> What kind of energy density could we get using it for energy storage?

Actually, not a lot. The are some very compelling uses of them for storing energy, but they are much more relevant for distribution grid stability and control than for raw energy storage.

There are people here are pushing some really non-compelling use cases (like long distance power distribution), but there are plenty of transformative ones.

(But the thing is that this one on the paper is much less useful than it could be. There is still some work on understanding why and fixing it.)