Mechanisms Matter

A post meant for analysis with friends got some attention, and I’ve got some feedback about the analysis from the non-battery community that generally asks:

If you trust the electrochemical data, why don’t you think this is the breakthrough that it claims to be?

Short answer: The way in which the battery works (the mechanism) is as important as the raw data itself. The data does not automatically indicate why the battery works. In fact it almost never directly indicates why the battery works. And understanding why the battery works allows us to predict and exploits its available performance, whether we seek to maximize the energy density, power density or cycle life.

Longer answer: Mechanisms matter because they tell us what fraction of the battery is undergoing electrochemistry and what fraction is support material. If we hypothesize a mechanism that neglects active or required material, we end up overestimating energy density, and then the popular press picks it up as a 10 x breakthrough and I get a lot of email asking me if it’s real and then I have to write a post to work it through.

Take this sketch:

When we build the battery, it has everything it needs for the reaction, and then a lot of support mass for electrolyte, current collectors, conductive additive, packaging, etc. We base experiments on what we think the active mass is, but this is an estimate of what we think is active and what is inert.

In the cell in the sketch, we have the same cell but two different proposed mechanisms, one where 40% of the mass is active, and the one where 80% of the mass is active. In both scenarios the absolute weight of the cell is 10 g, and the capacity is 1,000 mAh.

On the left hand side, we calculate the active material capacity to be 1/2 that of the right hand side. Only one can be correct, and this narrows the range of possible reaction mechanisms.

When reporting data in papers, we often use the active mass because in theory it is easier to reproduce the data on the active mass then having to reproduce every detail of the cell. The problem arises when we miscalculate the active mass, either because we 1) measure incorrectly or 2) we ascribe the wrong mechanism.

This happens all the time and there is nothing nefarious to it: there’s a lot of stuff in the battery required to enable the active materials to do what they’re supposed to do, and it isn’t supposed to be active, but many times it becomes active and it then has to be properly accounted for, because if that mass and chemistry wasn’t there you wouldn’t see the same effect.

This has been a particular challenge for the lithium air community.

So it is important to clearly prove the mechanism because the ultimate achievable performance of the battery depends on what is active, and the mechanism states what mass is active and what mass is passive.