Enhanced Energy-Storage Density and High Efficiency of Lead-Free

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Enhanced energy storage density and high efficiency of lead-free CaTiO3-BiScO3 linear dielectric ceramics Bingcheng Luo, Xiaohui Wang, Enke Tian, Hongzhou Song, Hongxian Wang, and Longtu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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ACS Applied Materials & Interfaces

Enhanced energy storage density and high efficiency of lead-free CaTiO3-BiScO3 linear dielectric ceramics Bingcheng Luo1, Xiaohui Wang1*, Enke Tian2, Hongzhou Song3, Hongxian Wang1, Longtu Li1* 1

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China 2

3

School of Science, China University of Geosciences, Beijing 100083, P. R. China

Institute of Applied Physics and Computational Mathematics, Beijing 100094, PR China

ABSTRACT: A novel lead-free (1-x) CaTiO3-xBiScO3 linear dielectric ceramic with enhanced

energy storage density was fabricated. With the composition of BiScO3 increasing, the dielectric constant of (1-x)CaTiO3-xBiScO3 ceramics firstly increased and then decreased after the composition x>0.1, while the dielectric loss decreased firstly and increased. For the composition x=0.1, the polarization was increased into 12.36 µC/cm2, 4.6 times higher than that of the pure CaTiO3. The energy density of 0.9 CaTiO3-0.1 BiScO3 ceramic was 1.55 J/cm3 with the energy storage efficiency of 90.4 % at the breakdown strength of 270 kV/cm, and the power density was 1.79 MW/cm3. Comparison with other lead-free dielectric ceramics confirmed the superiority potential of CaTiO3-BiScO3 ceramics for the design of ceramics capacitors for energy storage applications. First-principles calculations revealed that Sc subsitution of Ti-site induced the atomic displacement of Ti ions in the whole crystal lattice, and lattice expansion was caused by variation of the bond angles and lenghths. Strong hybridization between O2p and Ti3d was

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observed in both valence band and conduction band, the hybridization between O2p and Sc3d at high conduction band was found to enlarge the band gap, and the static dielectric tensors were increased, which was the essential for the enhancement of polarization and dielectric properties. KEYWORDS: Lead-free ceramics; energy storage; first-principles calculations; ferroelectric; dielectric.

1. INTRODUCTION Dielectric materials with high power and energy storage density have been investigated due to their potential application in power electronics, weapons and electric vehicles. Comparing to the Li-ions battery and electrochemistry capacitors, dielectric materials have been exhibiting advantages in higher working voltage, better thermal stability and faster charging-discharging. However, relatively lower energy-storage density of dielectric materials than battery is still one problem demanding prompt solution. The stored energy density in dielectric materials can be calculated by integrating the electric field with respect to the ferroelectric displacement, that is U= ∫EdD, where U is the energy density, E is the electric field intensity and D is the electric displacement. To maximize the energy density, the dielectric breakdown strength and dielectric polarization should be optimized. The common dielectric materials used to store energy is ferroelectric ceramics, polymers and their nanocomposites. Usually, the energy density of pristine polymer is limited by low dielectric permittivity, which is usually 2. For instance, the commercially high-pulse discharge polymer capacitors, biaxially oriented polypropylene (BOPP), showed low energy density of 2-3 J/cm3 due to low dielectric constant (~2) although the dielectric

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breakdown strength is higher than 600 MV/m.1 Similarly, many other polymer including polystyrene, polyethylene, polyvinyl chloride, and polycarbonate show high dielectric breakdown strength (higher than 400 MV/m) and low dielectric permittivity (2~5 at 1000 Hz).2 Incorporating inorganic nanoparticles with high dielectric constant have been widely studied in order to obtain high energy density. Dielectric capacitors of phosphonic acid modified Barium titanate/ polycarbonate and P(VDF-HFP) nanocomposites exhibit energy densities of 3.9 J/cm3 and 6.1 J/cm3 at the dielectric strength of 2100 kV/cm.3 Barium titanate-copolymer/polystyrene composites showed energy density value of 9.7 J/cm3 due to the formation of diblock copolymer shells on BT particles, which acted as insulating layers and afforded good dispersibility in nonpolar medium.4 By enhancing the interfacial polarization, higher energy density of 20 J/cm3 was achieved in poly(vinylidene fluoride) nanocomposites containing BaTiO3@TiO2 nanofibers.5 Combining metallic nanoparticles into polymer matrix can also enhance the energy density of polymer nanocomposites. Michael et al6 fabricated metallic aluminum-polypropylene composites with recoverable energy storage density of 14.4 J/cm3, and results showed these composites obeyed the percolation law for two-phase composites due to the high contrast in the complex permittivities and conductivities between the metallic aluminum nanoparticles and the polymeric polypropylene matrix. In addition, uniaxially stretching in blend films could significantly improve the dielectric breakdown strength, energy density, and mechanical properties due to conversion of the nonpolar to polar phases of ferroelectric polymers, which increased the remnant polarization and polarization hysteresis.7It was demonstrated energy density over 20 J/cm3 was achieved in uniaxially stretched PVDF based polymer by means of a zone drawing process to five times their original length.8–10 Unfortunately, the uniaxially stretching devices require large polymer dimensions for production purposes and very expensive for industrial application.11 Moreover, polymer dielectric and its composites suffer relatively low working temperatures and are unable comply with requirement for electric industries under extreme conditions.12 Li and co-

