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. 2020 Sep 4;12(39):43942–43949. doi: 10.1021/acsami.0c13057

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

Huijing Yang †,‡, Zhilun Lu †,§, Linhao Li †, Weichao Bao ∥, Hongfen Ji †,⊥, Jinglei Li #, Antonio Feteira , Fangfang Xu ∥, Yong Zhang ∇, Huajun Sun ∇, Zhichao Huang ¶,○, Weichao Lou ¶,○, Kaixin Song ○, Shikuan Sun †, Ge Wang †,*, Dawei Wang †,*, Ian M Reaney †
PMCID: PMC7586291  PMID: 32885648

Abstract

graphic file with name am0c13057_0007.jpg

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.

Keywords: energy storage, capacitors, lead-free, BaTiO3, dielectric, ceramics

1. 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

1. 1

recoverable energy density

1. 2

and energy conversion efficiency

1. 3

for nonlinear dielectric capacitors are obtained from the integration of polarization–electric field (P–E) loop, where Pmax is maximum polarization and Pr is remanent polarization. Therefore, both large ΔP (Pmax – Pr) and maximum applied electric field (Emax) are desirable for achieving high Wrec and η.

The Wrec and η for ferroelectrics (FEs) are restricted because of low ΔP and Emax. Instead, weakly nonlinear relaxor-FEs (RFEs) and anti-FEs (AFEs) are generally proposed as potential candidates for high energy density ceramic capacitors, with compositions commonly based on BaTiO3(BT),16−25 Na0.5Bi0.5TiO3(NBT),26−29 BiFeO3(BF),30−37 NaNbO3(NN),38−40 K0.5Na0.5NbO3(KNN),41−44 and AgNbO3 (AN) ceramics.13,45,46 Even though excellent Emax (>400 kV cm–1) and η (>80%) have been achieved in RFE ceramics, Wrec is still limited by low Pmax (<45 μC cm–2) which requires further improvement to achieve Wrec > 4 J cm–3. AFEs possess larger Pmax at an intermediate electric field (∼300 kV cm–1) but saturation of the polarization restricts Wrec and they often exhibit low η (<70%) because of the field induced transition to the FE state.

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 Wrec (2.3 J cm–3 at 225 kV cm–1) for BT-based compositions was 0.7BT-0.3BiScO3 (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 Wrec compared with commercial X7R (poor temperature stabilities) and C0G (low Wrec and Emax) capacitors.

Stoichiometric Bi(Mg2/3Nb1/3)O3 (BMN) is a commonly reported third end-member or dopant in perovskite solid solutions and has been shown to optimize Wrec by promoting a weakly nonlinear relaxor state.25,48,49 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 (Bi3+, Ti4+ and Nb5+), reducing Pr 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 Pmax.50

This strategy has been adopted in many lead-free systems to effectively enhance ΔP and Wrec, for example, 0.10 BMN-BT (1.18 J cm–3),51 0.06 BMN-BF-BT (1.56 J cm–3),52 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 Emax.33,36,43,53 However, the role of A-site vacancies (VA) is rarely addressed in “active solid solutions”.

In this study, a solid solution of BT with A-site deficient Bi2/3(Mg1/3Nb2/3)O3 (B2/3MN) has been synthesized and the role of VA in optimizing Emax and Wrec is investigated. Small concentrations of VA have been reported previously to improve the conductivity of lead-free dielectrics, such as SrTiO3 (ST) and BT.54−56 In addition, we postulate that VA 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 xB2/3MN-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 Emax ∼ 520 kV cm–1 (∼270 kV cm–1 for BMN-BT) and results in Wrec ∼ 4.55 J cm–3 and η ∼ 92% in compositions with x = 0.06. Although several materials have similar Wrec,17−19 A-site deficient xB2/3MN-BT compositions were also shown to be compatible with Ag–Pd metal, suggesting potential for commercialization as high-voltage, high-temperature MLCCs.

2. Experimental Procedures

A-site deficient xBi2/3(Mg1/3Nb2/3)O3–(1 – x)BaTiO3(xB2/3MN-BT) ceramics, with (x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) solid solution were synthesized using a conventional solid–state reaction with analytical-grade raw powders of BaCO3 (>99.5%), TiO2 (>99.9%), MgO (>99.9%), Nb2O5 (>99.5%), and Bi2O3 (>99.9%). Excess 0.5 mol % Bi2O3 was added to compensate for Bi-loss during processing and Li2CO3 was used as a fluxing agent to reduce the sintering temperature.57−59 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 xB2/3MN-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.

