Some years ago, I designed a battery chemistry. Then, I got a semi-related chemical industry job. Then, in 2020, I did some testing at a university lab. Here's the basic design:

As far as I was able to test it, the battery design worked, but the project died. I haven't been able to do much with that design since.

The current name for this battery chemistry is "SMAC" for "sodium metal aqueous cathode". If you want, you can ask about:

  • technical details
  • experimental results
  • why I didn't do X
  • the response from X types of people
  • other battery chemistries

I could certainly provide more details here, but I'm curious what people are most interested in.

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13 comments, sorted by Click to highlight new comments since: Today at 6:35 PM

What were your goals in designing this battery chemistry?

The goal of the battery design was to be something suitable for electric cars, with similar specific energy (that's watt-hours/kg) to Li-ion, less flammability, and lower cost.

If you mean "why was I designing a battery", I guess I just like thinking about designs for new technology.

Why these particular pieces to achieve those goals? (Or, insofar as there were degrees of freedom where you just arbitrarily chose a piece, where were those degrees of freedom and what do the alternatives look like?)

See this blog post for an initial answer.

Eh, I don't think I like AMA style posts without much detail in the post itself. If you're going to reply, why not write it beforehand and include it in the post? This seems like it might have been interesting if you had done so.

  • That would be pretty long. The intro might be something like this.
  • It's hard to do multiple levels of technical detail in one post.
  • I want to know how to order things if I'm talking to a LW-ish investor-ish person.

I don't think it would be TOO long, I happily read through very long posts on here.

However, that said, I was curious enough to read that blog post, and that's about the length and level of detail I expect in a normal short-to-medium size LW post, but it also stopped short of where I wanted it to. I hope that helps calibrate a little? I don't know how "typical" I am as an example LW reader though.

Oh, and because I know it annoys me when people get distracted away from the main question by this sort of stuff, question is "Can you share the experimental results with just enough explanation to understand the methodology", because I think everything else will flow naturally from questions about the experiment and the results.

Sure, but first off, professors don't typically let people work on whatever they want in their lab, so to be able to work on my own thing, there were some...compromises. Plus COVID was happening. So, equipment availability wasn't quite what I'd hoped for.

With batteries, perhaps the main thing you do is assemble test cells and do electrical testing on those. The first thing I tested for was water leakage. That's one of the big obvious questions people had about my design:

> Won't water rapidly diffuse across the electrolyte layer, and either react with Na or get electrolyzed directly?

Normally, that's true, which is why the SMAC design needs special "very salty" water. I estimated the activation energy for water migration to the electrolyte layer being high enough, but I wanted to get experimental validation. So, I did a test with no electrolyte salt, and here was the cyclic voltammetry plot:

You can see current is negligible and very linear, no nonlinearity showing water electrolysis. The little spike is from me bumping a wire.

Then I started making proper test cells - well, manually assembled test cells with graphite electrodes and a teflon foam gasket, not airtight commercial ones. The problem was, this is obviously air-sensitive, and I didn't have a glovebox available. Towards the end I managed to borrow a glovebox for a bit, but even then I didn't have properly degassed liquids. So, all my CV data was affected by oxygen contamination, but I guess I'll show some CV plots anyway.

How do you distinguish oxygen contamination from other problems, and what's the mechanism by which oxygen causes problems?

Oxygen reacts spontaneously with sodium metal. (And various other things.) That causes current to go in and not come back out, but other things can cause that too.

When that happens, telling why can be complicated, but one way to tell if oxygen is the problem is to take away the oxygen and see if that helps.

How do you think discharge rates would affect the battery? Would it behave like a LFP that basically outputs mostly the same rate of Amps in the full spectrum of charge (Voltage varies little with discharge %)  

Do you think an approach like this generates bateries with long lifetimes? 

Did you expect balancing of these particular batteries to be particularly complicated? 

Basically, tell us more! 

Li-ion batteries have solid particles that Li ions migrate into and out of. This can cause particles to break up, especially at high charge/discharge rates. Because there are fewer ions to migrate, fast discharge at low charge is bad for battery lifetime and gives lower voltage.

SMAC batteries have solid particles that dissolve and form as the battery is operated. It doesn't matter if those break up. To some extent, the maximum discharge rate would decrease as some smaller particles disappear during discharge. There's also some Ostwald ripening that happens, which decreases discharge rate a bit over time, until the next charge cycle, but the extent is limited.

Li-ion batteries are limited largely by SEI growth from electrolyte-Li reaction. Charging and discharging accelerates SEI growth because it causes cracking in the existing SEI, especially at high rates.

SMAC battery lifetime would probably be limited by water migration, with charge cycles being irrelevant and only time & temperature being important, but the long-term lifetime isn't clear at this point. Yes, there is a SEI in SMAC batteries, but it's a thin SEI that works for Na but not Li, with less surface area, so it wouldn't cause much capacity loss.

The relative charge rate of Li-ion vs SMAC depends on the thickness of the electrolyte layers, which depends on the manufacturing process rather than the chemistry. The experimental data I got doesn't really indicate this because an insulating oxide layer was forming, and because the test cells used much thicker layers than commercial cells would. But I'd expect it to be similar, meaning max charge rates between 0.1C and 10C.

I'm not sure why balancing would be different.

Personally I would be interested in a longer post about whatever you have to say about the battery and battery design. You could make a sequence, so that it can be split into multiple posts.

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