Dry Process - Deep Dive
Dry Process - Deep Dive
Engineering and scaling a novel electrode manufacturing process.
The process of producing electrodes for batteries is considering a revolutionary shift. For three decades these electrodes were made by a wet paint style application, but now the industry is considering whether a novel process is ready for commercialization. In 2004 a start-up called Maxwell Technologies filed a patent around producing electrodes through a dry process technique. After years of validation and efforts they were acquired by Tesla in February of 2019 giving significant credence to the technology.
The wet electrode is traditionally made by mixing conductive material, lithium metal oxides, and a binding agent in a solvent. This yields a suspension or slurry that can be painted onto either aluminum or copper foils (step 1). Although this method has been optimized to the nth degree the process uses undesirable and hazardous solvents.
Over the past few years there has been a strong global push toward sustainability in all facets of life and industry. Maxwell’s dry process technology will be a key driver for future battery manufacturing, because it eliminates the use of solvents.
Dry Process
The dry process (dry painting) involves mixing the conductive material, lithium oxides, and binding agent without solvent. This mixture is electrostatically sprayed onto the current collector and fed through a heated rolling process to effectively press the paste onto the current collector. The picture below walks you through the process from step A to E. It is great that the process is more sustainable and eco-friendly, but researchers also found that it yields electrodes with performance enhancements.
Let’s dive into a paper that describes these benefits and why the industry should consider this process for future electrode manufacturing.
Sustainable and Cost Reducing
The process uses no solvent, so this eliminates exposure and drying needs. This will have a huge impact on the long term environmental impact of the battery industry, and may actually save companies money in the process. In fact, an analysis based on an Argonne battery performance and cost (BatPac) model suggests that the dry process could save ~15% on labor, capital equipment, and plant area/layout (Table below) versus the conventional process.
Performance Enhancements
Researchers also found that the dry process effectively changes the way that the materials are dispersed and then oriented on the current collector. Their orientation has positive impacts on the cohesive strength of the electrode as well as its electrochemical performance.
The author of Solvent-Free Manufacturing of Electrodes for Lithium Ion Batteries compared a 90/5/5 LCO/Carbon/PVDF formulation in both the conventional (wet slurry) and dry process techniques. The conventional method yielded a bonding strength of ~80kPa while the dry process with pressing yielded a increased bonding strength of ~140kPa.
With higher pressing temperatures it was found that the bonding strength could be increased even further. This is most likely due to softening the binding material so that it connect more carbon and active material particles.
Electrochemical Performance
It was found that rate capability, cycle life, and resistance were all impacted positively. In both LCO (lithium cobalt oxide) and NMC (lithium nickel manganese cobalt oxide) cells the higher C-rate performance is considerably better suggesting that more active material is readily available to accept charge. In LCO cells cycled at 3C (charge/discharge in 20 minutes) the cell yielded ~10% higher capacity at cycle number 10.
LCO cells were cycled at .5C (charge/discharge every 2 hours), and after 50 cycles the dry painted electrode retained about 70% of initial capacity while the conventional only retained 58%.
Finally, a method called electrochemical impedance spectroscopy (EIS) was employed to analyze the impedance of LCO half cells. The Nyquist plot displays useful data for showing the collective processes that contribute to a cell’s resistance. The intercept with the Re(Z) axis at high frequency refers to the total amount of Ohmic resistance, which includes electrolyte and electric contact resistance. These cells have low ohmic resistance compared to the semicircle that defines the electrode-electrolyte interfacial impedance. It is clear that the dry process electrode has low interfacial resistance than the conventional electrode.
Lower resistance and better cycling performance is ideal for future batteries, but why is a process change so impactful? The authors analyzed microscopic images of the surface of conventional and dry process electrodes. As you can see the concentration of carbon at the surface is much lower on dry process electrodes. This suggests that there is more active material available to accept electric charge. In turn, this is why dry process electrodes are able to attain higher capacity, better cycle life, and higher rate capability.
Conclusions:
The battery industry is undergoing dramatic innovation in a period of rapid expansion. Companies are pursuing strategies of implementing more sustainable manufacturing techniques by limiting the use of undesirable solvents. The dry process is a potential solution. In fact, Tesla mentioned the efforts that the team is doing on dry process during Battery Day and other companies may follow suit. Stay tuned as it will be groundbreaking news when the first dry process mass production line is installed and used to make electrodes for batteries.
P.S. Shoutout to The Limiting Factor on YouTube for a great synopsis of this technology. Check it out for an even deeper dive along with great visuals.
References:
Solvent-Free Manufacturing of Electrodes for Lithium Ion Batteries. Ludwig, B. March 16th, 2016
Polymer Binders: Characterization and Development toward Aqueous Electrode Fabrication for Sustainability - A. Cholewinski
Tesla Dry Process Discussion starts at 2:00 mark.