Dielectric materials have critical applications in many fields. Compared with electrochemical energy storage batteries, ceramic capacitors show outstanding competitive power performance in actual energy storage applications, such as diesel engine starters, camera flashlights, spacecraft, pulsed power weapons, and medical devices due to their ultra-fast charge/discharge capability and high-power density. However, with the development of electronic device integration and miniaturization, capacitors are required to have a high effective energy storage density (W_rec) under low electric field. The existing dielectric ceramic capacitors are difficult to meet the corresponding requirements, so it is urgent and significant to develop dielectric ceramic capacitors that can obtain high energy storage density under low voltage. In recent years, the co-design strategy has been used as a typical and effective method to enhance the energy storage performance of BNT, which is achieved by adding different kinds of ABO₃ perovskites and various doping ions. Co-design strategy improves energy storage properties by compositional design to induce the formation of polar nanoregions (PNRs) as a result of A/B site ion disordering or structural phase transitions. In this paper, KNN was added to BNBST ceramics to modify the internal crystal structure, and the effect of KNN on the phase structure, microstructure, dielectric properties, energy storage properties, ferroelectric stability and fast charge-discharge characteristics of BNBST ceramics were investigated.

As we know, high P_max, low P_r and high breakdown strengthen E_b are required for dielectric ceramics to achieve high W_rec. To obtain high energy storage density under a low electric field, we chose Ba_0.105Na_0.325Bi_0.325Sr_0.245TiO₃ (BNBST) with large P_max and high Curie temperature as the research object, the co-design strategy with the introduction of K_0.5Na_0.5NbO₃ was adopted to further destroy the ordered arrangement of A/B position ions, reduce their residual polarization intensity P_r, and optimize their electric polarization behavior.

Using solid-state method prepared Ba_0.105Na_0.325Bi_0.325Sr_0.245TiO₃+x%K_0.5Na_0.5NbO₃ (BNBST-x% KNN, mole fraction, x=0, 2, 4, 6, 8, 10, 12) ceramic samples. Bi₂O₃ (99.999%), K₂CO₃ (99%), BaCO₃ (99%), SrCO₃ (99%), Na₂CO₃ (99.8%), TiO₂ (99%), Nb₂O₅ (99.95%) were used as synthetic raw materials to prepare samples according to the following process: 1) Accurately weigh the ingredients according to the corresponding stoichiometric ratio, put them in the nylon ball mill tank for 24 h, dry, grind, and pass 80 mesh sieve; 2) Pre-burning after pressing into large sections, heating up to 850 ℃ with a heating rate of 5 ℃/min and holding for 3 h to exclude CO₂ and pre-synthesize the powder; 3) The large pieces are ground into powder, passed through an 80-mesh sieve, ball milling again for 24 h, drying, adding 5% concentration of PVA adhesive for granulation, pressing into a small disc with thickness of about 1 mm, diameter of about 13 mm (pressure (120±10) MPa); 4) The small disc was heated to 650 ℃ for 3 h to remove organic matter, and then heated to 1 240 ℃ for 2 h to sintering; 5) After polished and silvered, heating up 30 min at 650 ℃, and then various electrical properties are tested.

All BNBST-x%KNN ceramics have a single perovskite pseudo-cubic phase structure, and no second phase is generated. The grain distribution of all ceramics is uniform and the density is good. When x=6, the optimized energy storage characteristics are obtained only at the low electric field of 140 kV/cm with W_rec=1.8 J/cm³ and η=86%. Whatmore, BNBST-6% KNN ceramics have TCC=±15% high dielectric constant (ε_r=3 128@125 ℃) and excellent temperature, frequency, cycle stability and fast charge-discharge characteristics in the temperature range of -8-215 ℃. The results show that BNBST-6%KNN ceramics can be used as a candidate material for pulsed power capacitors, and has obvious advantages in low electric field application.

The main innovation points of this paper are summarized as following. A collaborative design strategy with the introduction of K_0.5Na_0.5NbO₃ was used to further destroy the ordered arrangement of A/B ions and promote the formation of polar nanodomains (PNRs), under a certain electric field, PNRs can be transformed into long-range ordered ferroelectric domains, resulting in larger P_max. When the electric field is removed, the ferroelectric domains formed by PNRs will quickly break back to the initial state, resulting in smaller P_r. More importantly, "premature saturation" is delayed due to the appearance of PNRs, which can obtain hight larger P_max at low electric field, resulting in a high W_rec=1.8 J/cm³ at the low electric field of 140 kV/cm.