Designing Antiferroelectrics Based On Rare-earth-substituted Bismuth Ferrite For Energy Storage

Nowadays energy storage technology has a big impact on industry and daily life.Dielectric capacitors,as one type of energy storage technology,are widely used in circumstances that require a fast charging/discharging rate and a high power density.The currently commercialized dielectric capacitors often come with a low energy density,caused by a small dielectric constant or a low intrinsic breakdown field,limiting its further application.However,antiferroelectrics,which have a zero spontaneous polarization and can shift into a ferroelectric phase under an externally applied electric field,may possess a large polarization.Furthermore,antiferroelectrics can undergo a phase transition back to the original antiferroelectric phase when the applied electric field reduces to zero.For antiferroelectrics,this behavior endowed them with a distinctive double hysteresis,which may yield a high energy density.Thus,developing antiferroelectric materials suitable for energy storage has now drawn much attention.As an excellent example,multiferroic bismuth ferrite BiFeO3 can be tuned via doping with rare-earth elements to favor an antiferroelectric phase as its ground state,and shift into ferroelectric phase under an applied electric field,implying an enormous potential as a suitable material for energy storage.In this thesis,we focus on the energy storage performance of rare-earth-substituted bismuth ferrite.Using an effective Hamiltonian method based on first principles,we simulate the effects of introducing different dopants,doping composition and orientations of applied electric fields,to find a feasible way of fulfilling an excellent performance of rare-earth-substituted bismuth ferrite.In addition,we analyzed how these factors affect the shape of P-E loops,and consequently the energy storage performance,based on a simple model.The rare-earth dopants that have been taken into consideration are La,Nd,Sm,Gd,Dy and Tm.Doping composition varies from 0.1 to 1.0 with an interval of 0.1.Four directions of externally applied electric fields are the pseudocubic of BiFeO3[001],[100],[110]and[111].Different dopants come with different radii,and a greater atomic number implies a smaller ionic radius.This radius is the direct factor that has impact on the behavior of Bi1-xRxFeO3(BRFO).For pure BFO,the ground state is the ferroelectric R3c.According to the phase diagram of BRFO,we know that the composition has to reach a certain extent for the ground state to be antiferroelectric Pnma,and the extent varies from a rare-earth element to another rare-earth element.It’s only when the ground state is antiferroelectric that the polarization can have a leap at the critical electric field,and thus higher energy density.Under electric fields with different directions,the ferroelectric phase at high fields varies.For[001]and[100]fields,the ferroelectric phase is P4mm;For[110]fields,the ferroelectric phase is Amm2;For[111]fields,the preferred phase after transition is R3c.The two ferroelectric phases under[110]and[111]fields are more stable than those under[001]and[100]fields.There are two key quantities that quantify the energy storage performance:energy density and efficiency.As for energy density,it generally increases as the doping composition increases.But as the dopant changes from La to Tm(the ionic radius decreases),the energy density doesn’t display an obvious trend.For La substitution,the system tends to stay in the ferroelectric phase for low composition;For Tm substitution,a phase transition to the ferroelectric phase can not happen even under the intrinsic breakdown field for high composition.From Nd to Dy substitution,the energy density doesn’t change very much.As for the efficiency,its reaction to doping composition depends on the direction of the applied electric field.For[001]and[100]fields,increasing doping composition will improve the energy efficiency significantly.However,for[110]and[111]fields,the efficiency is not sensitive to composition.Considering the effects of dopant types,when it changes from La to Tm,the energy efficiency will continually decrease.As for the orientations of electric field,it determines the ferroelectric phase at high field and hence the relative energy between the antiferroelectric phase and the ferroeletcric phase.Overall,energy storage performance can not be improved by simply adjusting one single variable;instead,all mentioned factors shall be well balanced.Nevertheless,exceptions might happen.For example,the critical field for antiferroelectric-ferroelectric phase transition is greater than the intrinsic breakdown field,or the system remains ferroelectric even at zero field after discharging.Remarkably,we predict very promising storage performance in such system;good energy density and efficiency can be achieved at the same time.In particular,we obtain a maximum density of 239 J cm-3 when the dopant is Tm,with the composition of 0.6,and the electric field is applied along[110].The corresponding efficiency is generally above 80%.We show that through rare-earth doping,the performance of bismuth ferrite for energy storage can be improved significantly.This approach has a great potential in developing novel materials for energy storage.