One of the best ways for long-term energy storage is to take something (like water) and move it to a higher spot.
We can take advantage of the decay of an atom to generate electricity, but if we want to use that energy later, we push water up a hill.
Although, depending on your local geography, you could just dig a deep hole.
Also, you can use existing deep holes which were dug for other reasons - for instance, open-cut mine pits tend to end up flooded once they're no longer mined.
It's pretty huge, able to produce more than a gigawatt for ~8 hours.
Not sure why they use those balloons though, seems like a maintenance nightmare. A diving bell would work just as well and be much simpler.