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

Superparaelectric\n(SPE) relaxor ferroelectrics are emerging as\nthe primary candidates for electrostatic dielectrics due to their\nsuperior energy storage capabilities. However, there is a lack of\nsystematic studies on the intrinsic mechanisms that enhance energy\nstorage performance. Here, by controlling the annealing temperature\n(<i>T</i><sub>an</sub>), we comprehensively analyzed the\nevolution of domain structures in (Ba<sub>0.985</sub>,Na<sub>0.015</sub>)(Ni<sub>0.05</sub>,Ti<sub>0.95</sub>)O<sub>3</sub> (BNTN) dielectric\nfilms to explore the internal mechanisms enhancing the energy storage\nperformance. Our results demonstrate that lowering <i>T</i><sub>an</sub> reduces the film’s crystallinity, transforming\ninternal domain structures from microdomains to large polar nanoregions\n(PNRs) with some nanodomains and then to small PNRs and finally to\nthe nonpolar state, thereby nearly eliminating polarization switching\nhysteresis. Simultaneously, the specific <i>T</i><sub>an</sub> for the SPE state enables the presence of hexagonal BNTN, endowing\nthe films with high polarization. Additionally, the unique nanocrystalline\ngrains and disordered amorphous-like phase lead to a high breakdown\nstrength of 6.4 MV cm<sup>–1</sup> for the BNTN films annealed\nat 650 °C. These factors synergistically contribute to an excellent\nenergy storage density (92.8 J cm<sup>–3</sup>) and ultrahigh\nefficiency (90%) in SPE films. Furthermore, the SPE films also exhibit\nexcellent fatigue reliability (10<sup>7</sup> cycles) and temperature\nstability (30 to 200 °C), as well as outstanding discharge properties.\nThis study offers new insights for achieving high energy storage performance\nin monolithic thin films.