Polyethylene Glycol–Na⁺ Interface of Vanadium Hexacyanoferrate Cathode for Highly Stable Rechargeable Aqueous Sodium-Ion Battery

Vanadium hexacyanoferrate (VHCF) with an open-framework crystal structure is a promising cathode material for rechargeable aqueous metal-ion batteries owing to its high electrochemical performance and easy synthesis. In this paper, vanadium hexacyanoferrate cathodes were first used for constructing rechargeable aqueous sodium-ion batteries (VHCF/WO₃) and tested in the new-type electrolyte (NaP-4.6) consisting of a polyethylene glycol (PEG)/H₂O/NaClO₄ electrolyte with a low H⁺ concentration (molar ratio of [H₂O]/[Na⁺] is 4.6), which has high stability at a high current density as high as 1000 mA g^–1 with a capacity retention of 90.3% after 2000 cycles at high coulombic efficiency (above 97.8%). To understand their outstanding performance, the proton-assisted sodium-ion storage mechanism and interphase chemistry of VHCF are investigated by solid-state NMR (ssNMR) technology. It is suggested that the H⁺ storage reaction is accompanied by the redox of vanadium atoms and Na⁺ intercalation is accompanied by the redox of iron atoms. It is also observed that the complex of polyethylene glycol (PEG) with Na⁺ (PEG–Na⁺) exists on the VHCF surface, which facilitates the stability of VHCF and promotes the alkali-ion transfer at a high current density. The results of the ssNMR study offer new insights into the intercalation chemistry of Prussian blue analogues with open-framework-structured compounds, which can greatly broaden our horizons for battery research.