Dielectric capacitors are widely used in pulsed power and electronic systems due to their high power density, long lifespan, and low cost. However, their energy density is considerably lower than that of batteries. Dielectric materials are the core components for enhancing capacitor performance. Although conventional lead-based ceramics (e.g., PZT) exhibit the superior properties, their toxicity drives the development of environmentally friendly lead-free alternatives as a major research focus. Bismuth-based perovskite materials, due to the similar electronic configurations of Bi³⁺ and Pb²⁺, demonstrate ferroelectric properties comparable to those of lead-based materials. Among them, bismuth potassium titanate (i.e., K_0.5Bi_0.5TiO₃, KBT) stands out as a highly promising lead-free energy storage candidate due to its combination of high theoretical polarization and a wide bandgap. However, a pure KBT is a typical ferroelectric with a high remnant polarization, which limits its energy storage performance. In general, introducing a paraelectric material into a ferroelectric to form a relaxor ferroelectric can disrupt a long-range order and refine ferroelectric domains, thereby improving energy storage characteristics. This study selected quantum paraelectric SrTiO₃ (ST) to form a solid solution with KBT. The effect of ST content on the crystal structure, microstructure, dielectric, and ferroelectric properties was investigated. The ceramics maintained a pure perovskite phase with increasing ST content, showing a shift of the dielectric peak to lower temperatures with an enhanced frequency dispersion and gradually slimmer polarization-electric field hysteresis loops. The optimal composition (i.e., 0.6KBT-0.4ST) achieved a recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70% in an electric field of 210 kV/cm, along with good temperature and frequency stability. The results indicated that KBT-ST is a promising class of high-performance lead-free dielectric materials for energy storage applications.

In this study, (1-x)KBT-xST ceramics (where x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a conventional solid-state sintering method. The starting raw materials were Bi₂O₃ (≥99%), K₂CO₃ (≥99%), TiO₂ (≥99%), and SrCO₃ (≥99%), from Aladdin Co., China. Prior to weighing, K₂CO₃ powder was dried in a muffle furnace at 200 ℃ for 2 h to remove absorbed moisture. The raw materials were then weighed according to the stoichiometric ratio and placed in a nylon milling jar with zirconia grinding balls. Isopropanol was added as a medium in grinding, and the raw materials were wet ground for 12 h. After grinding, the slurry was poured out, dried, and calcined at 850 ℃ for 2 h. The calcined powder with isopropanol was further ground for 22 h. Afterwards, the slurry with 0.2% of binder was ground for additional 2 h. The slurry obtained after further grinding was dried, sieved, and pressed into green ceramic pellets with a diameter of 10 mm and a thickness of 1-2 mm. The green pellets were placed in a muffle furnace, heated at a rate of 2 ℃/min to 600 ℃, and held for 6 h to allow binder burnout. After debinding, the pellets were embedded in an alumina crucible with calcined powder of the same composition (without binder) and sintered at 1050-1400 ℃ (with higher sintering temperatures corresponding to higher ST content) for 2 h. The sintered ceramics were then ground and polished. Silver paste was applied on the both sides of the ceramics and heat-treated at 850 ℃ for 20 min to form silver electrodes. Alternatively, gold electrodes were deposited on the both sides by an ion sputtering coater. The phase composition of the ceramic powders was analyzed by a model Smartlab X-ray diffractometer (XRD) (Rigaku Co., Japan). The Raman spectra were determined on well-polished ceramic samples by a model Alpha 300R Raman imaging microscope (WITec Co., Germany). The microstructure of the materials was characterized by a model Regulus 8230 high-resolution cold-field emission scanning electron microscope (Hitachi Co., Japan). The temperature-dependent dielectric properties were measured by a model IM3536 LCR meter (HIOKI Co., Japan) equipped with a model HCP421V-MP heating stage (INSTEC Co., USA). The polarization-electric field (P-E) hysteresis loops of the ceramics were tested by a model Multiferroic II ferroelectric analyzer (Radiant Co., USA).

The results show that the (1-x)K_0.5Bi_0.5TiO₃-xSrTiO₃(KBT-ST) lead-free dielectric ceramic system is synthesized by a conventional solid-state reaction method. The effect of SrTiO₃ incorporation on the microstructure, phase evolution, and electrical properties of the materials is systematically investigated.

The XRD patterns and SEM images confirm the formation of a single perovskite phase and a dense microstructure for all the compositions. The temperature stability of the dielectric constant is significantly enhanced, accompanied by a shift of the dielectric peak towards lower temperatures, as SrTiO₃ content increases. The gradual slimming of polarization-electric field hysteresis loops indicates a transformation from a typical ferroelectric to a relaxor ferroelectric state. The optimal composition (i.e., 0.6KBT-0.4ST) exhibits a high recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70 % in an electric field of 210 kV/cm. This work demonstrates that KBT-ST ceramics constitute a highly promising lead-free dielectric system for high-performance energy storage applications.

In this work, KBT-ST lead-free ceramics were prepared by a solid-state sintering method. Their structural evolution, properties, and potential for dielectric energy storage applications were systematically investigated. Single-phase

perovskite-structured KBT-ST solid solutions were successfully synthesized. The lattice contraction occurred, and the incorporation of Sr²⁺ effectively introduced A-site compositional disorder as the ST content increased. The addition of ST effectively suppressed a long-range ferroelectric order, thus inducing a transition from a ferroelectric to a relaxor ferroelectric state. This transition was evidenced by enhanced frequency dispersion of the dielectric peak, slimmer polarization-electric field hysteresis loops, and improved temperature stability of the dielectric constant. The optimal energy storage performance was achieved in the composition with x of 0.4, which exhibited the optimal overall energy storage properties. The introduction of Sr²⁺ produced a synergistic effect. It enhanced a local polarization, and suppressed a long-range ferroelectric order, promoting the formation of an ergodic relaxor state. The effects both contributed to a reduction in (P_max-P_r). The high breakdown strength of 0.6KBT-0.4ST, combined with the high polarization contributed by the hybridization between Bi 6s and O 2p orbitals, collectively yielding a high recoverable energy density of 3.64 J/cm³ and a high energy efficiency of 70 %. Furthermore, this composition demonstrated superior cycling stability and temperature stability in its energy storage performance. In summary, A-site modification of KBT ceramics through SrTiO₃ solid solution was an effective strategy for optimizing their energy storage properties. This study could present a promising lead-free energy storage ceramic system and provide a valuable reference for designing high-performance relaxor ferroelectrics via tuning local structural disorder.