High Energy Storage Performance in BNTN Superparaelectric Capacitors with Tetragonal/Hexagonal Phases

Superparaelectric (SPE) relaxor ferroelectrics are emerging as the primary candidates for electrostatic dielectrics due to their superior energy storage capabilities. However, there is a lack of systematic studies on the intrinsic mechanisms that enhance energy storage performance. Here, by controlling the annealing temperature (Tan), we comprehensively analyzed the evolution of domain structures in (Ba0.985,Na0.015)(Ni0.05,Ti0.95)O3 (BNTN) dielectric films to explore the internal mechanisms enhancing the energy storage performance. Our results demonstrate that lowering Tan reduces the film’s crystallinity, transforming internal domain structures from microdomains to large polar nanoregions (PNRs) with some nanodomains and then to small PNRs and finally to the nonpolar state, thereby nearly eliminating polarization switching hysteresis. Simultaneously, the specific Tan for the SPE state enables the presence of hexagonal BNTN, endowing the films with high polarization. Additionally, the unique nanocrystalline grains and disordered amorphous-like phase lead to a high breakdown strength of 6.4 MV cm–1 for the BNTN films annealed at 650 °C. These factors synergistically contribute to an excellent energy storage density (92.8 J cm–3) and ultrahigh efficiency (90%) in SPE films. Furthermore, the SPE films also exhibit excellent fatigue reliability (107 cycles) and temperature stability (30 to 200 °C), as well as outstanding discharge properties. This study offers new insights for achieving high energy storage performance in monolithic thin films.