They do that already with pumped-storage.
Pumped storage has an RTE (round-trip-efficiency) of about 80%. Modern li-ion batteries are over 90%. Pumped storage requires very specific geography (two reservoirs separated by a hill). Batteries will work anywhere.
There are also some liquid batteries.
The most common "flow" batteries are based on vanadium redox, and have an RTE of 65-75%.
Li-ion is just too expensive and maintenance-intensive to use grid scale.
Well, the point of this announcement is that Li-ion is getting cheaper. Li-ion grid storage still won't make sense in the middle of America, where power is cheap, and grids are wide. But it make make sense in places like Hawaii ($0.40 / kw-hr), where grid stability is already a problem.