Nonlinear Polarization And Energy Storage Characteristics Of SrTiO₃ Based Lead-free Thin Films

With the widespread use and rapid development of pulse power technology,people's need for high energy storage density materials has been stimulated.SrTiO₃is classic incipient ferroelectrics and has non-ferroelectric properties at room temperature;therefore,SrTiO₃ has larger dielectric breakdown strength and efficiencity than other perovskite ferroelectrics.However,its relatively low polarization limits the development and application of SrTiO₃ as an energy storage material.In order to solve this problem,this paper regulates the polarization behavior of SrTiO₃ thin films and increases the polarization by constructing defect complexes and“ionic pairs”.Based on the first-principles calculation,the effects of Bi³⁺-V_Sr^??-Bi³⁺defect complex and Bi³⁺-Li⁺ionic pairs on the crystal structure were discussed and analysis the source of ferroelectricity of SrTiO₃-based films.The purpose of optimizing the energy storage performance of SrTiO₃-based films is achieved by regulating the polarization behavior.Bi³⁺doped SrTiO₃ thin films with pre-designed Sr vacancies form the Bi³⁺-V_Sr^??-Bi³⁺defect complex and optimize the polarization behavior of SrTiO₃-based films.It was found that 5 mol.%and 10 mol.%Bi-doped thin films exhibited ferroelectricity at room temperature.10 mol.%Bi-doped thin film obtained a maximum storage energy density of 24.4 J/cm³ at 1982 kV/cm and good temperature stability was also achieved from 30 to 110 ^oC.First-principles calculations illustrate that Bi³⁺and V^??_Srr can form Bi³⁺-V_Sr^??-Bi³⁺defect complex and induce the rotation of TiO₆-octahedron,giving rise to the appearance of ferroelectricity.By donor-acceptor co-doping,Bi³⁺and Li⁺are introduced at the A position of SrTiO₃ to form Bi³⁺-Li⁺ionic pairs in the crystal lattice,which increases the polarization of the film.The ferroelectric test results show that the Bi³⁺-Li⁺co-doped SrTiO₃ thin film has room temperature ferroelectricity.Moreover,the thin film with 2.5 mol.%Bi³⁺-Li⁺content obtained maximum energy storage density of47.7 J/cm³ at 3307 kV/cm and maximum energy storage efficiency of 67.0%.This is mainly because the formation of ionic pairs breaks the symmetry of the crystal,causing local distortion of the crystal lattice.In addition,the displacements of cations and anions surrounding the ionic pairs,resulting in the local polarization in co-doped SrTiO₃ thin films.Based on the different type of defect,Bi³⁺-Li⁺-X(Na⁺/K⁺/Y³⁺)doped SrTiO₃thin films were prepared and the effects of different kinds of ternary composite doping on the energy storage performance of the films were investigated.The results show that Bi³⁺-Li⁺-K⁺doped SrTiO₃ film and Bi³⁺-Li⁺-Na⁺doped SrTiO₃ film have good ferroelectricity at room temperature.The Bi³⁺-Li⁺-Y³⁺doped SrTiO₃ thin film has the lowest P_r value with good pressure resistance.As a result,the Bi³⁺-Li⁺-Y³⁺doped SrTiO₃ thin film reached a maximum of 21.1 J/cm³ at 3600 kV/cm with the maximum energy storage efficiency of 71%.