Lithium ion batteries are actually a little endothermic when charged. The problem with heat is that you need a lot of energy to charge the battery.
If the battery is 4Ah at 4v, then if the charging cycle is 100% effective, you need to supply 4v for one hour at 4A.
If you want to charge 4Ah at 4v at 15m, you need to supply 16A at 4v (of course these values must have at least 0.25 ohm of internal resistance of the battery to be possible.) 16A is a lot of current, and will generate a lot of heat in the circuit. .
Practically though, they increase the voltage when charging the battery and not the current, because it is much colder. I don’t know how they do it without increasing the resistance of the battery.
USB-C actually supports up to 20V.