The ideal grid storage battery may be EV's. Especially EV fleets used by government agencies, the Post Office, etc etc.
I have a professional interest in these matters, and without labouring the point, the economics don't work, and even the most optimistic assumptions don't do enough to solve grid balancing issues. They can certainly be actively managed to optimise charging and minimise their own impact on the system, but solving wider grid problems, not a chance.
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
Reuse of vehicle batteries is old hat - Nissan in the UK will already sell you a home battery using recycled car cells. Unfortunately, because the actual cells are only about a quarter of the price of the installed system, and the recovered cells aren't free (recovery costs, inspection & QC, warranty) the savings of a part used battery pack are fairly minimal. For a commercial system you might have a better case because replacing failed cells or banks can be achieved within scheduled maintenance, but you're still buying an asset that has significantly lower life expectancy than using new cells.