(606e) Electrochemically Active ZnO Discharge Product Formed in Rechargeable Zn-Alkaline Batteries: Performance Effects and Mechanistic Insights

Rechargeable alkaline batteries with zinc metal (Zn) electrodes are being studied for use in the next generation of energy storage systems because of their high energy density, low cost, inherent safety, and environmental friendliness. However, Zn electrodes have historically been used mainly in single-discharge applications because cyclability is typically low at high utilization of the Zn. In Zn-alkaline batteries, high utilization results in the accumulation of a zinc oxide (ZnO) discharge product, which is thought to contribute to electrode failure. Despite decades of research, however, the composition, electrochemical behavior, and performance effects of the ZnO discharge product are still not fully understood.

In this work we show the ZnO discharge product is electrochemically active via a proton-coupled electron transfer process, in which electrons and protons are simultaneously and reversibly inserted into the ZnO as a function of electrode potential. Electrochemical impedance spectroscopy (EIS) shows the electrical conductivity of the ZnO changes by over 1000x in a 0.6 V electrochemical window, which contributes to the failure of Zn paste electrodes via loss of conductivity at high utilization. Additionally, in operando UV-vis spectroscopy reveals the ZnO is an electrochromic material and exhibits charged-impurity assisted free carrier absorption, which causes a stark color change from white to blue accompanying electron insertion. This establishes a link between color, conductivity, and electrochemical potential of ZnO, which can be exploited for the development of better control systems and cycling protocol in Zn-alkaline batteries.

To better understand the underlying physical properties and electrochemical processes enabling this behavior in ZnO, we used in operando X-Ray diffraction (XRD) and confocal Raman spectroscopy to reveal the ZnO formed in Zn alkaline batteries is proton-doped and that oxygen vacancies are electrochemically active defects. We also used quantitative solid-state magic angle spinning (MAS) ¹H single-pulse nuclear magnetic resonance (NMR) measurements, in conjunction with 2D ¹H{¹H} EXchange SpectroscopY (EXSY) and radio frequency driven recouping (RFDR) experiments to gain insights into the nature of the proton defect environments. This work highlights the importance of the ZnO discharge product to the performance of Zn-alkaline batteries and provides insights into the physical defects and mechanisms that enable ZnO electrochemical activity. We hope these discoveries and associated methods will aid in the development of the next generation of batteries for electric vehicles and grid-scale energy storage systems.