We worked on that for hybrid vehicle energy storage.
Vacuum seals that last for years on rotating shafts is a tough one to do economically.
Battery electric seems to be the winner right now.
I had a team that worked on flywheel energy storage. Those were tough problems. I think the best we ever did was make compact units the could go underground (for safety) to power traffic lights through occasional power outages. I’ve been out of that business for a bit over 20 years now.