Aliovalent Doping Engineering for A- and B-Sites with Multiple Regulatory Mechanisms: A Strategy to Improve Energy Storage Properties of Sr_0.7Bi_0.2TiO₃-Based Lead-Free Relaxor Ferroelectric Ceramics

Sr_0.7Bi_0.2TiO₃ (SBT) is a promising pulse energy storage material due to minor hysteresis, but its low maximum polarization (P_max) is bad for energy storage. K⁺-Bi³⁺ defect pairs were introduced into the A-site of SBT to obtain Sr_0.35Bi_0.35K_0.25TiO₃ (SBKT) with larger P_max. Through first-principles calculations, we determined that the introduction of defect pairs destroys the paraelectric order phase and increases local polarization, resulting in more and larger polar nanoregion (PNR) formation. On this basis, doping NaNbO₃ (NN) in A- and B-sites of SBKT increases the cationic disorder and ferroelectric destabilization, further destroying the long-range order structure and forming more PNRs with smaller sizes. This enhances relaxation and decreases remnant polarization, and the broadened dielectric peak enables 0.85SBKT-0.15NN to meet the X7R specification. Furthermore, the decreased grain size and oxygen vacancy, increased thermal conductivity, and weakened local electric field (simulated by COMSOL) increase the dielectric breakdown strength (BDS). As a result, 0.95SBKT-0.05NN exhibits a high energy storage density (W) of 2.45 J/cm³ with a high efficiency of 93.1%, a high pulsed discharge energy density of 2.1 J/cm³, and a high power density of 54.1 MW/cm³ at 220 kV/cm. The energy storage properties show excellent stability of temperature (-55 to 150 °C), frequency (10-500 Hz), and cycling (10⁵ cycles). Notably, for the pulse charge-discharge properties, 0.95SBKT-0.05NN shows great fatigue resistance during 10⁵ cycles under 25 and 150 °C, accompanied by excellent thermal stability. Moreover, the BDS and P_max of 0.95SBKT-0.05NN sintered in O₂ further enhance. A higher W of 2.92 J/cm³ with a high efficiency of 89% at 250 kV/cm is achieved. Therefore, 0.95SBKT-0.05NN shows great application potential for pulse energy storage. In this work, we provide a novel strategy and systematic in-depth study for improving the energy storage properties of SBT.