The role of PN-like junction effects in energy storage performances for Ag2O nanoparticle dispersed lead-free K0.5Na0.5NbO3-BiMnO3 films

Capturing Carriers and Driving Depolarization by Defect Engineering for Dielectric Energy Storage

The inevitable defect carriers in dielectric capacitors are generally considered to depress the polarization and breakdown strength, which decreases energy storage performances. Distinctive from the traditional aims of reducing defects as much as possible, this work designs (Fe_Ti^' - V_o^••)^• and (Fe_Ti^″ - V_o^••) defect dipoles by oxygen vacancy defect engineering in acceptor doped Sr₂Bi₄Ti_(5-x)Fe_xO₁₈ layered perovskite films with n-type leakage conductance. It is shown that oxygen vacancies effectively capture electrons (carriers) in n-type dielectrics to enhance the breakdown strength. Meanwhile, defect dipoles provide a driving field for depolarization to engineer the generation energy of domains and the domain wall energy, which effectively lowers the residual polarization P_r but not at the expense of the maximum polarization P_max as relaxor ferroelectric regulations. Such defect engineering effectively breaks through the limitation, in which the energy storage density suffers from the trade-off relationship between polarization and breakdown strength. The Sr₂Bi₄Ti_4.92Fe_0.08O₁₈ film with the proper oxygen vacancy content achieves a high energy density of 110.5 J/cm³ and efficiency of 70.0% at a high breakdown strength of 3915 kV/cm. This work explores an alternative way for breakthroughs possible in the intrinsic trade-off relationship to regulate dielectric energy storage by defect engineering.

Oxygen polyhedral dipole-dipole interaction induced domain reconstruction and relaxor behaviors in layered perovskite films for dielectric energy storage

We define AO₁₂ and BO₆ oxygen polyhedra in layered perovskite films as A-O* and B-O* polyhedral dipoles, respectively, which are responsible for the spontaneous polarization and the construction of domains. Based on the dipole-dipole interaction among such polyhedral dipoles, we propose and deduce the forming domain energy G_k, which decides the evolution of domains. Furthermore, A-O* polyhedral dipoles of three-layered Bi₄Ti₃O₁₂ are regulated by respectively inserting two layers of SrTiO₃ and Sr_0.92Ba_0.08TiO₃ into it to form Sr₂Bi₄Ti₅O₁₈ (SBT) and Ba_0.16Sr_1.84Bi₄Ti₅O₁₈ (BSBT) films. It is shown that the domains are reconstructed to form smaller domains due to the decrease of the forming domain energy, which is also embodied in the relaxor characteristics. Likewise, interestingly, the maximum polarization of a BSBT film is enhanced due to the lattice extension derived from A-O* dipole regulation, which brings about improved energy storage performances with the density reaching as high as 102.9 J cm^-3 and the efficiency increasing to 64.8%. This work provides a preferred alternative strategy to regulate the relaxor behavior by domain reconstruction for dielectric energy storage.

Superior Electrostrictive Effect in Relaxor Potassium Sodium Niobate Based Ferroelectrics

Electromechanical actuators with toxic lead-based electrostrictive materials dominate the market of high-precision electronics devices, so one of urgent aims becomes how to explore the new generation of ecofriendly electrostrictive materials with superior electrostriction behaviors. Herein, a strategy of modifying the active space for the B-site in lead-free relaxor ferroelectrics arouses the potential capacity of electrostriction, including K_0.5Na_0.5NbO₃ (KNN). Through co-doping of Bi³⁺ and Ni²⁺, typical relaxor ferroelectrics (1 - x)K_0.5Na_0.5NbO₃-xBiNi_2/3Nb_1/3O₃ (KNN-xBNN, 0.03 ≤ x ≤ 0.07) are constructed because of the incorporation of dopants ions on the A- or B-site promotes structural disorder. Importantly, a limited active space for the B-site is obtained owing to the contracted oxygen-octahedron with smaller A-site dopants induced by lattice distortion. Benefiting from the design strategy, a remarkable enhancement of electrostrictive coefficient Q₃₃ of ∼0.0456 m⁴/C² is achieved, which is higher than those of KNN-based materials and also twice as large as those of lead based materials. The Q₃₃ also exhibits a relatively good electric field and is temperature-independent. A giant Q₃₃ of 0.0512 m⁴/C² is gained at a high frequency of 100 Hz, meeting the requirement of some commercial actuators for fuel injectors. Such a strategy may help us design high-performance electrostrictive materials.