Effect of Li ions on reduction of Fe oxides in aqueous alkaline medium

Abstract It has long been known that the performance of the Fe negative electrode in Fe/Ni or Fe/air batteries is improved by the presence of lithium ions in the electrolyte. This work therefore investigated quantitatively the effect of Li + on the reduction of some Fe II and Fe III oxides, as possible intermediates of Fe reduction/oxidation cycles, by comparing the extent of the reduction in pure 6.0 M KOH with that in lithiated alkaline media. Fe oxides were studied as pressed powder samples embedded in Ni foam. It was found that, in 6.0 M KOH, only FeO is electrolytically reduced at room temperature, whereas Fe 2 O 3 and, to an even greater extent Fe 3 O 4 , are very recalcitrant to the reduction process. The situation dramatically changes in, for instance, 4.0 M KOH + 2.0 LiOH electrolyte, in which the extent of the reduction, even at room temperature, becomes significant for all compounds. This behaviour is very probably due to the reduction of Li + within the oxide lattice to produce Li x Fe y O z intercalation-compound intermediates, which are then reduced to metallic Fe and Li hydroxide.