An Sr doping 0.65(Bi0.5Na0.5) TiO3-0.35 (Sr0.7+x+ Bi0.2) TiO3 Ceramic with Tunable Crystal Structures and Energy Storge Performances

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Introduction
Supercapacitors are the crucial units in the advanced pulsed power systems such as the pulsed power weapons, hybrid electric vehicles, and et al., where the key dielectrics of the supercapacitors should exhibit the unique characters including a large recoverable energy density, a high releasing efficiency, a giant power density in highfrequency pulses, and especially a good charging/discharging cycling stability [1][2][3][4][5] .
According to the dielectric and polarization features of stability, dielectrics can be divided into the linear and nonlinear materials.Different from linear dielectrics BOPP that exhibits a remarkable stable dielectrics permittivity under gradually evaluate electric field, most of dielectric polymers, ceramics, and single crystals et al are nonlinear dielectrics, which reveals an unstable electrical polarization at evaluated electric field [6][7][8][9] .And even the nonlinear dielectric exhibits the poor temperature stability and cycling characteristics.
Besides, these nonlinear dielectrics are often composed of randomly large crystal ferroelectric domains which is believed to cause a large remnant polarization (Pr).Therefore, although these nonlinear dielectric polymers or ceramics possess a relatively high dielectric properties, their nonlinear features caused by large ferroelectric domain will lead to a part of undischarged energy.For these reasons, the effective energy storage density of nonlinear dielectrics is suitable to calculate by the polarizationelectric field (P-E) hysteresis loops, and the evaluation parameters are shown in the following equations ( 1)-(3) [10] .(2)

Edp
where W, Wrec and η are the energy storage density, recoverable energy storage density and energy storage efficiency [11][12][13] , while the E and Pmax represent the applied electric field and maximum polarization, respectively.Thus, to improve Wrec as well reduce Wloss under the high electric breakdown strength (Eb), a modified crystal grain size should be consideration.Simultaneously, through the reduction the crystal grain size the ferroelectrics will convert into relaxor ferroelectric which favors of a high discharged energy storage density.
Recently, in the perovskite-structured ceramics having high electric polarization value, PZT, PLZT, and PLZST et al. are commonly investigated for their high Pmax and low Wloss.By the introduction of PbO-B2O3-SiO2, Chen found a microwave sintering anti-ferroelectric ceramics (Pb, Ba, La) (Zr, Sn, Ti) O3 (PBLZST) possesses a Wrec of 2.3 J cm -3 and a high ƞ of 76.8% [14] .Wang et al. obtained a high energy density of 10.4 J cm -3 in a (Pb0.98La0.02)(Zr0.55Sn0.45)0.995O3anti-ferroelectric ceramics [15] .Moreover, in order to achieve high Wrec, Zhang et al. explored a PBLZST ceramics by doping 0.75 mol% yttrium (Y), and the ceramics exhibited a Wrec of 2.75 J cm -3 and an ƞ of 71.5% [16] .However, these lead-based materials are often accused of being harmful to the environment.Thus, some environment friendly lead-free ceramics, especially Bi-based ABO3 structured perovskite materials, draw a lot of attentions in the electronic devices research areas.In these lead-free Bi-based ABO3 ceramics, Bi0.5Na0.5TiO3(BNT)based has been reported as one of the most promising candidate materials for high energy density applications.Because BNT possesses a similar lone pair electronic 6s 2 configuration of Bi 3+ ion and Pb 2+ ion, which will generate a distorted perovskite structure, a large ferroelectric polarization, and a macroscopic relaxor characteristics [17][18][19] .Later, researchers proposed that through a doping such as a displacement in A site element the distorted perovskite structure of BNT has a phase transition from ferroelectric to relaxor ferroelectric phase, which is responsible to the discharging energy density [20][21][22] .
More recently, therefore, in order to reduce energy barrier, the modifying of structural heterogeneity is taken into consideration since it has the contributions to relaxor ferroelectric state in the BNT-based materials.According to the literature [23][24][25] , dielectric properties of a pure BNT ceramic exhibits a good temperature stability.
However, BNT usually shows a typical ferroelectricity, which has a large Pr.Thus, to reduce its Pr, it requests a suitable doping material to modify the structural heterogeneity of BNT.At this time, a Sr0.7Bi0.2TiO3(SBT) ceramic attracts our attention, which has been reported as a new lead-free ferroelectric relaxor and is believed to show a diffused dielectric maximum over a wide temperature range [26][27][28][29][30] .
In this work, SBT was employed to compound with BNT and construct a new solid BNT-S0.7+xBTusing simple solid-state sintering method [31] .The obtained BNT-S0.7+xBT is supposed to reach a high energy storage performance and a high discharging energy efficiency.Simultaneously, we expect that the BNT-S0.7+xBTexhibits a good temperature stability or endurance as well as moderated frequency-dependent.In addition, this work is expected to exploit a suitable method to tailor energy storage in BNT-S0.7+xBTceramics for dielectric capacitor application using the adjustments to the A-site stoichiometry, and to provide a valuable strategy in designing a relaxor ferroelectric doping ceramics.

