Thermal Runaway of Li-Ion Batteries and Energy Measurements in Space

Ann Nguyen:
Hi. I'm Ann Nguyen, Senior Associate Conference Producer with Cambridge EnerTech. Welcome to this podcast for the Battery Safety conference taking place this November 3-4 in Bethesda, Maryland.

Today I'm interviewing one of our speakers, William Walker, Heat Transfer Analyst in the Thermal Design Branch at NASA Johnson Space Center.

Thanks for your time today, Will.

William Walker:
Oh, no problem. I'm excited to be here. I appreciate you taking the time to do this podcast.

Ann Nguyen:
You'll discuss “New Understanding of Energy Distributions Exhibited during Thermal Runaway of Commercial Lithium-Ion Batteries Used for Human Spaceflight Applications” on November 3. Can you describe what's novel about this work and how you came to conduct these experiments?

William Walker:
Sure. I think that it's no new news to anyone in the audience that this conference is oriented towards that thermal runaway is an issue that we deal with when we're talking about lithium-ion batteries. Naturally, because we use these batteries for so many different applications, space included, we have a safety concern.

This study wanted to focus on, what is the total energy content coming out of the cell? The reason we want to do this is because thermal runaway can be such a catastrophic event, we actually ended up reevaluating our certification processes for battery assemblies. We decided that to design safe battery assemblies, we needed to evaluate the thermal runaway concern of said battery assembly, and we need to evaluate whether or not cell-to-cell propagation would happen for that design.

One of the key things you need to know, for that type of design process is how much energy is coming out of these cells so that we can take that energy and go in to do analysis or small-scale testing of various designs, but you have to know how much energy that you're trying to mitigate. When we looked at some of the numbers that were out there in literature, we just thought the numbers were a little low. We also noticed that when you do a standard ARC test of a lithium-ion battery, you're not really able to take into account the fact that while we have ejecta leaving the cell, we have electrolyte leaving the cell, watts of energy leaving via radiation, minimal conduction to the apparatus.

Really we thought that maybe what was out there wasn't the total energy content of the cell, simply because it was just taking measurement from the cell body. We designed an apparatus that allowed us to measure the total energy change of not only the cell body but the gaseous material inside the apparatus as well as the energy change of the apparatus. All of this was placed inside the ARC chamber, which enabled us to determine the total energy content that's really stirring runaway. As we expected, we found nearly double the energy coming out of these cells than previously reported in literature. Why this is significant is if you're basing your design and analysis on a number that is too small, you may wind up with a thermal management system that's not robust enough to prevent cell-to-cell propagation, and then what should have been a single-cell thermal runaway event turns into a catastrophic chain reaction.

Ann Nguyen:
What's the takeaway theme you'd like your presentation to convey to your audience?

William Walker:
The takeaway theme, and I would say this is the main conclusion from the study, is if you're trying to measure any test form, if your goal is to measure how much energy is coming out of a cell and to take that energy count and put that into your design process, it is not significant to base that tally on temperature measurement taken on the cell body alone. Whether you have to take into account the remainder of energy, whether you do a standard ARC test and you imply an approximation factor, if you're familiar with ARC testing, there's an approximation you're going to apply called bifactor. If you ... whatever you do, cell measurement alone is not enough if you're trying to get total energy content. That is what I would say is the main takeaway theme.

Ann Nguyen:
How might your work on thermal runaway and accelerated rate calorimetry related to the space environment and its unique conditions -- like microgravity and vacuum conditions -- enhance the design of safe lithium-ion batteries for our more earthbound consumer and commercial applications?

William Walker:
I would not say that we were necessarily looking at microgravity or vacuum conditions to enhance the safety of the lithium-ion battery, nor do I expect any enhancement in the safety. I think one could consider the differences we would see in the combustion behavior when you're in a vacuum environment. There have been some studies that have shown that there are enough gaseous products created during a vaporization and degradation process at the electrolyte that combustion still happens to an extent because it produces its own combustion products. Nonetheless, even with that I don't think that we would notice a significant change.

Now, where we do enhance the design of safe lithium-ion batteries is at an assembly level. As I was stating earlier, we have to, for space applications now, part of NASA's new certification requirement is that we assess our assembly-level design and whether or not it safely mitigates cell-to-cell propagation. It forces us to examine thermal runaway behavior. What we get from that is the design of new and innovative thermal management systems, whether it be active cooling of some type or some type of heat spreader system, we come up with a design that assumes at some point in time runaway might happen. It takes that assumption and it mitigates the energy that's distributed from that in a way that we don't end up with cell-to-cell propagation.

Ann Nguyen
Well, thank you again Will. We'll wrap up for now, but we'll look forward to an even deeper look at your projects and findings in early November.

William Walker:
Thank you.

Ann Nguyen

That was William Walker of NASA. He'll be speaking during the Battery Safety conference happening November 3-4 in Bethesda, Maryland. To learn more from him, visit www.knowledgefoundation.com/battery-safety for registration info, and enter the keycode "Podcast."

This is Ann Nguyen. Thanks for listening.


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