I believe it was the superior reserve power capacity of the Lithium battery rather than weight which led Boeing to their battery choice.
This all electric a/c is fine whilst in the air, even with say a double engine failure, as windmilling engines, RAT, an APU (if it starts) will provide sufficient electrics to power high load items like flying controls.
On the ground its a different matter.
It is interesting the A380 has a very high pressure hydraulic system 5000psi? in order to provide the necessary power for high load items.
Boeing decided electric cables were lighter? & simpler to transfer high load items. However unlike hydraulics which tend to leak in failure mode, electric cables if overloaded tend to catch fire.