This is happening because it is a rechargeable battery. This is standard fair for these types of batteries. The battery will get warm when you charge it and when you use the phone as well.
I used to use rechargeable "AA" type batteries in a professional camera flash unit. These batteries would get downright hot to the touch--almost too hot to hold--while recharging. I've been using those batteries for well over five years without a lick of trouble.