(Invited) Manganese Dioxide Positive Electrodes for Aqueous Zinc-Ion Batteries

Abstract

Tremendous work has been made in a relatively short period of time on developing the rechargeable aqueous zinc-ion battery (ZIB). The ZIB using inexpensive/abundant raw active materials is a cheaper and safer alternative to Li-ion batteries (LIBs), particularly where portability is not of primary importance. The ZIB is also considered by many to be the most promising rechargeable zinc battery technology because it addresses many of the short-comings of traditional zinc battery chemistries. By using a near-neutral pH electrolyte as compared to a strong alkaline solution, cycling of the zinc metal electrode is improved drastically. The coulombic efficiency of zinc plating/stripping is increased and internal short circuits due to dendritic growth of zinc metal are suppressed. The slightly acidic (pH 4-6) electrolyte also allows for new reversible (de)intercalation mechanisms to occur at the positive electrode. This has led to the development of several novel classes of electrode materials which store zinc cations.One such electrode material which is capable of intercalating Zn2+ cations is manganese dioxide. Compared to the alkaline Zn-MnO2 battery in which both the zinc negative electrode and the manganese dioxide positive electrodes undergo conversion reactions, the chemistry of the ZIB is simplified. Zn2+ is stripped from the negative electrode and intercalates into the positive electrode during discharge. Upon charge, Zn2+ cations de-intercalate from the positive electrode active material and plate back onto the negative electrode as zinc metal. In this work, various forms of manganese dioxide were explored in detail for use as the positive electrode active material for zinc intercalation. Different crystal structures of MnO2 were studied and several additional factors, such as particle size and morphology were found to impact the performance of Zn2+ intercalation. The diffusion coefficients were calculated and each material was cycled in a zinc-ion battery to determine its suitability as a Zn2+ intercalation electrode in a near-neutral aqueous system. Through this structure-properties correlation, general material design rules can be drawn, guiding future R&D efforts on ZIB chemistry. Figure 1