Ionic conductivity and dielectric characteristics of a Li incorporated PVA electrolyte membrane and a study of a fully solid-state electrochromic device based on it

Abstract

The construction and study of a fully solid state electrochromic (EC) device with cathodic coloration exhibiting very fast response times (< 1s) is reported using a WO3 EC layer and a poly (vinyl alcohol) (PVA)- Lithium acetate membrane electrolyte. Broadband dielectric studies on the polymer membranes with different Li concentrations show multiple relaxation mechanisms out of which a segmental relaxation prominent beyond a certain composition is strongly correlated with Li-ion conduction yielding better ionic conductivity and reversible coloration in the EC device. The relatively high Li conductivity in PVA has a direct correspondence with the lowering of the crystallinity of PVA and a decrease of the glass transition temperature, Tg with increasing Li addition. A simple model of the coloration process suggests that the short electrolyte diffusion length coupled with a high electric field and an interfacial emf that varies as ≈t−0.6 results in the swift current response of the film with an optimal performance at 20 wt.% Li in PVA, exceeding which irreversible coloration can occur on the surface. The transient current is crucial for the observed swift switching characteristics. Ageing studies for almost a month indicate potential stability of the EC device for over 150 cycles.