Abstract: Ceramic capacitors designed for energy storage demand both high energy density and efficiency. Achieving a high breakdown strength based on linear dielectrics is of utmost importance. In this study, we present the remarkable performance of densely sintered (1-x)(Ca_0.5Sr_0.5TiO₃)-xBa₄Sm_28/3Ti₁₈O₅₄ ceramics as energy storage materials, with a measured energy density (W_rec) of 4.9 J/cm³ and an ultra-high efficiency (η) of 95% which is almost optimal in linear dielectric that has been reported. To unravel the underlying mechanisms, we conducted a systematic investigation on the influence of adding paraelectric Ba₄Sm_28/3Ti₁₈O₅₄ (BST) on both microstructure and macroscopic electrical properties of Ca_0.5Sr_0.5TiO₃ (CST). Notably, the addition of BST effectively reduces the grain size of CST. The conduction mechanism is primarily governed by grain boundaries, where high-density grain boundaries act as barriers to charge carrier transport due to their elevated resistivity. Moreover, the activation energy associated with grain boundaries increases with rising resistivity, implying a lower concentration of free vacancies within these regions. The increased barrier height for oxygen vacancy migration at grain boundaries compensates for the grain boundary defects, thereby resulting in enhanced breakdown strength. This characteristic offers a substantial advantage in terms of thermal and frequency stability (25-175 °C, 1-100 Hz). This work introduces a candidate material with outstanding comprehensive energy storage properties.

Key words: Linear dielectric ceramic, Ca_0.5Sr_0.5TiO₃, Energy storage, Grain boundary