Enhanced energy storage properties of Bi0.5Li0.5TiO3 modified Sr0.1Bi0.45Na0.45TiO3 based ceramics

Lead-free (1−x)Sr0.1Bi0.45Na0.45TiO3−xBi0.5Li0.5TiO3 (x = 0−0.4) ceramics were successfully prepared by a solid-state reaction technique. The effects of amount of Bi0.5Li0.5TiO3 on structure and electrical properties were examined. The X-ray diffraction (XRD) analysis revealed that all the investigated specimens have a perovskite structure. An obvious change in microstructure with the increase of Bi0.5Li0.5TiO3 concentration was observed. This study demonstrated that relaxor could be stabilized in Sr0.1Bi0.45Na0.45TiO3 based ceramics by lowering the tolerance factor and electronegativity difference. Besides, a dielectric anomaly related to thermal evolution of crystallographic symmetry was emerged at the depolarization temperature. Upon incorporation of 26 mol% Bi0.5Li0.5TiO3, the specimens were able to withstand an electric field intensity of 106.9 kV/cm with an energy density of 0.88 J/cm3 and an energy efficiency of 65%.


I n t r o d u c t i o n
In response to the current environmental regulations against the use of lead in daily electronic devices, ceramics with a perovskite structure have been of great interest to the community. The presence of interstitial sites and relatively large spatial tolerance for substitution atoms are beneficial for chemical modifications and enabling perovskite structure to tailor electrical properties [1]. Among various perovskitetype materials, Bi 0.5 Na 0.5 TiO 3 (BNT) is considered as one of the most competitive alternatives for lead based ceramics. BNT is a typical perovskite structure ferroelectric material with 1:1 ratio of Na + and Bi 3+ at A-site, which causes high polarization due to the existence of stereo-chemically active long pair electrons [2]. Therefore, it is also a candidate for energy storage dielectric. However, pure BNT ceramic is a typical ferroelectric (FE) substance suffering for large polarization loss which limits its application in practice.
Recently, binary and ternary system relaxor ceramics have been investigated for their high energy storage application, e.g., BNT-BaTiO 3 [3], BNT-BaTiO 3 -K 0.5 Na 0.5 NbO 3 [4][5][6], BNT-BaTiO 3 -NaNO 3 [7], BNT-CaTiO 3 [ 8 ] , B N T -S r T i O 3 -BaTiO 3 [9], BNT-K 0.5 Bi 0.5 TiO 3 -BaTiO 3 [10], etc. The decreased remnant polarization (P r ) value, which is favored to receive larger recoverable energy density compared with pure BNT, is found in these systems. As reported, the discharged energy density (J d ) of BNT based ceramics  is ranged from 0.6 to 0.9 J/cm 3 if no glass phase is added. Among these systems, Sr 0.1 Bi 0.45 Na 0.45 TiO 3 (SBNT) has attracted much attention due to its strong dispersion of the permittivity with a relaxor-like behavior [11][12][13]. Besides, the alkali metal ion of Li + , which is in the same group as Na + and K + in periodic table of elements, has the similar properties. Based on the above, in this paper, (1x)Sr 0.1 Bi 0.45 Na 0.45 TiO 3 -xBi 0.5 Li 0.5 TiO 3 (SBNT-xBLT) are selected as lead-free energy storage materials and investigated for the dependence of structure and electrical properties on composition (x). In addition, the energy storage properties of SBNT-xBLT (0 ≤ x ≤ 40 mol%) ceramics are demonstrated in detail.

