What is happening?
Telephone battery keep energy, sun ray make the car hot and increase pressure in the car, this can make telephone battery explode. On the condition close tightly the temperature in the car can increase vastly, and can make bad effect to the phone cell battery. Actually is not very clear why the energy on the battery merges and changes to very high energy. This explosion make the car roof broken, this is not usual explosion. Certainly should be any certain chemicals on the battery, so can create new energy that merge very high.
To avoid this explosion you better don’t left your phone cell in the car, especially under hot sun. Phone cell battery become improves on and on, so the battery kind that is used for your phone cell may will react if temperature too hot and can make an explosion. For normal battery, it may not happen but for developed battery I don’t what the kind of battery materials.