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>were more efficient in (relatively) colder environments

this is definitely false; While almost everything (electronics/mechanics/hydraulics/pressure vessels) hates the heat, the operation temperature for Li-Ion cells is not too low, either. In very layman terms: the chemistry is not effective at lower temperatures.

As an experiment: have a two year phone and leave it at -20C, it might shut down on its own even at higher charge (voltage) as the battery cannot provide the needed current.