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 using an Agilent E4980A impedance AC analyzer from room temperature (RT) to 800 °C at 25 °C intervals on heating. Impedance data (Z″ and M″) were normalized by a geometric factor (thickness/surface area) and fitted using ZView software (Scribner Associates, Inc., Southern Pines, NC), as reported previously.60−62

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 B2/3MN 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. Full-pattern 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).

Figure 1.

Figure 1

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(a) X-ray powder diffraction patterns in a 2θ range from 15 to 70° with representative {111}p and {200}p XRD reflections for xB2/3MN-BT. (b) Bipolar P–E loops and (c) temperature-dependent εr (Solid line) and tan δ (Dashed line) data for xB2/3MN-BT ceramics with x = 0.00–0.10. (d) TCC value for xB2/3MN-BT ceramics with x = 0.00–0.10 in a temperature range from −55 to 125 °C.

Bipolar P–E loops obtained at 100 kV cm–1 for xB2/3MN-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 Pmax ∼ 20 μC cm–2 and Pr ∼ 12 μC cm–2. Both Pmax and Pr reduce gradually but ΔP increases with x, indicating a FE to RFE transition. The temperature-dependent relative permittivity (εr, solid line) and dielectric loss (tan δ, dashed line) at 100 kHz for xB2/3MN-BT ceramics are shown in Figure 1c. The sharp anomalies for BT at ∼135 °C (εr ∼ 7000), 22 °C (εr ∼ 1750), and −70 °C (εr ∼ 950) correspond to the cubic–tetragonal, tetragonal–orthorhombic, and orthorhombic–rhombohedral phase transitions, respectively.63 As x increases, the maximum εr decreases continuously reaching 1000 for x = 0.10, which shows a rather temperature independent εr. Temperature-independent permittivity were reported for conventional BMN-BT solid solution by Reaney and co-workers with temperature coefficient of capacitance (TCC) < ±15%.64 Here, TCC values for xB2/3MN-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 xB2/3MN-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 (Tc) but a frequency dispersion is observed in εr – T curve for x > 0.02, indicating relaxor behavior.

3.2. Microstructure

SEM images of thermal-etched surfaces for xB2/3MN-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 tweed-like domains surrounded by a largely featureless pseudocubic shell (Figure 2b).

Figure 2.

Figure 2

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(a) Thermal-etched SEM surface image for 0.06B2/3MN-BT ceramic. (b) Bright-field TEM image of grains in 0.06B2/3MN-BT ceramics. (c) High-resolution TEM images with ⟨110⟩ diffraction patterns. (d) BSE surface micrographs of Ag–Pd co-fired 0.06B2/3MN-BT ceramics. (e) EDX mapping distribution of Ag, Pd, Ba, and Ti elements.

Most research into BT-based MLCCs with superior Wrec utilize Pt as inner electrodes; however, the use of such expensive noble metal precludes their commercial exploitation in mass production applications.17−19,24,65 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(O2) required for co-firing with Ni internal electrodes.66,67 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 Bi2Mo2O9 which reacts with Ag electrodes to form Ag-molybdate compounds.68 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 0.06B2/3MN-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 0.06B2/3MN-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 Emax. The low ΔP and Emax (<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 xB2/3MN-BT (0.02 ≤ x ≤ 0.08) at Emax are shown in Figure S5a–d (Supporting Information), with corresponding Emax and ΔP displayed in Figure S6a (Supporting Information). The highest Emax ∼ 520 kV cm–1 and ΔP ∼ 25 μC cm–2 are both obtained for ceramics with x = 0.06, as shown Figure 3a. The Wrec and η for compositions with 0.02 ≤ x ≤ 0.08 are calculated and displayed in Figure S5e–h (Supporting Information). The highest Wrec ∼ 4.55 J cm–3 and η ∼ 90% are obtained for 0.06B2/3MN-BT at Emax ∼ 520 kV cm–1 (Figures 3b and S6b, Supporting Information), exhibiting the highest Wrec values compared to all reported BT-based lead-free ceramics to date (Figure 3c,d). Other systems have recently shown higher Wrec 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 BF36 compounds.

Figure 3.

Figure 3

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(a) Unipolar P–E loops under Emax and (b) calculated Wrec and η at different electric field for 0.06B2/3MN-BT ceramics. (c) Comparisons of electric field vs Wrec and (d) Wrec vs η among lead-free dielectric ceramics, including BT-based,24,25,51 NBT-based,26,27,29 KNN-based,38,41,43,44 AN-based,13 and BF-based.30,31,36,52 *The author only used data points from the references to compare with our reported data and there is no any graphics that are reproduced from any other resource.

