Flexible Crosslinked Poly(ionic liquid)s for All-Solid-State High-Performance Self-Powered Ionic Pressure Sensors

Despite the progress achieved in ionic-based sensors, the growing demand for soft and flexible devices has highlighted several key requirements for the next generation of materials: (1) complete elimination of liquid phases that may evaporate or leak; (2) sufficient mechanical robustness for stable operation over repeated loading cycles; (3) facile device assembly; (4) high voltage output and sensitivity; and (5) a broad operational pressure range. To address these challenges, two complementary ionic liquid-like monomers (ILMs) were designed and synthesized: a cationic (M-BIM) and an anionic (M-TFSI) monomer, each incorporating a butylimidazolium (BIM⁺) or bis(trifluoromethylsulfonyl)imide (TFSI^-) fragment covalently bonded to a methacrylate group together with their complementary counterions (respectively, TFSI^- or BMIM⁺). Their copolymerization with poly(ethylene glycol) monomethacrylate (PEGM) and dimethacrylate (PEGDM) and optimization of the composition afforded flexible, self-standing, highly ion-conductive films combining good mechanical properties (E' = 0.4-1.0 MPa) with high ionic conductivities (up to 2.3 × 10^-6 S cm^-1 at 25 °C). Systematic optimization of film thickness, interfacial electrode layer (IEL) geometry, and IEL metal type revealed that sputtered gold IELs effectively suppressed signal drift and shortened response times, while platinum IELs markedly enhanced voltage generation. Comparison among sensors with mobile anions, mobile cations, and mixed carriers demonstrated that the charge-carrier type critically governs both sensitivity and voltage amplitude. The optimized sensor, based on the anionic M-TFSI formulation, featuring a 0.35 mm-thick film and rectangular Pt IEL exhibited a linear potential-pressure dependence across the entire range (0-80 kPa), an ultrafast recovery time of 0.2 s, and an exceptional output voltage of 190 mV, maintaining stable operation over 1000 loading cycles.