The energy density is NOT 31% greater. The volume is 31% greater. The energy is 31% greater. The energy density is roughly the same as the traditional 18650 cells used in the Model S and X when using the same chemistry. With new chemistries the energy density will increase in small increments, but the size does not change the energy density. A big box full of 18650 cells would have the same energy and energy density as the same box filled with the 21 x70 cells.