Novel BaTiO3-Based, Ag/Pd-Compatible Lead-Free Relaxors with Superior Energy Storage Performance

Ceramic dielectrics are reported with superior energy storage performance for applications, such as power electronics in electrical vehicles. A recoverable energy density (Wrec) of ∼4.55 J cm–3 with η ∼ 90% is achieved in lead-free relaxor BaTiO3-0.06Bi2/3(Mg1/3Nb2/3)O3 ceramics at ∼520 kV cm–1. These ceramics may be co-fired with Ag/Pd, which constitutes a major step forward toward their potential use in the fabrication of commercial multilayer ceramic capacitors. Compared to stoichiometric Bi(Mg2/3Nb1/3)O3-doped BaTiO3 (BT), A-site deficient Bi2/3(Mg1/3Nb2/3)O3 reduces the electrical heterogeneity of BT. Bulk conductivity differs from the grain boundary only by 1 order of magnitude which, coupled with a smaller volume fraction of conducting cores due to enhanced diffusion of the dopant via A-site vacancies in the A-site sublattice, results in higher breakdown strength under an electric field. This strategy can be employed to develop new dielectrics with improved energy storage performance.


INTRODUCTION
Energy storage technologies such as lithium-ion batteries and electrolytic super-capacitors have been the focus of much recent research. 1,2 Batteries provide long-lasting energy/power through a continuous slow discharge rate whereas supercapacitors charge and discharge more rapidly and are primarily used in kinetic energy recovery systems. 3−10 However, their polymeric components mean that they have limited temperature stability. 11 In contrast, ceramic dielectric capacitors do not offer such high energy density but are stable above 100°C and are finding applications in high temperature, high power electronics in electric vehicles, and in pulsed power and laser applications. 12−15 The total energy density recoverable energy density = ∫ W E d P P P rec r max (2) and energy conversion efficiency for nonlinear dielectric capacitors are obtained from the integration of polarization−electric field (P−E) loop, where P max is maximum polarization and P r is remanent polarization. Therefore, both large ΔP (P max − P r ) and maximum applied electric field (E max ) are desirable for achieving high W rec and η.
BT-based ceramics are commercially the most attractive candidates for high energy density storage since they are already utilized for consumer electronics at low fields as filters and de-couplers. The first example of improved W rec (2.3 J cm −3 at 225 kV cm −1 ) for BT-based compositions was 0.7BT-0.3BiScO 3 (0.7BT-0.3BS) bulk ceramics, whose properties were enhanced to 6.1 J cm −3 in multilayer ceramic capacitors (MLCCs). 25 The same research group reported 0.7BT-0.3BS MLCCs which exhibited much better temperature stability (<15% from 0 to 300°C) and W rec compared with commercial X7R (poor temperature stabilities) and C0G (low W rec and E max ) capacitors.
Stoichiometric Bi(Mg 2/3 Nb 1/3 )O 3 (BMN) is a commonly reported third end-member or dopant in perovskite solid solutions and has been shown to optimize W rec by promoting a weakly nonlinear relaxor state. 25,47,48 Solid solutions which incorporate a range of ion sizes and valences on the A and B sites of the perovskite structure disrupt coupling between polarisable species (Bi 3+ , Ti 4+ and Nb 5+ ), reducing P r but simultaneously creating an "active solid solution" of local disordered regions within a pseudocubic matrix, which can be addressed by an electric field leading to high P max . 49 This strategy has been adopted in many lead-free systems to effectively enhance ΔP and W rec , for example, 0.10 BMN-BT (1.18 J cm −3 ), 50 0.06 BMN-BF-BT (1.56 J cm −3 ), 51 0.15 BMN-NN (2.8 J cm −3 ), 38 and 0.10 BMN-KNN (4.08 J cm −3 ). 41 Such dopant strategies are often accompanied by an increase in electrical homogeneity and reduction of grain size/ porosity, leading to enhanced E max . 33,36,43,52 However, the role of A-site vacancies (V A ) is rarely addressed in "active solid solutions".
In this study, a solid solution of BT with A-site deficient Bi 2/3 (Mg 1/3 Nb 2/3 )O 3 (B 2/3 MN) has been synthesized and the role of V A in optimizing E max and W rec is investigated. Small concentrations of V A have been reported previously to improve the conductivity of lead-free dielectrics, such as SrTiO 3 (ST) and BT. 53−55 In addition, we postulate that V A reduces the concentration of Bi based alloying addition required to induce a weakly nonlinear relaxor state, thereby enhancing compatibility with conventional Ag−Pd electrodes which react with Bi at high temperatures.
