Lithium-ion batteries, because they’re lightweight and compact, have enabled smart phones to get slimmer and electric vehicles more practical. Like all batteries, they work by means of chemical reactions that send electrons and ions from one electrode to another. Lithium-ion batteries, like the e-book battery shown here, require safety measures such as a fuse to prevent them from catching fire. Battery configurations vary, but generally the electrodes must be thin enough to allow lithium ions to move readily in and out of them. Electrode materials are deposited on foil that collects electrons and conveys them out of the battery. The electrodes don’t store much energy by area, so long strips of them are folded or rolled up to boost the battery’s capacity.

A typical e-book battery (above left) contains a copper foil coated with black graphite, the negative electrode material (middle). Peeling back more layers reveals the dark-gray positive electrode and the white electrolyte-soaked separator material (far right), which provides a path for lithium ions to travel between the electrodes but blocks electrons.