Are LFP (lithium iron phosphate) batteries more fragile than we thought? A recent study shows that repeated cycles at a high charge level can damage the cells over time.
Fragile LFP batteries?
It is often advised not to recharge the battery of an electric car beyond 80%, or only in a fairly exceptional manner. And this for various reasons: the recharge curve collapses and the battery capacity can be affected in the long term. However, LFP batteries are supposed to not be affected by this phenomenon. In any case, this is what car manufacturers have been implying to consumers until now.
A new study co-authored by Tesla battery researcher Jeff Dahn and published in the Journal of The Electrochemical Society points out this weakness in LFP batteries. The authors have managed to show to what extent charging at a level higher than 75% can be (very) harmful to the capacity of an LFP battery. This is bad news for Tesla, and all the others who use these cells.
A weakening of capacity in the long term
As with NMC cells, the voltage of LFP cells increases when the battery is at a high charge level. But with a different curve. If a high temperature is added to this (for example during fast charging), a reaction in the electrolyte can form harmful compounds. According to the researchers, certain parts of the anode surface are no longer available for the desired electrochemical reaction. To put it simply, this means that "the usable capacity of the cell decreases because the lithium ions can no longer dock properly".
Jeff Dahn explains that “our results show a correlation between the state of charge of the battery and the rate of capacity decline. In concrete terms, the lower the state of charge, the longer the lifespan. Therefore, it is essential to reduce the recharge between 75 and 100% to preserve the health of the battery”. This study therefore contradicts the recommendations of car manufacturers who suggested that it is possible to regularly charge an LFP battery to 100%. This is also what is written in the Tesla Model 3 and Model Y manual.
But there is a surprising way to extend its lifespan
Conversely, the results of the study show that recharging an LFP battery to a level between 0 and 25% “can extend the life of the cells”. However, the authors specify that “it is not realistic to recommend that motorists stop charging at 25%”. Indeed, that would not make any sense. They therefore recommend "not changing charging habits". Jeff Dahn nevertheless believes that "the simple fact of being aware of this phenomenon should push some owners of electric cars to be more vigilant".
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