Experiments
A group of 0.65(Bi0.5Na0.5)TiO3-0.35(Sr0.7+x+Bi0.2)TiO3[abbreviated as BNT-S0.7+xBT](x=-0.1,-0.05, -0.01, 0, 0.01, 0.05, 0.1, and 0.15, respectively) lead-free piezoelectric ceramics was prepared by the conventional solid-state reaction method, where SrCO3 (99%), Na2CO3 (99%), Nb2O5 (99.9%),Bi2O3 (99.9%), and TiO2 (99.9%) were used as the raw materials.The related oxides were weighted according to the chemical formula of BNT-S0.7+xBT and milled with alcohol and zirconia ball media for 12 h.With complete drying the powders were pre-sintered at 850 o C for 2 h (about ) in air.Then the calcined powders were ball-milled again and dried.After that, the pellets with 10 mm diameter and about 1.5 mm in thickness were pressed under a pressure of 150 MPa using a 5% polyvinyl alcohol binder.The organic binder was burned out at 600 o C for 2 h (about 1 o C•min -1 ) and sintered in a covered alumina crucible at 1120 o C for 2 h (about 3 o C•min -1 ).For the electric performance characterization, the pellets were brushed silver paste as the electrode on both surface sides.
The crystal structure of BNT-S0.7+xBTceramics was examined by XRD (D/ MAX 2400, Japan) at a scan step of 0.02°.The microstructural morphology of the ceramics was prepared by grinding, polishing and thermal etching, followed by examination under SEM (Quanta F250, Germany) with energy dispersive X-ray spectroscopy (EDX) detector.Specimens for dielectric measurement were painted silver paste first as electrode, followed by a firing procedure at 600 o C for 0.5 h.The temperature-dependent and frequency-dependent dielectric properties were measured in a frequency range of 0.1-1 MHz and a temperature range of -20-300 o C, respectively, using a broadband dielectric spectrometer (Novocontrol alpha-A, German).To obtain the unipolar polarization-electric field (P-E) and strain-electric field (S-E) loops at 10 Hz, the thickness of the ceramic was polished to 200 µm and characterized by a ferroelectric test system (Premier Ⅱ, Radiant, USA) at room temperature.

