Sodium-rich nickel–cobalt hexacyanoferrates for enhanced energy storage performance in aqueous electrolytes†

Prussian blue analogues (PBAs) are promising positive electrodes in sodium-ion batteries because of their three-dimensional structure, high capacity, and low-cost. In this work, we synthesize sodium-rich nickel–cobalt hexacyanoferrate (Na⁺-rich Ni_xCo_yHCF) and study the influence of Na⁺ incorporation on the structure and electrochemical properties of the resulting material alongside Ni_xCo_yHCF, NiHCF, and CoHCF. The Na⁺ incorporation introduces minor lattice distortions but retains the fundamental crystal structure of Ni_xCo_yHCF and facilitates Na⁺ diffusion. Na⁺-rich Ni_xCo_yHCF exhibited a significantly enhanced specific capacity of 303.15 F g⁻¹ at a current density of 1.0 A g⁻¹ in an aqueous electrolyte compared to its non-Na⁺ counterparts. The assembled hybrid device delivered a high energy density of 166.15 μW h cm⁻² at a power density of 1800 μW cm⁻² and 83.7% capacity retention over 2000 cycles at 5 mA cm⁻². These results indicate that Na⁺ incorporation is an efficient strategy to prepare high-performance electrodes for aqueous Na-ion storage.