Early electrode and cell manufacturing leaders have scaled up their volume of production by duplicating existing production lines to meet the increasing demand for batteries in electric vehicles. As a result, in some instances, yield has been sacrificed to reduce time to market or increase the number of battery cells supplied.
End-of-line electrical cycle testing of completed cells offers partial assurance of the safety and performance of the final product. However, it is not a foolproof solution. With the increasing adoption of electric vehicles worldwide, the number of product recalls due to battery failures has also risen.
As market competition intensifies, maximizing yield is becoming essential for long-term profitability. However, failure rates of 5 % to 30 % are commonly observed, particularly during the early stages of production scale-up. The electrode coating process is especially challenging and often accounts for the most significant yield losses.
This article introduces a novel metrology technique, in-line mass profilometry, which has the potential to significantly enhance electrode coating quality and improve process yield.
Industrial Metrology
Before the mid-1900s, the thickness or basis weight (mass per unit area) of flat sheet materials produced on an industrial scale could only be measured through destructive contact methods performed after production.
However, as physicists began exploring the applications of X-ray, radioisotope, and other electromagnetic energy-based sensors, manufacturers across various industries gained access to non-contact, non-destructive measurement instruments. (See Figure 1.)
Figure 1. Early beta-ray basis weight gauge. Image Credit: Thermo Fisher Scientific – Production Process & Analytics
The functionality and features of these early instruments evolved as process engineers demanded more real-time data on the mass profile and dimensional properties of the materials being produced.
To obtain thickness data across the sheet, a sensor was mounted on a frame equipped with a motor to drive the measurement head from one edge of the strip to the other (see Figure 2). This fundamental approach remains in use today on manufacturing lines for all types of flat sheet materials, including battery electrodes.
Figure 2. 1952 patent for profile thickness gauge. Image Credit: Thermo Fisher Scientific – Production Process & Analytics
Battery Electrode Production
Modern gigafactory electrode manufacturing lines are optimized for continuous mass production. Ideally, slot-die coaters operating at carefully controlled flow rates deposit precise amounts of active electrode material, suspended in a slurry, onto metal foil substrates. The wet coating is then passed through a long oven to dry the slurry.
Once the top side of the sheet is coated and dried, it is directed through a second coating station, where the same application process is performed on the bottom side. (See Figure 3.)
This process results in a coated electrode "mother roll," which is then slit into narrower formats to be combined with its opposing electrode and a separator film, eventually forming the stacked or rolled final cell.
Traditionally, during the electrode coating process, the mass per unit area of active material—commonly referred to as mass loading—is monitored using a multi-frame gauging system.
In this setup, each frame is equipped with a single-point sensor that moves across the sheet in a synchronized motion, following the measurement path of the previous sensor.
Figure 3. Typical double-sided electrode coating line. Image Credit: Thermo Fisher Scientific – Production Process & Analytics
This synchronized movement enables the system software to determine a differential measurement of the top or bottom layer separately. This information is beneficial to line operators for observing the slot die gap and slurry pump flow. From a quality viewpoint, it only presents calculation data on 2–4 % of the electrode material. (see Figure 4)
Scaling high-volume production has been instrumental in reducing the cost per kilowatt-hour (kWh). However, there is an increasing need to enhance yield while maintaining uncompromising standards of battery cell quality, safety, and performance.
Innovations in in-line metrology provide manufacturers and cell development teams with new insights into the electrode coating process. Real-time analysis of mass loading across the entire electrode at full production speeds is also revolutionizing quality assurance, enabling faster process qualification and development while elevating production standards.
Figure 4. Traversing single point sensors only measure 2% to 4% of the electrode material (note red trace of measurement spot). Image Credit: Thermo Fisher Scientific – Production Process & Analytics
Mass Profilometry
The term mass profilometry represents a new paradigm in in-line metrology, accurately reflecting the capability of an advanced analyzer to deliver real-time mass loading data across the full width of the electrode sheet.
By providing an instantaneous measurement profile, 100 % of the electrode material is monitored. This equips line operators with a comprehensive data set for precise control of coating stations and offers process engineers valuable insights for optimizing process parameters and conducting design studies.
Figure 5. Mass profilometry measurement. Image Credit: Thermo Fisher Scientific – Production Process & Analytics
From a quality and traceability standpoint, the mass profilometer surpasses single-point sensors by detecting loading defects that might otherwise go unnoticed. With its high-resolution, high-speed capabilities, it can identify defects in-line that previously required time-consuming offline destructive analysis.
Issues such as high-frequency oscillations in the coating application, excess coating, scratches, and high edges can all affect the localized loading of active electrode materials, potentially disrupting the critical anode-to-cathode balance.
The example of visualized mass profilometry data in Figure 6 provides a full mapping of an electrode patch, clearly highlighting high-edge defects (indicated by orange and red along the left and right edges) and coating streaks. Pass/fail thresholds and alarm parameters can be customized to notify operators of any changes in process conditions
Figure 6. Mass loading heatmap of a cathode patch. Image Credit: Thermo Fisher Scientific – Production Process & Analytics
By presenting the loading uniformity (or lack thereof), the mass profilometer segregates out-of-tolerance material or coating parts with high or low spots.
Detecting these faults early in cell production is cost-effective in downstream processes, including slitting, stacking, and electrical testing.
Additionally, sections of the electrode that slightly deviate from the target loading can be paired with similarly loaded areas of the opposite electrode material, helping to maintain the ideal anode-to-cathode ratio for optimal battery cell performance.
Compared to the time and length of material that goes under a traversing scanner before a complete edge-to-edge profile measurement is accessible, the immediate profile data from the mass profilometer allows users to drastically decrease the time to target when initiating a new production run.
Summary
Existing single-point gauges utilized for electrode mass loading measurement only measure 2–4 % of the electrode material.
This technology has been in use for many decades and has now reached its peak, making it difficult to achieve the meaningful improvements required by the rapidly evolving battery manufacturing industry to enhance overall yield and meet the reliability and performance levels demanded by consumers.
In-line mass profilometry provides effective, real-time quality assurance, allowing for improvement in the areas of:
- Identifying small non-uniformities and dimensional errors
- Reliable balancing of anode and cathode mass loadings
- Faster identification to lower scrap and reduce downtime
- Complete data traceability and failure analysis
- Responsive and advanced process control
With access to extensive loading data, pilot plants and gigafactories can meet production targets while ensuring optimal cell quality and safety.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Production Process & Analytics.
For more information on this source, please visit Thermo Fisher Scientific – Production Process & Analytics.