(592d) Molecular-Level Insights into the Defect Structures and Dynamic Electronic Properties of ZnO Formed in Rechargeable Zn-Alkaline Batteries

Rechargeable alkaline batteries using zinc metal (Zn) electrodes have received significant interest for energy storage applications because of their high energy density, low cost, environmental friendliness, and inherent safety. Zn metal electrodes, however, have low cycle lives at high depths-of-discharge, in part because zinc oxide (ZnO) forms during discharge and accumulates within the electrode. ZnO buildup increases resistance and promotes passivation, eventually leading to cell failure. However, despite decades of research, the compositions, structures, and properties of ZnO formed in alkaline electrolytes are not fully understood. In this work, we use in operando microscopy, spectroscopy, and diffraction to study the composition and electronic properties of the ZnO discharge product as a function of electrode potential. In addition, we use ex situ solid-state ¹H, ²H, and ⁶⁷Zn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy on ZnO discharge products to characterize the different hydrogen and zinc environments.

In operando XRD establishes no crystalline phases other than ZnO are present in the discharge product regardless of electrode potential. In operando electrochemical impedance spectroscopy (EIS) and ultraviolet-visible (UV-vis) spectroscopy show that the conductivity, band gap, and color of ZnO reversibly change as a function of electrode potential. Coulometry shows that the ZnO discharge product inserts electrons as electrode potential decreases, suggesting that the ZnO becomes blue in color from the electrochromic effect. In operando confocal Raman spectroscopy suggests the ZnO is disordered, natively hydrogen-doped, and contains oxygen vacancies, and that protons reversibly insert as a function of potential. Solid-state ¹H and ²H MAS NMR spectroscopy establish that a change in the electrode potential results in a change in hydrogen environments due to proton insertion, while solid-state ⁶⁷Zn MAS NMR spectroscopy show that local zinc environments are distorted compared to those in crystalline ZnO synthesized via conventional routes. 2D ¹H-¹H exchange (EXSY) NMR measurements demonstrate mobility of protons between defect environments and ¹H-¹H dipolar-mediated double-quantum-filtered NMR measurements establish through-space molecular-level proximities between proton sites. Overall, the combination of in operando measurements and solid-state MAS NMR enables a deeper understanding of the compositions, electronic and optical properties, and behavior of ZnO not only in alkaline Zn batteries, but also in next-generation electronic devices utilizing ZnO.