Lithium ion batteries are commonly used in devices such as laptops and phones because they are small, light, store energy efficiently and are slow to lose their charge.
Lithium is a highly volatile substance, so any number of changes can cause the liquid to overheat.
When a battery's temperature hits around 100C (200F), its materials begin to break down. This starts a chemical chain reaction that releases its own energy and accelerates the heating in a snowball effect.

In a “thermal runaway", the temperature can keep increasing until the battery catches fire.

http://www.telegraph.co.uk/technology/2016/10/11/why-is-the-samsung-galaxy-note-7-catching-fire-the-lithium-ion-b/