Investigation of capacity recovery effects due to rest periods during high current cyclic ageing tests in Li-ion cells and their influence on lifetime
This is a review of “Investigation of significant capacity recovery effects due to long rest periods during high current cyclic aging tests in automotive lithium ion cells and their influence on lifetime” (2019) by Epding, Rumberg, Jahnke, Stradtmann, and Kwade.
An important but often neglected fact is that privately owned cars spend only a small fraction of their lifetime driving. Long, continuous rest periods of hours (at night, while at work) or even days (over the weekend) are part of most realistic usage scenarios.
I agree. For example, in “Closed-loop optimization of fast-charging protocols for batteries with machine learning”, cells were cycled non-stop to make lifetime predictions and choose the best fast charging protocol.
Frequent rest recovers capacity and prolongs a cell's life
A cell that is rested every 50 cycles for 2 days degrades slower than a cell that is rested every 100 cycles:
The authors found that the recoverable capacity during a rest period depends on the current usable cell capacity (i. e., state-of-health: "UCC" on the chart). You can notice this above: the lower the UCC, the more capacity is recovered during rest periods, and the recovered amount at the same UCC levels are comparable for the two analysed cells.
The recoverable capacity also depends on the length of the rest period. There is a limit to how much capacity can be recovered which the cell approaches exponentially, so after 5 days very little capacity can be recovered:
On this chart, ΔC is the capacity recovery. UCC stands for "Usable Cell Capacity" relative to the capacity of a new cell; hence, black squares represent the recovery when a cell is almost new (50-200 cycles, rest periods every 50 cycles), red circles - when the cell is already close to the end-of-life limit in terms of remaining capacity (250-400 cycles), green triangles and blue stars - when the cell is already beyond the normal end-of-life and is cycled to death (450-800 cycles).
The authors found that the recoverable capacity during a rest period depends on the length of the period (as discussed above) and the current usable cell capacity (i. e., state-of-health), as you can notice on the chart above.
The authors found that the temperature at which a cell is rested doesn't significantly affect the speed or the amount of recoverable capacity.
When the authors analysed only the cells that were rested every 50 cycles, they found that the recoverable capacity roughly linearly depends on the lost capacity since the previous rest period, and that about 40% of that lost capacity can be recovered. However, this seems to me to be a false relationship because on the chart above, we can see that while the cell which is rested every 50 cycles recovers roughly 40% of lost capacity, the cell which is rested every 100 cycles recovers only 10-25% of the lost capacity.
Reasons for cell capacity recovery during rest
When a cell is rested after non-stop cycling, its capacity can recover for multiple reasons:
Anode overhang effects (see below)
Change in the homogeneity of Lithium distribution in the anode
Reversible Lithium plating (see section “Cell capacity fade accelerates when Lithium deposition becomes irreversible”).
The authors suggest that more frequent rest leads to slower degradation because Lithium plating is a self-reinforcing process, therefore, preventing it early leads to compounding benefits.
Anode overhang effects can increase measured cell capacity after resting at 100% SoC
Regular rest periods or characterizations during accelerated cyclic ageing tests lead to an increase in the measured cell capacity as well as to a longer lifetime of the cells. A capacity increase is measured even if the cell is left at 100% SoC during the rest period. This excludes the anode overhang as the source of the capacity increase.
I don't see how exactly resting at 100% SoC excludes anode overhang effects as the possible reason for capacity recovery.
As far as I understand, when a cell is cycled at a fast pace, the overhanging areas of the anode achieve the equilibrium SoC which is approximately the average SoC of the non-overhanging parts of the anode during cycling. Thus, when a cell is rested at 100% SoC, some of the Lithium from non-overhanging parts can move to the overhanging parts.
If we measure the capacity of the cell as the amount of charge that we can extract from it immediately after the rest period, then resting at 100% should decrease the capacity (or at least not increase it), indeed. However, if there is still some Lithium in the cathode (or excess Lithium in the electrolyte) while the cell was rested at 100% SoC, and we measure the cell capacity over a full charge-discharge cycle (e. g., following the precise cell capacity measurement protocol), then it might appear that the cell capacity has increased. This can happen because on top of the full "baseline" discharge capacity, the anode overhang can supply additional Lithium to be released into the electrolyte while the cell is discharging.
And, in fact, the authors write that they were measuring capacity during the second discharge phase after a rest period. To me, this seems to be compatible with the theory that anode overhang effects can increase the measured cell capacity.
“Bumpy” voltage relaxation indicates Lithium plating
According to the literature, a shoulder appears in the relaxation voltage curve if Lithium plating occurred in the charge step due to the coexistence of Lithium metal (0 V vs. Li/Li+) and graphite (0.08 V vs Li/Li+) on the anode side .
The following chart confirms the proposition that Lithium deposition takes over SEI growth as the leading capacity fade mechanism at 1C rate and 25 °C from the review of Lithium plating: