18650 vs. 21700 Li-ion cells – A direct comparison of electrochemical, thermal, and geometrical properties

Abstract

Li-ion cells of the classic 18650 format are directly compared with the new 21700 format regarding electrochemical, thermal, and geometrical properties. Both types of cells were reproducibly built on pilot scale with the same electrodes, separator, and electrolyte allowing a direct comparison for the first time. Internal temperature sensors give insights on the heating behaviour. Similarities are found in the voltage curves and capacity fade at 1C cycling at 25 °C. The capacity and energy increase per cell is ~50% from 18650 to 21700 for discharge in the range of 0.5C–3.75C. The results are compared with calculations of electrode areas based on Archimedean spiral's arc lengths and geometrical data extracted from X-ray computed tomography (CT) measurements. Two evaluation methods for extraction of the number of electrode windings from CT images are compared. Calculations are made for the winding number as a function of the periodic winding distance (thicknesses of anode, cathode, and 2x separator) as well as on electrode curvature for both cell formats. The influences of intended extensions of the cell diameter by few μm (as observed in commercial cylindrical cells), as well as larger formats such as 30700 on additional electrode windings and cell capacity are estimated.