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workers

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fabricated crosslinked polymer nanocomposites with high thermal stability by

incorporating boron nitride nanosheets, and obtained a discharged energy density of 1.8 J/cm3 at 250 ℃. Comparing to polymer dielectrics, ceramic energy storage capacitors have been shown potential for industrial production due to high energy storage efficiency and better temperature stability, such as temperatures that range from -100 °C to 500 °C. However, the key issue is the lower energy density of ferroelectric ceramics caused by the low dielectric breakdown strength. In order improve the energy storage density of lead-free ceramics, a lot of researches have been reported recently.14–17 Relaxor ferroelectric ceramics BaTiO3–Bi(Mg2/3Nb1/3)O3 (BT-BMN) showed an energy density of about 1.13 J/cm3 at the dielectric breakdown strength of 143.5 kV/cm.18 Zhang et al showed energy density of Ba0.4Sr0.6TiO3 ceramics with 5 vol% glass addition was enhanced to 0.89 J/cm3 comparing with energy density of 0.37 J/cm3 of pure Ba0.4Sr0.6TiO3 ceramics.19 Antiferroelectric (PbLa)(ZrTi)O3 ceramics demonstrated an energy density of 1.85 J/cm3 at the electric field of 65 kV/cm, while the lead-free antiferroelectric ceramics 0.89 Bi0.5Na0.5TiO3–0.06 BaTiO3–0.05 K0.5Na0.5NbO3 showed an energy density of 0.59 J/cm3 under 56 kV/cm in the stable antiferroelectric phase region.20,21 In order to produce higher energy density of dielectric ceramics, multilayer ceramic capacitor (MLCC) technologies have been showing potential for manufacturing products. Energy density of about 6.1 J/cm3 at a field of 730 kV/cm was achieved in weakly coupled relaxor behavior 0.7 BaTiO3–0.3 BiScO3 ceramics via tape casting methods. A C0G type linear dielectric 0.8 CaZrO3–0.2 CaTiO3 ceramic fabricated through multilayer ceramic capacitor technologies yields energy densities of up to 5 J/cm3, breakdown strengths of 650–1750 kV/cm,22 while higher energy density of 9.5 J/cm3 was achieved in Mn-Doped 0.8CaTiO3–0.2CaHfO3 ceramics up to 200 °C.23 Thus, developing novel dielectric ceramics is urgent for future MLCC technologies. In this work, lead-free linear dielectric CaTiO3-BiScO3 ceramics with high energy storage efficiency were studied experimentally and theoretically for the first time. CaTiO3-BiScO3 ceramics were fabricated using

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conventional solid-state ceramic processing methods. Dielectric, ferroelectric, and energy storage properties of CaTiO3-BiScO3 ceramics were characterized and discussed. First-principle calculations were adopted to investigate the electronic and structural properties, together with the dielectric and phonon properties.

2. EXPERIMENTAL SECTION 2.1 Preparation of lead-free CaTiO3-BiScO3 dielectric ceramics CaTiO3-BiScO3 ceramics were fabricated using conventional solid-state reaction methods. Analytical reagent CaCO3, TiO2, Sc2O3 and Bi2O3 were used as starting materials, which were purchased from Sinoparm Chemical Reagent Company (Beijing, China). The raw materials were weighed according to the stoichiometric ratio and ball-milled in deionized water using zirconia balls for 24 h. After drying, all powders with different competition were calcined at 950 ℃ for 3 h to obtain the target perovskite phase. Then, the calcined powders were granulated with 5 wt% PVA binder and pressed into disks with the size of 10 mm in diameter and around 0.4 mm in thickness under a pressure of 2 MPa from the dry pressing machine. All disks were sintered at 1250 ℃ for 3 h in air after burning out PVA binder at 600 °C for 2 h.

2.2 Characterization X-ray diffraction (XRD) patterns of all ceramic samples were collected at room temperature on a Bruker D8 Advance A25 X-ray diffractometer using Cu Ka radiation of wavelength λ = 0.154056 nm. X-ray scans were performed over a wide range of 2θ (10 ° ≤ 2θ ≤ 90 °). Morphology analysis was performed with a field emission scanning electron microscope (FESEM, Merlin, ZEISS, Germany). To characterize the dielectric properties, silver pastes were coated on both two sides of samples and the disks were sintered at 550 °C for 30 min. The frequency dependence of dielectric constant and loss tangent of the nanocomposites was measured by

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employing a precision impedance analyzer (Agilent HP4194A, Agilent Technologies, Santa Clara, California, USA) at room temperature at a frequency from 40Hz to 10 MHz.

