LAUSR

Abstract Rechargeable aqueous batteries using zinc metal anode offer high safety and low cost. However, the narrow electrochemically stable voltage window of water and lack of high voltage cathode materials limit the energy density. Herein, we demonstrate a high voltage polyanion cathode and realize its reversible deep charge in Na–Zn hybrid aqueous batteries. The exfoliated layered VOPO4·2H2O cathode contains partially reduced vanadium centers and presents a high open circuit voltage of 1.8 V (vs. Zn). The effective oxidation of VIV to VV is realized in the water-in-tri-salt electrolyte of 0.5 m Zn(ClO4)2/16 m NaClO4/3 m NaOTf as confirmed by electrochemical analysis, X-ray diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, as well as density of state calculations. The process retrieves more VV/VIV redox centers for energy storage and provides an extra reversible capacity of 38 mAh g−1 at 2 V. The electrode delivers an overall capacity of 140 mAh g−1 with excellent stability and coulombic efficiency. Further analysis demonstrates that Na+ is the major active cation in the cathode reaction, while Zn2+ de/intercalation provides additional capacity. Our work reveals the energy storage mechanism of vanadyl phosphate cathode and proposes an efficient strategy towards high performance Na–Zn hybrid batteries.