Experiment
SBNT-xBLT (x = 0.00, 0.20, 0.24, 0.26, 0.30, and 0.40, abbreviated as N1, N2, N3, N4, N5, and N6, respectively) ceramics were prepared by a mixed oxide route from appropriate quantities of high purity (≥ 99.9%) Li 2 CO 3 , SrCO 3 , Bi 2 O 3 , Na 2 CO 3 , and TiO 2 . The starting materials were mixed and grounded in alcohol for 9 h with ZrO 2 balls. Then, the mixture was dried and calcined at 870 ℃ for 2 h. The resultant powders were mixed with 5 wt% of polyvinyl alcohol as binder and pressed into pellets of 10 mm in diameter and 1 mm in thickness by uniaxial pressing. Then, dense pellets were obtained by sintering in air at 1020 ℃ for 4 h. The crystal phases of the sintered ceramics were analyzed using an X-ray diffractometer (Model D8-Advance, Bruker, Germany). The microstructural observation of the crystallized samples was carried out using a field emission scanning electron microscope (FE-SEM, Model S-4800, Hitachi, Japan). For electrical measurements, each of pellet faces was coated with Ag electrode. The polarization-electric field hysteresis loops were measured using a ferroelectric tester (Precision Premier II, USA) with a frequency of 1 Hz, and the energy density was estimated from the P-E curves by integrating the area enclosed within the polarization axis and the discharged curve. The temperature-dependent dielectric properties were measured at 1 kHz by an Agilent HP4294A analyzer, with heating and cooling controlled conditions in an oven. The breakdown strength (BDS) was determined by a DC bias source (Model MARX, Tianjin Dongwen Company, China). For each composition, at least 6 samples were measured to obtain the average BDS.

