[donny2018]

Another issue is round trip efficiency. Iron-air, I believe, is somewhere at 50-60% (you lose almost half of the energy that you pump in).

While LFP and the Sodium battery mentioned in the title, is 95-97%.

[pfdietz]

True, although for a battery system with a given economic lifespan, the "cost of inefficiency" is inversely proportional to the average storage time. So, it's 7x less important for weekly vs. daily storage.