High-performance aqueous copper-ion batteries based on iron hexacyanoferrate cathodes for enhanced energy storage

The integration of renewable energy sources, such as solar and wind, requires efficient energy storage systems. Aqueous batteries, with their safety, low cost, and flexibility, have gained attention as promising solutions for energy storage. In this study, we developed an aqueous copper-ion storage device based on an iron hexacyanoferrate (FeHCF) cathode, which offers high capacities of 190 mA h g^-1 at 1 A g^-1 and 102 mA h g^-1 even at 3 A g^-1, with a discharge plateau at 0.59 V <i>vs.</i> SHE and a low polarization voltage of 0.2 V. <i>In situ</i> XRD, Raman, and XPS characterization techniques show that copper-ion insertion induces structural changes in FeHCF, leading to a valence state transition between Fe²⁺ and Fe³⁺, with a partial conversion of Cu²⁺ to Cu⁺. To improve the working voltage, we replaced the Cu²⁺/Cu⁰ anode reaction with the lower potential Zn/Zn(OH)₄^2- reaction, achieving an aqueous battery with a voltage range of 1.6-2.5 V. These findings highlight FeHCF-based aqueous batteries' potential for high-performance and safe energy storage.