In-Situ Raman and DFT Analyses of ZnO Formation Reaction at Zn Negative Electrodes during Discharge Processes

Abstract

Zn is of interest as a promising material for a negative electrode to be implemented in next generation rechargeable batteries. However its morphological changes, such as mossy-like structural growth or dendritic growth, which bring about serious degradation of the battery, are still a major issue. Such changes of the electrode surface are accompanied with the discharge reaction, which is expressed as Zn + 2OH- → ZnO + H2O + 2e-. This indicates that molecular level mechanisms of ZnO formation reaction are necessary to be investigated, to solve the issue of the undesirable electrode surface changes. We have recently shown the growth behavior of ZnO deposits by using in-situ Raman spectroscopy1. This paper describes the ZnO formation behavior during the discharge process in detail, based on the results of the Raman spectroscopy with DFT.In-situ Raman spectroscopy was performed during the chronoamperometry for the ZnO formation. Our measurement cell is made up of a three electrodes system, in which all the electrodes is Zn. As the system approached the passivation, a strong peak around 400 cm-1 appeared, which represents the ZnO crystal structure formation. This suggests ZnO leads to the passivation of electrode surface. Furthermore, it turned out that the peak of ZnO crystal structure is not so stable: the peak could show blinks within several tens of seconds. This indicates that ZnO formation should go through atomic level structural changes with high flexibility after the deposition. The DFT calculation explains this from the molecular level viewpoint. It suggests Zn electrode significantly decreases the surface diffusion barrier of chemical species. The trend seen here is also shown in our other DFT results, in which Zn surface structure itself is also variable. The character of Zn, which allows flexible structural changes, might govern the growth behavior on Zn electrode surface.This research was partially supported by the “Development of Systems and Technology for Advanced Measurement and Analysis”, and the “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING 2)” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.1. T. Otani, T. Yasuda, M. Kunimoto, M. Yanagisawa, Y. Fukunaka and T. Homma, Electrochim. Acta, 395, 90 (2019).