The science behind exploding phone batteries | The Verge
'... To understand what makes a battery safe, it’s helpful to know how they work. There are two electrodes, or electrical conductors, on opposite sides. One electrode holds positively charged ions and is called the cathode. The cathode is filled with lithium and that is where the “fuel” is stored. The opposite electrode holds negatively charged ions and is called the anode.
During charging, lithium ions move from the cathode to the anode. When the battery is in use, the lithium moves in the opposite direction. In between are chemicals called electrolytes that conduct the current by helping ions move more easily between the two sides. But even though ions need to move from side to side, the anode and the cathode themselves should never touch because they’ll redirect energy to the electrolytes. To prevent this from happening, battery makers insert separators in between.
That’s what went wrong with the Galaxy Note 7: the separators were flawed and let the two electrodes touch. “This is considered the worst possible failure because it will most certainly lead to fire and possibly even an explosion,” says Archer. When the electrodes come into contact, all the energy being pumped into the battery goes directly to the electrolytes in the middle instead of in the electrodes on the side.