Reaction mechanisms for electrolytic manganese dioxide in rechargeable aqueous zinc-ion batteries

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 ZnMn(2)O(4) (spinel-type) and Zn(2+) is intercalated into the tunnels of the γ- and ε-MnO(2) phases, forming Zn(x)MnO(2) (tunnel-type). When a critical concentration of Mn(3+) in the intercalated Zn(x)MnO(2) species is reached, a disproportionation/dissolution reaction is triggered leading to the formation of soluble Mn(2+) and hydroxide (OH(–)) ions; the latter precipitates as zinc hydroxide sulfate (ZHS, Zn(4)(OH)(6)(SO(4))·5H(2)O) by combination with the ZnSO(4)/H(2)O electrolyte. During charge, Zn(2+) is reversibly deintercalated from the intergrown tunneled phases (γ-/ε-Zn(x)MnO(2)), Mn(2+) is redeposited as layered chalcophanite (ZnMn(3)O(7)·3H(2)O), and ZHS is decomposed by protons (H(+)) formed during the electrochemical deposition of chalcophanite.