This study addresses a fundamental challenge posed by hexavalent iron electrochemical irreversibility, which had slowed practical development of reversible Fe(VI) cathodes; nanolayers circumvent fundamental passivation challenges, and are electrodeposited from solution phase FeO₄²⁻, forming 601 mAh g⁻¹ Li, and 485 mAh g⁻¹ Na, Fe(III/VI) alkaline cathodes, the latter exhibiting 100–200 storage cycles at 80% depth of discharge, providing pathways towards metal hydride batteries with twice the cathode capacity.