Results and discussion
Figure 1(a) provides the XRD patterns of various BNT-S0.7+xBTceramics.Clearly, all the samples exhibit a pure perovskite phase and without any obvious impurities or secondary phases.Nevertheless, the Sr 2+ addition or reduction has a little influence on crystal structures of BNT-S0.7+xBTcomposite ceramics.And the splitting peaks of ( 200) and (002) shown in Fig. 1 (b) indicate the presence of a typical tetragonal phase in these ceramics [32] .Moreover, it is easy to identify from Fig. 1(b) that the enlarged diffraction peaks around 46 o is related to the entrance of overmuch Sr 2+ ions change the host lattice in these perovskite ceramics.The SEM micrographs shown Fig. 2 are the microstructures of all BNT-S0.7+xBTceramics specimens, and all images illustrate the high-temperature sintering ceramics have the highly dense and compact morphology.However, it can be observed that addition or reduction of Sr 2+ ions affects grain growth of BNT-S0.7+xBT.Significantly, with increasing x, the grain size of BNT-S0.7+xBTfirst decreases and then increase.
When x increases to 0.05, the BNT-S0.75BTexhibits minimum average grain size.It should be pointed that oxygen vacancy concentration contributes a lot to the crystal grain growth in BNT-S0.7+xBT.And the roughening or faceting of the grain boundary structure depending on the oxygen vacancy concentration results in the change of crystal grain size, as reported in recent literature [33][34][35] .As mentioned above, the introduction of appropriate Sr 2+ ion facilitates diffusion, homogenizes the microstructure, and significantly suppress the grain growth of BNT-S0.7+xBT.A smaller crystal grain size and denser surface usually correspond to a greater dielectric breakdown strength [36] .Moreover, the tunable microstructure of BNT-S0.7+xBTshould have a considerable effect on the electromechanical properties through minor adjustments to the A-site stoichiometry.Because the introduction of Sr 2+ often attributed to the increment of the site disorder and charge fluctuation.Consequently, this will lead to the increases of dielectric loss in Sr 2+ ions abundant BNT-S0.7+xBTsamples.As illustrated in Fig. 3, BNT-S0.85BTwith more Sr 2+ ions content shows a larger dielectric loss of 0.11 at 1 MHz.Besides, εr at Tc point also increases with the increasing of the Sr 2+ content, as shown in Fig. 4(i).When x increases up to 0.05, BNT-S0.7+xBTceramic expresses a giant εr (5000) while reaches to an unfriendly low loss (0.12) at 1 MHz.Clearly, the Asite defect resulting from Sr-excess or Sr-deficiency play an important role for the poor temperature stability of dielectric properties in this BNT-S0.7+xBTceramics.Meanwhile, with further increasing Sr 2+ content, the disordering is enhanced.As shown in the yellow curve of Fig. 4(i), when the conducted temperature is increased, a maximum εr of ~5400 is obtained in BNT-S0.85BTceramic sample at higher temperature Tc, which is associated with the symmetry transition from polar to non-polar structure in PNRs [37] .These results indicate that the temperature stability of dielectric properties can be For characterizing the polarization and strain behavior under the high electric field, unipolar P-E hysteresis loops and strain-electric field (S-E) hysteresis loops for BNT-S0.7+xBT(-0.1 ≤ x ≤ 0.15) ceramics are displayed in Fig. 5(a-b).Clearly, besides that of BNT-S0.7+xBTsamples with negative x, the P-E loops of the Sr excessing BNT-S0.7+xBTceramics always maintain pinched shapes under different electric fields, and even become slimmer and slimmer with increasing of x from 0.01 to 0.15.This indicates the compositional disorder is caused by the addition of Sr 2+ , which further destroys the existing ferroelectric macrodomains and enhances the relaxor characteristics.The influence of x on the dielectric properties is consisted with the explanations of XRD and SEM results.Moreover, there has a large unipolar strain response in these ceramic/ceramic composites due to polarization and strain coupling [38][39][40] .As shown in Fig. 5b, S-E loops of all the composites show the negligible remnant strains (less than 0.05%), implying a high cycle reliability in these BNT-S0.7+xBTsamples.The residual internal stresses caused by shrinkage during sintering and thermal expansion mismatch or interdiffusion across the interface will have a considerable influence on the macroscopic electromechanical property [41][42][43] .By referenced equations ( 1)-(3), the Wrec, Wloss and η of the BNT-S0.7+xBTceramics with different compositions at 100 kV•cm -1 are calculated using the data of P-E loops from Fig. 5(a), and presented in Fig. 5(c).When x is 0.05, the BNT-S0.7+xBTceramics has ultrahigh Wrec and favorable η, which corresponds to an excellent energy storage performance.As such, an ultrahigh Wrec (9.81 J•cm -3 ), a Wloss (4.07 J•cm -3 ) together with a high η (70.7%) are achieved in BNT-S0.75BTsample, respectively.In addition, Wrec and η of the BNT-S0.75BTceramic increase with the applied electric field from 10 kV•cm -1 to 100 kV•cm -1 , as shown in Fig. 5(d).Attractively, the increasing electric strength has little influence on the discharging efficiency of the BNT-S0.75BTceramics, indicating a favorable electric field stability.BNT-S0.75BTreaches to 42µC•cm -2 under 100 kV•cm -1 , which is higher than BNT-S0.8BTceramic.Besides, under the same electric field, Pr of BNT-S0.75BT is as low as 4.5 µC•cm -2 .The slim P-E loops of BNT-S0.75BT with a small remnant polarization indicate that BNT-S0.75BT is a favorable relaxor ferroelectric.This relaxor behavior of BNT-S0.75BTceramic attributes to its PNRs including a smaller size than those of macroscopic domains since PNRs are more easily to align and switchback under the applied electric field.Therefore, slim P-E loop including high Pmax, low Pr, and large DBS resulting from the highly dynamic PNRs are likely to responsible for the high energy storage density and low energy loss during the process of charge/discharge, as shown in Fig. 6(a).Furthermore, the random field, together with nanosized domains, will further facilitate the distribution of electric field-induced ferroelectric order out of the nonergodic relaxor state and hence enhance the strain rate.As shown in Fig. 6 (b), the electric field-induced strain of BNT-S0.75BT is risen along with the increasingly pronounced relaxor features and reaches to 0.28% under 100 kV•cm -1 .In practical environments, the service conditions of energy storage devices are often accompanied by temperature fluctuations in frequent charging and discharging process.What is more, the variation in Wrec over the operating temperature range should be less than 15%.Therefore, the temperature stability of energy storage properties of ceramics becomes a critical parameter to evaluate the quality of devices.
and a strain of 0.26% do not show an obvious vibration, as presented in Fig. 7(a) and (b), which indicates that these ceramics have the favorable discharging energy density and a reliable temperature stability.

