Design And Mechanism Of High-Performance Lead-Free Relaxor Ferroelectric Energy-Storage Ceramics

Dielectric energy-storage ceramics have become the research frontier in the field of materials and chemistry in recent years due to their ultrahigh power density,ultrafast charge and discharge rates,and excellent energy-storage stability.However,as the core component of pulsed power capacitors,their low energy-storage density and efficiency greatly limit the development of devices towards miniaturization,intelligence,and integration.In this paper,the lead-free perovskite type relaxor ferroelectric ceramics are taken as the research object.Combining composition and structural design,various new design strategies have been proposed and a series of excellent new lead-free energy-storage materials have been developed.Advanced characterization and theoretical calculation methods are used to systematically explore the composition-structure-performance correlation,revealing the origins of excellent energy-storage performance.Through a multi-scale synergistic design strategy,a high energy-storage density of about 7.57 J/cm3 is achieved in Bi0.5K0.5TiO3-based ceramics,which is 2.4 times the highest value previously reported.The main sources of the excellent energy-storage performance have been revealed through first principles calculations,piezoelectric force microscopy,and transmission electron microscopy,which are:(1)the introduction of ions with different valence states at the atomic scale breaks the long-range ferroelectric order,promoting the transformation of large-size domains at the mesoscopic scale into polar nanoregions of about 2-4 nm;(2)The introduction of high-bandgap NaNbO3 at the atomic scale significantly increases the bandgap of the sample,preventing the transition of carriers from the top of the valence band to the bottom of the conduction band;(3)At the grain microstructure scale,combined with the introduction of sintering aid,high-energy ball milling is applied to significantly reduce the grain size to 180 nm and increase the density of high-resistance grain boundaries,significantly improving the sample quality and achieving an ultrahigh Vickers hardness of about 8.63 GPa.Excellent energy-storage and mechanical properties have been achieved in BaTiO3-based ceramics via hetero structure design,and the synergistic strengthening mechanisms including solid solution strengthening,denseness strengthening,grain boundary and twin boundary strengthening have been preliminarily established.Introducing Bi0.5N0.5TiO3 and CaZrO3 into BaTiO3 matrix to control the phase transition temperature for stabilizing the tetragonal phase and the rhombohedral phase,respectively.A heterostructure of the coexisting rhombohedral-tetragonal ergodic polar nanoregions with the size of about 2-4 nm is successfully established at room temperature,which can be found by spherical aberration transmission electron microscopy.The heterostructure can enhance the random field,break the long-range ordered ferroelectric state,delay polarization saturation,achieving a high energy-storage density of about 9.04 J/cm3 and a high efficiency of about 87.2%in the sample.The enhancement of energy-storage properties by the presence of twin boundaries is reported for the first time,and the synergistic strengthening mechanisms between energy-storage and mechanical properties have been revealed in combination with excellent mechanical properties.The correlation between configuration entropy and energy-storage characteristics is studied,and a new avenue for local diverse configuration to regulate energy-storage characteristics is developed.Taking Bi0.5Na0.5TiO3 as a matrix,the gradual evolution of materials from low entropy,medium entropy,high entropy to maximum entropy is constructed by gradually introducing Fe3+ and Nb5+without changing the configuration of perovskite A-site ions.The(Bi0.5Na0.5)(Ti1/3Fe1/3Nb1/3)O3 ceramic with the highest entropy value exhibits an ultrahigh energy-storage density of about 13.8 J/cm3 and a high efficiency of about 82.4%.Its energy-storage density has achieved a nearly tenfold increase compared to the low-entropy sample.Through piezoelectric force microscopy,transmission electron microscopy,and finite element simulation,the intrinsic mechanism of the enhanced energy-storage performance with the increasing entropy is deeply revealed.Subsequently,under the guidance of two-dimensional Gibbs free energy calculations,a high-entropy superparaelectric material with the coexisting rhombohedral-orthogonal-tetragonal multiphase polar nanoregions in a cubic matrix is constructed in the classic BaTiO3-Bi0.5Na0.5TiO3-NaNbO3 ternary system.By analyzing the spherical aberration electron microscopy data,it is found that their sizes are about 1-3 nm and their average polarization displacements are only 6-8 pm,forming a local diverse polarization configuration,which can reduce the potential barrier of polarization rotation.Compared to the binary systems with single tetragonal-phase ferroelectric,relaxor ferroelectric,and relaxor antiferroelectric polarization configurations,the polarization and energy-storage properties of the sample with a local diverse polarization configuration are significantly improved,achieving an ultrahigh energy-storage density of about 10.59 J/cm3 and a high efficiency of about 87.6%.A new design concept of "high-entropy ferroelectric energy-storage" is proposed.Based on the correlation between energy-storage performance and entropy,a high-entropy strategy is designed to introduce a variety of ions with different valence states and radii in K0.2Na0.8NbO3 to enhance the local random electric field and stress field,control the polycrystalline phase transition temperature,realizing the coexistence of rhombohedral-orthogonal-tetragonalcubic multiphase nanoclusters and multiple oxygen octahedral distortion types at room temperature,namely,local polymorphic distortion.This structure effectively regulates polarization fluctuation and saturation behaviors under electric field,reduces loss and grain size,optimizes sintering,and significantly improves breakdown strength.The mechanism of multi-directional regulation of highentropy strategy to improve energy-storage performance has been systematically revealed,and it is the first time to achieve a performance breakthrough in lead-free ceramics with energy-storage density higher than 10 J/cm3 and efficiency higher than 90%.From the perspective of composition and structural design,this paper focuses on the study of the relationship between composition,structure,and energy-storage performance of new lead-free ferroelectrics,which will provide a theoretical basis for the development of excellent dielectric energy-storage materials in terms of composition development,structural design,and precise performance control,promoting their development towards practical applications.