We demonstrate that A-site deficient xB 2/3 MN-BT exhibits a bulk and grain boundary response similar to conventional BMN-BT ceramics but the total conductivity is at least one order of magnitude lower. The lower conductivity leads to an enhancement of E max ∼ 520 kV cm −1 (∼270 kV cm −1 for BMN-BT) and results in W rec ∼ 4.55 J cm −3 and η ∼ 92% in compositions with x = 0.06. Although several materials have similar W rec , 17−19 A-site deficient xB 2/3 MN-BT compositions were also shown to be compatible with Ag−Pd metal, suggesting potential for commercialization as high-voltage, high-temperature MLCCs. Excess 0.5 mol % Bi 2 O 3 was added to compensate for Bi-loss during processing and Li 2 CO 3 was used as a fluxing agent to reduce the sintering temperature. 56−58 Mixed powders were ball milled for 16 h, dried, and calcined 2 h at 900°C. After calcination, the mixed powder was ball milled for 16 h, dried, and uniaxially pressed into 10 mm diameter pellets, followed by sintering 4 h from 1050−1200°C. The density of sintered ceramic pellets was evaluated using the Archimedes principle, yielding relative densities >95% of theoretical. To investigate the chemical compatibility of A-site deficient xB 2/3 MN-BT with the Ag−Pd (70−30%) electrode, 20 wt % of Ag−Pd electrode ink was mixed with ceramic powders and co-fired 4 h in air at 1100°C. X-ray diffraction (XRD) was performed using a D2 phaser X-ray diffractometer on crushed pellets, annealing for 5 h in air at 550°C to eliminate residual stress. Specimens for scanning electron microscopy (SEM) were ground, polished, thermally etched at 1080°C for 30 min, and carbon coated. Thermally etched ceramics were evaluated using an FEI Inspect F50 SEM, equipped with backscattered electron (BSE) and energy-dispersive X-ray spectroscopy (EDX) detectors. Samples for transmission electron microscopy (TEM) were ground manually to ∼50 μm, followed by polishing to electron transparency using an Argon ion mill (PIPS II 695, Gatan, USA). Samples were examined with a JEOL JEM 2100F (JEOL, Tokyo, Japan) operated at 200 kV.

EXPERIMENTAL PROCEDURES
Ceramic pellets for electrical measurements were ground to 0.2 mm, gold sputter-coated for 1 min. FE P−E measurements were performed using an aixACCT TF2000E system with a 1 Hz triangular signal. Temperature-dependent permittivity and loss were examined using an Agilent 4184A precision LCR meter from −100 to 200°C from 1 kHz to 1 MHz. The electrical microstructure was evaluated  3. RESULTS AND DISCUSSION 3.1. Crystal Structure, Dielectric, and FE Properties. The crystal structure of ceramic powders was examined using XRD data collected in 15−70°2θ range, as shown Figure 1a. A single-phase perovskite is observed for compositions with x ≤ 0.06. A secondary phase (peaks labelled in Figure 1a) is presented in x = 0.08 and x = 0.10, indicating that the solubility limit of B 2/3 MN in BT has been reached. Doublets merge into single peaks as x increases, suggesting a transformation from tetragonal into pseudocubic symmetry, in which the correlation length of polar order decreases. Fullpattern refinement of the diffraction data for all single-phase compositions was carried out, Table S1 (Supporting Information), confirming a pseudocubic phase for x > 0.02, Figure S1 (Supporting Information).
