Building a Better Battery
Anyone with a cell phone can appreciate what lithium-ion batteries have done for portable electronics: no “memory effect,” reliable performance, long life. But in a future that includes large scale wind and solar power installations and electric cars, these often-taken-for-granted powerhouses will have to live up to bigger and bigger expectations. Boosting the lifespan of lithium-ion batteries from a couple of years to a couple of decades is key, and scientists around the globe are finding ways to do this that could work. The trouble is, how can one test a 30-year lifespan in a matter of days? Dalhousie University’s Jeff Dahn, Canada Research Chair and the NSERC/3M Canada Ltd.’s Industrial Research Chair in Materials for Advanced Batteries and Materials Science, has the answer.
ACCN: Most people think of lithium-ion batteries in their computer or cell phone. Are these batteries up to the challenge for things like electric cars and storing energy from wind or solar?
J.D.: I think lithium-ion batteries are up to the challenge. There are a number of things that need to be improved — costs needs to be reduced, the lifetime wants to be increased, especially for storing energy from wind or solar where you’d like the batteries to last 30 years or more — but I think those are problems that can be solved.
ACCN: How do lithium-ion batteries work?
J.D.: Lithium-ion batteries use intercalation compounds as electrode materials. An intercalation compound is a compound that has atomic-scale voids inside its structure that little atoms like lithium can fit inside. Because those voids are pre-existing, when a lithium-ion battery operates, lithium-ions are transferred from one electrode to the other, causing virtually no structural damage to the electrode materials. This is one of the reasons it has a high charge-discharge cycle life.
ACCN: You mentioned that increasing the lifetime of lithium-ion batteries is important for things like wind energy; what determines how long a battery will last?
J.D.: The user uses a lithium-ion battery how he or she wants. You might have an application like a power tool, where the battery would undergo five or six charge-discharge cycles through a workday. On the other hand, in an application like a computer, you might discharge only for an hour and then not use your laptop again for a number of days. The question is: What is it that leads to the decline of the capacity to store energy of a lithium-ion battery? Is it the number of charge-discharge cycles, or is it the time that the battery has been charged after initial assembly?
What you learn is that there are a number of parasitic reactions that are going on inside a lithium-ion battery all the time. Whether the battery is being charged or discharged or just sitting there, these reactions are happening. So there’s a pretty strong component to the failure of a lithium-ion battery that’s basically just time-dependent. These parasitic reactions involve reactions between the charged electrode materials — either the negative or the positive — and the electrolyte. And temperature has a huge impact on these parasitic reactions; they’re all exponentially activated so when you crank up the temperature, they go much faster.
ACCN: How do you measure the rates of these reactions?
J.D.: Coulombic efficiency is the ratio of the amount of charge obtained from the battery during the discharge, compared to the amount of charge that was stored in the battery during the charge. A perfect lithium-ion battery would have a coulombic efficiency of exactly 1.0000000. That would occur if there are none of these parasitic reactions that I’m speaking about. So if it is possible to measure the coulombic efficiency very accurately, one can probe for the presence of these parasitic reactions at a very fine scale.
ACCN: So you did it with off-the-shelf equipment, but you’re the first ones to put it all together?
J.D.: A lot of people in the battery industry have realized the importance of the coulombic efficiency measurement, but they’ve always been limited by the testing equipment. If you’re trying to measure coulombic efficiency, you put on two identical cells and one of them has a coulombic efficiency of 99.7 per cent, and the other one has a coulombic efficiency of 100.1 per cent. Which is impossible. It’s just because of the error in the current controls of that equipment. So, the battery community basically threw up its hands and said, ‘It’d be nice to do it, but our equipment can’t do it, so therefore we’re not going to try to do it.’ I was trying to think of a way that as university researchers we could make an impact on lithium-ion batteries, which are going to need to have a lifetime of 30 years for grid energy storage. How the heck do you tell something’s going to last for 30 years unless you test it for 30 years? Well, these coulombic efficiency measurements can tell you the health of a battery pretty quickly, if you can measure it precisely enough.
