To store energy for days, which chemical process does a new type of grid battery use?

Unlike lithium-ion batteries that last 4 hours, iron-air batteries can provide power for over 100 hours. They work by absorbing oxygen to turn iron into rust (discharging) and using electricity to turn rust back into iron (charging). Using cheap, non-toxic iron makes them 1/10th the cost of lithium systems, ideal for balancing wind and solar grids.