When It Goes Wrong: Runaway, and the Film That Tries to Stop It

Every rung in this batch has been about getting more energy into a smaller space and taking it out faster. This one is about what that means when it stops being under control. A lithium cell in thermal runaway is not a fire that started near a battery. It is the battery itself, generating its own heat faster than it can shed it, carrying its own oxidiser, and therefore indifferent to being smothered. The chemistry that made the cordless tool possible is the same chemistry that makes this possible. The hero image is the failure the whole design was meant to avoid: lithium dendrites growing from the anode, through the separator, to the cathode. This rung works out the energy arithmetic, watches a separator film do its one defensive trick on a hot plate, and sets out what to actually do — with cells, with packs, and when one has already gone wrong.