Modern mobile devices use lithium ion (sometimes called Li-ion) batteries, which consist of two main parts: a pair of electrodes and the electrolyte between them. The materials that these electrodes are made of varies (they can be lithium, graphite, or even nanowires), but they all rely on the chemistry of lithium. It's a reactive metal, which means that it has a tendency to combine with other elements. Pure lithium is so reactive, it can catch fire in the air, so most batteries use a safer form called lithium cobalt oxide. Between the two electrodes is the electrolyte, which is usually a liquid organic solvent that allows electrons to flow between them. When a lithium ion battery is charged, the lithium cobalt oxide molecules capture and hold electrons, which they then release when the battery is in use, such as when it is running your cell phone.