A High-Quality Monoclinic Nickel Hexacyanoferrate for Aqueous Zinc-Sodium Hybrid Batteries

Aqueous rechargeable batteries (ARBs) are expected to be used in future energy storage systems (ESSs) because of their nontoxicity, nonflammability, and ultrahigh ionic conductivity. However, their low energy density and poor cycle stability are two main drawbacks restricting their application. In this work, we synthesized a highly crystalline monoclinic nickel hexacyanoferrate (NiHCF) and used it in aqueous zinc-sodium hybrid batteries. Because of its low Fe(CN)6 vacancy amount and low water content, the monoclinic NiHCF exhibits high specific capacity and excellent cycle and rate performance. The assembled AZSHBs show excellent electrochemical performance, with an average operating voltage up to 1.464 V, high energy density of 99.1 Wh kg-1, and good cycling stability with 91% capacity retention after 1000 cycles.