Q&A: Innovations in Battery Manufacturing — Comparing Dry & Wet Electrode Processing
This Q&A is from a 2024 DryPro webinar with the same name.
Recently, Powder & Bulk Solids presented “Innovations in Battery Manufacturing — Comparing Dry & Wet Electrode Processing” as part of its DryPro webinar series.
Huda Ashfaq, lead process engineer at Sila Nanotechnologies Inc., discussed the traditional methods and innovative techniques of manufacturing electrodes. She talked about the critical differences between wet and dry electrode processing. Wet electrode processing, the conventional method, and dry electrode processing, which is a promising alternative, eliminating the use of solvents and associated drying steps.
Huda Ashfaq, lead process engineer, Sila Nanotechnologies Inc.
You can also view the dry & wet battery production webinar on demand here.
Here are the questions and answers from this webinar.
Q. How would you determine the uniformity of the mix (wet or dry): the method and specification?
The uniformity of the electrode is definitely key to the performance. There are different methods that are generally used. One is the SEM method (Scanning electron microscopy). SEM can be used to examine the surface morphology of the materials at high magnification. By analyzing the SEM images, one can assess the distribution of the active materials, the conductive additives, and binders. Uniformity can be judged based on the evenness of the particle distribution and advances and in the absence of agglomerates.
Another method is energy dispersive X-ray spectroscopy, which is EDS. Often coupled with SEM, EDS provides elemental mapping of the electrode materials. The technique offers help in determining the uniformity of the mix by showing distribution of the different elements across the sample.
Another way is XRD, and that can be used to analyze the crystalline structure of the electrode materials, and uniformity is inferred from the consistency of the diffraction patterns, which indicates a homogenous distribution of material. There are so many methods. There's even FTIR, which is Fourier transform infrared spectroscopy, which can detect the chemical bonds and the functional groups, and the electrode mix, and uniformity can be evaluated by comparing the spectra of different sample areas to ensure consistent chemical composition.
And there's Thermogravimetric analysis (TGA), which measures the change in weight and response to temperature changes and can help assess the uniformity of the binder distribution by showing consistent decomposition profiles across different samples. And there's also things like particle size analysis. So, you can actually use laser particle size analysis since most likely it's a smaller powder form. And it just measures the PSD across the electrode mix and determines whether it's a narrow mix or very different across particle size. There are also electrical conductivity measurements. But when you're actually doing studies with the mixer, and you're wanting to ensure that the mixer is creating a very homogenous batch of material, one way you could do it is doing spot samples throughout the mixer and seeing, um, like at time zero, and then after maybe two minutes and after four minutes and actually see how that distribution becomes more and more narrow over time. Or the variance in ecology is not distribution. Um, much the variance, uh, decrease until you would consider it statistically similar.
Q. What is the typical CV obtained with graphite NMC cell after electrolyte filling and formation?
The CV, which is the open circuit voltage of a graphite NMC (nickel magnesium cobalt) cell after electrolyte filling in the formation typically falls under a specific range. It depends on several factors: the state of charge and the specific formulation of the NMC material. There are typical CV values. One is at a fully charged state. In a graphite cell or NMC cell, the typical CV when fully charged is around 4.2 to 4.3 volts. This is more like an upper limit to ensure longevity and safety for the cell. And there is the fully discharged state. So that typically drops down to 3 or 3.2 volts and ensures that the cell is not over discharged, which can lead to degradation and potential safety issues. And the third is just the nominal voltage. The nominal voltage is the average operating voltage, which is around 3.6, 3.7V for graphite or NMC cells. During the actual formation process, the initial charge and discharge cycle that helps in the formation of the solid electrolyte interface layer CV will vary as the cylinder goes. These different stages of charging and discharging and after the formation process and electrolyte filling, the cell usually is brought to a mid-state of charge to stabilize the AC layer. These are just the general ranges.
Q. What do you see as the biggest hurdle to dry electrode adoption?
One of the major things I hear people have issues with is actually the homogenization process. Generally speaking, the wet process and mixing wet material leads to more homogenous mixtures. But a lot of people are having issues with the mixing of dry materials. However, I have worked firsthand with a lot of great dry mixers — as long as you're looking at high shear applications and getting creative depending on what your material is. Sometimes people even add fluid inside the mixers, but then that leads to things like having back false filters. But having a uniform material distribution is usually a bigger challenge. There is also the distribution of the binder, because usually with solvents the binder can get mixed very thoroughly. And if you're using the spray drying method and other methods, it's quite a hurdle to look at these new technologies. Usually there's this specialized equipment that's like high shear and that can help with the uniformity usually involve needing very high initial investment in the specialized equipment. The company has to commit to that and then scale up, from the laboratory or pilot scale to full scale. When it comes to bulk powder solids it is a little bit difficult. But as long as you're well prepared and you have ways you can optimize it; it should be doable.
