The environmentally friendly dielectric capacitors based on lead-free relaxation ferroelectrics have attracted recent attention. The material (Bi_0.5Na_0.5)TiO₃ (BNT) exhibits an intrinsic high saturation polarization due to the hybridization between Bi 6s and O 2p orbitals, surpassing conventional lead-free relaxor ferroelectrics. However, a low breakdown strength (BDS) of BNT results in a relatively poor energy storage performance, restricting its further applications. Therefore, improving the BDS of BNT-based materials is a necessity towards achieving a high energy storage density, and one effective measure is a compositional modification. Sr_0.7Bi_0.2TiO₃ (SBT) as an emerging relaxor ferroelectric material has a higher dielectric constant due to the ion substitution-induced local charge imbalance and dipole fluctuations in nanoscale. It is anticipated that introducing SBT as a solid solution component into BNT matrix films can enhance the BDS of the films, optimizing their energy storage performance. In this paper, the impact of SBT/BNT solid solution ratio on phase structure, microstructure, dielectric, and energy storage properties of the SBT modified BNT-based film was investigated.

The precursor solutions for (1-x)(Bi_0.5Na_0.5)TiO₃-xSr_0.7Bi_0.2TiO₃ (BNT-xSBT, x=0.30, 0.35, 0.40, 0.45, and 0.50) were prepared by a sol-gel method. The precursor solutions prepared were aged via static aging for 24 h before spin-coating. Subsequently, the films were heat-treated on a hot plate and rapidly annealed at 630 ℃ for 2 min. The film preparation process, including spin-coating, heat treatment, and rapid annealing, was repeated for 8 times to complete the film fabrication.

The crystal structure of the films was analyzed by a model PANalytical X'Pert PRO X-ray diffractometer (XRD). The surface and cross-sectional morphologies of the samples were determined a model Ultra Plus scanning electron microscope (SEM). To evaluate the electrical properties of the samples, platinum electrodes with a diameter of 0.2 mm were deposited on the film surface using magnetron sputtering. The leakage current density of the films was obtained by a model 6517A electrostatic/high-resistance meter. The ferroelectric properties of the samples were tested by a model PolyK CPE1801 ferroelectric workstation. The dielectric properties of the samples were analyzed by a model 4294A precision impedance analyzer.

All the BNT-xSBT films exhibit a single pseudocubic phase structure. The unit cell volume initially increases and then decreases as the SBT content increases, which is affected by a combined effect of Sr²⁺ doping at the A-site and vacancies. The films demonstrate a well-crystallinity, clear grain boundaries, good adhesion to the substrate, and a thickness of approximately 140 nm. The introduction of SBT results in a reduction in the dielectric constant of the film, improving the temperature stability of the dielectrics. At 2 000 kV/cm, the maximum polarization (P_max) and remnant polarization (P_r) of the films both decrease as the SBT content increases, which is consistent with the dielectric performance. The incorporation of an appropriate amount of SBT increases the BDS of the films from 2 975 kV/cm for BNT-0.3SBT to 3 622 kV/cm for BNT-0.40SBT, which is consistent with the leakage current density. Therefore, the optimal composition BNT-0.40SBT film achieves a high recoverable energy storage density (W_rec) of 85.99 J/cm³ and an efficiency of 74% at the breakdown electric field (E_b). This film demonstrates some advantages, compared to other film capacitors. In addition, the BNT-0.40SBT film exhibits a superior high-frequency stability, a heat resistance, and a fatigue resistance, showing a promising potential for applications in dielectric capacitors.

All the films exhibited a well-crystallinity, having a single perovskite phase. With the introduction of SBT, the unit cell volume initially increased and the decreased, and the BDS firstly increased and then decreased. The dielectric constant and polarization intensity gradually decreased, while dielectric loss remained stable. The optimal component BNT-0.40SBT film achieved a high BDS of 3 622.25 kV/cm and a high energy storage density (W_rec) of 85.99 J/cm³. The superior energy storage density and stability indicated that BNT-0.40SBT thin films could be promising candidates for pulse power and power electronics applications in capacitors.