Dr. Kai-Philipp Kairies working in a meetingDr. Kai-Philipp Kairies working in a meeting

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Partial Transcript

I want to start with what is plating and why is it worth investigating in the first place. And then we are going to look at when and why [lithium plating] happens in the lithium-ion battery. And as the third part we are going to look at the effects that plating has on the cell.

So plating increases aging and safety risks in the battery due to lithium loss, dendrite formation - which causes safety problems we will see in more detail how this looks later - and SEI [Surface Electrolyte Interphase] re-formation that happens strongly during plating. So the question is: what is it? It is the deposition of metallic lithium on the surface of the anode and which we normally don't want to have, because we want the ions to go into the anode and non stay on the surface of the anode. It is an unwanted process that happens at low temperatures [or] at very high charging currents. So this is just the basic overview of what plating is and now we go into more detail when exactly it happens and what are the exact effects.

Plating occurs when the potential at the anode drops below zero volt versus lithium. Normally, when the cell is fully charged the potential of the the golden material that we've just seen is at around 2 volts versus lithium. So plating can theoretically not happen, but it can happen because of overpotentials in the cell.

As soon as the overpotentials in the cell are larger than this small gap that we have between the voltage of pure lithium versus the graphite that is integrated with lithium, then we can have lithium plating. These overpotentials can caused by the low solid state diffusion inside of the anode material [or] it can be caused by overcharging. Of course, if I try to put more lithium into my graphite than it can take, then this will also cause lithium plating. And it can happen due to an overabundance of lithium-ions at the anode, for example, if we try to charge the cell too fast.

Looking at these three conditions, for example normal diffusion looks like this: so we have the diffusion and they're all going inside the graphite and it's fast enough for all ions to intercalate into the anode. But when the cell reaches 100 percent SOC [state of charge], we can see they are piling up on the surface - very figuratively speaking - and this is what plating is. So the lithium-ions have no more space inside of the anode and they will accumulate on the surface in the form of metallic lithium instead, and this can be normal, or is normally prevented by oversizing the anode. So this should not happen if the cell is manufactured properly, because the anode is always larger than all lithium content in the cell can intercalate into.

Then, if we have very low solid state diffusion speeds, which could be caused by low temperatures, we can see that even though the anode is not full, there is still an accumulation of lithium-ions on the surface. This means the diffusion is too low for the ion influx and, in this case, plating can occur even at low SOCs because - as we saw - the anode wasn't even full. I mean, let's say it was at 50 percent SOC or something and there was still plating at the surface of the anode.

Or it can happen when we have very high charging speeds, which basically causes the same problem: the diffusion speeds are too low. Not because it's too cold, but because there are too many ions arriving at the surface and for this reason [it's] the same as for the low temperatures: the particle surface approaches 100 percent SOC very fast, which causes a high overpotential due to the SEI voltage drop. This again can cause plating even at low SOCs.

So, now, we've seen why plating can happen basically it's all happening because the surface of the particle is fully saturated with ions either because the actual particle was completely full or because the ions inside of the particle cannot move away fast enough for the new particles to arrive.

Now we're going to look at why is it even problematic...

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