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.