Hierarchically polar structures induced superb energy storage properties for relaxor Bi0.5Na0.5TiO3-based ceramics

Electrostatic capacitors are fundamental components in electronics and electric power systems because of their inherent superiorities of charging/discharging speed and power density. However, energy storage properties (ESPs) of the top promising relaxor ferroelectric (RFE) are intimately entwined symmetry of local lattice and dynamics of nano-scaled polar structure. Herein, hierarchically polar structure and oxygen octahedral tilting can be induced in the 0.7Bi0.5Na0.5TiO3-0.3Na0.91Bi0.09Nb0.94Mg0.06O3 (BNT-NBNM) matrix with the addition of CaZrO3 (CZ) to achieve outstanding comprehensive ESPs. The chemically induced coexisting triple phases of tetragonal P4bm, orthorhombic P21ma and Pnma2 bring forth hierarchically polar structure, featuring nanodomains and slush-like polar clusters embedded in stripe submicro-domains, as indicated by morphological observations, splitting of diffraction spots and polar state distributions. Accordingly, an ultrahigh recoverable energy storage density Wrec = 7.5 J cm−3 concurrent with a high-efficiency η = 94.5 % at a breakdown electric field of 510 kV cm−1 can be attained in BNT-NBNM-0.05CZ RFE ceramic. In addition, the optimal composition also displays excellent stability (Wrec = 5.1 ± 0.1 J cm−3, η = 92.7% ± 1%, 20–170 °C and Wrec = 5.2 ± 0.2 J cm−3, η = 92.1% ± 1.3%, 5–500 Hz) at 410 kV cm−1. The present work provides a new perspective for designing lead-free RFE ceramics with ultra-high ESPs that benefit from hierarchically polar structures.