Tuning Piezoelectricity and Pyroelectricity in Poly(vinylidene fluoride) through Ionic Liquid Anion-Size Directed Polymorph and Interface Engineering

Despite the widespread deployment of ionic liquids (ILs) in poly(vinylidene fluoride) (PVDF) to amplify piezoelectric properties, systematic anion-specific structure-property correlations governing crystallinity, microstructure, wettability, and pyroelectric transduction remain strikingly absent. Here, we introduce a homologous [Emim]+-IL series featuring gradually increasing size and polarizable anions X (Cl-, NO3-, BF4-) to demonstrate stereotypical ion-dipole interactions at the electrophilic •CH2 and nucleophilic•CF2 sites of PVDF, as validated from the gas-phase density functional theory calculations. Extensive analyses reveal that an anion-driven effect maximizes H···X templating that yields refined spherulites (R ≤ 1.4 μm) and polarity-mediated work-function modulation across ferroelectric phases. Unlike empirical priors, we establish predictive correlations linking anion polarizability to nucleation kinetics and lamellar polarity, delivering peak-to-peak open-circuit voltage (VPP ∼ 6.3 V), ultrasensitive low-pressure detection and rapid capacitor charging (τ ≤ 26 s). The flexible device also demonstrated real-time responsiveness to dynamic changes for effective wireless IoT-enabled gesture and cardiovascular monitoring. Furthermore, we leverage electrode-interface optimization to record pyroelectric responsivity under varying thermal gradients, establishing anion-precise IL engineering as a transformative paradigm, surpassing limitations for wearable health monitors, therapeutic implants, and IoT sensors.