This is the third in a series of posts from the IEEE Spectrum article, “How to Build a Safer, More Energy-Dense Lithium-ion Battery,” authored by Ashok Lahiri, Nirav Shah, and Cam Dales of Enovix. The article describes and illustrates how thermal runaway can occur in a conventional Li-ion battery. The polymer separator is an inactive material and has to be physically longer and wider than the electrodes to make sure