Enabling high-performance organic copper metal batteries via ultralow-concentration iron ion redox

Abstract

The stable coordination of chloride ions in aqueous CuCl2 electrolyte results in the coexistence of Cu2+ and Cu+, rendering it a promising electrolyte capable of facilitating various redox reactions. However, the significant challenge arises from the strong combination reaction between aqueous CuCl2 electrolyte and metallic copper, hindering the development of cost-effective copper metal batteries based on variable-valence copper ion electrolytes. Herein, utilizing the synergistic effect of electrolytes, a novel copper metal battery is constructed with a 1 m Cu(OTf)2 + 0.05 m FeCl3 organic electrolyte based on the N-methyl-2-pyrrolidone (NMP) system. Mechanism investigations reveal that the organic electrolyte achieves three-step stable redox reactions in both Cu||PANI battery and Cu||CuFe-PBA battery. Intriguingly, the coordination of ultralow-concentration chloride ions effectively stabilizes Cu+ and enables two independent redox reactions (Cu2+/Cu+ and Cu+/Cu0). Concurrently, ultralow-concentration of Fe3+ facilitates the mutual conversion to Fe2+, which is very rare redox process in electrochemical energy storage. Consequently, the designed organic copper ion electrolyte with 0.05 m FeCl3 additive exhibits outstanding thermodynamic stability towards metallic copper, enabling the Cu||Cu symmetric battery to maintain a cycle lifespan of 4000 h (∼167 days) at 0.5 mA h cm−2, which is the highest one among copper-ion batteries up to date. Benefitting from the redox of Fe3+/Fe2+, the Cu||PANI battery exhibits a high capacity retention of 85.60 % after 2000 cycles. This work opens up unprecedented possibilities of optimizing electrolyte formulations for copper metal batteries.