Realizing the highly reversible Zn2+ and Na+ dual ions storage in high-crystallinity nickel hexacyanoferrate microcubes for aqueous zinc-ion batteries

Prussian blue analogues (PBAs) with the 3D open framework are regarded as promising cathode candidates for aqueous Zinc ion batteries (ZIBs). Among various PBAs, nickel hexacyanoferrate (NiHCF) has attracted considerable attention because of its high operating voltage and economic merit. However, the cyclability of NiHCF is unsatisfactory due to poor structural stability during Zn2+ ions insertion/deinsertion. Moreover, the ion storage mechanism of NiHCF in aqueous electrolytes has not been fully revealed yet. Herein, high-crystallinity NiHCF (HC-NiHCF) microcubes with improved structural stability and larger crystal plane spacing are synthesized. For the first time, highly reversible Zn2+ ions and Na+ ions co-insertion/extraction are achieved for the HC-NiHCF microcubes in mixed aqueous electrolyte, as evidenced by various observations including two separated discharge plateaus and sequential changes of Na 1s and Zn 2p signals in ex-situ X-ray photoelectron spectroscopy (XPS). As a result, a high specific capacity of 73.9 mAh g−1 is obtained for the HC-NiHCF microcubes at 0.1 A g−1, combined with enhanced cycle stability (75% vs. 16.4%) over 1000 cycles at 2 A g−1. The reversible Zn2+ ions and Na+ ions co-insertion in HC-NiHCF microcubes reveals a new ion storage mechanism of Ni-based PBAs in aqueous electrolytes.