ACCN: And nobody else has a device like this?
J.D.: At this exact stage that’s the case. I first talked about it in August of 2009. At that point we weren’t quite finished building it all but we had prototypes and preliminary data. When I spoke about it, there was quite a bit of interest and three companies started efforts to build one. I just heard from one of them. They have their prototype close to being ready, they’ve given it a test drive, and they can measure to a precision of 0.0016 per cent. That’s about a factor of seven better than what we can do with our first-generation machine. Our second-generation machine will be able to match theirs. But my hope is that someone will make available this kind of instrumentation at a very affordable cost for everybody in the industry to use.
ACCN: How have you used it so far?
J.D.: We collaborated with 3M and a battery producer to test electrolyte additives in commercial lithium-ion cells. The battery producer knows already the cycle life of these cells with various combinations of electrolyte additives, which are added to the electrolyte to slow down the parasitic reactions between the electrolyte and the electrode materials. In experiments that lasted about two weeks, we screened 20 different electrolyte additive combinations in triplicate. We were able to show a one-to-one correlation to the coulombic efficiency; the closer it was to 1.0000000, the better the long-term cycle life was. The cycle life testing had taken over a year for the battery producer to accumulate. So it was kind of like a proof of principle experiment. That’s the whole goal, so that researchers like me can make a change to the cell and stick it on a device like this, and know in a couple of weeks whether it’s going to have an impact in cycle life improvement on a many-year time frame.
ACCN: Coulombic efficiency tells you only whether or not it’s working; it doesn’t describe exactly what’s going on inside the battery. How do you get to that?
J.D.: Well, the key point at the moment is that nobody really knows what particular additives are doing. I have a chart on my office wall, which I’ve put there because it was impossible to view on my computer screen. It’s a big chart of the electrolyte components — the primary solvents, the electrolyte solvents, and the additives — in about 80 different types of lithium-ion batteries. You need about four feet by three feet on your wall at 12 point font to show the whole thing. That’s because there’s so many different types of electrolyte additives that various battery makers are using.
The electrolyte additive business at this moment really is a black art, there’s only a few that are well understood. Again, I can’t think of a better way to learn about the impact of electrolyte additives than having a rapid way to screen the various combinations.
ACCN: : Do you think the general public takes batteries for granted?
J.D.: The general public definitely takes batteries for granted, and I’ll tell you why: it’s because they work really well. For your phone or a computer, you might [at some point] need a new battery, but until that point the thing always works. You just have to put it on the charger from time to time, and it does its job perfectly. Most people don’t have any real perception of how it works, because they don’t have to think about it.
ACCN: What drives you to this kind of research ?
J.D.: I think the goal here is to really see renewable energy take its role in the world on a big scale. Lithium-ion batteries are the best way to store wind and solar. People say that battery storage is too expensive. I counter with: what if the battery lasted 100 years? Then you could pay a big up front cost. And this is not outrageous, because all these degradation reactions I speak of accelerate with temperature, so all you have to do is just bury the lithium-ion battery under the ground. It will sit there at 10 degrees C, almost the optimum temperature for it, and last a long time. I bet current technology would last 30 years if it was at 10 degrees C all the time.
ACCN: How long will it take for us to invent the batteries that we’ll need for electric vehicles and renewable energy storage?
J.D.: You know, you get this all the time as a battery scientist: people talk about Moore’s Law for computers, where everything gets better and smaller and faster by a factor of two every eighteen months. Then they say, “You battery guys took from 1991 to 2011 to improve the lithium-ion battery by a factor of two. You guys must be dumb!” It’s not that we’re dumb, it’s that the tests take a long time. If we can speed up the testing turnaround, maybe we can start to approach more Moore’s-like behaviour.
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