Results and discussion
The structures of all the samples were characterized by X-ray diffraction (XRD), as presented in Fig. 1, and the results are identified by a pseudocubic symmetry (PDF Card No. 81-2200) for x ≤ 0.2 within the sensitivity of XRD [14]. Meanwhile, a secondary phase indexed as Li 2 TiO 3 appears with x ＞ 0.2, although its content is found only in trace amount. The FE-SEM micrographs of the samples are shown in Fig. 2. The average grain size for the pristine ceramic is much less than the others. Besides, with the increase of BLT concentration, the grain size distribution gradually becomes uniform. The result suggests that a certain amount of additive of BLT will result in increasing grain size and affect their electrical properties accordingly.
Tolerance factor (t) and electronegativity difference (X) can be used to assess the rational design of stability in perovskite. Lead-containing ceramics possess double P-E loops with both low t and X. Compared to complex lead perovskite compounds, lead-free perovskite phase is more stabilized via solid solutions when t or X is increased. On the contrary, a relaxor phase, which exhibits a pseudocubic symmetry in average with the presence of non-polar below the detection limit of XRD technique, can be stabilized by decreasing the value of t [15], e.g., Nd-doped BiFeO 3 displays double P-E loops that is attributed to a decrease in t [16,17]. The Goldschmidt tolerance factor t and electronegativity difference X, are given by Eq. (1) and Eq. (2) respectively: where R A , R B , and R O are ionic radii for the A-and B-site cations and the oxygen anion, respectively. X AO and X BO are the electronegativity difference of A cation and B cation with oxygen anion, respectively. Figure 3 shows a linear relationship between t a n d X f o r SBNT-xBLT compositions. With the increase of the BLT concentration, both t and X decrease gradually and SBNT-xBNT ceramics, therefore, are expected to approach a square P-E loop. Meanwhile, the relaxor phase will become more and more stable, which is manifested in the follow-up experiments. A set of temperature-dependent dielectric measurements on SBNT-xBLT ceramics, given in Fig. 4, confirms that the changes of dielectric constant induced by compositional modifications are apparent. The dielectric properties of pure SBNT ceramic are featured with a distinct dielectric anomaly at T m (~250 ℃), as marked in the figure. Compared to the pristine ceramic, the BLT-added ceramics exhibit another dielectric anomaly with 0.2 ≤ x ≤ 0.3, locating at depolarization temperature T f (~90 ℃). The corresponding dielectric loss in the inset (a) of Fig. 4 also possesses a peak, which can be ascribed to that the polar nanoregions (PNRs) transform into microsized domains at T f [18]. And PNRs are also the origin of relaxation behavior for SBNT-0.2BLT ceramic around T f , as seen in the inset (b) of Fig. 4. Simultaneously, the dielectric constant decreases and the curves become more flat with increasing BLT concentration. Previously, this anomaly was believed to be attributed to thermal evolution of ferroelectric polar and weakly polar [19] or non-polar symmetry [7].     where E i is the specific breakdown value of each specimen in the experiments, n is the sum of specimens, i is the serial number of the specimens, and the specimens are arranged in ascending order of BDS values so that E 1 ≤ E 2 ≤ … ≤ E i ≤ … ≤ E n . Obviously, all the slopes (m) of the Weibull curves are linear, and m > 1; it indicates that the BDS can be analyzed by the Weibull model. The values of BDS as a function of BLT addition are shown in the inset of Fig. 5. As the BLT concentration increases up to 26 mol%, the breakdown strength of the ceramic samples increases from 48.1 (x = 0) to 106.9 kV/cm (x = 0.26). With the further increase of BLT content, the values of BDS decrease gradually. It was reported that the breakdown strength is closely related to activation energy (E a ) and grain size [20][21][22]. According to the experiment results, E a is the most likely driving force for breakdown strength variation, and the change of grain size is unnoticeable when the BLT concentration exceeds 20 mol% (as seen in Fig. 2).
The ener gy storage behavior of the SBNT-xBLT ceramics with the same electric field is investigated in terms of the polarization-electric field (P-E) hysteresis loops. Figure 6 displays hysteresis loops of SBNT-xBLT ceramics measured with a frequency of 1 Hz at room temperature. A massive alignment of randomly-oriented ferroelectric domains occurs in BNT ceramics along the field direction by switching its polarity towards one of the energetically equivalent directions [1]. As mentioned before, a reduction in tolerance factor and electronegativity difference can contribute to the lessening of the relative stability of the FE structure. As a result, a greater fraction of reoriented ferroelectric domains switch back with the removal of the applied electric field, leaving the material reach a reduced remanent state. And this corresponds to the   Fig. 6. However, the so called "non-polar phase" is too stable to induce a ferroelectric state out of a relaxor state under the same external applied electric field in N5 and N6, as shown in Figs. 6(e) and 6(f), respectively.
The discharged energy density is obtained by integrating the area between the polarization axis and the discharge curve in the first quartile of P-E hysteresis loops [23]. According to the definition of discharged energy density, saturated polarization and remnant polarization have a direct impact on it. It is observed that with the increase of the BLT concentration, the saturated polarization (P s ) decreases gradually. Meanwhile, the remnant polarization first decreases sharply and then almost remains constant. Figure 7 shows the discharged energy density of the samples sintered at 1020 ℃ as a function of electric field at room temperature. The nonlinear behavior, with increasing electric field, is observed for all the cases. Obviously, higher applied electric field is more conducive to energy storage capability. Pure SBNT ceramic has the lowest discharged density, owing to the rather slow kinetics of reoriented domains back switching. The increasing concentration of BLT leads to a sharp drop in remnant polarization, which improves energy storage properties gradually. Consequently, a maximum energy storage density of 0.88 J/cm 3 under an applied field 95 kV/cm is achieved when the increasing content of BLT up to 26 mol%. Thereafter, the energy storage properties deteriorate as BLT further increases. The energy efficiency (η) is defined as the ratio of the energy density discharged to the energy density charged (J c ). As presented in Fig. 8, the energy efficiencies of BLT-added specimens are obviously enhanced, showing the values in the range of 60% -75% at 60-90 kV/cm. The polarization retention behavior is believed to be the scenario to explicate the difference between J d and J c in a charge-discharge cycle. For the BLT-added specimens, reoriented ferroelectric domains switch back rapidly depending on their energetic stability with the removal of the applied electric field, resulting in a depressed poling process in the grains. Consequently, the BLT-added specimens achieve higher energy efficiencies as compared to the pristine specimen. However, the increase of the external applied electric field will expand the gap between J d and J c , and the corresponding energy efficiencies of SBNT-0.26BLT specimen initially at around 88% then gradually decrease to ~65%.

Conclusions
(1x)Sr 0.1 (Bi 0.5 Na 0.5 ) 0.9 TiO 3 -xBi 0.5 Li 0.5 TiO 3 c e r a m i c s with different content x were fabricated by solid-state reaction method. The effects of x values on the microstructure, ferroelectric properties (including the energy storage density), and dielectric properties were studied. XRD results of all the samples exhibited a typical perovskite structure. In the system, the BLT showed a strong influence on the microstructure, dielectric constant, BDS, and energy density. Besides, a maximum recoverable energy storage density of 0.88 J/cm³ with a high BDS of 106.9 kV/cm was achieved. These results showed that SBNT-BLT ceramics are a promising candidate dielectric material for high voltage, high energy density capacitors.