Cheapest battery when space is of no concern

If the amount of space used is absolutely no concern at all, what are the cheapest options out there right now for solar battery storage? I’ve read a lot about iron/air/rust batteries, but haven’t really found anything commercially available yet.

From my direct experience, and from what I have seen and read, Lithium iron phosphate (LFP) batteries are still the cheapest option available. This is especially true when considering round-trip efficiency, performance and lifespan.
Most new battery technologies still seem to be in the development phase, except for redox flow batteries which are quite economical on a large scale (500kWh or more).