Grid storage technology is already there with pumped hydro storage(PHS). As 90% of built and newly building grid storage is already PHS. It is political problems and inertia that's holding it back. To be clear, I'm referring to off the river closed-loop PHS, which doesn't need a huge river check dam, flood zone, huge population relocation or ecological issues. All it needs is two places with area of 1sqkm each some hundred meter elevation apart. There is ANU study that claims we have enough of such sites for needed storage. Look at how fast China is building PHS recently. I think PHS approvals, even for closed-loop, in US are as hard to get as nuclear, which is holding it back.
https://www.pv-magazine.com/2021/01/05/sustainable-pumped-hy...
https://www.pv-magazine-australia.com/2021/10/05/aussie-scie...