Certainly iron is the metal to look at if you're going for the lowest cost, but the results so far are not inspiring.<p>There are about nine orders of magnitude in between these results and utility-scale energy storage, but it doesn't seem like that's what they're aiming for—the existing flow batteries they mention in the paper would be a better fit. They seem to be aiming at home applications, but they are seven orders of magnitude away from even the “tens of kilowatts” they cite in their abstract as typical for such applications.<p>However, both the negative results they mention in passing and the positive ones are major contributions to the progress of such low-cost battery designs.<p>Iron costs around US$0.03 per kg, while lithium is more like US$300 and nickel is US$30. In the intervening 3 orders of magnitude, cheaper than nickel but not as cheap as iron, are aluminum, antimony, arsenic, cadmium, carbon, cerium, chromium, copper, lead, manganese, samarium, silicon, tin, titanium, vanadium, and zinc.<p>Some of these are unpromising precisely because they are too electronegative; zinc, as they said, cannot be reduced electrolytically in water; the same problem applies to titanium, aluminum, and carbon—but maybe a different electrolyte could solve that problem. Others, like lead, are already in common use for batteries.<p>Chromium, titanium dioxide, vanadium, tin and tin oxide, copper, manganese and its oxides, cerium, samarium, and of course lead seem like plausible candidates. Indeed at least zinc-cerium batteries exist.