Bipolar P−E loops obtained at 100 kV cm −1 for xB 2/3 MN-BT ceramics are displayed in Figures 1b and S2 (P−E loop for BT at 60 kV cm −1 , Supporting Information). A FE P−E loop is observed for x = 0.00 and 0.02, with P max ∼ 20 μC cm −2 and P r ∼ 12 μC cm −2 . Both P max and P r reduce gradually but ΔP increases with x, indicating a FE to RFE transition. The temperature-dependent relative permittivity (ε r , solid line) and dielectric loss (tan δ, dashed line) at 100 kHz for xB 2/3 MN-BT ceramics are shown in Figure 1c. The sharp anomalies for BT at ∼135°C (ε r ∼ 7000), 22°C (ε r ∼ 1750), and −70°C (ε r ∼ 950) correspond to the cubic−tetragonal, tetragonal−orthorhombic, and orthorhombic−rhombohedral phase transitions, respectively. 62 As x increases, the maximum ε r decreases continuously reaching 1000 for x = 0.10, which shows a rather temperature independent ε r . Temperature-independent permittivity were reported for conventional BMN-BT solid solution by Reaney and co-workers with temperature coefficient of capacitance (TCC) < ±15%. 63 Here, TCC values for xB 2/3 MN-BT (x ≥ 0.04) at 100 Hz were calculated, as shown Figure 1d, with x = 0.08 and x = 0.10 exhibiting TCC < ±22% from −55 to 125°C, corresponding to an X7S specification. Frequency-dependent dielectric properties for xB 2/3 MN-BT ceramics are shown in Figure S3 (Supporting Information). A frequency-independent dielectric peak occurs at ∼135 and 102°C for x = 0.00 and x = 0.02, respectively, corresponding to the Curie temperature (T c ) but a frequency dispersion is observed in ε r − T curve for x > 0.02, indicating relaxor behavior.
3.2. Microstructure. SEM images of thermal-etched surfaces for xB 2/3 MN-BT ceramics are shown in Figure 2a (x = 0.06) and Figure S4 (Supporting Information). The average grain size reduces with increasing x from 25 μm for x = 0.00 to ∼2.8 μm for x = 0.06, Table S2 (Supporting Information). Secondary phases are observed for x = 0.08 and x = 0.10 at the grain boundary. TEM images of a ceramic with x = 0.06, as shown in Figure 2b,c, revealed some cores with FE or tweedlike domains surrounded by a largely featureless pseudocubic shell (Figure 2b).
Most research into BT-based MLCCs with superior W rec utilize Pt as inner electrodes; however, the use of such expensive noble metal precludes their commercial exploitation in mass production applications. [17][18][19]24,64 It is therefore, essential to evaluate compatibility of potential MLCCs dielectric layers against lower cost electrode systems such as Ni, Ag, or Ag−Pd. In the case of Bi-based or containing compounds, reaction with Ni is a well-known phenomenon, which is often accompanied by decomposition at the low p(O 2 ) required for co-firing with Ni internal electrodes. 65,66 The sintering of Bi-based compounds with pure Ag electrodes is also problematic and limited to co-firing at <850°C because of melting of Ag. Even for systems which can co-fire at <850°C , the reaction of Bi containing compounds with Ag is common depending on the thermodynamic stability of the Bi compound in the presence of Ag. This is exemplified by Bi 2 Mo 2 O 9 which reacts with Ag electrodes to form Ag- Research Article molybdate compounds. 67 The sintering temperature of compositions with x = 0.06 is > 850°C, and therefore, the use of pure Ag can be ruled out but alternatively Ag−Pd alloys can be employed at higher temperatures. In this study, we have therefore investigated the compatibility of 0.06B 2/3 MN-BT with Ag−Pd. SEM images and EDX mapping do not reveal chemical interaction between Ag−Pd particles and ceramic grains, as shown in Figure 2d,e, indicating that 0.06B 2/3 MN-BT is a promising material for the commercial fabrication of MLCCs.
3.3. Energy Storage Performance. The energy storage properties are obtained from unipolar P−E loops under the E max . The low ΔP and E max (<200 kV cm −1 ) of BT gave a poor response, as predicted, and the energy storage properties are not illustrated in this contribution. The unipolar P−E loops of xB 2/3 MN-BT (0.02 ≤ x ≤ 0.08) at E max are shown in Figure  S5a−d (Supporting Information), with corresponding E max and ΔP displayed in Figure S6a (Figure 3c,d). Other systems have recently shown higher W rec but this is accompanied by either poor efficiency (<70%) such as AN or cannot be co-fired with internal electrodes other than Pt, for example BF 36 compounds.
