An 18650 battery is a cylindrical lithium-ion battery common in electronic devices. The batteries measure 18mm in diameter by 65mm in length, giving them the name 18650.
Panasonic first developed the 18650 battery "in 1994 when there was a growing need for ever smaller and lighter storage batteries that could be used for ever longer periods of time". They are now commonly used in power tools, electric bicycles, laptops, and electric vehicles.
18650 batteries are most commonly lithium-based, although sodium-ion variants are available for purchase. Lithium-ion batteries consist of an anode, typically made of graphite and a cathode, often of lithium-metal oxide.