So only looking at the top Samsungs. The only one that showed any capacity fade after 1.5 years stored at 4.20V was the one stored at 35*C.
Yes, the 26H LCO cell. Do note however, that the newer-generation 29E NCA chemistry cell, which better represents later cells available, shows some clear capacity fade at 4.20V AND even at 4.00V at room temp (note how small the difference between 4.20V and 4.00V is - 3.0% vs. 2.3% capacity fade per year - hence my earlier comments, you need to go lower for real gains! And, I have seen a paper where a Panasonic cell stored slightly
worse at 80% than at 100%!), and you need to go down to 3.60V to get "completely" rid of the capacity fade. I'm sad I'm lacking data points inbetween, it would be nice to see the result at 3.9V and 3.8V. My
guess based on some papers I have seen but not stored for reference is that it starts getting considerably better already at 3.9V.
Was this battery charged at 35*C or charged at room temp and then stored at 35*C?
All charging and discharging was performed at the same room temp approximately 22 degC. For DC ESR measurement, it's especially important all samples are handled in similar conditions, including using the same holder so that the thermal impedance to the surroundings during the test keeps unchanged. Hot/cold cells were brought back to room temp (for 24hrs, IIRC) for post-storage measurement. So only the storage conditions are part of the equation.
Charging termination was C/40 instead of "standard" C/20 to reduce variability of environmental variables vs. SoC. Measurements were automated using a calibrated Agilent 6632B supply and this (kludgy) control code:
https://github.com/siwastaja/battestIf it was charged at 15*C and then stored at 35*C then it's voltage would have risen above 4.20V and stayed there until it self discharged rapidly initially, but could have done the damage shown, no?
I think you are misunderstanding something, or mixing up with lead acid I guess? There is no such voltage-temperature relationship in li-ion, at all (or it is at completely meaningless level). Open-circuit voltage is a function of SoC, simply. Of course, DC ESR is related to the temperature so at different charging temperatures, you end up at a slightly different SoC for a certain charge termination condition, but there's no way neither SoC or voltage would shoot up when the cell is heated. Voltage always goes down to the open-circuit voltage, by approximately Vdrop = I_charge_termination * R_dc_esr (ignoring some possible second-order effects). This is why termination current is kept fairly low (such as C/20, or C/40 in this test), to standardize the end-result SoC and thus OCV, regardless of variations in R_dc_esr (caused by temperature differences and cell aging).
The first one stored at normal conditions at 4.20V seemed to gain capacity, though it's ESR rose(?).
Yes, do note however that the "gained" capacity is just 0.2%. This is likely just noise and measurement inaccuracy, not any real capacity gain. This is, as you said, difficult to measure accurately. Expect some +/- 0.5% error margin. So basically the conclusion is, Samsung 26H LCO cell does not lose capacity in storage, but does increase its DC ESR; I think this was an interesting, but not unexpected result. This is caused by the excess thickening of the SEI (solid electrolyte interface), kind of passivation layer during storage at high voltage. This passivation layer actually prevents self-discharge, but also slows down ion transfer, which can be seen as increased ESR.
But the 29E NCA cell, on the other hand, both loses capacity, and increases the DC ESR.
I'm still not 100% convinced enough to discharge Lithium Ions I store for months as I may need them to be fully charged
I agree, looking at my data, the worst performing (LG HE2) capacity fades 4.9%/year fully stored at room temperature. So it would take 6 years to reach -30% End-Of-Life. This isn't too bad. I only discharge cells to 30-50% if I expect them to sit for
years unused and I'm sure I want to use them after that long storage.
edit: removed broken quote