21700
Direct Answer
A 21700 Li-ion cell measures 21.1mm in diameter and 70.4mm in length. Offering 4000 to 5000 mAh capacity at 3.6V nominal, it provides up to 50% more energy density than an 18650 cell.
Source: IEC 61960
Specs
| Diameter | 21.10 mm |
| Length | 70.40 mm |
| Capacity | 4000 - 5000 mAh |
History & Origin
The 21700 was jointly developed by Panasonic and Tesla, with Tesla announcing mass production of the new format on January 4, 2017, at Panasonic's Gigafactory Nevada facility. It was designed explicitly to push past the energy limits of the 18650 cells Tesla had relied on for its earlier vehicles, packing roughly 35% more capacity per cell into a package that is only modestly larger.
Other major cell manufacturers followed quickly -- Samsung SDI introduced its own 21700 cells within days of Tesla's announcement -- and the format is now produced by most of the major lithium-ion manufacturers for use in electric vehicles, power tools, and high-drain consumer electronics.
Why the Larger Size Matters
Moving from an 18mm to a 21mm diameter increases a cell's internal volume by roughly 35%, which lets manufacturers pack in more active material without redesigning the underlying chemistry. Reported energy density moved from around 250 Wh/kg in earlier 18650 cells to around 300 Wh/kg in 21700 cells. For battery-pack designers, fewer, larger cells also means fewer welds and interconnects for a given total capacity, which can simplify manufacturing and improve pack reliability.