Electrochemical Deposition of Zn–Ni Hexacyanoferrate on Spherical Glassy Carbon for a High‐Voltage Aqueous Sodium–Zinc Ion Hybrid Battery

Abstract Aqueous zinc‐ion batteries (AZIBs) are attractive for next‐generation energy storage due to their safety, low cost, and environmental compatibility. However, achieving high‐voltage output is challenging due to the onset of the water electrolysis potential at 1.23 V versus RHE. This study presents a novel, additive‐free cathode enabling AZIB operation at an elevated voltage of 2.5 V. The cathode comprises zinc–nickel hexacyanoferrate (Zn–NiHCF) electrochemically deposited onto spherical glassy carbon (SGC) powder, forming a composite (SGC@Zn–NiHCF). The battery operates in a hybrid electrolyte of 0.1 m NaCl with 1 m m Zn 2 ⁺, effectively suppressing zinc dendrite formationa major limitation in conventional AZIBs with high Zn 2 ⁺ concentrations. Remarkably, this low‐concentration electrolyte allows stable performance with high overpotential to hydrogen evolution reaction. In situ EQCM and SECM are used to probe ion intercalation and reaction dynamics. The system delivers a high specific capacity of 170.8 mAh g −1 and a power density of 1.75 kW kg −1 at 0.5 A g −1 , with excellent cycling stability—retaining 80% of its capacity over 5000 cycles. This work demonstrates a viable pathway toward a high‐voltage, sustainable aqueous sodium and zinc ion hybrid battery without the need for electrolyte additives.