The research I've seen points to an unfortunate conclusion that technically speaking any amount of charge in Li-Ion causes degradation. So the longest life is storing the battery at nearly 0% SOC (not 0V), and the shortest life is storing the battery at 100% SOC. Obviously the battery has a job to do, so it's been determined that a happy medium is cycling Li-Ion in the middle of its range, such as perhaps 30% to 70%. The reason they didn't recommend going any lower is because there is a risk of fully discharging a cell when in a multi-cell configuration. So avoid going too low.
Also, I have seen various laptops offer an option to extend the battery life by stopping the charging at 80% SOC.
I do not recall and could not find any research that discussed floating the voltage of the cell at some reduced voltage. Is there some degradation caused by this alone, or is it always just the fact that the cell is storing a charge, per the other research?
Even perhaps decades ago I recall discussions on the Candle Power forums where heavy users of Li-Ion reported a much longer cell life by halting the charge at perhaps 80% or 90% SOC instead of 100%. So it's been a thing for a very long time. However, battery chemistries have also changed and we even do have higher voltage Li-Ion cells available on the market that top out at 4.30V or 4.35V, so stopping at 4.2V would be a similar concept.
So a valid conclusion could be that given the low cost of Li-Ion and the continued developments, it is probably not really an issue to just go with whatever solution is safe/easy/inexpensive to implement and so what if the cells last 8 years instead of 10.
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