Origin of electrochemical activation on vanadium hexacyanoferrate cathode for aqueous zinc-ion battery

Herein, we investigate the phase evolution and reactions of vanadium hexacyanoferrate (VHCF) cathode in aqueous Zn-ion battery. X-ray diffraction, in situ Raman spectroscopy, scanning electron microscopy, time-of-flight secondary-ion mass spectrometry, and electrochemical methods reveal electrochemical activation of VHCF electrode in multiple charge–discharge cycles that eventually converts VHCF into zinc hexacyanoferrate and vanadium oxides (VOx). The emerging VOx redox behavior enhances the specific capacity of the electrode from 77 to 165 mAh g−1. Operando UV–vis absorption spectra of the electrolyte in the vicinity of the electrode during electrochemical reaction verifies the presence of decavanadate anion — the intermediate of VHCF transformation into the electrochemically active vanadium oxide. Analysis of the electrodes under various electrochemical conditions and experiments in aqueous and non-aqueous media demonstrate that vanadium of VHCF is redox inactive in the pristine material but products of VHCF transformation start to contribute to the specific capacity of the electrode upon the formation of vanadium oxide.