 Metal batteries can fail over time because of tiny fibers called dendrites. When you use or discharge a battery, metal ions move from the anode to the cathode. The ions return when the battery is being charged. Repeated charging can cause metal dendrites to form on the anode. These dendrites can grow until they finally reach the cathode causing the battery to short circuit. Here is a typical zinc battery with dendrites that have grown long enough to cause a short. To prevent shorting, Stanford engineers used an insulator to trap the dendrites on the anode so they never reach the cathode. Making rechargeable zinc batteries is difficult thanks to the buildup of dendrites. But with the new insulator design, the Stanford team successfully isolated dendrite growth, allowing them to charge and discharge an experimental zinc battery more than 800 times.