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Relaxor ferroelectrics usually possess low remnant polarizations and slim hystereses, which can provide high saturated polarizations and superior energy conversion efficiencies, thus receiving increasing interest as energy storage materials with high discharge energy densities and fast discharge ability. In this study, a relaxor ferroelectric multilayer energy storage ceramic capacitor (MLESCC) based on 0.87BaTiO3‐0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 (BT‐BZNT) with inexpensive Ag/Pd inner electrodes is prepared by the tape casting method. The MLESCC with two dielectric layers (layer thicknesses of 5 µm) sintered by a two‐step sintering method exhibits excellent energy storage properties with a record‐high discharge energy density of 10.12 J cm−3, a high energy efficiency of 89.4% achieved at an electric field of 104.7 MV m−1, a high temperature stability of the energy storage density (with minimal variation of <±5%), and energy efficiency (>90%) over a range of −75 to 150 °C at 40 MV m−1. These results suggest that the BT‐BZNT relaxor ferroelectric ceramic material can provide realistic solutions for high‐power energy storage capacitors.

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A series of ferroelectric ceramic models with grain and grain‐boundary structures of different sizes are established via Voronoi tessellations. A phase‐field model is introduced to study the dielectric breakdown strength of these ferroelectric ceramics. Afterward, the relation between the electric displacement and electric field and the hysteresis loop are calculated using a finite element method based on a classical and modified hyperbolic tangent model. The results indicate that as the grain size decreases, the dielectric strength is enhanced, but the dielectric permittivity is reduced. The discharge energy density and energy storage efficiency of these ferroelectric ceramics extracted from the as‐calculated hysteresis both increase along with a decrease in their grain size at their breakdown points. However, under the same applied electric field, the ferroelectric ceramic with a smaller grain size possesses a lower discharge energy density but a higher energy storage efficiency. The results suggest that ferroelectric ceramics with smaller grain sizes possess advantages for applications in energy storage devices.

Inorganic/organic dielectric nanocomposites have been extensively explored for energy storage applications for their ease of processing, flexibility, and low cost. However, achieving simultaneous high energy density and high efficiency under practically workable electric fields has been a long‐standing challenge. Guided by first‐principles calculations of interface properties and phase‐field simulations of the dynamic dielectric breakdown process, superhierarchical nanocomposites of ferroelectric perovskites, layered aluminosilicate nanosheets, and an organic polymer matrix are designed and simultaneous high energy density of 20 J cm−3 and high efficiency of 84% at a low electric field of 510 MV m−1 are achieved. This is the highest energy density of all the state‐of‐the‐art dielectric polymer nanocomposites with energy efficiency > 80% at a low electric field of <600 MV m−1. Strong atomic hybridization, large ionic displacement, the enhanced breakdown strength through forming charge‐blocking layers, and the superhierarchical microstructure with gradient interfaces are responsible for the high performances. This superhierarchical structuring modulation strategy is generally applicable to composites for different functionalities and applications.