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The electrochemistry of the Zn–VPO 4 F system has been studied for the first time in both wet non-aqueous and aqueous zinc-ion electrolytes. It is shown that H + ions present in the electrolyte insert into the cathode host in preference to sluggish Zn 2+ ion insertion. Furthermore, it is demonstrated that while H 2 O as a source of H + is essential for proper cell operation, an aqueous electrolyte is detrimental to cathode stability due to extensive dissolution. 

The zinc-ion battery (ZIB) has been a system of particular interest in the research community as a possible alternative to lithium-ion batteries (LIB), and much work has been devoted to finding a suitable host material. In this article, monoclinic V 2 (PO 4 ) 3 is investigated as a host material for reversible insertion of Zn 2+ . Initial chemical assessment via a facile microwave-assisted chemical insertion method indicates the possibility of Zn 2+ insertion into the host. Electrochemical assessment, however, exhibits a significant capacity fade. In-depth analysis on the average and local structure of Li 3 V 2 (PO 4 ) 3 , the empty host V 2 (PO 4 ) 3 , and the Zn-inserted V 2 (PO 4 ) 3 reveals that heavy distortion is induced upon Zn 2+ insertion into the V 2 (PO 4 ) 3 framework, which is believed to be a result of a strong host–guest interaction jeopardizing the structural integrity. This is further supported by the dissolution of most of the material during the chemical oxidation of the Zn-inserted V 2 (PO 4 ) 3 . The underlying structural inadequacy poses difficulties for monoclinic V 2 (PO 4 ) 3 to be a viable reversible host for Zn-ion batteries. This work suggests that not only the electrostatic repulsions of multivalent ions in a structure during diffusion, but also the structural stability of the host upon insertion of multivalent ions, must be considered for a better design of suitable host materials for multivalent-ion batteries.