Occurrence of shape change in rechargeable alkaline zinc electrodes observed by operando confocal optics and X-ray diffraction

Abstract

Alkaline secondary batteries with zinc negative electrodes are attractive power sources for large-scale energy storage systems. However, low cyclabilities of zinc electrodes prevent their wide applications. While shape change has been known as one of the predominant degradation modes, its origin has not been clarified yet. In this study, we analyze the initial charge-discharge behavior of ZnO composite electrodes using a unique “laboratory” operando observation method combining confocal optics and X-ray diffraction to capture the occurrence of shape change and elucidate the mechanisms. It is shown that the ZnO composite electrodes in 4.0 mol dm −3 KOH solution, which show relatively short cycle life, suffers from uneven reactions caused by spatially concentrated metallic zinc depositions on charging and significant ZnO dissolutions on discharging, whereas those in 1.0 mol dm −3 KOH solution and the ionomer-modified electrodes in 4.0 mol dm −3 KOH with improved durability exhibit uniform formation of metallic zinc and ZnO precipitation during charge-discharge cycles. We thus conclude that the occurrence of the shape change processes limiting the cycle life is essentially ascribed to the spatially uneven reactions in the zinc electrodes. • Initial charge-discharge behaviors of ZnO composite electrodes are analyzed. • Operando confocal optics and operando X-ray diffraction are combined. • The occurrence of the shape change is ascribed to the spatially uneven reactions.