A new design of an insulating material that separates the electrodes in lithium-ion batteries may help prevent a rare but potentially disastrous scenario that can cause these batteries to suddenly go up in flames. A small number of burning-battery incidents, including two last year involving Boeing 787 Dreamliner airplanes, have kept lithium-ion batteries in the safety spotlight. Under circumstances that are not thoroughly understood, charging a battery sometimes drives lithium metal to accumulate on one electrode in the form of dendrites. As these tiny wiry structures grow, they can extend through microscopic pores in the protective polymer separator to the other electrode. That process short-circuits the battery and can cause it to quickly overheat. A team led by Stanford University’s Hui Wu and Yi Cui has demonstrated that by sandwiching a 50-nm-thick copper film between polymer layers they form a bifunctional separator. The hybrid material keeps the electrodes isolated and the battery functioning normally, yet it also serves as a detector for early stages of dendrite growth. Dendrites that grow halfway through the film contact the copper layer and cause a rapid voltage change that signals the need to replace the battery long before the situation turns hazardous (Nat. Commun. 2014, DOI: 10.1038/ncomms6193).