Redox flow batteries already exist, are proven and in use as grid-storage.
The current tech appears to be mostly based on vanadium and using iron instead makes it much less problematic in case of spill and handling and more importantly cheaper to build.
Energy density is low, but is totally no issue when we talk about grid-storage solutions.
To put a number (from the linked Wikipedia article) here: iron-iron based redosx flow batteries have an average fluid energy density of 20 Wh/l.
Or in other words: you need 500 litres to save 1 kWh of electric energy.

Low price and durability (in terms of cycles) look very promising!