Heat increases battery temperature and decreases battery life. Here are some tips from Apple to increase battery life
Using the iPhone or iPad when it is very hot can damage the battery, permanently. The same Apple says in a published on its online help, confirming what is already known for some time a bit 'for all electronic devices powered by lithium-ion batteries.
The devices of the bitten apple, however, cost a lot 'more than average and, considering that now the batteries are all integrated into the device and to replace them you have to go through technical assistance, perhaps it would be appropriate to read (and apply) well the advice of Apple to safeguard the life and durability of the batteries of our phones and tablets. The damage caused by heat, in fact, are not repairable: when the cool weather returns, therefore, our battery will not be as before. In the worst cases, a heat stroke can make the battery completely unusable.
Why heat damages batteries
All lithium-ion batteries, unlike solid-state batteries (which are the future, not the present), contain fluids inside them. In hot weather these fluids can partly evaporate and, even if they do not escape from the battery (which would damage it irreparably), they can create small damages inside the battery. Damage that remains there and is added to that caused by previous heat waves. The results are twofold: it decreases the maximum charge that the battery can accumulate and worsens the overall performance of the battery.
How to protect iPhone battery from heat
The problem, therefore, is all related to heat and consequently you can eliminate it only by avoiding overheating the battery. Certainly we must not leave the smartphone or tablet in the sun, or in a very hot place. But we must also avoid running very heavy applications (such as video games or watching high-resolution movies) during the hottest hours of the day. It is also useful to turn off the chips that are not strictly necessary: if we are at home we do not turn on the data connection and, even if we are out, we disable Bluetooth. All this will limit the heat produced by the smartphone and, consequently, the internal temperature of the battery.