The name 21700 comes from battery dimensions. Let’s break down the name of this battery: 21 is the diameter in millimeters (mm), and 70 is the length in millimeters (mm). Last digit, Zero (0) represents shape/type of the cell which in this case is Cylindrical. 21700 rechargeable Lithium-ion battery energy density is higher than in 18650. Important benefits and improvements are: Very high energy density ratio, Higher Capacity, High safety, High output, Lower production cost.
21700 rechargeable batteries are available in two sizes and two types: 1. Protected (electronic advanced safety protection built-in) 21700 button top battery 2. Unprotected bare flat top cell (without any externally added safety protection)
Keep in mind that Protected 21700 Li-ion battery is longer than 70 mm. It is because of added PCB (Printed Circuit Board and el. parts soldered to it) safety protection and button top (made of Nickel) that is welded to the bare battery cell.
20700 Li-ion 3.7V cells are also available (size:20mm x 70mm), and 21700 backward compatible. 1mm smaller in diameter but very powerful.