Characterizing electrode coatings in lithium-ion battery production

Online metrology represents an efficient and accurate means of ensuring quality, safety and consistency – all of which are critical considerations in the battery industry. These tools are particularly useful in terms of the electrode coatings used in lithium-ion batteries.

This interview with [name] from Thermo Fisher Scientific looks at some of the available tools and techniques for the online characterization of electrode coatings, highlighting the advantages and disadvantages of each approach.

They also provide a summary of the theoretical basis of metrology measurements, as well as providing useful insights into the best ways to optimize these tools for use with battery materials.

Could you start by introducing the concept of the electrode coating line and the typical costs involved?

An electrode coating line is an expensive piece of equipment, both in terms of direct production costs and indirect costs. This tool needs to be used regularly and with a high degree of productivity in order to ensure a return on investment.

Production costs include the line itself and the raw materials. The substrate and coating materials can be expensive, as are the utilities to run the line and the people to support, monitor and control this. These direct costs add up when making the material itself.

Indirect costs may occur if the line goes down, in the form of an opportunity cost. The cost of quality should be considered to ensure that the material meets your customer’s specifications and any maintenance that needs to be done to address wear and tear on the equipment.

How is electrode quality currently measured, and what are the advantages and disadvantages of leveraging an online metrology approach instead?

Electrode producers currently check their quality at the end of the line, roll or sheet. They do this by cutting a section and using a hand mic to measure the thickness of the coded section and the uncoded section.

The hand mic is very user-dependent and position-dependent, which will only measure the high points. Due to this, some concerned producers and QA departments are using a waste strip method, whereby they punch a coupon out, weigh it, remove the coating and then weigh it again.

The difference between the first and the second weight gives the basis weight, for example, the loading in grams per square meter of the coded material on top of the substrate.

This is more user-independent, and it provides better resolution and accuracy. This method has specific tolerances relative to the dimension you are cutting out and the scale resolution.

The drawbacks of both methods are that they take place after the product has been produced, and each involves a tiny sample size relative to the coded area itself. You are running many meters of material, but you are only checking a small section at the end.

Online metrology brings optimized electrode loading to the production process and increases productivity in terms of time savings.

As you are making these measurements online, if there is a problem, you can correct it in real-time to maintain high quality and ensure customer confidence.

This approach also ensures dependable results, and data is available to support claims around the high repeatability of these online tools. You can also do regular offline validation of measurement reference samples, meaning that if you are concerned about a CPK value, you can go and check that instrument to make sure that the tolerances are as expected.

Another advantage of online metrology is data analytics.

When measuring, you scan back and forth across the strip. This means that more data is collected than scanning just a few data points at the end of the sheet. Scanning covers the entire width and from head to tail, giving you a tremendous amount of data for analytics or machine learning that you might want to implement concerning coating control.

What specific advantages does this approach and these measurements offer to battery manufacturers?

This degree of measurement and control enables a faster response to any changes in the material. You can implement a control, whether this is automatic or manual, due to being able to detect defects at a higher resolution.

You also have access to measurements of the entire area you are sampling, allowing you to generate much more relevant statistics than can be collected from a few data points at the end of the process.

Precise measurements are possible because the sensors are calibrated to give you the most optimum online measurement.

Finally, you have an effective user interface that allows the operator to visualize any changes. You can see if the measurement tool is not where it needs to be and make adjustments. A flexible PLC link also enables remote data storage or automated control.

What sort of sensor technology is used in metrology for electrode coating lines?

The two leading metrology technologies we use – beta and X-Rays – offer non-contact, non-destructive measurements. Both use a radiation source and a detector, and as you place material into the beam, the intensity of radiation that the detector sees drops based on how absorbent the material is.

These measurements can be plotted on a graph, and if the material changes in terms of its density or absorption, there will be an offset in that curve.

This works according to a basic formula called Beer’s Law, which sees the exponent of that attenuation coefficient multiplied by the thickness. These two exponents dictate how the shape of this curve will behave as a function of thickness.

X-Rays are a form of electromagnetic radiation. When we talk about the electromagnetic spectrum, we start with very low frequency, long wavelength, low energy radio waves that can travel many miles.

As we work our way across this electromagnetic spectrum, we move towards higher frequency, higher energy, much shorter wavelengths from microwave, infrared and visible to X-Rays and gamma rays. These wavelengths have much higher energies and the ability to pass through materials.

Electrode measurement uses X-Rays in the range of around 5 kV to 25 kV, depending on what you measure. If you know the photon’s wavelength being used, you can convert that to the energy and electrons in the form of volts.

X-Rays are typically produced via an X-Ray tube, which sees high-speed electrons from a filament directed towards a target. As they strike the target, they convert their energy into X-Ray emissions from the target itself.

