SECONDARY BATTERIES | Super-Iron Batteries

Abstract

Super-iron batteries are based on an unusual Fe(VI) redox couple and multiple electron cathodic charge storage. Such Fe(VI) cathodes can be ‘green’, cost effective, and store considerably higher charge than conventional cathode materials. For example, the Fe(VI) salt K2FeO4 is readily prepared, holds 406 mA g−1 through the three-electron reduction to Fe(III), and this ferric oxide discharge product is environmentally benign. Fe(VI) salts that have been synthesized and demonstrated as super-iron cathodes include a range of alkali and alkali earth Fe(VI) salts, as well as the transition metal Fe(VI) salt Ag2FeO4. Four classes of super-iron battery have been demonstrated: super-iron primary alkaline batteries, super-iron primary lithium batteries, super-iron rechargeable alkaline batteries, and super-iron rechargeable nonaqueous batteries. The alkaline batteries exhibit twofold higher capacity than conventional alkaline batteries, but have development challenges regarding storage of the cathode when in direct contact with the alkaline electrolyte. This has been addressed by coating the Fe(VI) cathode salts with an overlayer, such as zirconia, which is alkaline stable and shuttles hydroxide between Fe(VI) cathode and electrolyte. Thin-film Fe(VI/III) cathodes are rechargeable in both alkaline and nonaqueous electrolytes. Reversibility challenges arise for rechargeable Fe(VI)/metal hydride and Fe(VI)/Li batteries due to the decreasing conductivity of thicker cathodes.