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 BaTiO₃-CeO₂ (B_{1- x}S_xT-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_xT-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 BaTiO₃-based films from ∼ 80% to ∼ 90%, while simultaneously boosting the energy storage density (U_e) from 57.6 J/cm³ to 81.2 J/cm³. Specially, at x = 0.8, the film achieves a high η of ∼ 85% and a superior U_e of 81.2 J/cm³. Remarkably, all B_{1- x}S_xT-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.