Abstract
This study reports the phase transformation behaviour associated with electrolytic manganese dioxide (EMD) utilized as the positive electrode active material for aqueous zinc-ion batteries. Electrochemical techniques, including galvanostatic charge–discharge and rotating ring-disk electrode measurements, and microstructural techniques, using X-ray powder diffraction, scanning electron microscopy, and transmission/scanning transmission electron microscopy, were utilized to characterize the positive electrode at different stages of discharge and charge of zinc-ion cells. The results indicate that, during discharge, a fraction of EMD undergoes a transformation to ZnMn2O4 (spinel-type) and Zn2+ is intercalated into the tunnels of the γ- and ε-MnO2 phases, forming ZnxMnO2 (tunnel-type). When a critical concentration of Mn3+ in the intercalated ZnxMnO2 species is reached, a disproportionation/dissolution reaction is triggered leading to the formation of soluble Mn2+ and hydroxide (OH–) ions; the latter precipitates as zinc hydroxide sulfate (ZHS, Zn4(OH)6(SO4)·5H2O) by combination with the ZnSO4/H2O electrolyte. During charge, Zn2+ is reversibly deintercalated from the intergrown tunneled phases (γ-/ε-ZnxMnO2), Mn2+ is redeposited as layered chalcophanite (ZnMn3O7·3H2O), and ZHS is decomposed by protons (H+) formed during the electrochemical deposition of chalcophanite.
Highlights
This study reports the phase transformation behaviour associated with electrolytic manganese dioxide (EMD) utilized as the positive electrode active material for aqueous zinc-ion batteries
The purpose of this study is to investigate the mechanisms associated with the use of EMD as the positive electrode in aqueous Zn-ion batteries (ZIBs)
It is clear from the energy dispersive X-ray (EDX) data that the composition of the pristine electrode is similar to that of raw EMD; the carbon peak in the electrode is from carbon black and/or the graphite substrate used as a current collector for these electrodes
Summary
This study reports the phase transformation behaviour associated with electrolytic manganese dioxide (EMD) utilized as the positive electrode active material for aqueous zinc-ion batteries. Electrochemical techniques, including galvanostatic charge–discharge and rotating ring-disk electrode measurements, and microstructural techniques, using X-ray powder diffraction, scanning electron microscopy, and transmission/scanning transmission electron microscopy, were utilized to characterize the positive electrode at different stages of discharge and charge of zinc-ion cells. The purpose of this study is to investigate the mechanisms associated with the use of EMD as the positive electrode in aqueous ZIBs. Cycled batteries are disassembled to characterize the EMD electrodes at different potential stages using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission/scanning transmission electron microscopy (TEM/STEM). There is reversible Z n2+ intercalation into the tunnels of γ-/ε-MnO2 (in EMD) in addition to dissolution/precipitation side reactions
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