Sodium-rich Nickel-Cobalt Hexacyanoferrates for Enhanced Energy Storage Performance in Aqueous Electrolyte

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 NixCoyHCF) and study the influence of Na+ incorporation on the structure and electrochemical properties of the resulting material alongside NixCoyHCF, NiHCF, and CoHCF. The Na+ incorporation introduces minor lattice distortions but retains the fundamental crystal structure of NixCoyHCF and facilitates Na+ diffusion. Na-rich NixCoyHCF exhibited significantly enhanced specific capacity of 303.15 F/g at a current density of 1.0 A/g in aqueous electrolyte compared to its non-Na+ counterparts. The assembled hybrid device delivered a high energy density of 166.15 μWh 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 the Na+ incorporation is an efficient strategy to prepare high-performance electrode for aqueous Na-ion storage.