An innovative active material alone does not constitute a good battery. The interactions between the electrode and the electrolyte determine its service life and performance. The right particle morphology and selected additives are crucial for the production of electrodes. The cell design, quality and speed of production processes determine the cost and quality of the product. In this regard a comprehensive understanding of all processing steps under real production conditions is an absolute necessity. Furthermore, new materials or components must be classified before the commercial production and verified in sample lines.
The research portfolio encompasses the development of lithium-ion cells in various standard formats (18650, pouch cells and PHEV-1 cells) as prototypes and sample lines as well as all the processing steps up to industrial production.
New materials make advances in battery performance possible. By processing the new materials into electrodes and full cells, their properties and also their interaction with other parts of the cells can be extensively understood and their possible uses can be analysed. For this purpose, ZSW is developing compositions and new processing methods that will allow the positive properties of the materials to be transported into the cells. Extensive facilities with analysis and test equipment enable a well-founded evaluation of the impact of the processing method on both the material and the properties of the electrodes and cells.
At ZSW, single-layer pouch cells, stacked pouch cells (e.g. 2 Ah), 18650 cells and prismatic cells (PHEV-1) can be produced.
During cell development, the requirements regarding the surface density of electrode coating as well as its microstructure vary according to the use of the lithium-ion cells. Electrodes for high-performance cells (high power) are given a thinner coating and, owing to the increased need for conductive additives, they normally exhibit a low proportion of active mass. In contrast, electrodes for use in high energy applications are generally coated more thickly and the proportion of additives in the composition is reduced to the bare minimum.
At the same time, the porosity of the electrodes determines their kinetics, energy density, adhesive strength and stability. Examinations using the scanning electron microscope (SEM), mercury porosimetry and the electrochemical analysis of the electrodes make it possible to identify the optimum electrode structure.
The cell is set up for an intended performance by balancing the cathode and the anode. This enables the cycle stability, discharge current or a specific operating temperature range to be prioritised.
The results of electrochemical investigations of the electrodes in half cells provide important initial indications for suitably balancing electrodes. The electrochemical investigations of various electrode combinations in full cells with reference electrodes enable a better insight into the interrelationships.
At ZSW full cells of single-layer pouch cells, stacked pouch cells (e.g. 2 Ah), 18650 cells and prismatic cells (PHEV-1) can be produced.
ZSW is, in addition to prototypes of smaller standard cells on a pre-industrial scale, researching on prismatic coil cells that are used in electric vehicles or to store renewable energy sources. Key aspects include the industrial production of cells and the optimisation of individual production processes. With the "research platform for the industrial production of large lithium-ion cells (FPL)", the gaps in the transition from laboratory-scale production to mass production have been closed.
Since operations began in autumn 2014, the first PHEV-1 cells were able to be presented after just 4 months. The aims of the industrial production and process research are:
All processes up to the finished lithium-ion cells, from the preparation of compositions and the electrode production to the cell assembly and formation (charging of the cells), can be developed, optimised and classified under close-to-production conditions using the modular system. The platform is designed for one PHEV-1 cell (20+ Ah) per minute with series-compatible processes.
With this global beacon, the blueprint for commercial cell production in Germany was established at ZSW location in Ulm. The German Federal Ministry of Education and Research (BMBF) provided 25.7 million euros to fund plant equipment and the Baden-Wuerttemberg Ministry of Finance and Economics (MFW) provided six million euros for the building extension.
ZSW is collaborating with industry partners to ensure that the best batteries for electric vehicles or for storing wind and solar energy shall come from Germany in the future. The aim is to continually optimise and research production processes using new materials or improved system components in order to gain important pre-competitive production expertise.
However, new materials can place completely new demands on the production process. For example, if the organic solvent NMP (N-Methyl-2-pyrrolidone), which is extremely questionable in regard to health and the environment, is substituted by water, then the use of alternative water-compatible binders and a new process for coating anodes is necessary.
At ZSW, electrodes and complete cells can be developed and produced from the laboratory to the industrial scale.
When the electrodes are produced, the active material is processed into a dispersion (electrode slurry) with conductive additives, binders and, if required, other additives, and is coated on a collector film. In this process, all substances in the composition affect both the usability and the properties of the finished electrodes, such as the adhesive strength and current carrying capacity.