Q. Why is this dry spraying method bringing a better and more regular distribution of the materials?
That's a great question because sometimes people have issues with the distribution of the material, but it does help with the lack of having issues like cracking happening sometimes. The dry spraying method, specifically dry electrostatic spray deposition, can actually bring about more of a regular distribution of materials due to several key factors that improve uniformity and performance. One is the electrostatic forces, so the mechanism is in dry spraying method. Electrostatic forces are actually used to charge the dry particles of active materials and conductive materials, and then these charged particles are then sprayed onto the substrate, and the benefit is that the electrostatic charge actually helps that even dispersion and the particles across the substrate to ensure a very uniform coating, and it reduces the likelihood of agglomeration. And having better control over your particle size distribution. The dry spraying method also has precise control over the SDE and ensures that homogenous mixture. And then just the removal of solvents. Solvents can sometimes lead to, like we mentioned before, cracking, delamination, things like that.
Q. Is a carbon-coated current collector needed for dry processing?
The use of carbon-coded current collector and dry processing methods can provide significant benefits, but it's not strictly necessary. The decision to use a carbon-coded current collector depends on several factors, including specific requirements of the electrode material and the desired performance characteristics of the battery. Key benefits that you could have if you do decide to use it. So, it's not necessary. But if you decide to use it, it can help with conductivity. It can help with adhesion. It can help with corrosion resistance. In dry processing, the enhanced adhesion provided by the carbon coating can be particularly more beneficial. Um, in this case. But there are certain situations where you may not need it. So high quality binders and advanced mixing techniques. If the dry processing method uses advanced binders and mixing techniques that really ensure excellent adhesion and uniformity, the carbon coating may not be necessary. I have seen a lot of great technologies where you do get great mixing happening and heat and adhesion, so you don't really need it.
Q. Is there any difference in cycle time between dry and wet electrode?
There is a difference in cycle time, because the electrode process includes the mixing and coating, then the drying, and then it goes into calendaring and slitting. Meanwhile, the dry electrode process you generally have lower mixing times. And the extrusion is not time consuming either. There is also no drying phase, whereas in the wet process there's significant long drying times. The calendaring and slitting are actually pretty similar to the wet process. I guess you don't have to think about that portion for cycle time. So there definitely is, I would say, a 20% to 30% or so difference, depending on your process. It’s a reduction in cycle time going to dry processing.
Q. Do you see a clear trend between the wet and dry processing methods, depending on the final application of the battery mass?
Usually processing trends are for applications for consumer electronics. And that's because higher energy density and stability performance are critical. Um, so and also in EVs and grid storage. But I'm still seeing a trend where even consumer electronics and EVs are shifting to dry. Grid storage solutions may still continue it, but they do because they do require longer cycle life and stability. But for dry processing trends, it's more cost-effective applications, environmentally focused applications, and just new emerging technologies. I don't know if the end mass is necessarily influencing whether they should go for wet, dry, or even possibly long-term storage.
Q. How does dry coating compare to wet coating, if you're trying to coat both sides simultaneously?
Generally speaking, I think that the difficulty of the dry process is significantly less because you don't have to worry about the thermal stresses. Just the complexity that comes around the stresses applied when heating and then coating. So, coating both sides of the electrode presents a really unique challenge in both cases. I think they both have different complexities. When it comes to dry coating usually mixing and uniform material distribution is a challenge, but there's definitely those solutions where the advanced techniques of the electrostatic spray deposition or dry powder rolling that can still ensure you're getting even coating on both sides. I think they're comparable when it comes to that. They'll both have similar challenges but dry might be slightly easier.
Q. What is the expected timeframe you anticipate that dry process becomes the method of choice?
Considering a lot of major facilities, I think within this past year has created this shift to the dry processing methods, I would guess, in the next 5 to 10 years. But like I said, it does involve that initial CapEx. So, the company does have to make that move and decide to invest in that. In the next upcoming few years, we’re already going to start seeing it everywhere.
Q. What do you see as the most promising dry electrode processing method?
One of the best methods I think out there is the dry electrostatic spray deposition method. Just because that method actually allows for a wide range of active materials, conductive additives, and binders to be used, it's a very versatile method that can accommodate various chemistries. So, including the traditional lithium-ion materials, but also emerging alternatives as far as electrode adhesion, cohesion. Like the electrostatic spray method (ESD) enhances the adhesion of the particles due to the substrate due to the electrostatic forces applied during that spray process. This leads to better cohesion within the electrode layer and reduces the likelihood of that delamination process. And as far as form factor, like whether it's thick or thin, ESD does offer precision control over the thickness of the electrode layers, so it's capable of producing both thick and thin electrodes, making it suitable for a variety of battery applications from high-energy density cells to high power cells. That is the one I would say is the leading method.
Q. When will we see dry silicone electrodes?
These silicon electrodes have garnered significant interest in the battery industry due to their high theoretical capacity compared to traditional graphite anodes. However, their particular implementation has been quite a challenge due to issues like volume expansion and structural integrity. The development of dry process methods for silicon electrodes aimed to address these challenges. But I don't know what kind of timeline we may see it in. I would say for commercialization, short-term pilot projects could be in the next few years. I think scale-up could be in the next five or so years, and we’ll see some widespread adoption in five plus years.
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