Observation\nof Structural Decomposition of Na₃V₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃ as Cathodes\nfor Aqueous Zn-Ion Batteries

Abstract

Na\nsuperionic conductor (NASICON)-type compounds have been recently\nconsidered to be some of the most attractive candidates for aqueous\nZn-ion batteries (AZIBs) due to their large ionic channels and fast\nkinetics. However, in this work, our findings demonstrate that NASICON-type\ncompounds are maybe not suitable for AZIBs due to their structural\ninstability. Herein two typical NASICON structures, Na₃V₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃, as cathodes for\nAZIBs are investigated. Surprisingly, it is found that both cathodes\nundergo structural decomposition in 1 M Zn­(CF₃SO₃)₂ electrolyte during repeated cycling. Na₃V₂(PO₄)₃ degrades into Zn₃V₂O₈, V₂O₅, and VO₂ after 200 cycles, while Na₃V₂(PO₄)₂F₃ decomposes into dominant phases\nof V₂O₅, VPO₅, and Zn₃(OH)₂V₂O₇·2H₂O,\nwhich are demonstrated by a combination of galvanostatic charge and\ndischarge cycling, X-ray diffraction (XRD), scanning electron microscopy\n(SEM), transmission electron microscopy (TEM), and X-ray photoelectron\nspectroscopy (XPS) measurements. The possible decomposition mechanisms\nof both materials are not only associated with the inherent instability\nduring Zn²⁺ ion (de)­intercalation but are also affected\nby the coinsertion of H⁺ and solvation effect of H₂O, which accelerates the structural decomposition. This work\npresents insights on the structural evolution of NASICON-structured\ncathodes for AZIBs.