An Ethylene-Vinyl Acetate Copolymer Based Solid Polymer Electrolyte via Water Modification for Laminated WO3-NiO Electrochromic Devices

The ethylene-vinyl acetate (EVA) copolymer, a popular transparent adhesive interlayer material for solar cell encapsulation, can be formed by a simple extrusion process at a relatively low temperature without the aid of an organic solvent. However, the inherently poor ionic conductivity (σ) restricts its application as a solid polymer electrolyte (SPE) for laminated WO₃-NiO electrochromic devices (ECDs). Here, we propose a strategy to improve the σ by a water modification in the EVA matrix. The results demonstrate that the sample treated at 50 °C and 25% relative humidity (RH) for 1 h exhibits a higher σ of 3.80 × 10^–4 S cm^–1, as well as a visual transmittance of more than 90%, a tensile strength of 1.61 MPa, and an excellent thermal stability up to 219 °C. Using this kind of EVA-based SPE (EVA-SPE) as the interlayer, WO₃-NiO based ECDs with sizes varying from 2.5 × 5 to 20 × 20 cm² have been successfully laminated and exhibit favorable EC performances. Besides, the water modification is conducive to an enlarged light modulation range (ΔT) for the laminated ECD in the near-infrared zone, ensuring a high energy efficiency when the device is used as a smart window in buildings.