Unveiling the Charge Storage Mechanism in Nonaqueous and Aqueous Zn/Na₃V₂(PO₄)₂F₃ Batteries

In search of a suitable cathode host for aqueous Zn-ion batteries (ZIBs), polyanionic materials have been explored due to their open-framework structure that is believed to improve Zn-ion diffusion. Among them, Na3V2(PO4)2F3 was recently shown in the aqueous ZIB to exhibit attractive electrochemical performance, and the charge storage mechanism was attributed to reversible Zn2+ insertion into the cathode. Here, however, we investigate the puzzling differences in the electrochemical behavior of Na3V2(PO4)2F3 as a cathode material between nonaqueous and aqueous ZIBs. Ex situ analyses of the cathode after cycling in both systems unveil that the observed disparity in the electrochemical behavior stems from the difference in the charge storage mechanism. In the nonaqueous ZIB, guest ions are determined to be both Zn2+ and Na+ initially that gradually shift to be pure Zn2+. In the aqueous ZIB, however, H+ is found to be the guest ion species rather than Zn2+. This explains the attractive electrochemical performance in the aqueous ZIB as H+ insertion and diffusion would be extremely facile unlike Zn2+. Moreover, even with Zn(CF3SO3)2 electrolyte, the formation of zinc salt byproduct on the cathode after discharge further supports that H+ ions are inserted into the cathode, as observed with the aqueous ZnSO4 electrolyte. This byproduct formation on the cathode in the aqueous ZIB system calls for careful analyses to be performed to categorically elucidate that Zn ions are indeed the guest ions at the cathode when investigating cathodes for “Zn-ion” batteries.