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

Na superionic conductor (NASICON)-type compounds have been recently considered to be some of the most attractive candidates for aqueous Zn-ion batteries (AZIBs) due to their large ionic channels and fast kinetics. However, in this work, our findings demonstrate that NASICON-type compounds are maybe not suitable for AZIBs due to their structural instability. Herein two typical NASICON structures, Na3V2(PO4)3 and Na3V2(PO4)2F3, as cathodes for AZIBs are investigated. Surprisingly, it is found that both cathodes undergo 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 of V2O5, VPO5, and Zn3(OH)2V2O7·2H2O, which are demonstrated by a combination of galvanostatic charge and discharge cycling, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) measurements. The possible decomposition mechanisms of both materials are not only associated with the inherent instability during Zn2+ ion (de)intercalation but are also affected by the coinsertion of H+ and solvation effect of H2O, which accelerates the structural decomposition. This work presents insights on the structural evolution of NASICON-structured cathodes for AZIBs.