The ideal grid storage battery may be EV's. Especially EV fleets used by government agencies, the Post Office, etc etc. There are some fleets that plug into V2G systems. The company that runs the cars manages how much charge they need to have and when so they don't have flat batteries and the grid can use them for balancing when they need to. I think I saw that on "The Fully Charged Show" on YouTube. It might have been Transport Evolved, can't remember. The idea is that with EV's, power can be moved around to take stress off the the grid and base load + intermittent power can be used far more efficiently. Something that can't be done with petrol. I can have a contract that says I'm happy to take and pay for all of the power my EV will accept and the grid can buy back anything they want that doesn't leave me with less than a certain percent of charge unless I indicate that I want a full charge for a trip I'm going to take.
In Japan there is already a company that has designed and implemented the hardware to take a certain brand of used vehicle batteries, stack them in a storage container and use them as storage at solar farms so they can sell power when it fetches the best price rather than being constrained to only being able to sell power when the sun is up. I have seen other startups that do the same to turn used Prius batteries into home storage batteries. That should get more prevalent in the future. A 60kWh battery pack from a Bolt that has lost 1/3 of its capacity isn't great to have in the car, but 40kWh is a ton of power for a home back up including being able to juice up the EV with the new battery pack fitted with power that was harvested during the day from solar panels on the roof.