, G. Hinds, D. J. Brett, E. Darcy, D. P. Finegan, P. R. Shearing, Cell Reports Physical Science 2021 , 2 (3), 100360.
About Fraunhofer FEP
The Fraunhofer Institute for Electron Beam and Plasma Technology FEP works on innovative solutions for vacuum coating and the treatment of surfaces, liquids and gases.
Based on its core areas of expertise in electron beam technology, magnetron sputtering, and plasma-assisted surface processes, Fraunhofer FEP develops resource-saving and efficient process technologies for the strategic fields of energy and sustainability, life sciences, environmental technologies, smart buildings, and digitalisation.
Fraunhofer FEP offers a wide range of research, development and pilot production options for the development and scaling of both corresponding processes and customised hardware systems. Together with its partners, the institute develops tailor-made and industry-ready solutions and processes for forward-looking coating solutions.
The institute develops technologies for energy efficiency and sustainability, including solutions for photovoltaics, hydrogen and batteries. Key topics include biofunctional coatings, electron beam sterilisation, smart coatings for buildings, and environmentally friendly solutions for mobility, packaging and agriculture. Future focus will be on digitalisation, sputter epitaxy, and plasma chemistry to promote innovation in a wide range of industries.
Fraunhofer FEP develops suitable processes and technologies as well as appropriate prototypes, devices, and key technological components for its industrial customers and partners in research, science, and the public sector worldwide.
