A pack temperature between 15-35degC is optimal.
Keep in mind this is not ambient, but cell/pack temperature, so you need to take into account both ambient and internally derived heating (due to internal resistance). Also this range applies for both charge and discharge cycles.
This doesn’t mean you can’t run a pack higher than 35deg or cooler than 15degC; but doing so will affect longevity and reliability.
Most EV designs follow this range when designing their cooling/heating systems around the cells.