Lithium trapping as a degradation mechanism of the electrochromic properties of all-solid-state WO3//NiO devices

Abstract

There has been keen interest for years in the research of all-solid-state transmittance-type electrochromic (EC) devices due to their various applications especially in “smart windows”. A step forward has been taken in the successful preparation of full multilayered devices with enlarged optical contrast and fast switching response. However, limited durability remains a severe issue. Upon cycling, EC devices suffer from decline of charge capacity as well as optical modulation while the detailed degradation mechanisms remain poorly understood. Here, we demonstrate unambiguous ion-trapping evidence to interpret the charge density decay of the EC device induced using various voltammetric cycling protocols, namely long-term cycling and accelerated cycling. Pronounced comparable ion trapping occurs in cathodically colored WO₃ films whatever the cycling procedure is, suggesting the existence of the trapping “saturation” phenomenon. From second-ion-mass-spectroscopy analysis, the ⁷Li⁺/¹⁸⁴W⁺ ratio in the degraded WO₃ films is more than 100 while it is almost zero in the as-prepared films. In contrast, for anodically colored NiO, a larger number of trapped cations is determined in the long-term cycled films than in the accelerated ones. In combination with X-ray-photoelectron-spectroscopy, variable bonding energies indicate that the ions are trapped at different types of sites, depending on the cycling procedure, and they can reside in the structural channels or break the network chains to form new chemical bondings, thus resulting in a significant color difference. In addition, a clear upward trend in the trapped Li concentration along with depth is observed. All our findings provide a deep insight into the degradation phenomenon taking place in electrochromic films as well as in full devices and offer valuable information for the understanding of micro mechanisms.