18650 battery was introduced in 1991 as standardized lithium-ion battery model designed to reduce production costs. Mass-produced by Sony, it was initially used in laptops, power tools, and other small electronic devices. The name “18650” refers to the battery’s dimensions: 18mm in diameter, 65mm in length, and the “0” indicates a cylindrical shape. This design strikes clever balance between volume and energy density, though it was primarily intended for compact electronics.

In 2016, 21700 battery was introduced and mass-produced. With significantly larger diameter of 21mm, its single-cell capacity increased to 5Ah. By increasing the size of the cell, the number of individual cells required in battery pack was reduced by 30%, effectively simplifying the battery pack structure. Compared to 18650 batteries,  21700 greatly reduces the number of cells needed, effectively lowering production costs and improving energy efficiency and range.

In both 18650 and 21700 cylindrical cells, “tabs” (metal strips) are welded from the positive and negative electrodes to connect the cell to the external circuit. In this design, current must travel from the edge of the electrode to the tab, leading to high localized current density. This can cause the battery to heat up, especially during fast charging. Tabs also occupy internal space and require additional welding, adding to the battery’s weight and cost. Moreover, the welding points between the tabs and electrodes are prone to detachment from vibration or expansion, or can suffer from oxidation over time, leading to poor contact and shortened battery life.

The internal resistance of a single-tab 21700 cell is about 30mΩ. The evolution from 18650 to 21700 batteries reflects abroader technological trend focused on increasing energy density, enhancing charging speed and safety, and reducing costs. With ongoing innovation, the future may bring solid-state batteries—or even miniature reactor batteries.