Ferroelectric thin-film capacitors exhibit significant applications in pulsed power systems due to their high power density and ultrafast charge/discharge capability. However, the inherent trade-off between polarization and breakdown strength in ferroelectrics fundamentally imposes challenges for co-optimization of recoverable energy density (W_rec) and efficiency (η), while also compromising energy storage stability. To address these challenges, a gradient nano-columnar crystallite engineered BaTiO₃/Ba(Zr_0.2Ti_0.8)O₃/BaTiO₃ sandwich film was constructed enabled via a low thermal budget process of 200 ℃. This approach facilitates an unique microstructure featured by gradient-distributed nano-columnar crystallites in the amorphous-dominated matrix via a spatially-separated manner throughout the sandwich film, thereby effectively modulating polarization behavior, mitigating dielectric nonlinearity, and consequently boost the energy storage performance remarkably. Experimental results demonstrate that this sandwiched structure is endowed with an enhanced maximum polarization (P_max) while a small remnant one (P_r), and a much-delayed polarization saturation, which are accountable for a W_rec ~ 100.6 J/cm³ with an ultrahigh η ~ 90.9% at 6.1 MV/cm. Furthermore, these sandwich films displayed broad operating temperature (RT~150℃) and frequency (100 Hz ~ 10 kHz) stability, and especially robust cycling-reliability (2 × 10⁹ cycles). Through an innovatively engineered sandwich heterostructure design coupled with a low thermal budget, simultaneous achievements of low-temperature compatibility and superior energy storage characteristics lays a foundation for those integrated energy storage devices.

Owing to the complex composition architecture of these solid solutions, some fundamental issues of the classical (1−x)Bi_1/2Na_1/2TiO-xBi_1/2K_1/2TiO₃ (BNT-xBKT) binary system, such as details of phase evolution and optimal Na/K ratio associated with the highest strain responses, remain unresolved. In this work, we systematically investigated the phase evolution of the BNT-xBKT binary solid solution with x ranging from 0.12 to 0.24 using not only routine X-ray diffraction and weak-signal dielectric characterization, but also temperature-dependent polarization versus electric field (P-E) and current versus electric field (I-E) curves. Our results indicate an optimal Na/K ratio of 81/19 based on high-field polarization and electrostrain characterizations. As the temperature increased above 100 °C, the x = 0.19 composition produces ultrahigh electrostrains (> 0.5%) with high thermal stability. The ultrahigh and stable electrostrains were primarily due to the combined effect of electric-field-induced relaxor-to-ferroelectric phase transition and ferroelectric-to-relaxor diffuse phase transition during heating. More specifically, we revealed the relationship between phase evolution and electrostrain responses based on the characteristic temperatures determined by both weak-field dielectric and high-field ferroelectric/electromechanical property characterizations. This work not only clarifies the phase evolution in BNT-xBKT binary solid solution, but also paves the way for future strain enhancement through doping strategies.

Sr_0.7Bi_0.2TiO₃ (SBT) by increasing the proportion and size of polar nano-region. Meanwhile, the BDS remains a high level with x ≤ 0.38 attributed to the addition of KBT with a large band gap. As a result, the 0.62SBT-0.38KBT exhibits a high energy storage density of 2.21 J/cm³ with high η of 91.4% at 220 kV/cm and superior temperature stability (−55 ~ 150 °C), frequency stability (10 ~ 500 Hz) and fatigue resistance (10⁵ cycles). Moreover, high pulsed discharge energy density (1.81 J/cm³), high power density (49.5 MW/cm³) and great thermal stability (20 ~ 160 °C) are achieved in 0.62SBT-0.38KBT. Based on these excellent properties, the 0.62SBT-0.38KBT are suitable for pulsed power systems. This work provides a novel strategy and systematic study for improving energy storage properties of SBT.