Heat is only part of the problem. Lithium-ion batteries are also stressed when their voltage is too high (100% charged) for long periods of time, which causes the batteries to degrade faster. See here:
https://batteryuniversity.com/articl...ased-batteries
The worst case scenario is storing the battery at 100% charge in a hot environment. Even though you're not using the battery at all, the capacity will rapidly decline.