Ultrahigh energy storage density in glassy ferroelectric thin films under low electric field

The current approach to achieving superior energy storage density in dielectrics is to increase their breakdown strength, which often incurs heat generation and unexpected insulation failures, greatly deteriorating the stability and lifetime of devices. Here, a strategy is proposed for enhancing recoverable energy storage density (W_r) while maintaining a high energy storage efficiency (η) in glassy ferroelectrics by creating super tetragonal (super-T) nanostructures around morphotropic phase boundary (MPB) rather than exploiting the intensely strong electric fields. Accordingly, a giant W_r of ≈86 J cm⁻³ concomitant with a high η of ≈81% is acquired under a moderate electric field (1.7 MV cm⁻¹) in thin films having MPB composition, namely, 0.94(Bi, Na)TiO₃-0.06BaTiO₃ (BNBT), where the local super-T polar clusters (tetragonality ≈1.25) are stabilized by interphase strain. To the knowledge of the authors, the W_r of the engineered BNBT thin films represents a new record among all the oxide perovskites under a similar strength of electric field to date. The phase field simulation results ascertain that the improved W_r is attributed to the local strain heterogeneity and the large spontaneous polarization primarily is originated from the super-T polar clusters. The findings in this work present a genuine opportunity to develop ultrahigh-energy-density thin-film capacitors for low-electric-field-driven nano/microelectronics.