Operando Observation of Zinc Negative Electrode Using Confocal Optical System and X-Ray Diffraction

Abstract

Alkaline secondary batteries using zinc negative electrodes are attractive candidates for large-scale energy storage systems since they potentially satisfy low cost, high safety standard and high energy density. However, the short cycle life of the zinc electrodes hinders their practical applications. To overcome this problem, it is necessary to understand the degradation mechanism. In this work, we applied the combination of operando confocal optical system and operando x-ray diffraction (XRD) to alkaline zinc electrode systems to investigate the mechanism of the degradation from the physical and the chemical points of view. Operando confocal optical system is a confocal-optics-based microscopy system that enables acquisition of all-in-focus high definition color image on uneven surface by vertical scanning of observation surface. It also enables time-resolved observation of morphological and color change of electrodes during charge-discharge cycle by periodical scanning. We previously applied this to lithium-ion battery systems and successfully visualized local reaction distribution.[1] Since the morphological change and the local reaction distribution mainly cause the degradation of zinc electrode[2], the operando confocal optical system possibly supplies important information about the degradation mechanism. On the other hand, it is difficult to analyze the chemical properties with the optical system. To compensate the chemical aspect, we also carried out operando XRD. Nakata et al. applied operando synchrotron XRD to zinc electrode systems and successfully quantified ZnO and Zn phases.[2] In this work, we expanded synchrotron XRD into laboratory XRD, which has higher versatility and higher availability.The optical measurements and the XRD were separately employed with a confocal optical system (ECCS, Lasertec) and XRD system (SmartLab, Rigaku), respectively, but the same electrochemical cell and operating conditions were applied. The electrochemical cell consists of ZnO composite electrode filled in Cu foam (working electrode), Hg|HgO electrode (reference electrode), Zn wire (counter electrode), poly(propylene) film (observation window) and 1.0 and 4.0 mol dm–3 KOH solutions saturated (sat’d) with ZnO (electrolyte solution).Figure (a), (b) shows parts of operando confocal optical images and operando XRD patterns of the cross-section of the ZnO composite electrodes in 1.0 and 4.0 mol dm–3 KOH solutions sat’d with ZnO. Zn deposited to form clusters at around the Cu current collectors at the charge in 4.0 mol dm–3 KOH sat’d with ZnO while relatively uniform Zn deposition was observed at the charge in 1.0 mol dm–3 KOH sat’d with ZnO. The diffraction pattern of ZnO was hardly observed after the discharge in 4.0 mol dm–3 KOH sat’d with ZnO. In contrast, ZnO110 peak was clearly observed and bluish blacked deposition was uniformly observed in the optical image after the discharge in 1.0 mol dm–3 KOH sat’d with ZnO. Charge-discharge measurements using three-electrode full-cells with Ni(OH)2 counter electrodes showed that the ZnO composite electrode in 1.0 mol dm–3 KOH exhibited about 4 times longer cycle life than that in 4.0 mol dm–3 KOH. These results indicated that higher solubility of [Zn(OH)4]2– in 4.0 mol dm–3 KOH caused local deposition of Zn and ZnO followed by the degradation due to the shape change.