A lithium-ion battery is a light, high-power battery used in computers and mobile phones. It comes in several shapes, although a flat rectangle is most common. It is lighter than the nickel cadmium battery and the nickel metal-hydride battery. That makes it useful for devices that should be lightweight. Lithium ion batteries work by the movement of lithium ions through a membrane (thin sheet that allows some substances to pass through). They are different from lithium batteries though. Lithium batteries contain lithium metal and are not rechargeable (primary cells). Lithium-ion batteries do not contain lithium metal and are rechargeable (secondary cells). They also don't last forever; each will have an average of 600 charge cycles.