Optimizing the grain size and grain boundary morphology of (K,Na)NbO3-based ceramics: Paving the way for ultrahigh energy storage capacitors

Relaxor dielectric ceramic capacitors are very attractive for high-power energy storage. However, the low breakdown strength severely restricts improvements to the energy storage density and practical application. Here, a strategy of designing small grain sizes and abundant amorphous grain boundaries is proposed to improve the energy storage properties under the guidance of phase field theory. 0.925(K0.5Na0.5)NbO3–0.075Bi(Zn2/3(Ta0.5Nb0.5)1/3)O3 (KNN–BZTN) relaxor ferroelectric ceramic is taken as an example to verify our strategy. The grain sizes and grain boundaries of the KNN–BZTN ceramics are carefully controlled by the high-energy ball milling method and two–step sintering strategy. Impedance analysis and diffusion reflectance spectra demonstrate that KNN–BZTN ceramics with a small grain size and abundant amorphous grain boundary exhibit a lower charge carrier concentration and higher band gap. As a consequence, the breakdown electric field of KNN–BZTN ceramics increases from 222 kV/cm to 317 kV/cm when the grain size is decreased from 410 nm to 200 nm, accompanied by a slightly degraded maximum polarization. KNN–BZTN ceramics with an average grain size of ∼250 nm and abundant amorphous grain boundaries exhibit optimum energy storage properties with a high recoverable energy density of 4.02 J/cm3 and a high energy efficiency of 87.4%. This successful local structural design opens up a new paradigm to improve the energy storage performance of other dielectric ceramic capacitors for electrical energy storage.