prismatic lithium battery cell

https://www.sokbattery.com/

A cylinder or sphere makes the best use of the outer covering vs internal volume. This can maintain more heat and allow the battery to react at a higher temperature, which speeds up the reaction. These may not be a better shapes if the container is part of one of the poles of the battery because it reduces the amount of viable surface for electron transfer. A flat cell has the baility to disspate more heat from the increases surface area per output.