...and doesn't require specific large-scale geography to implement.
This doesn't have to get a whole lot more efficient before it could become profitable. If you look at the wholesale electricity market in the UK, the peak cost of electricity is more than double the minimum cost over a week. So something that can store electricity at minimum cost and sell it back into the grid at peak cost only needs to be 50% efficient to be making money. Of course, that's ignoring the capital cost, but still, this is not too far off being profitable.
It's a pretty sad indictment of the state of energy storage, really. It only needs to be 50% efficient to be profitable. If it's profitable, people will do it. Therefore conclude that every practical energy storage system is less than 50% efficient (at least where it doesn't require geography).