ESS recently went public and has some good sized deals for their iron-flow batteries. If they're what's claimed, they ought to be a pretty scalable technology without too many materials bottlenecks.

This is pretty much a press release, but a bit of a summary: https://www.utilitydive.com/news/ess-sb-energy-softbank-reac...

That said, it's entirely out of my realm of expertise for vetting how good their tech or plans are.