During the development of the composition, a material composition is developed that makes a reproducible processing process possible and with which the required material properties can be analysed.
Rheometers with rotation and oscillation modes, density measuring devices and a zeta potential measuring device are available to analyse the dispersions, while the impact of the composition on the electrochemistry can be determined with the help of laboratory electrodes in half cells or full cells.
In order to produce coating dispersions (electrode slurries), the active material and the other substances must initially be processed into a dispersion that is as homogeneous and as stable as possible without being damaged in the process. In this process, each material has an influence on the process and the coating result. For example, in the case of the active materials the particle morphology plays an important part.
ZSW is equipped with dissolvers and planetary mixers of different sizes as well as a homogenizing mixer and an agitator bead mill in order to devise a suitable process. Altogether, a range from 250-ml to 60-litre slurry coatings can be produced.
At ZSW, the dispersions can be applied with various methods, using a comma bar, flat die or roll-to-roll on a collector film. Firstly, this enables the use of a professional machine to process materials in the development stage, which are only available in small quantities, and thus produce preliminary demonstration cells. Secondly, with larger quantities of materials this makes it possible to develop high quality electrodes with considerable homogeneity, defined surface density coatings and coating patterns (intermittent coating).
Our coating methods encompass laboratory scale (s. fig.), and the industrial scale with 500 mm thick electrode coating on both sides at 30 m/min. belt speed, using various application systems, and 20 m drying tunnel.
After coating and drying, the density and porosity of the electrodes are specifically set. On the one hand good contacting between the active mass particles needs to be established by compressing, but on the other hand the remaining porosity must enable a sufficient electrolyte influx into the pores. The exact setting of the electrode microstructure determines the dominant transport mechanisms and therefore also the kinetic properties of the electrodes. Furthermore, the energy density of the electrodes is increased and their adhesive strength and stability are influenced by compressing during calendering.
The compressed electrodes are either cut for assembly in 18650 cells or PHEV-1 cells, or die-cut for assembly in pouch cells. The normal electrode width for 18650 cells is 57 mm, while standard format pouch electrodes are 63 mm x 38 mm or 65 mm or 40 mm in size. Other pouch electrode dimensions can be achieved upon request. In the industrial cell production department, electrodes for PHEV-1 cells can be precisely tailored using a roll cutter with four different cutting units (s. picture).
The tape cutting machines used at ZSW guarantee a sharp, burr-free cut edge and also a highly precise spooling and gentle treatment of the material thanks to web edge guide control and movement control.
At ZSW, single-layer pouch cells, stacked pouch cells (e.g. 2 Ah), 18650 cells and prismatic cells (PHEV-1) can be produced. The institute not only assembles the cells but also performs detailed analyses of the cell components. These analyses are very important, in particular for the development and optimisation of new cells.
For assembly, the material is channelled into ZSW’s dry room that has a thawing point of less than -60 °C. In this room, first the anodes and then the cathodes are dried in the vacuum furnace according to the ZSW method. On a semi-automatic coiling machine, each anode and cathode is precisely positioned, aligned in parallel and coiled with a separator in a controlled movement. Depending on the material used, special requirements may arise. For example, in the case of very thin separators, the focus is on avoiding wrinkles while at the same time ensuring tight coiling in a parallel track. The outer layers are formed by surplus separators. After being encased in the container, the following processes are carried out:
ZSW is equipped with a professional batch coiling machine. It can be used to produce reproducible pouch cells from electrodes with edge lengths in the range of 40 mm x 65 mm. Capacities of up to 2 Ah can be demonstrated using standard materials. We can also develop special formats on request.
The assembly of prismatic winding cells is completely automated in a 200 m2 dry room with a dew point of -60 °C that is designed for testing new assembly technologies. The fully automated systems for winding, assembling, and filling of prismatic cells run with a cycle time of 1 cell/min.
An oxygen-reduced room with 240 temperature-controlled cyclisation stations and 2,016 storage spaces is available in a 70 m2 area for the likewise fully automated cell formation (s. picture).