How to build a better battery – by Jeremy Meyers, Ph.D., Technology Development Leader, Black Diamond Structures

This is the first in a series of blogs authored by the technical and scientific team of Black Diamond Structures and Molecular Rebar Design. Dr. Meyers has an extensive background in battery storage systems and energy technology. He received his BS in Chemical Engineering from Stanford University, followed by a Doctor of Philosophy in Chemical Engineering from U.C. Berkley. He is currently the Technology Development Leader for Energy Storage with Molecular Rebar Design.

As a battery researcher, it has been rewarding to consider all the variables at play in building a quality battery. It has led me to reflect on the various batteries I’ve worked on in my career. I’ve worked on ultracapacitors, lead-acid batteries, fuel cells, flow batteries, lithium-ion batteries, and a few other emerging (and not-so-emerging) chemistries along the way. I’ve come to realize that all batteries present some common challenges.

There are unique and subtle problems to solve for every battery chemistry, to be sure, but listed here are the problems and solutions that I’ve had to face every time I’ve worked on a new battery. This is also a good overview of the approach that our team takes to solve any unique customer problem:
· Get yourself a Pourbaix diagram
· Figure out what your side reactions are.
· Anticipate when and where phase changes will occur.
· Don’t skimp on sealing.
· Get familiar with test data.
· Design your own experiments.
· Do just enough modeling to help you see inside your battery.
· Figure out how you’re going to manufacture it; the sooner, the better.

A Pourbaix diagram

“If you don’t know where you are going, you’ll end up someplace else.” –Yogi Berra

A Pourbaix diagram isn’t a perfect map, but it’s a useful one. It’s a great way to orient yourself when you’re working on a new electrochemical system. These diagrams show the equilibrium potential of electrochemical reactions of interest vs. the pH of the adjacent electrolyte. If you have a non-aqueous electrolyte, you’ll need a different concentration scale instead of pH.

Battery test data generally only reveal cell potentials. They don’t tell you which materials are stable in the vicinity of each electrode. It’s tremendously helpful to visualize the potentials at which your main reactions occur, and to see what other reactions can be promoted if you overcharge or overdischarge your battery.

The Pourbaix diagram can help you to find the most robust operating windows for potential and composition at each electrode. If you’re working on a novel battery chemistry, the diagram will be incomplete. Even so, it’s a good framing device for figuring out what you need to know.

Figure out what your side reactions are. Shut ’em down.

It’s important to minimize your side reactions in any energy storage system. You have only a finite quantity of active material in your system, and you’ll want to get as much out of it as you can.

Your battery can be useful even if your coulombic efficiency is well below 100%. Consider the practical reality, though, that any unchecked side reaction degrades your battery a little bit at a time. Are you gassing hydrogen and oxygen in a lead-acid battery? You’re boiling away your electrolyte. If you run out of electrolyte in a “maintenance-free” battery, you run out of battery. Does a non-uniform distribution in a fuel cell induce platinum dissolution or carbon corrosion? You’re going to run out of places to run your reactions. That’s an effective way to kill your fuel cell fast.

You can curtail side reactions with materials or with potential control. A single well-considered change to design or operating condition can extend lifetime dramatically. To make a proper change, you must understand the side reaction and its driving forces.

To boost your power and energy density, you need to study and optimize the processes that control your main reactions. If you want to improve the lifetime and robustness of your battery system, you need to halt the reactions that don’t contribute to charge or discharge.

Anticipate when and where phase changes will occur. Then either stay away from those conditions, or figure out how to get the phases to co-exist peacefully.

Phase changes imply nonlinear responses. Nonlinear responses are harder to control than linear ones.

When it comes to phase changes, you have two options. The first option is to ensure that you stay in a region where only a single phase will exist. The other option is to design your system to accommodate the presence of both phases. For example, you could try to design a fuel cell operating system that keeps reactants exclusively in the vapor phase. Alternatively, you could include hydrophilic pores in the catalyst and diffusion layers, to allow liquid to move freely when it condenses. You can do either, but if you don’t know where phase transitions will occur, your system will likely behave erratically.

You’ll be better off addressing phase changes in your design or operating conditions. Doing so at that stage in development will make your system robust and easier to control.

Don’t skimp on sealing

Ugh. I’m an electrochemical engineer, not a mechanical designer. Sealing problems are uninteresting to me, and I wish I didn’t have to deal with them.

However, you’ve got to get sealing right if you want to take your battery design out of the lab and into the marketplace. It’s even more critical if you have to worry about thermal expansion or flow systems in your battery. Letting air into your lithium battery will kill your battery. Letting hydrogen out of your fuel cell could endanger end-users. You might not even be able to operate for long enough to confirm your product’s lifetime unless you can seal reliably.

There are books on the subject, but I learned more from the experts I worked with than I learned from the texts. If you’ve got a sealing expert on your team, consider yourself lucky and be nice to them. They’re going to keep you from chasing down leaks every day of your development program. Chasing down leaks is a good way to slow your progress and decrease your job satisfaction.

You can demonstrate basic battery chemistry on the benchtop, but you need to have a good seal to prove that you have a durable product. If you’ve got a robust battery chemistry, you don’t want to mask that success with an inadequate seal.

Seal design doesn’t have to be something that you control if it’s not your core competency. If it’s not, you’ll need a partner who can provide a seal that’s robust to the operating environment.

To be continued next week