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The boom in high-power-density electronics and advanced pulsed power systems has led to a requirement for high-energy-density dielectric capacitors, for which the key enabler is the availability of dielectric materials with high energy densities and high efficiencies. Although antiferroelectric ceramics are promising dielectric materials with high energy densities, they have low efficiencies. In this study, we address this problem through the core–shell structure design. A phase-field model is developed by considering the core as antiferroelectric and the shell as linear dielectric, and the polarization hysteresis loops are determined. The results show that the polarization–electric field loop of the core–shell sample is slanted, with a delayed saturation polarization, decreased maximum polarization, and declined hysteresis loss compared with the pure sample. This phenomenon becomes more distinct with increasing shell fraction and decreasing shell permittivity, and vanished hysteresis is achieved in samples with a high shell fraction and a low shell permittivity. Through deconvolution, it is determined that the underlying mechanism of energy storage is the difference in the antiferroelectric polarization contribution of various shell parameters. It is found that a giant energy density of 15.5 J/cm3 and an ultrahigh efficiency of 99.7% at the saturation polarization can be achieved concurrently for a certain core–shell sample; these values considerably exceed the corresponding values (5.0 J/cm3 and 52.8%) for the pure sample. The findings of this study can serve as guidance for engineering core–shell structures, thus paving the way for enhancing the energy-storage performance of antiferroelectric ceramics. [ABSTRACT FROM AUTHOR]