Viologen-Embedded Cross-Linked PVB Composite as the Integrated Solid Polymer Electrolyte for High-Performance Electrochromic Devices

Traditional liquid electrolytes suffer from inherent limitations, including poor shape adaptability, inferior interfacial stability, low safety, and narrow operational voltage windows. Moreover, the risks of electrolyte leakage complicate device packaging and increase its cost. In this work, we fabricate an integrated electrochromic-solid polymer electrolyte (EC-SPE) material based on viologen derivatives and cross-linked polyvinyl butyral (PVB) via a solution-casting process. The EC-SPE is laminated between transparent conductive indium tin oxide (ITO) electrodes to form an ordered ITO/EC-SPE/ITO assembly. The cross-linked PVB-based SPE (denoted as PK-10) exhibits high ionic conductivity (3.48 mS·cm –1 ), strong adhesion (1.32 MPa), and excellent thermal stability. Tailored viologen derivatives (V1 and V2) show significantly improved solubility in organic solvents and, through π–π stacking during solvent removal, achieve uniform dispersion while effectively suppressing crystallization. The resulting EC-SPE films (PKV1 and PKV2) demonstrate nearly 99% initial transmittance, high optical modulation (Δ T = 65% and 60%), and fast switching (coloration/bleaching times of 8.2 s/2.5 s and 3.9 s/2.4 s, respectively). Notably, they retain 91.5% and 85.7% of their initial Δ T after 10,000 and 5000 cycles, respectively, indicating outstanding cycling stability. This work provides a promising strategy for developing high-performance integrated EC-SPE devices.