This is a hard thing to accomplish. It requires high energy density. The more energy dense something is the more dangerous it is.
Remember the first cell phones widely available looked like bricks and half or more of the phone was battery. Even some of the early phones with high tech batteries had problems overheating and catching fire.
With smartphone is packing more stuff into such a small space, the battery requires frequent recharging. It is already a common annoyance for people using smart phone. Everywhere they go, they first check if they can recharge their phone battery.
My first cell phone was the old bag phone, used mostly as a car phone. A car charger was a necessity. I don’t recall it overheating but maybe it was big enough to shed heat.