The "Iron Air" battery seems to be a legitimate prospect:Durability is a plus, though there are Edison batteries 50 years old, and maybe some 100 yo NiFe batteries still exist. A bit strange they went out of fashion, though supposedly some companies are starting to make them again.
Flywheels are great for short-term buffering, to level out daily fluxations, but don't seem to be used much beyond handling peak-demand intervals, and absorbing the energy of decelerating trams (when electric grids can't handle such loads).
Wonder what the round-trip energy loss for a flywheel is, after one week or one month. Can't be great, and at some point it's zero.
As interesting as flywheels are, it seems like some inexpensize battery chemistry breakthru will eventually obviate them.
In fact, googling reveals a new "Iron Air" battery. Let the battery battles continue!
"Form Energy was founded in 2017 by Mateo Jaramillo, former head of battery development for Tesla, and MIT professor and battery scientist Yet-Ming Chiang.[3]"
Form Energy - Wikipedia
en.wikipedia.org