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 ZnMn₂O₄ (spinel-type) and Zn²⁺ is intercalated into the tunnels of the γ- and ε-MnO₂ phases, forming Zn_xMnO₂ (tunnel-type). When a critical concentration of Mn³⁺ in the intercalated Zn_xMnO₂ 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-Zn_xMnO₂), Mn²⁺ is redeposited as layered chalcophanite (ZnMn₃O₇·3H₂O), and ZHS is decomposed by protons (H⁺) formed during the electrochemical deposition of chalcophanite.