Because the autumn was flooded with news of Galaxy Note 7 terminals exploding due to the batteries used by Samsung, some researchers thought to analyze the problem a little. Not that of the Galaxy Note 7, but the problem of batteries that swell and explode after enough stress is applied to them during charging.
The researchers took several batteries and scanned them with X-rays before putting them through tests designed to push the batteries to the manufacturer's predicted endurance limits. After each series of tests, the researchers scanned the batteries each time to see how the layers inside them change, and the image below shows us everything as well as possible.
In the first picture you can see the battery before starting the tests, in the second the battery after some tests, and in the last picture you can see the swollen battery, before it explodes. From the first to the last image, there are many differences regarding the spaces occupied by the air inside the battery after the complex tests.
After the long testing of the batteries, several "air gaps" were formed inside it between the layers of the battery, suggesting that it is going to explode. Basically, due to overheating, the liquid inside the battery turns into gas, and thus it swells and emits flames, destroying the terminal, but maybe even more than that.
You can find more details in this work published by researchers.
