Breakthrough in Lab-Scale R2R Coating: Optimizing Drying With Dual-Sided Sensor
A New Way to See Drying: Confocal Sensors in Battery Electrode Production
A recent study titled “In-line monitoring of a multi-stage drying process for battery electrodes: vol. 2—advancing measurement techniques with chromatic confocal sensors” explores an innovative solution to one of the most critical bottlenecks in battery production: electrode drying. Conducted by researchers at the Karlsruhe Institute of Technology, the study introduces a new in-line monitoring approach that uses chromatic confocal sensors to improve quality assurance and process control during drying.
In lithium-ion battery production, drying inconsistencies can lead to electrode defects, increasing rejection rates and wasting valuable materials. While previous research (vol. 1) focused on scattered light sensors, this follow-up work examines how chromatic confocal sensors perform in a dynamic lab-scale roll-to-roll setup. By using dual-sided confocal sensors, the researchers achieved accurate, real-time thickness and surface roughness measurements—even under belt vibrations.
The study not only shows that chromatic confocal sensors match the performance of scattered light sensors for detecting critical drying stages such as pore-emptying, but it also reveals new possibilities for correlating thickness data with porosity and microstructure evolution.
Keep reading to see how this could influence your lab-scale R2R setup—and help scale smart.
Key Highlights
Chromatic confocal sensors provide real-time, in-line thickness measurements during electrode drying.
Dual-sided sensor setup enables sub-micron accuracy even on vibrating substrate belts.
Surface roughness can be quantified using the standard deviation (3σ) of thickness signals.
The sensors successfully detect the onset of pore-emptying, a critical point in staged drying processes.
Chromatic confocal results closely match those of scattered light sensors used in previous studies.
Combining sensor data with gravimetric porosity analysis offers insights into drying mechanisms.
Sensor readings show potential for correlating with pore structures, though further study is needed.
The compact, heat-tolerant design of the sensors supports integration into lab and industrial dryers.
The Laboratory Roll-to-Roll Coater is the ideal coating machine for scaling up to roll-to-roll processing of battery electrode materials or for testing and optimizing battery electrode slurries.
Why This Study Matters
The drying step in battery electrode manufacturing is notorious for being difficult to control. Inconsistent drying can lead to defects such as binder migration, delamination, and poor porosity distribution—all of which can compromise battery performance.
Traditional control strategies rely on pre-set drying profiles, but real-time feedback is limited. That’s where in-line monitoring becomes valuable. This study builds on earlier work by demonstrating that chromatic confocal sensors can be used to monitor drying progress and surface properties with high precision.
For engineers and lab managers working with roll-to-roll coaters, these findings could make a real difference. With more accurate data from the dryer, you can better tune your process settings and avoid defects before they happen.
Chromatic Confocal Sensors: The Basics
Confocal chromatic sensors work by projecting a polychromatic light beam that is split into different wavelengths. Each wavelength is focused at a different distance along the optical axis. When this light hits a surface, it reflects back and is analyzed to determine the exact distance to the surface. This allows for non-contact, high-resolution thickness measurements.
The dual-sided setup used in this study places two sensor heads on either side of the coated web. This configuration compensates for substrate vibration by summing the signals from both sides, maintaining sub-micron precision in the total thickness measurement.
This setup also measures variability in the thickness signal, which serves as a proxy for surface roughness. This is crucial because roughness tends to increase as drying progresses, especially during pore-emptying—a phase when solvent is removed from the microstructure of the electrode.
The Slot-die Coater is an excellent choice for researchers focused on developing and optimizing battery electrode slurries at the laboratory scale.
Detecting the Onset of Pore-Emptying
A key goal of this study was to identify whether chromatic confocal sensors can detect the onset of pore-emptying—a stage where the solvent leaves the internal pores of the electrode film. This is a critical point in multi-stage drying processes where drying conditions typically need to change (e.g., from fast to slow drying) to prevent structural defects.
By comparing the confocal sensor data with scattered light sensor data, the researchers found similar trends. Both sensors showed a stable signal during the early drying stages, followed by a sharp increase in roughness and a drop in reflected light intensity at the onset of pore-emptying.
Interestingly, the chromatic confocal sensors displayed a secondary increase in reflected light intensity later in the drying process. The authors attribute this to a decrease in solvent vapor in the air above the film, rather than changes in the film itself—highlighting the importance of understanding both the measurement environment and the sensor's limitations.
Porosity, Film Shrinkage, and Drying Mechanisms
To push the method further, the researchers conducted gravimetric experiments where they interrupted the drying process to cut out film samples. They measured area weight and calculated porosity using known formulas based on dry mass and film thickness.
By correlating this with confocal sensor data, they tracked how porosity evolves during drying. Initially, porosity is zero. As the film dries and shrinks, porosity begins to increase—first slowly, then more rapidly as film shrinkage ends and evaporation from pores dominates.
The chromatic confocal sensors were able to capture the onset of pore-emptying reliably but were less useful for tracking later stages like film shrinkage completion or the final drying phase. The reflected light signal was influenced by airflow conditions rather than film properties alone.
Still, this approach offers valuable insight into the drying mechanisms and could support more advanced control strategies for staged drying.
Double-sided battery coating on a Laboratory Roll-to-Roll Coater.
A Step Toward Smarter Drying Control
While chromatic confocal sensors may not replace all characterization methods, their ability to detect pore-emptying and measure surface roughness in-line makes them a powerful tool for lab and pilot-scale R2R processes.
Their compact sensor heads and fiber-coupled design make them suitable for integration inside technical dryers, even in high-temperature zones. This opens new opportunities for smarter, sensor-driven drying control in real time.
The study also lays the groundwork for future research into pore size distribution and deeper microstructural analysis using sensor data. While such goals require advanced techniques and calibration (e.g., mercury porosimetry), the initial results are promising.
Future Outlook
There are several directions for expanding this research:
Developing sensor calibration methods to link signal variability with pore size distribution
Integrating airflow monitoring to isolate gas-phase effects on sensor readings
Automating real-time adjustments in multi-stage drying profiles based on sensor input
Scaling the method from lab-scale to industrial roll-to-roll setups
As battery technologies and manufacturing standards evolve, so too must our tools for monitoring and quality control. This study provides a glimpse into how advanced sensing can make drying more predictable, efficient, and scalable.
Conclusion
This work demonstrates that chromatic confocal sensors are a robust and practical option for in-line monitoring of electrode drying in lab-scale roll-to-roll setups. They match scattered light sensors in detecting the onset of pore-emptying and offer additional insights into surface roughness and film thickness.
Although limitations exist in tracking the full progression of drying or porosity development, the sensors still deliver actionable data that can improve drying strategies and reduce material waste. For labs and pilot lines focused on optimizing lithium-ion battery electrodes, chromatic confocal sensors are worth serious consideration.
Authors
Jonas Mohacsi (Karlsruhe Institute of Technology)
Kevin Ly (Karlsruhe Institute of Technology)
Jannes Hohlweck (Karlsruhe Institute of Technology)
Philip Scharfer (Karlsruhe Institute of Technology)
Wilhelm Schabel (Karlsruhe Institute of Technology)
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