Cause Of Voltage Fade In NMC Batteries Determined, Reversal Possible


Can’t beat the heat!

The quest to build better electric vehicles is basically about better battery engineering. While there are a number of promising chemistries being worked on in the lab, improvements are still being made to existing chemistries. Take this work on NMC (nickel manganese cobalt) cells, for instance. The University of California San Diego has just published in Nature Energy detailing how they pinpointed the cause of voltage fade in the cells and how they seem to have fixed it.

Apparently, when NMC cells are charged, small, nano-sized defects occur in the cathode (where the charged ions collect) and the amount of energy it can hold decreases with every cycle (charge and discharge) and its voltages fades. The precise dynamic, according to postdoctoral researcher and study lead author Andrej Singer, is down to this: “The dislocations are extra atomic layers that don’t fit into the otherwise perfectly periodic crystal structure.” (Of course! Why didn’t we think of this?)

With the problem identified, the next step was applying a solution. In this case, the key ingredient was heat. When the degraded cathode was heated to some extent, the defects disappeared. To test the treated cathode, it was then placed in a new cell. Tested at a range of voltages, all the way up to 4.7 volts, it was discovered that its storage and voltage potential had returned. This would seem to be a Eureka type moment, but alas, it’s more complicated than that. According to researchers, this approach can not scale. Still, there is reason to be hopeful the research will be helpful.

This is because of the methods used to discover the defects happening: in situ Bragg coherent diffractive imaging technique. Basically, the scientists can now see what is happening inside the cells on the nanoscale during charging and discharging events. It is also hoped that, because of their relatively high voltages, these heat-treated cathode materials can be used for solid-state cells in the future.

Source: Green Car Congress, UC San Diego

Categories: Battery Tech

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12 Comments on "Cause Of Voltage Fade In NMC Batteries Determined, Reversal Possible"

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Good eye! Actually, the source article has magnesium, and it slipped past me.


As I mention to “anon” above, the source article has magnesium and it slipped past me.

How much are we talking about heating the cells to? Do we actually need to remove them from the battery? Can we just short-circuit the battery and have it magically fix itself?

I believe the paper talks about heating the cathode alone, outside of the cell. They don’t state a heat treatment temperature, but I suspect it is well above the maximum temperature for the fully assembled cell. Otherwise, we would see this sort of annealing happening during normal use.

(⌐■_■) Trollnonymous

Heat used to make “the defects disappeared”?

This will work perfect an a Nissan Leaf!!!!!!

I don’t think you can help the LEAF batteries by applying more heat to the cathode. They already wilt from the heat and more heat can’t be good. If that were the case you would think doing a lot of DC Fast Charging would help but it makes it worse.
Liquid cooling to prevent lose in the 1st place seems the best. Just look at the results on Chevy Volt,SPARK EV and Volts. I have 2 SPARK EV in the HOT Phoenix -Chandler area with no problems. The same with the Tesla S and X. Don’t compare the Roadsters since they didn’t have as good of a cooling system as the S and X and 3. Our 3 has been Super but we have only had it a few months.

Obviously this solution won’t work for batteries already installed in a battery pack, but I wonder if there would be an advantage to heat-treating the cathodes before they are installed in cells? It certainly should be worth the experiment to find out!

Yup, it is that “why didn’t I think of that” moment. We already know LiIon moves in clumps, so it’s only natural that fade has something to do with different sized clumps not fitting as well. Heat treating is one way, but I wonder if it’s possible to make more malleable cathode material. That will have LiIon lasting pretty much forever (if cathode malleable forever).

Another approach might be to have smaller grained charge carriers, but that might be tougher (or is it?).

What this fails to mention (and the source article only mentions way down under “Research Details”), is that this is not about the NMC materials used in batteries today, but rather about lithium-rich NMC — which was a promising material that has seen a lot of research attention in the past, but was ultimately abandoned because of the voltage fade problem.

(The Green Car Congress article gets it right, though.)