You can affect the maximum energy of the photons leaving the X-Ray tube by adjusting its potential.

If you want to increase or decrease the amount of signal, you have to adjust the current through the tube and the target material. Even at a low electron or a low X-Ray tube current level, the photon output is around one thousand times higher than a typical isotope emission.

Beta particles are much higher in energy because they are physical particles, unlike photons. These give up a lot more energy as they pass through any material, meaning that you need higher beta energy than you would with photons because photons are massless and will pass right through the material in terms of scatter or absorption.

This means that beta particles are not giving us the same signal relative to an X-Ray tube, but they do provide a much better measurement in terms of compositional variation.

For example, when using X-Rays to investigate an anode with a copper substrate and carbon or graphite coating, you need to consider the difference between the atomic numbers of copper (29) and carbon (6).

For typical electrode coating measurements, this would result in two orders of magnitude in terms of the attenuation coefficient in the photon range that you’re looking for.

Therefore, when you see a minimal change in copper, it will look like a significant change in graphite. This is because the absorption ratio has a difference of around 40. Any variation in this coating is important because that is part of the electrode itself and is responsible for the battery’s energy density.

How can users optimize the resolution of their online metrology measurements?

Ensuring high resolution with a traditional round beta source where radioactive material is deposited in this small cylinder can be achieved by taking that active material and positioning it in a line. You can achieve a much higher resolution when you have a line source.

Some technologies and manufacturers will try and mask that round beam, but that absorbs some of the active material and reduces the amount of radiation that your detector sees. This means you see a lot more of the active material, and the radio ICEP itself is absorbed by the mask, resulting in a lower signal-to-noise ratio that affects your measurement.

The line source allows you to have a much more narrow beam of around three millimeters of pass line relative to a round beam masked down. Most beam sizes are defined by their full width at half maximum.

Depending on the pass line and the geometry of the source, it is possible to achieve a very narrow or very wide radiation measurement. A narrower X-Ray or beta beam provides a better resolution.

Where the signal-to-noise ratio is approaching one, it will be more challenging to separate the signal from that noise. There are two ways to do that. The first is to use a higher source or a higher power X-Ray, and the other technique is to start slowing things down, averaging and filtering them.

This provides higher counting statistics, which allows you to differentiate your signal from your noise.

Both of those approaches are useful in certain circumstances, but the end result of slowing your signal down and averaging it is that you end up blurring what is going on in terms of any step changes or localized defects.

A high-speed measurement is necessary, ensuring higher activity rather than averaging. This allows us to detect defects on the order of one millimeter or less. Any defect on the one-millimeter level could be critical for a battery.

Using a highly active source and scanning across the coating in the order of three seconds will provide a sufficient volume of data to ensure a good indication of what is occurring. It is also essential to ensure that measurement performance is consistent throughout the entire loading range.

How vital is repeatability to metrology for electrode coating lines?

Regardless of the resolution or speed of measurements, it is vital to have a highly repeatable measurement. Repeatable measurements require a highly active source, and you need to ensure very precise measurement via a sensitive detector.

Another thing to consider when scanning back and forth is the sensor lag, which can be minimized by using a faster sensor and not relying on averaging, which can exaggerate shifted or smeared data.

Is there any other advice you would give to companies looking to implement online metrology in their battery production operations?

Whether you are looking at the substrate, the coated material or the press line, it is essential to consider the relative strengths of beta ray and the X-Ray tools and understand where each may fall short in specific applications.

Beta rays are good in terms of their applicability for several different materials. X-Rays have an excellent signal and resolution, but this may be more sensitive to compositional effects.

Once you have all that data, you have a great tool to visualize what is going on in your process. You can look at your product’s uniformity and quality and get information on where the loading might be high in terms of its X, Y position, allowing you to control your process and improve your process yield.

A higher resolution line source can provide insight into what is going on at those strip edges, such as bunny ears, a sliding edge, a scratch, a wrinkle or a fold.

You can get a better indication of what is going on with your product every step of the way, thanks to a timestamp telling you exactly when the measurement is taking place and the ability to trace this in terms of lot number.

Regardless of what sensor technology is being used, various options are available to archive and store the data locally or in a third-party cloud.

New battery applications are emerging almost every day in terms of materials and performance required to meet the needs of the end product that the battery is powering. You need to guarantee quality, safety and battery performance – all of which can be improved via online metrology.

Overall, quality cannot be sacrificed for productivity, but at the same time, productivity should not be sacrificed for quality. That is why online dimensional measurement is critical to battery production.

About Thermo Fisher Scientific – Materials & Structural Analysis

Thermo Fisher Materials and Structural Analysis products give you outstanding capabilities in materials science research and development. Driving innovation and productivity, their portfolio of scientific instruments enable the design, characterization and lab-to-production scale of materials used throughout industry.

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