Giant Capacitive Energy Storage in Lead‐Free Super‐Paraelectric Relaxor Ferroelectric Films With Simple Composition

This study demonstrates giant capacitive energy storage in Sr‐doped BaTiO 3 ‐CeO 2 (B 1‐ x S x T‐C, 0.0 ≤ x ≤ 1.0) lead‐free epitaxial films via a simple compositional design. By systematically tuning Sr doping level, the polarization behavior of B 1‐ x S x T‐C films transforms from relaxor ferroelectric to super‐paraelectric state, achieving optimized polarization behavior with large maximum polarization, low remnant polarization, and high breakdown strength. Structural analysis reveals that Sr doping induces disordered local structures, forming mixed‐phase nanodomains with local symmetry breaking, suppressing hysteresis while maintaining high polarization. The Sr doping significantly enhances the energy storage efficiency ( η ) of BaTiO 3 ‐based films from ∼ 80% to ∼ 90%, while simultaneously boosting the energy storage density ( U e ) from 57.6 J/cm 3 to 81.2 J/cm 3 . Specially, at x = 0.8, the film achieves a high η of ∼ 85% and a superior U e of 81.2 J/cm 3 . Remarkably, all B 1‐ x S x T‐C films exhibit excellent frequency stability (10 Hz—2 kHz) and thermal stability (25°C–160°C), demonstrating their reliability and stability in energy storage applications. This work provides a practical strategy for designing compositionally optimized and environmentally friendly energy storage films.