I was using a desktop wireless charger this morning and picked up my phone and felt it being noticeably hot to the touch. I measured the back of my phone about a minute or so after removing from the desk and it measured under an IR thermometer as 101F (38.33C).
I recall reading someplace about certain car manufacturers putting the car battery in the trunk compartment to keep it out of the hot engine bay because heat degrades a battery's lifetime apparently. Could the same phenomena apply to phone batteries getting heated up via wireless chargers? The constant heat of an engine during the vehicle's operation isn't the same as an infrequent wireless battery charging of course but perhaps there's some amount of loss?