Lithium-ion batteries have a high energy density and thus 1.5 to 2 times the capacity of NiMH batteries of the same weight and very low self-discharge. As a result, they have a long shelf life and high storage capacity.
The rigidity and robustness of the round cells due to their housing made of - as a rule - nickel-plated steel ensures a high degree of stability and insensitivity.
The 21700 cell was introduced to the market as a consistent further development of the cylindrical 18650 cell. It achieves up to 30% capacity gain by 21mm instead of 18mm in diameter and 70mm instead of 65mm in length. The better size ratio to the active material usually results in a higher gravimetric energy density.
With a typical capacity of 5000mA, the number of cells required can be reduced compared to the 18650 round cell. This also reduces the necessary contact or welding points. This means better stability and reliability of the whole pack, as well as lower manufacturing costs.
Therefore, 21700 batteries are increasingly used not only in vehicles, but also in chargers, e-cigarettes, flashlights, electric bicycles and other devices.