Elevating kinetics of passivated Fe anodes with NH₄Cl regulator: Toward low-cost, long-cyclic and green cathode-free Fe-ion aqueous batteries

The environment benignity and battery cost are major concerns for grid-scale energy storage applications. The emerging dendrite-free Fe-ion aqueous batteries are promising due to the rich natural abundance, low cost and non-toxicity for Fe resources. However, serious passivation reactions on Fe anodes and poor long-term cyclability for matched cathodes still stand in the way for their practical usage. To settle above constraints, we herein use NH₄Cl as the electrolyte regulator to elevate the reaction kinetics of passivated Fe anodes, and also propose a special cathode-free design to prolong the cells lifetime over 1,000 cycles. The added NH₄Cl can erode/break inert passivation layers and strengthen the ion conductivity of electrolytes, facilitating the reversible Fe plating/ stripping and Fe²⁺ shuttling. The highly puffed nano carbon foams function as current collectors and actives anchoring hosts, enabling expedite Fe²⁺ adsorption/desorption, Fe^II/Fe^III redox conversions and Fe^III deposition. The configured rocking-chair Fe-ion cells have good environmental benignity and decent energy-storage behaviors, including high reactivity/reversibility, outstanding cyclic stability and far enhanced operation longevity. Such economical, long-cyclic and green cathode-free Fe-ion batteries may hold great potential in near-future energy-storage power stations.