(591c) Revealing Multi-Ion Charge Transfer Mechanisms in the Iron Redox System in Alkaline Sulfide Containing Solutions

The development of grid-scale battery systems is important to store and utilize the energy produced by intermittent sources like wind and solar power. Lithium-ion batteries are not preferred for grid storage due to their high cost and safety concerns, making rechargeable aqueous batteries a viable alternative due to the use of nonflammable electrolytes. Particularly, alkaline iron batteries are an attractive option due to the low cost and high abundance of iron. Iron redox in weak alkaline solutions (pH<13) follows an all-solid-state mechanism without any dissolution steps which makes dendrite formation issue less prevalent unlike its other counterparts including alkaline zinc batteries.

While iron redox offers various advantages, parasitic hydrogen evolution reaction (HER) is a major barrier while reducing iron oxides to metallic iron, resulting in poor efficiency. From the Fe-H₂O Pourbaix diagram, the standard reduction potential of HER is around 50 mV more positive than Fe(OH)₂àFe reduction. Utilizing the FeOOH/Fe(OH)₂ redox where the effect of HER is less pronounced is a better option instead of relying on the Fe(OH)₂/Fe redox couple. The major challenge in FeOOH/Fe(OH)₂ redox is the accumulation of Fe₃O₄. Being a close-packed phase, Fe₃O₄ is electrochemically less active and its accumulation across cycles results in poor cycling performance.

Anion-intercalated layered double hydroxide (LDH) phase, so-called “green rust (GR)”, having a chemical formula of [Fe²⁺_1−x Fe³⁺ _x(HO⁻)₂]^x+(A^n-_x/n )^x−, where anions (Aⁿ⁻) are intercalated between the Fe−O interlayers can be used to bypass Fe₃O₄ accumulation during redox as it connects the FeOOH and Fe(OH)₂ phases in mildly alkaline solutions. To combat the parasitic hydrogen evolution during reduction, silicate employed as an additive in the electrolyte, suppresses HER by slowing down the ionic and water transport.

Sulfide salts (Na₂S, K₂S, etc.) have been widely used as electrolyte additives to mitigate hydrogen evolution in conventional Fe-air batteries involving Fe(OH)₂/Fe or Fe₃O₄/Fe redox couples. It is believed that during the charging process, the presence of sulfide anions helps in the formation of a protective FeS layer on the anode surface which mitigates hydrogen evolution due to the high overpotential for HER on the FeS layer. Though Na₂S has been reported to improve the charging performance, its effect on oxidation, especially in weak alkaline solutions (pH around 12), is not well studied. In this work, we report that the addition of Na₂S assists the Fe²⁺/Fe³⁺ conversion by facilitating a multi-ion redox mechanism. We found that Na₂S helps in charging by reducing Fe₃O₄ into Fe(OH)₂ while suppressing HER and possibly hydrogenating the anode, and improves the discharge by (i) the de-hydrogenation of the hydrogenated anode to give additional capacity and (ii) forming a mixture of Fe₃O₄ and sulfide intercalated green rust phases. This multi-ion charge storage mechanism mediated by proton, hydroxyl ion and sulfide ion helped us achieve a specific discharge capacity of 300 mAh/g at 0.1 A/g.