Source: https://www.researchgate.net/publication/328396021_Energy_Density_of_Cylindrical_Li-Ion_Cells_A_Comparison_of_Commercial_18650_to_the_21700_Cells The energy content per cell can be higher by ~50% from 21700 compared to 18650. Therefore, for certain applications, less cells have to be built and used to deliver the same amount of energy. The higher energy content on cell level leads to potentially lower effort and costs in the production of 21700 compared to 18650 type cells and their packs. ... Rivian claims they can cool the cells by one end more efficiently than Tesla's coolant channel that passes between the cells. Thermal cooling is engineering 'black magic' so I don't have a strong opinion one way or the other. However, I've wondered for a long time if heat-pipe technology could be used in pack construction. A heat pipe uses the phase change from liquid-to-gas to transport heat away from the hot side. Then the cold side condenses the gas back to a liquid which via capillary action returns to the hot side. No pumps or cooling channels needed as the porous mat covering the hot side handles liquid transport. A whisker fuse coated with an impervious coating would be needed for cell-to-buss. The module/pack would need to be gas and pressure tight. Bob Wilson
