Lithium-ion batteries power our laptops, cellphones, and electric cars. They are compact and rechargeable, but with a significant drawback: Occasionally, the batteries burst into flames. In 2013, Boeing grounded an entire fleet of planes after the lithium-ion batteries on two aircrafts caught fire. Now, material scientists have found a clever way to alert users of damaged batteries before any hazard occurs, they report online today in Nature Communications. A typical lithium-ion cell consists of a lithium oxide cathode and a graphite anode, separated by a thin, porous polymer sheet that allows ions to travel between the electrodes. When the cell is overcharged, microscopic chains of lithium, called “dendrites,” sprout from the anode and pierce through the polymer separator until they touch the cathode. An electrical current passing through the dendrites to the cathode can short-circuit the cell, which causes overheating and, in some cases, fire. Attempts to stop dendrite formation have met with limited success, so the researchers tried something different. They built a “smart” separator by sandwiching a 50-nanometer thin copper layer (shown above) between two polymer sheets and connecting the copper layer to a third electrode for voltage measurement. When the dendrites reach the separator, the voltage between the anode and the copper layer drops to zero, alerting users that they should change the damaged battery while it is still operating safely—disaster averted.