(Invited) Electrochemical “Zinc-Ion” Storage at Nanostructured Manganese Oxides: Mechanistic Insights from 2D Interfaces to Advanced 3D Electrode Architectures

Abstract

Alkaline Zn/MnO2 cells remain a popular energy-storage technology due to their low cost and benign components, yet they lack the long-term rechargeability and high-power capability required in many present and emerging applications. The benefits inherent to the coupling of zinc and manganese oxide (MnOx) electrodes can be translated to a new generation of rechargeable “zinc-ion” batteries that incorporate mild-pH Zn2+-containing electrolytes. By pairing such electrolytes with particular nanostructured forms of MnOx that undergo reversible Zn2+-based redox reactions, the rechargeability of Zn/MnOx systems can be extended to hundreds or thousands of cycles. We are exploring two particular MnOx polymorphs—layered, birnessite-type and cubic-spinel ZnMn2O4—for zinc-ion storage, where these oxides are expressed as nanoscale coatings on 3D carbon nanofoam (CNF) architectures. Such electrode designs enable high oxide-normalized capacity delivered at moderate-to-high rates and over hundreds of charge–discharge cycles. When Na+ or Li+ salts are added to the Zn2+-based aqueous electrolyte, pseudocapacitive reactions at the MnOx become active to provide pulse-power functionality at time scales approaching those for electrochemical capacitors [1]. Electrochemical charge storage in zinc-ion electrolytes may follow multiple pathways, including direct Zn2+ insertion/association at MnOx or via proton-insertion reactions that drive the formation of Znx(OH)y(SO4)z salts at the MnOx surface. We use ex situ characterization (diffraction, spectroscopy, and microscopy) of electrochemically conditioned MnOx–CNF electrodes to show that the latter reaction is dominant for birnessite-type MnOx, where the reversibility of Znx(OH)y(SO4)z formation/dissolution is promoted by the pore–solid architecture of the CNF [2]. These findings are further confirmed by scanning-probe microscopy and quartz-crystal microbalance techniques at planar MnOx–carbon electrodes that mimic the interfaces in MnOx–CNF. Insights from these investigations inform the design of advanced electrode architectures for high-performance, rechargeable zinc-ion batteries. [1] “Combining Battery-Like and Pseudocapacitive Charge Storage in 3D MnOx@Carbon Electrode Architectures in Zinc-Ion Cells.” J.S. Ko, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, Sustain. Energy Fuels, 2 (2018) 626–636. [2] J.S. Ko, M.D. Donakowski, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, MRS Comm., in press (2019).