2.3 First-principles calculations Superlattice structure of CaTiO3-BiScO3 was modeled with 40 atoms based on 1x1x2 CaTiO3 supercell, which has PNMA space group with the lattice parameter a = 5.44 Å, b = 7.639 Å and c = 5.38 Å for one formula unit per unit cell.24 All calculations presented in this study were performed within first-principles calculations based on the density functional theory (DFT) framework25,26. The pseudopotentials used for CaTiO3-BiScO3 superlattice models were constructed by the electron configurations as Ca 3s23p64s2 states, Sc 3s2sp63d14s2 states, Ti 3s2sp63d24s2 states, Bi 5d106s26p3 and O 2s22p4 states. The generalized gradient approximation (GGA) with the PBEsol exchange-correlation functional for solid implemented in Vienna ab initio simulation program (VASP) were used for the initial CaTiO3-BiScO3 superlattices.25,27–29 After careful converge tests for the total energy calculations, cut-off energy and Monkhorst-Pack mesh grid were chosen to be 600 eV and 8×8×8 special k-point, respectively.30 The Born effective charges and dielectric tensors were calculated by using density functional perturbation theory.31,32 The energy tolerance was 1×10−8 eV/atom, while the force tolerance was set to 0.001 eV/Å.

3. RESULTS AND DISCUSSION 3.1 Microstructure and morphological analysis Figure 1 showed the X-ray diffraction patterns of lead-free CaTiO3-BiScO3 dielectric ceramics measured at room temperature. The patterns exhibited the ceramics are crystallized into perovskite orthorhombic phase and no obvious traces of secondary phase can be detected in composition range between x=0 and x=0.1. The diffraction peaks were indexed using standard powder diffraction PDF files with the number JCPDS#22-0153. The XRD patterns around 47.5° 6


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were magnified in order to gather information about the lattice distortion. It can be observed that the diffraction peaks at 2θ from 47.3 °to 47.8 ° primarily shifted toward lower degree with the composition increase of BiScO3, which reveals the expansion of the lattice cell. Changes in lattice parameters and volume with increasing BiScO3 content in CaTiO3-BiScO3 ceramics were shown in Figure 1(c). It can be seen that the volume increases with the increase of BiScO3 contents. Table 1 listed the measured physical density of (1-x)CaTiO3-xBiScO3 ceramics with different composition. The density of the pure CaTiO3 ceramics was 3.92g/cm3, while the theoretical density of CaTiO3 is 4.0398g/cm3.24 This resulted a relative density of ~97%. After the addition of BiScO3, the density was increased. Furthermore, the effective ionic radii of A-site Ca2+ and Bi3+ ions were reported as 1.34 Å and 1.38 Å, while the B-site ionic radii of Sc3+ and Ti4+ ions were reported as 0.745 Å and 0.605 Å, respectively.33 It can be concluded that the lattice expansion was caused by the substitution of a large Sc ion in the B-site as shown by following substitution:

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which agrees with the data in BaTiO3-BiScO3 ceramic and BiScO3-Ba(Mg1/3Nb2/3)O3.34,35 Effects of the substitution on the crystal structure were investigated by the following first-principles calculations.

Figure 1 (a) XRD patterns of (1-x)CaTiO3-xBiScO3 ceramics with different composition. (b) enlarged XRD profile from (c) Changes in lattice parameters and volume with increasing BiScO3 content in CaTiO3BiScO3 ceramics Table 1 Physical density of 1-x)CaTiO3-xBiScO3 ceramics with different BiScO3 composition.

Density, Samples

x g/cm3

CTBS-1

0

3.92

CTBS-2

0.05

4.19

CTBS-3

0.1

4.28

CTBS-4

0.2

4.24

CTBS-5

0.3

4.42

Figure 2 presented the SEM images of CaTiO3-BiScO3 ceramics sintered at 1250 ℃ with different BiScO3 content. It can be observed that the doping elements causes significant changes in the microstructure. For the pure CaTiO3 ceramics as shown in Figure 2(a), a less dense

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microstructure was formed with small holes. The average grain size of pure CaTiO3 ceramics was determined to be 0.62 µm as shown in Figure 2(f), but the composition with x=0.05 showed an increased grain size of 0.7 µm grain size. The cross section of ceramics showed the dense microstructure as shown in (g-i), which also showed a densifier structure mporphology The grain sizes for the composition x=0.1 is 0.55 µm as seen in the supporting information. With the BiScO3 content increasing, the grain sizes for the composition with x=0.2 and x=0.3 were determined to be 0.55 µm and 0.63 µm, respectively. The difference of grain sizes is attributed to the ionic diffusion and particle migration during the sintering process.

