Improvement in cycling stability of Prussian blue analog-based aqueous sodium-ion batteries by ligand substitution and electrolyte optimization

In this study, we investigated the effects of ligand substitution in Prussian blue analog (PBA) cathode materials on the performance of aqueous sodium-ion batteries. NaCu[Fe(CN)₆] (NaCuHCF) and ligand-modified PBAs, Na_xCu[Fe(CN)₅(C₆H₄N₂)] (Na_xCuCNPFe), and Na_xCu[Fe(CN)₅(CH₃C₆H₄NH₂)] (Na_xCuTolFe) were tested in different electrolytes. The Na_xCuCNPFe and Na_xCuTolFe cathodes exhibited the best capacity retention of ∼50% after 2000 cycles in 1 M Na₂SO₄, which is much higher than that of the NaCuHCF cathode (0% capacity remained after 2000 cycles). To understand the charge–discharge mechanism of PBA cathodes, in situ synchrotron X-ray absorption spectroscopy and X-ray diffraction were performed. To demonstrate practical energy storage applications, PBAs were tested in full-cell configurations with an anode made of sodium titanium phosphate (NTP) coated with reduced graphene oxide and carbon (NTP@C@RGO). The Na_xCuCNPFe//NTP@C@RGO and Na_xCuTolFe//NTP@C@RGO full cells in 17 m NaClO₄ aqueous electrolyte exhibited high power densities of up to 4338 W kg⁻¹ (with an energy density of 18.11 Wh kg⁻¹) and 4742 W kg⁻¹ (with an energy density of 11.87 Wh kg⁻¹), respectively. Our study demonstrates the potential of optimizing organic ligands in PBAs and electrolytes for the improvement of the cycling stability of high-power aqueous sodium-ion batteries.