Really interesting architecture, using electric turbopumps instead of engine-driven ones. The pumps are powered by a battery capable of a peak discharge of 1MW, and so their weight is non-negligible and for the second stage they are disposed of once fully discharged.
They use lithium-ion which is probably a cost/availability consideration. A primary (non rechargeable cell) would likely exceed the energy density of the lithium ion pack, but would cost more and require more engineering effort. I guess if they intend to recover the packs eventually, then Li-Ion could be important.