3.4. Electrical Microstructure

Complex impedance plane plots and spectroscopic plots of Z″ and M″ data for xB2/3MN-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.60−62 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 xB2/3MN-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, xB2/3MN-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 xB2/3MN-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 xB2/3MN-BT is at least one order of magnitude larger, which explains the enhancement of Emax.51

Figure 4.

Figure 4

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Z* plots of xB2/3MN-BT ceramics with (a) x = 0.00 at 400 °C and (b) x = 0.06 at 550 °C. Spectroscopic plots of Z″ and M″ for x = 0.06 at (c) 550 and (d) 350 °C.

Figure 5.

Figure 5

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(a) Spectroscopic plots of M″ for xB2/3MN-BT ceramics (0.02 ≤ x ≤ 0.06) at 350 °C. (b) Arrhenius plot of conductivity of different components in xB2/3MN-BT (0.00 ≤ x ≤ 0.06).

The conductivity of different components in xB2/3MN-BT are summarized in an Arrhenius plot, as shown in Figure 5b. The conductivity of the core, σb,core, of all three samples (x = 0.02, 0.04, and 0.06) are similar. However, with increasing x, the conductivity of the shell, σb,shell, and grain boundary, σgb, decreases by 2 and 1 order of magnitude, respectively. The activation energy, Ea, of both core and shell remains relatively unchanged at ∼ 0.51–0.62 eV for σb,core and 1.32–1.36 eV for σb,shell. σb and σgb of BT is lower than the σb,core of xB2/3MN-BT ceramics but higher than the σb,shell with an Ea of 0.98 and 1.35 eV, respectively.

For BT (x = 0.00), σgb is lower than σb, especially around RT because of the higher activation energy of σgb compared to σb. 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 Emax in xB2/3MN-BT (especially for x = 0.06) is attributed to the following three facts: (i) the total conductivity, σtotal, decreases with increasing x doping level. The σtotal 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 (σb,shell in x = 0.06) and grain boundary response is higher in x = 0.00 than x = 0.06. The difference in Ea is 0.32 and 0.22 eV compositions with x = 0.00 and x = 0.06, respectively, which also means that the difference in σb,shell and σgb at RT is significantly smaller in 0.06B2/3MN-BT than BT. Despite the existence of some core–shell grains, the shell region constitutes ∼80% of the volume fraction of 0.06B2/3MN-BT and cannot be bypassed by the current. Thus, the voltage is more evenly distributed throughout the sample in 0.06B2/3MN-BT compare to BT which leads to a high Emax. (iii) the much smaller grain size of 0.06B2/3MN-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 xB2/3MN-BT is attributed to the greater diffusion rates of dopants through cubo-octahedral interstices in comparison with BMN-BT.

4. Conclusions

In summary, A-site deficient xBi2/3(Mg1/3Nb2/3)O3-BT (xB2/3MN-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 xB2/3MN-BT ceramics with increasing x. A record high Emax ∼ 520 kV cm–1 and Wrec ∼ 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 Emax for 0.06B2/3MN-BT ceramics was because of a reduction in the total electrical conductivity, with greater electrical homogeneity between different electrical components and an overall smaller volume fraction of cores. Compared to BMN-BT, the presence of VA in xB2/3MN-BT not only encouraged electrical homogeneity but also reduces the concentration of Bi on the A-site ensuring greater compatibility with Ag–Pd electrodes.

Acknowledgments

This work was supported by the Engineering and Physical Sciences Research Council (EP/L017563/1 and EP/N010493/1), Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P02470X/1 and EP/P025285/1 and National Natural Science Foundation of China (51602060 and 51402005). The authors are grateful for support provided by Functional Materials and Devices group from The University of Sheffield.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c13057.

  • Full-pattern refinement of xB2/3MN-BT ceramics with refined crystallography details; P–E loop for BT ceramics; frequency-dependent dielectric properties for xB2/3MN-BT (x = 0.00≤ x ≤ 0.10) ceramics; SEM image of thermal-etched surfaces for xB2/3MN-BT (x = 0.00≤ x ≤ 0.10) ceramics with average grain size information; and unipolar P–E loops under Emax and (e–h) calculated energy storage properties (Wrec, η, Emax and ΔP) at different electric fields for xB2/3MN-BT ceramics (PDF)

Author Contributions

⧫ H.Y., Z.L., L.L., and W.B. contributed equally to this work.

The authors declare no competing financial interest.

This paper was published on the Web on September 21, 2020. Reference 47 was removed from the paper, and the subsequent references renumbered. The corrected version was reposted on September 21, 2020.

Supplementary Material

am0c13057_si_001.pdf (658.3KB, pdf)

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