Electrochemical Control and Understanding of the Electronic and Optical Properties of ZnO Formed in Rechargeable Zn-Alkaline Batteries

Abstract

Rechargeable alkaline batteries using zinc metal (Zn) electrodes are of significant interest for next-generation energy storage because of their high energy density, low cost, environmental friendliness, and inherent safety. Rechargeable Zn-alkaline batteries, however, have not been widely commercialized in part due to the low cyclability of Zn electrodes. At high depths of discharge, zinc oxide (ZnO) discharge product accumulates in the electrode, passivating the zinc-electrolyte interface and ultimately leading to cell failure. While the ZnO discharge product has been studied for decades, there is no general consensus on its local structure, composition, and electrochemical properties, nor on how these properties are affected by the electrode potential or cycling conditions.In this work, we used in operando analytical techniques including X-Ray diffraction (XRD), confocal Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and ultraviolet-visible (UV-vis) spectroscopy to demonstrate that the discharge product is proton-doped ZnO with oxygen vacancies, and that the electrical conductivity, band gap, and color vary systematically as a function of electrode potential. To investigate local hydrogen and zinc environments within the ZnO discharge product, we performed ex situ solid-state 1H, 2H, and 67Zn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Solid-state 1H and 2H NMR measurements establish that hydrogen environments change as a function of potential, while solid-state 67Zn NMR spectra indicate that the local zinc environments are distorted compared to ZnO generated via conventional routes. We also performed 2D 1H-1H exchange (EXSY) NMR measurements to investigate proton mobility between defect environments and acquired 1D 1H-1H dipolar-mediated double quantum-filtered MAS NMR spectra to establish through-space molecular-level proximities between proton sites. This work provides insights into the dynamics of zinc electrodes in alkaline environments, as well as how to electrochemically control the defect structures and electronic and optical properties of ZnO in alkaline media, which are expected to aid the development of next-generation Zn alkaline batteries and electronic devices utilizing ZnO.