conclusion
Lead-free relaxor BNT-S0.7+xBTsystems are synthesized in this work using the solid-state sintering method, and followed that the structure, surface morphology, dielectric properties and energy density are comprehensively investigated.Through minor adjustments to the A-site Bi stoichiometry using the changing of Sr 2+ ions, the microstructure, dielectric and energy density properties of BNT-S0.7+xBTceramics were extensively tuned, which shows that a small excess of Sr 2+ can lead to a decrease in Asite vacancy concentration.When x is tuned to 0.05, the BNT-S0.75BTceramics shows an ultrahigh Wrec of 9.81 J•cm -3 and a good η of 70.7% under 100 kV•cm -1 .And more interestingly, BNT-S0.7+xBTexhibits a high cycle reliability and an excellent temperature stability, which is of great value to broaden the application field of this lead-free dielectric materials.

Figure 3
Figure 3 presents the frequency dependence of relative dielectric permittivity (εr) and dielectric loss for BNT-S0.7+xBTceramics.Clearly, εr of all samples gradually decreases while dielectric loss increases as the enhance of test frequency.With x

Fig. 3 .
Fig. 3. Dielectric permittivity and loss spectrum for BNT-S0.7+xBTceramics vs frequency range from 100 to 1000 kHz.To further express the temperature reliability of dielectric properties of these ceramics, the εr and dielectric loss for BNT-S0.7+xBT(-0.1 ≤ x ≤ 0.15) ceramics in the evaluate frequencies of 1k to 1M Hz were tested at a temperature ranging from 25 to 300 o C, as displayed in Fig4(a)-(h).For all the samples, εr tends to increase firstly, and then decreases gradually.The apparently broad peaks in various dielectric spectrums correspond to the Currie temperature (Tc) of BNT-S0.7+xBTceramics, which is caused by the reversion of the macro ferroelectric domains.Moreover, Tc of BNT-S0.7+xBTbecomes highly frequency dependent and it moves to high temperature value with the increases of frequency, as shown in Fig.4.(a)-(h), which is a typical character of the ferroelectric ceramics.
modified by A-site defect for BNT-based materials.

Figure 6 .
Figure 6.(a)-(d) present the details of unipolar P-E loops and S-E loops of the BNT-S0.75BTand BNT-S0.8BTceramics under evaluated electric fields.Clearly, Pmax of

Figure 7 .
Figure 7. (a)-(d) depict the diversity of P-E loops and their corresponding energy storage values for BNT-S0.7+xBT(x=0.05 and x=0.1) ceramics over a temperature range from 25 to 100℃.To avoid prone to thermal breakdown under continuous heating conditions, the test environments at 100 kV•cm -1 and 1 Hz frequency are set up.These P-E loops of BNT-S0.7+xBTceramic are similar to others at different temperature, and Pmax or Pr and do not show any obvious reduction, indicating a good performance of temperature stability.Moreover, the residual long-range-ordered ferroelectric phase of BNT-S0.7+xBTceramics gradually changes to a short-range ordered relaxor phase with increasing temperature, which can also be demonstrated by the reduction of Pr, as shown in Fig. 7. Compared to polarization and strain loops of BNT-S0.8BTshown Fig.7(c) and (d), the P-E and S-E loops of BNT-S0.75BT in 100 kV•cm -1 maintain a stable shape at evaluate temperature, and a Pmax of 40.9 µC•cm -2 , a Pr of 5.8 µC•cm -2 ,

Figure 7 Temperature
Figure 7