By Dr. Adrian Spillmann, Head of Market Segment Battery Solutions with Grinding & Dispersion Technologies at Bühler
The manufacturers of lithium-ion batteries are currently in the process of massively expanding their global capacity. These investments are being driven by rising demand for electric vehicles, among other things. By 2025, the market for such vehicles will grow from today’s two million vehicles to eight million. The light and compact energy storage devices, however, also drive innumerable portable electronic devices, from smartphones to laptop computers. Another application with strong growth potential are the stationary large batteries used for intermediate storage of wind and solar energy before it is fed into the electricity network.
The production of batteries is multi-staged and extremely complex. One of the central process steps is the wet treatment of the so-called electrode slurries. These viscous suspensions are coated onto metal foils. Then the coated foils are dried, calendered and then trimmed to fit the battery application. The quality of the electrode slurries has a direct influence on the electrochemical performance and thus the capacity of the finished battery.
Batch processing has limited capacity and is expensive
Powdered active materials, conductive additives and binders are gently dispersed in a liquid carrier medium in the production. Battery manufacturers usually use a batch process today. The raw materials are kneaded in a mixer and then diluted. This relatively expensive operation takes up to six hours to complete. In addition to the large amount of time it takes, another disadvantage is the limited capacity. No more than 2,000 liters can be processed in one continuous pass, which means that for producing larger quantities, many separate systems need to be installed. That adds expense to the batteries. This process also includes many manual operations that can negatively influence the quality.
A continuous mixing process for electrode slurries
Bühler has used this deficit as an opportunity to develop a continuous mixing process for the manufacturing of electrode slurries. Compared to the conventional procedure, this approach offers a number of advantages: With higher capacities and lower space requirements, the investment and operating costs are markedly lower; the seamless process control in the continuous mixer enables a very gentle and efficient dispersion as well as the possibility of processing a wide range of raw materials; the constant process and product parameter monitoring also improves the consistency and quality of the electrode slurries and helps reduce rejects.
Up to 60 percent lower investment and operating costs
The continuous production process is extraordinarily productive and has a throughput of up to 2,500 liters per hour. One single production line can therefore replace multiple batch systems. The 70 million liters of electrode slurry required by a giga factory per year can be handled with just six lines compared to at least 24 batch systems needed for the same amount. The investment costs are thus a total of 60 percent lower.
Great savings potential is also present through operating costs because the continuous process achieves the same product characteristics but with three times lower specific energy input. The energy consumption per liter of electrode slurry drops from 75 watt-hours for a batch process to 25 watt-hours. Because the equipment footprint is smaller, the energy expense for the dry room climate is reduced by 60 percent. Finally, thanks to the high degree of automation, personnel costs can also be lowered up to 50 percent.
A high degree of design flexibility in terms of raw materials
The continuous mixing process itself also offers various advantages: It can handle several process steps at once as an aggregate, such as mixing, homogenizing, dispersing and degassing. Thanks to the various dosing points for adding solids and liquids, the solids concentration and therefore the viscosity can be varied at any time. The mixing and dispersion process can be adjusted exactly to the requirements of the raw materials which can be sensitive in part. Controlling the shear forces ensures that the active materials and conductive additives are dispersed to the desired degree in the carrier material without overloading them.
In sum, the flexibility in the raw materials used is extremely high. For example, carbon nanotubes can be used as a conductive additive. This material is very difficult to disperse in conventional batch processes, which is why an additional processing step with a three-roll mill or an agitated bead mill is needed.
Fully automated process improves consistency and quality
Improved consistency and quality of the electrode slurries is also an important aspect. In continuous mixing, important process parameters, such as energy input, temperature, dosing rate of the raw materials or pressure, are constantly monitored and recorded. That allows the system to immediately respond to changes.
The inline quality control system QuaLIB also continuously monitors critical product parameters such as the viscosity, particle size distribution or density. Real-time monitoring allows an optimum control of the process and ensures a high level of traceability. Should the quality requirements not be met at some point, the electrode slurry can immediately be transferred to another container. That reduces the amount of electrodes and battery cells that have to be rejected because of defective production of the electrode slurry.
Bühler has comprehensive know-how and years of experience
To achieve the desired product properties with the continuous production process, however, extensive and comprehensive knowledge of the technology is needed. Bühler has used continuous mixing for many decades in various industries and for diverse applications. The company’s process engineers developed the continuous mixing process jointly with Lishen, one of the largest Chinese battery manufacturers, which then implemented the solution successfully in its battery production. Bühler also offers a global network of customer service to support battery manufacturers and ensure they have maximum availability of their systems.