From 2035, only cars equipped with an electric drive or powered by climate-neutral fuels (eFuels) can be registered in the EU. This not only increases production requirements, but also the demands on the cells themselves. What role quality management plays in this and what manufacturers should bear in mind when starting up production: An overview.
The production of battery cells is on the threshold of mass production worldwide and the corresponding companies have developed into key suppliers for various industries. According to expert estimates, demand will triple compared to today to over 3 terawatt hours (TWh) by 2030 (Fraunhofer ISI). At the same time, cell manufacturers, automotive OEMs, start-ups and joint ventures are increasingly announcing their intention to build up an annual production capacity of more than 10 TWh by 2028 (Fraunhofer ISI). These developments make it clear: The market for lithium-ion batteries (LIB) is booming globally - regardless of borders and crises. What they all have in common is the challenge of establishing a holistic and effective quality assurance system that builds on decades of proven standards from the automotive industry. This is the only way to reliably prevent errors in the production process, detect them at an early stage and minimize the consequences of any errors that do occur in order to ensure a smooth production start-up.
With the growth of the industry described above, the demands on manufacturing companies to produce a sufficient number of flawless products are also increasing. Effective and efficient quality management is becoming a crucial building block for competitiveness.
The aim is always to achieve the highest possible quality of results, i.e. a smoothly functioning battery cell that meets customer requirements, as well as high process quality, i.e. a fault-free production process that directly influences the quality of results. To ensure both factors, certification according to quality standards and the provision of statistical evidence is common practice, particularly in the automotive sector. In this context, numerous automotive manufacturers place high demands on their suppliers. Relevant topics here include not only certification according to quality management standards such as DIN ISO 9001, but also industry-specific certifications such as IATF 16949. In addition, the provision of statistical evidence of machine, process and measuring equipment capability within the framework of VDA procedures is also relevant. However, the complex and, in their entirety, unique challenges of battery cell production sometimes push these methods to their limits. It is therefore necessary for quality management and quality assurance, i.e. the planning, testing, monitoring and improvement of all operational processes that contribute to the manufacture of products, to be extended to include the requirements of battery cell production.
There are various definitions of the term quality. According to DIN ISO 9000, quality refers to the »degree to which a set of inherent characteristics of an object fulfils requirements«. This can be measured using the battery cell application from the end user's perspective based on four key requirement areas:
1. capacity
Car manufacturers are pushing ahead with the optimization of vehicle batteries, i.e. a higher energy density is intended to increase range and reduce costs at the same time. This increases the requirements for volumetric energy density - in short: a lot of (energy) capacity in a small space.
2. performance
Particularly in energy-intensive applications, such as electromobility, it is also important to be able to retrieve the energy stored in the cell in a short time or to be able to supply it again in reverse form as part of fast charging. The ability of each cell produced to fulfil these requirements in accordance with the specifications can also be considered from a quality perspective. In addition to the cell voltage, the main relevant parameters here are the maximum charging and discharging rate and the internal resistance.
3. reliability and service life
The requirements for a battery cell also include reliability. For example, the »State of Health« (SoH) reveals the current condition of the battery. The actual age of the cell and the frequency of charging cycles have an impact on the condition. In addition to normal calendar ageing, quality defects in the manufacturing process are also risk factors for the service life of a battery cell.
4. safety
External influences (e.g. traffic accidents) as well as product and process errors can also often be the cause of an uncontrolled reaction within the battery cell, which can lead to the cell catching fire. This is known in technical jargon as a »thermal runaway«. The issue of »safety« is therefore extremely important from a quality perspective. In addition to a robust product design, it is particularly important in production to avoid process-induced faults as far as possible through stable processes and to reliably recognize faulty products and remove them from production.
The production of high-quality battery cells requires the systematic identification of potential problem areas. At the same time, it is often complex to detect the associated fault patterns automatically, quickly and reliably in the batteries produced. Methodologically validated procedures such as Failure Mode and Effects Analysis (FMEA) help to identify faults at an early stage and enable the production of fault-free products as part of proactive risk optimization.
There are many possible errors along the process chain. For example, lumps, known as agglomerates, can form during the mixing process in electrode production. This can lead to errors in subsequent process steps and performance losses due to reduced capacity and service life. During separation, i.e. punching out the individual electrode sheets before stacking processes, it is also possible for particle contamination - in the form of chips, splashes or splinters - to occur in the cell during the cutting process, which then stick to the foil. This fault pattern is particularly critical in terms of safety: in the worst case, the conductive particles penetrate the separator and cause a short circuit, which can ignite the battery as part of the aforementioned "thermal runaway". If the electrode foil is not stacked or wound with sufficient precision, it is possible that in extreme cases the anode and cathode may inadvertently touch, causing a short circuit, which in turn poses a high risk of damage and safety.
There are various approaches for detecting particle contamination. Imaging methods are often used in automated defect detection. For example, the particularly critical particle contamination described above can be detected using camera- and X-ray-based methods. The latter allow quality inspection of defects that are no longer visible from the outside once the cell has been assembled, particularly at the end of the process chain. In addition to the existing 2D X-ray method, computed tomography (CT) is another method in the field of non-destructive product testing for quality assurance that is currently undergoing intensive further development. CT is often used as a reference method, particularly due to its ability to visualise internal structures in detail and in three dimensions. CT can be used to visualise various types of defects, such as incorrect placement positions, deviations in the number and sequence of cathodes and anodes as well as insufficient quality in assembly and particle contamination. Only with advanced imaging technology including camera, X-ray and/or the described CT inspection at various points in the process chain is it possible to detect defects. Inline-capable CT systems in the context of industrial cycle time requirements are currently still the subject of research and development. In addition, there are many defect patterns and corresponding conventional and innovative measuring and testing equipment in battery cell production.
The rapid development of the battery market requires advanced testing methods, particularly around data analysis. In this context, the use of testing systems based on artificial intelligence can accelerate the successful identification of defects to ensure higher product quality and reduce reject rates. The development of innovative concepts for efficient process monitoring thus helps to ensure that digital tools enable process control for zero-defect production in large industrial process chains in the long term.
Further information & sources:
Fraunhofer Research Institution for Battery Cell Production FFB