Indeed, to provide reasonable battery life, Nissan does not allow us to use 100% of the actual battery capacity. If the actual capacity is 24 kWh and the usable capacity is about 21 kWh, which seems consistent with experience described on multiple threads here, then we are allowed to use about 88% of the actual capacity. Of course, pinning down "actual capacity" is somewhat fluid, as it depends on the voltage thresholds chosen at the upper and lower ends. I'm just going with 24 kWh because that's what the car is advertised as having.
In any case, "100% charge" is apparently close enough to the battery's actual 100% that Nissan advises charging to 80% instead when possible; they explicitly refer to 80% as "long life mode". Whether a "100% charge" is truly 85% or 95% of the actual capacity, we know from the owner's manual that it is not recommended to leave the battery sitting at "100%" for long periods of time. If one does need to charge to 100%, finishing charging shortly before driving seems to be a good idea.
As to the question of temperature, yes, lower temperatures should help slow capacity degradation, to some degree, when charging to 100%. At the upper end of the charge range, "secondary" chemical reactions that gradually reduce the battery's capacity occur in greater numbers. However, in general, chemical reactions are slowed by cooler temperatures. This is also why cold batteries offer less range, as charging and discharging involve "primary" chemical reactions.
Finally, while the LEAF's battery chemistry is relatively very stable thermally, charging to 100% does tend to raise the battery temperature a bit more than charging to only 80%. Especially with our coarse-grained battery temperature gauge, this hardly seems noticeable. But it is true that any extra heating will help your range in cold weather. Definitely do not hesitate to charge to 100% if you need the range.
