Maintaining sustainable growth while preserving natural resources is one of the biggest challenges the world currently faces.
It is predicted that by 2040, we will be using 30% more energy than we are today, which will require constant technological advances in battery production.
The battery market is expected to see substantial growth over the next few years due to:
• A booming global automotive industry
• Growing demand for energy storage systems and UPS systems
• Growth of the electric vehicle market
• Increased applications in the industrial motive sector
• Growing consumption of rechargeable batteries in consumer electronics.
This means there is an increasing need for those working in this field to be innovative and create new solutions that can keep up with the pace of change across all these sectors.
What are the stages involved in battery production?
The battery production process can generally be broken down into the following steps:
1. Precursor formation
2. Chemical conversion of precursor
3. Electrode slurry preparation
4. Electrode manufacturing
5. Cell assembly
6. Cell formation and testing.
Each step is vital for the final performance and should be well characterized, optimized, and verified. Over the last few decades, innovation in this field is constantly increasing. The development and the improvement of battery efficiency we see today is due to innovative new materials and fabrication process. Therefore, the characterization of these materials is a key step to:
• optimize performance
• understanding the differences between materials
• acquiring the application knowledge for the process
Overall, there are three main aspects that should be controlled:
1. The stability: the best powder-liquid dosing accuracy accounts for superior process stability and constant slurry properties and avoid batch-to-batch variations.
2. Rheological proprieties of battery slurries: tweaking the rheological behaviour of battery slurries by adjusting the operating parameters without making any changes to the slurry formulation.
3. The film formation process: after slurry application on the electrode foil, determining the best drying protocol (time, temperature) to optimize the battery’s features and production.
How was this study carried out?
The experiments were performed by Rheolaser Coating high temperature on NANOMYTE BE-45 (NCA) slurry, Lithium Nickel Cobalt Aluminium Oxide powder from NEI Corporation.
The Rheolaser Coating high temperature measurement is based on an optical technology called Diffusing Wave Spectroscopy (DWS). A laser illuminates the coating, the laser photons travel through the coating thickness and interact with the coating's scatterers (particles, droplets, and polymers).
The different optical paths of the photons induce interfered backscattered waves forming an image on the camera composed of bright and dark spots = Speckle Image.
When the coating presents significant microscopic activity (liquid-like), the speckle image will be fluctuating, some bright spots become dark, and some dark spots become bright. The fluctuation speed is directly correlated to the motion of the “scatterers” and thus to the visco-elastic properties of the material.
With dedicated image analysis, it is possible to determine a characteristic frequency, the microscopic dynamics (mD), which directly correlated to the speckle image fluctuation.
A high value of mD indicates fast speckle image fluctuation, which corresponds to a liquid sample (fast particle motion). However, a low mD value is a sign of a slower evolution of the speckle image and indicates solid-like behaviour.
With this technology, we can also precisely monitor film formation and drying kinetics of the coating. During drying or curing steps, the coating changes from liquid to solid form, meaning the viscosity and elasticity increase significantly.
As a result, both particle mobility and speckle image fluctuations decrease. The values for this vary from high mD value (liquid) to a lower plateau value (solid and no evolution of the structure), as presented in figure 1 above.
What were the results?
Figure 3 below shows the microscopic dynamics (mD) versus time for different sample thicknesses (100 and 250μm) and different temperatures (50 ℃ and 100°C).
From the sudden decrease of the mobility (mD), the characteristic ‘open time’ can be determined. The ‘open time’ is influenced by the wet film thickness and temperature. At the same temperature (50 °C for example), the open time Tc of the thinner sample (100μm, green) is shorter than the Tc of the sample 250μm thick (blue). The same influence is valid at 100 °C.
This result shows that the instrument allows to monitor the battery’s slurries microscopic dynamics (mD) at different drying conditions providing a characteristic time, Tc, which could be tuned by the formulation or other process parameters (Sample Thickness, Temperature, Humidity, etc.) to fit the best with the fabrication process needs.
Another interesting information is the exact determination of the drying time, the time when the mobility (mD) of the sample reaches a steady state (a plateau). To do so, we will focus on the time when the mD reached a plateau at 10-3 (Hz) (as shown in Figure 4).
When the mD level reaches a plateau at 10-3 (Hz) the sample dynamics contribution is almost negligible, indicating that the sample is completely dry. So, it allows the measurement of the drying time Td.
Figure 4 shows that the drying time is influenced by the thickness and temperature. At the same temperature (100°C for example), the drying time (Td) of the thicker sample (250 μm, yellow) is longer than the Td of the sample 100 μm thick (red).
The same influence is valid at 50 °C. To facilitate the ranking of different formulations or other process parameters influence on the drying (Thickness, Temperature, Humidity...), the software allows the measurement of the microscopic dynamics evolution, mDE (Figure 5).
The curves (Figure 5) correspond to the areas under each mD curve (Figure 4) at a selected time window. The mDE is directly correlated to the drying speed. The faster the mDE reaches 100%, the faster the drying.
This result provides more statistics that allow us to rank the mobility of different formulations or other process parameters and their influence on drying time. Figure 5 shows a ranking in agreement with the drying times determined for the different samples (Figure 4).
The software also allows us to determine another characteristic time, ‘t90’, where the slurry’s microscopic mobility decreases by 90% (inset figure5). For this slurry, the ‘t90’ is very close to the ‘open time’ identified in Figure 3 (‘t90’ ≈ ‘Tc’) due to the significant decrease of the mobility (Figure 3).
The Rheolaser Coating high temperature presents an in-situ, non-invasive and useful method to better understand your different materials, which allows:
• The monitoring and precise knowledge of the curing and drying kinetics
• The determination of the characteristic times of the film forming process
• The evaluation of the formulation parameters impact on the film formation
• The optimization of the manufacturing protocol according to different substrate materials.
October 2016 Fullbrook Systems Ltd move to new premises in Hemel Hempstead. After being in the same offices for many years the company moved to more suitable premises