REN: A different view of how to improve batteries

2021.05.21 – Ian Page

This was stimulated by a paper in Joule 5 551-563 Mar 17, 2021. " A Reaction Engineering Approach of Non-Aqueous Battery Lifetimes", which is a lot more interesting than the title suggests.

I'll get to that in a minute after setting some context

In my view most of the focus on battery development has been on increasing the factors most important to EV’s: These are capacity per kg and charging speed.

However, when you look at LCOE, what's most important is lifetime. If a $10K battery cycles 600 times in its life rather than 300, its half the cost per used kilowatt hour. If that figure were to approach the cost of the electricity used for charging it would cease to be a major issue.

Now for a bit of science for context.

All chemical reactions are reversible. Thus, if A+B=>C+D then C+D=>A+B

Thus, a reaction will typically end up in a mix of the two sides, the proportions defined by thermodynamics.

To add to the fun, all chemicals are potentially catalysts or anticatalysts for some reaction. Not necessarily the one you want and there are unwanted side reactions e.g., C+D=>E+F, or A+C=>E+F

Thus, if a reaction is allowed to run to thermodynamic completion you can end up with a wonderful soup- especially since even the reaction vessel, air, impurities and stirrers can dump unwanted elements into the mix.

However physical changes are not always reversible. For example, if D was a gas and A, B, C were liquids, it will evaporate off and the reaction will then go to completion.

While research chemists generally don't worry too much about these things as long as they get enough of the C they want, industrial chemists have a century or more of expertise in making reactions go in the direction they want, and getting high yields of what they want, and then purifying it so that the resulting vat of C contains nothing else.

They have great expertise in two domains of reaction vessel: Batch, where the reactants are thrown into the vat and stirred until the thermodynamic ratios are achieved, and Flow, where reactants are constantly pumped in and the results are usually quenched to stop the reaction at some point along pipes. This gives a kinetic mix of products.

The difference between thermodynamic and kinetic can be seen by considering a downhill ski race. All the skiers will eventually arrive at the apre ski celebration (thermodynamic), but at some earlier point in time some will be stuck in trees, some broke their skis, some are in ambulances or hospital, and some just went the wrong way (kinetic)

Now to the paper and the insight.

From the industrial chemist's point of view batteries are a batch reaction chamber, and not only that, a really bad one. 

Batch reactions involve stirring to make sure that all the concentrations of reactants are evenly distributed, and usually a lot of monitoring of the state of the reaction and the temperature and pressure. 

Batteries work with none of that. For example, as a lithium ion moves during charging, it starts deep in a lump of nickel, has to find its way to the surface, then it has to find some electrolyte molecules to solvate it and stabilize its energy in solution. Then it has to move with its associated electrolyte molecules to the anode/electrolyte SEI layer and thread its way through to the surface of the anode. Then it has to sidle into the carbon anode spaces faster than it can convert to lithium metal and plate the electrode. It's obvious that ensuring that all these different movements are synchronized so that ions don't build up somewhere and delay the overall movement is critical, and also that there will be concentration gradients in the electrolyte and the electrodes greater than the thermodynamic end states. 

That means that unwanted side reactions can happen, that wouldn't if the ions flowed quickly from one place to another. The analogy is that people going into a football ground is OK as long as there are no interruptions in the flow, but if someone's ticket is not valid and the flow stops, the build-up of fans is likely to lead to arguments and hospital visits.

An additional issue with batteries is that they are not just a single reaction, they are repeated reactions as the battery is charged and discharged over and over again in the same "reaction vessel".

If an irreversible physical reaction occurs this builds up. E.g., suppose that the nickel electrode leaches a few atoms into the electrolyte each cycle. As they come out of solution again, they probably will not end up on the nickel electrode in the right places in the crystal structure but as a sludge at the bottom of the cell. 

If you imagine that in one battery cycle only 0.5% of the materials end up irreversibly in the wrong place then the battery will be dead in 200 cycles, and useless in 40 cycles.

Given that the best batteries achieve 500 cycles or more, this shows just how good they are, (I think that is better than 99.8% reversible), but also shows how hard it's going to be to get to 1000 cycles (better than 99.9% reversible)

The paper recommends using the vast number of tools and knowledge from 100 years of batch reactor design to drive things further.  Things since to date improvements have mainly been by Edison style research (exhaustive try everything with intelligent guesses), e.g., by making trial reactors to measure all the various kinetics, and the panoply of chemical analysis tools to discover the tiny quantities of side reactions and monitor their build up, to provide a theoretical basis for the next phase of lifecycle improvement.

I recently noted a nice paper where a battery was cut so that you could see it from the side, and the movement of lithium ions through the electrode was monitored by looking at the change in color as it passed through various physical locations in the crystal structure. This is one of the very few papers I have seen on the detailed kinetic behavior. There are also some papers on the static atomic level structure of the materials, but I don't think there was anything similar during the actual development of the battery under charging.

This approach to improving battery cycles is quite hard and painstaking, but probably gives the best chance of the sort of improvement that will make a huge difference.

This is not to ignore ways of just cheating! Extra lithium is added to batteries to allow for losses during operations. Elastomers and other metals are added to electrodes to prevent them breaking up as they expand and contract. Temperature controls are added to ensure they operate at the best temperatures, and so on. All approaches are valuable for what is a very hard problem as we approach an asymptote.

This is worth it because currently static batteries for grid and utility use are cost effective up to 1-4 hours of capacity. This is useful for smoothing out flows, removing peaks, and a small amount of extending the hours of availability, but the main limiting factor on cost in this use is the lifetime not the weight. Doubling the lifetime would halve the cost (roughly) and might allow batteries to handle 8 hours of capacity. At this lifetime, the major cost factor is how often the battery can be used per day (a battery used twice per day, costs half as much capital amortization per kilowatt hour as used once per day.)

As we see the cost factors for batteries are quite complex but extending the number of cycles has a great effect.