Reaction mechanisms for electrolytic manganese dioxide in rechargeable aqueous zinc-ion batteries
Thuy Nguyen Thanh Tran, Susi Jin, Marine Cuisinier, Brian D. Adams, Douglas G. Ivey
Abstract
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 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.