An in-situ electrochemical oxidation strategy of VPO4 and its performance as a cathode in aqueous Zn-ion batteries

Abstract

• VPO 4 is reported as a new cathode for aqueous Zn-ion battery for the first time. • Due to in situ electrochemical oxidation process, the assembled cell exhibits the excellent electrochemical performance. • The detailed electrochemical oxidation process and Zn 2+ storage mechanism is discussed by In-situ XRD, ex-situ TEM and XPS. • A clear electrochemical kinetics of the assembled cell was also analyzed. Benefiting from the security and low cost, aqueous rechargeable Zn-ion batteries (ZIBs) have currently become attractive candidate devices for energy storage. However, the scarcity of appropriate materials served as cathodes is one of the major issues that hinder the development of aqueous ZIBs. VPO 4 is considered as a promising cathode material for energy storage with huge potential windows and high specific capacity due to the stable structure caused by the stable PO 4 polyanionic group and the various oxidation states of V element. However, due to the low valence state of V, as Zn 2+ host structure, PO 4 could not be effectively discharged directly. Herein, by an in situ electrochemical anodic oxidation method, we successfully improved the valence state of V and a higher valence vanadium-based compound (Zn 0.11 VO(PO 4 )•2.5H 2 O) was obtained. As expected, due to the increased valence state of V element and obviously reduced particle size caused the in situ electrochemical oxidation, the assembled Zn/VPO 4 cells exhibits high reversible specific capacity of about 210 mA h g −1 at 50 mA g −1 and a good rates cycling stability. Furthermore, the efficient in situ electrochemical oxidation process of VPO 4 cathode and the reversible Zn 2+ ions insertion/extraction mechanism in the subsequent cycles have been investigated through in situ XRD diffraction, field emission scanning electron microscopy (FESEM) and ex situ XRD photoelectron spectroscopy (XPS). This work also provides a novel method to design the high performance Zn ion cathode material.