surfingslovak wrote:LG Chem has used a ceramic-coated separator, which forms the essence of the heat-resistant battery design, with great success in the Volt for several years now.
The ceramic coated separator serves mainly to keep the anode/cathode separated and improves the durability of the battery in extremely high temperatures by keeping the separator from breaking down leading to catastrophic failure. This does not seem to be an issue that the LEAFs cells have.
There are two primary modes of capacity loss in lithium batteries and that is the growth of the SEI layer on the negative electrode along with oxidation of the electrolyte.
As the SEI layer grows, this increases the internal resistance of the cell - this effect tends to be greatest when the cell is new and tapers off as the cell ages and the SEI layer thickens.
On the negative electrode charging the battery causes the electrolyte to oxidize - this effect tends to be linear (does not slow down as the pack ages) and is oxidation happens faster at high cell voltages and high temperatures.
In the LEAF, electrolyte oxidation is probably the major component of capacity loss given the high correlation to temperature and the fact that capacity loss does not appear to slow down at all over time.
All covered in the video referenced here:
Why do lithium batteries die and how to improve them?
Yes, thank you, Dave. I think we might be forgetting another aspect of ceramic-coated separators. It's their excellent wettability, which permits the use a high content of PC and EC in liquid electrolytes. That's very helpful when trying to increase the cycleability of lithium-ion batteries at high temperatures. The extreme thermal stability this separator offers directly leads to excellent temperature tolerance, which is critical to large-size Li-ion batteries.
Yes, temperature-related safety issues are mostly related to the dimensional shrinking or melting of the separator, but wettability is also important, since it facilitates electrolyte composition, which is better suited for high-temperature environments. We have already learned earlier, before Prof. Dahn's excellent lecture, that electrolyte additives can play a crucial role in improving life cycle peformance in high temperature environments. I believe that it was mwalsh, who has unearthed the fact that Nissan will likely transition to a ceramic-coated separator when the development of the "hot battery" was discussed.
That said, I don't think that it's appropriate to turn this thread into a discussion of the chemistry changes, which might or might not be coming. Based on the press reports, it appears that Nissan could have been working on this for some time. The point being, if and when a larger battery arrives, it could incorporate a number of other changes and tweaks, including those that would improve the performance in high-temperature environments. I would not automatically assume that a longer-range LEAF would simply contain more cells and have a physically larger battery pack. Is that fair enough to say?
drees wrote:In the LEAF, electrolyte oxidation is probably the major component of capacity loss given the high correlation to temperature and the fact that capacity loss does not appear to slow down at all over time.
I just noticed this, and have to disagree with the notion that capacity loss does not slow down over time. If you look at Stoaty's model, it implies dependency on square root of time, and it does slow down. A slight curve might be imperceptible to the casual observer, especially if it's not tracked or properly plotted, but this does not mean that the relationship between time and capacity loss was linear.
I appreciate your efforts to get more information on this, and help debunk this, but here I have to disagree. The whole purpose of collecting data and the work Stoaty has put into it, was to have a model based on empirical data, which would allow us to project the life cycle of the battery with greater accuracy.
2011/2012 LEAF Projected Capacity Loss over 14 Years (Stoaty's Model with Default Assumptions)