3.4. Electrical Microstructure. Complex impedance plane plots and spectroscopic plots of Z″ and M″ data for xB 2/3 MN-BT ceramics are given in Figure 4 for various temperatures. BT consisted of three components at 400°C: two semicircles and a low-frequency spike, as shown in Figure 4a. These data were interpreted based on an equivalent circuit comprising three parallel resistor−capacitor elements connected in series. 59−61 The capacitance extracted from these three components from high to low frequency are 30 pF, 20 nF, and 5 μF and are attributed to grain (bulk), grain boundary, and electrode responses, respectively. Contrary to BT, only one arc is observed in all xB 2/3 MN-BT samples, for example, x = 0.06 at 550°C, as shown in Figure 4b. However, the inspection of the combined Z″ and M″ spectroscopic plots at 550°C indicate this arc should consist of two semicircles representing two electroactive regions with similar resistivity, as shown in Figure  4c. The capacitances for high-and low-frequency arcs are 50 and 250 pF which correspond to grain and grain boundary contributions, respectively. In addition, one more M″ peak is observed at lower temperatures (350°C), as shown in Figure  4d. It has a corresponding capacitance of 200 pF which suggests it is also a bulk response. Therefore, xB 2/3 MN-BT ceramics exhibit an electrical core−shell microstructure, in agreement with TEM images, as shown in Figure 2b. The change in capacitance (C = 1/2M″) indicates a decreasing core and increasing shell volume fraction with increasing x, as shown in Figure 5a. Assuming the permittivity for the core and shell remains relatively similar for all xB 2/3 MN-BT ceramics, the volume fraction of the core/shell region decreases from 40/60 to 20/80 for x = 0.02 and x = 0.06, respectively. Similar bulk and grain boundary responses were also reported in conventional, yBMN-BT (y = 0.05−0.20) ceramics but total resistivity obtained from Z*-plot for xB 2/3 MN-BT is at least one order of magnitude larger, which explains the enhancement of E max . 50 The conductivity of different components in xB 2/3 MN-BT are summarized in an Arrhenius plot, as shown in Figure 5b. The conductivity of the core, σ b,core , of all three samples (x = 0.02, 0.04, and 0.06) are similar. However, with increasing x, the conductivity of the shell, σ b,shell , and grain boundary, σ gb , decreases by 2 and 1 order of magnitude, respectively. The activation energy, E a , of both core and shell remains relatively unchanged at ∼ 0.51−0.62 eV for σ b,core and 1.32−1.36 eV for σ b,shell . σ b and σ gb of BT is lower than the σ b,core of xB 2/3 MN-BT ceramics but higher than the σ b,shell with an E a of 0.98 and 1.35 eV, respectively.
For BT (x = 0.00), σ gb is lower than σ b , especially around RT because of the higher activation energy of σ gb compared to σ b . Under an electrical field, therefore, the voltage applied at the grain boundary is higher than the bulk which leads to a much higher local field. The enhancement of E max in xB 2/3 MN-BT (especially for x = 0.06) is attributed to the following three facts: (i) the total conductivity, σ total , decreases with increasing x doping level. The σ total of composition with x = 0.06 is ∼3 orders and >1 order of magnitude lower than BT and BMN-BT ceramics, respectively, which leads to a reduction in leakage current under at a high field. (ii) The conductivity difference between bulk (σ b,shell in x = 0.06) and grain boundary response is higher in x = 0.00 than x = 0.06. The difference in E a is 0.32 and 0.22 eV compositions with x = 0.00 and x = 0.06, respectively, which also means that the difference in σ b,shell and σ gb at RT is significantly smaller in 0.06B 2/3 MN-BT than BT. Despite the existence of some core−shell grains, the shell region constitutes ∼80% of the volume fraction of 0.06B 2/3 MN-BT and cannot be bypassed by the current. Thus, the voltage is more evenly distributed throughout the sample in 0.06B 2/3 MN-BT compare to BT which leads to a high E max . (iii) the much smaller grain size of 0.06B 2/3 MN-BT (∼2.8 μm) compared with BT (∼25 μm) and BMN-BT (∼6− 10 μm) ceramics yields a higher volume fraction of the grain boundary and consequently reduces local electrical fields. We postulate that the lower volume fraction of cores in xB 2/3 MN-BT is attributed to the greater diffusion rates of dopants through cubo-octahedral interstices in comparison with BMN-BT.

CONCLUSIONS
In summary, A-site deficient xBi 2/3 (Mg 1/3 Nb 2/3 )O 3 -BT (xB 2/3 MN-BT with x = 0.00−0.10) ceramics were successfully fabricated using the solid−state reaction. A phase transition from FE to RFE, associated with structural transformation from tetragonal to cubic, is observed in xB 2/3 MN-BT ceramics with increasing x. A record high E max ∼ 520 kV cm −1 and W rec ∼ 4.55 J cm −3 for BT-based compositions were realized in ceramics with x = 0.06 which may be co-fired with Ag−Pd without a chemical reaction. Impedance data revealed that the high E max for 0.06B 2/3 MN-BT ceramics was because of a