Ureidopyrimidone based Poly(ionic liquid) as All Solid-State Electrolyte for Electrochromic Devices

Abstract

The growing demand for flexible electrochemical devices has prompted the development of safer and more mechanically compatible electrolytes. Conventional liquid electrolytes often fall short to meet these requirements, particularly in flexible or wearable applications, due to issues such as leakage, volatility and poor mechanical integration. In response, all-solid-state polymer electrolytes have emerged as a promising alternative, offering safety, mechanical robustness and processability. However, achieving high mechanical performance in these materials often comes at the expense of ionic conductivity. In this study, we report the synthesis and characterization of novel anionic poly(ionic liquid) (PIL)-based terpolymers, developed as flexible, self-standing all-solid electrolytes. The terpolymer design relies on the synergistic combination of three methacrylate-based monomers: an ionic liquid monomer that provides a high concentration of dissociated ions, poly(ethylene glycol) (PEG) methacrylate to enhance free volume and promote ion mobility, and ureidopyrimidinone (UPy) moieties capable of forming strong, reversible physical crosslinks, imparting elastomer-like mechanical properties and enabling reversible processability without compromising conductivity. The terpolymers were synthesized through a free radical polymerization (FRP) with various monomers ratios and their compositions were confirmed by 1 H NMR. The optimized material demonstrated an ionic conductivity of 2.8 x 10 -6 S/cm, a Young modulus of up to 2 MPa and a strain at break exceeding 150 %, indicating a favourable balance between conductivity and mechanical properties. To evaluate practical performance, the terpolymer was integrated into an electrochromic device consisting of a purple electrochromic polymer (as the active layer), the solid electrolyte, and a counter-electrode. Upon application of 0.7 V, the all-solid-state device demonstrated a maximal optical contrast of 32.6 %, a coloration efficiency of 687 cm²/C and a color switch time of 30 seconds. These results confirm the potential of UPy-functionalized PILbased terpolymers as versatile solid electrolytes for next-generation flexible electrochemical devices.