Just like your pets or babies, your iPhone can be permanently damaged if you leave it in a hot car. It can also happen if you leave it in the sun.
Don't Leave Your iPhone in a Hot Car
Extreme heat can permanently damage your battery.
Exposing your iPhone battery to temperatures above 95° F (35° C) can permanently damage your battery. That is, your battery won’t power your device as long on a given charge. It’s even more harmful if you charge your iPhone in high temperatures.
Your iPhone will let you know when it's too hot. Don't use it until it cools down.
When using your iPhone in a very cold environment, you may notice a decrease in battery life, but this condition is temporary. When your iPhone warms up, your battery should be right back to normal.
According to Apple, your device is designed to perform well in a wide range of ambient temperatures, with 62° to 72° F (16° to 22° C) as the ideal comfort zone.