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.
Does it make sense to store energy as heat differentials for long term? Insulation can only work for so long, eventually it will come back to environment temp.