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Figure 2 SEM images of (1-x)CaTiO3-xBiScO3 ceramics with different BiScO3 composition: (a) x=0; (b) x=0.05; (c) x= 0.1; (d) x=0.2; (e) x=0.3. (f) grain size distribution counted from the SEM images. Cross view of CaTiO3-BiScO3 ceramics with different BiScO3 composition: (g) x=0.05; (h) x=0.2; (i) x=0.3.

3.2 Dielectric properties of lead-free CaTiO3-BiScO3 dielectric ceramics Figure 3 showed the frequency dependence of the dielectric constant and dielectric loss tangent of (1-x) CaTiO3-x BiScO3 ceramics with different composition measured in the frequency range of 40−10 MHz and at room temperature. The dielectric constant almost remained constant with the increase of frequency from 100 Hz to 10 MHz. It could be observed that dielectric constant of (1x) CaTiO3-x BiScO3 ceramics firstly increased, and then decreased with the concentration increase of BiScO3. For instance, the dielectric constant of 0.8 CaTiO3-0.2 BiScO3 ceramics was 172, which was 1.25 times higher than the dielectric constant of CaTiO3 ceramics. For the samples with composition x = 0.3, the dielectric constant was 90. The increase of the dielectric constant was considered to be the distortion of the oxygen octahedron and the ion relaxation polarization. The ion polarizabilities for Bi3+ and Ca2+ were 6.04 Å3 and 3.17 Å3, respectively.36 Moreover, Bi2O3 was reported to increase the dielectric permittivity in the Ag(Nb0.8Ta0.2)O3 ceramics.37 Higher content (usually >20mol%) of BiScO3 has been reported to decrease the dielectric permittivity of BaTiO3-BiScO3-PbTiO3 ceramics as well as the polarization.38 And low addition of BiScO3 could increase the ferroelectric transition temperature, while higher content could result in the obvious decrease of transition temperature. Meanwhile, the dielectric loss tangent of all ceramic samples are lower than 0.008 in the frequency range from 1000 Hz to 10 MHz. With the content of BiScO3 increasing, the dielectric loss firstly decreased and then increased slightly. The dielectric loss for samples with composition x = 0.1 is almost 0.003, while the loss tangent of composition x = 0.3 is 0.007. The increase of dielectric loss with more BiScO3 contents should be attributed to the microstructural factors such as the porosity, holes and even secondary phase. Undesired secondary phase was formed for ceramics samples with the 0.2 and

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0.3 BiScO3 as shown in Figure 1(a) and microstructure with many holes was observed in Figure 2(d). Thus, the dielectric loss was increased with more BiScO3 contents, which is different with cases of BaTiO3-based ceramics because the intrinsic dielelctric loss of BaTiO3 was much higher than that of CaTiO3 ceramics.39,40

Figure 3 (a) Frequency dependence of dielectric constant and dielectric loss of CaTiO3-BiScO3 dielectric ceramic with different composition. (b) Enlarged details of frequency dependence of dielectric loss of CaTiO3-BiScO3 ceramics with different composition.

3.3 Electrical polarization response of lead-free CaTiO3-BiScO3 dielectric ceramics Figure 4 illustrated the typical polarization-electric field loops of CaTiO3-BiScO3 ceramics with different composition measured under different external electric fields at fixed frequency of 1 Hz. It could be found that the shape of the polarization-electric field loops was different the characteristic for normal ferroelectric and relaxor ferroelectric.41 All the CaTiO3-BiScO3 ceramic samples exhibited the “ferroelectric-type hysteresis” with linear response, which was ascribed from the space charge located near the defect sites such as grain boundary or electrode/bulk interface.42 Thus, in this work, we describe the CaTiO3-BiScO3 ceramic samples as linear dielectrics where the polarization increases with the increase of external electric field owing to

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the alignment of dipoles. The hystersis loops shows a growing trend of maximum polarization and coercive field for samples with 0~0.1BiScO3 composition, indicating the enhanced the polarization. In addition, the enhancement of the dielectric breakdown strength for the copmposition x=0.1 and x=0.2 might be attributed to the ceramic grain refining. It could be seen from the SEM images that the averaged grain size for the copmposition x=0.1 and x=0.2 were 0.55μm. From Figure 4, it can be observed that both spontaneous polarization and dielectric breakdown strength of CaTiO3-BiScO3 ceramics were notably higher than that of pristine CaTiO3 ceramics. For instance, the spontaneous polarization for samples with composition x = 0.05 is 7.60 µC/cm2 at external electric field of 223 kV/cm, while the polarization of pristine CaTiO3 ceramics is 2.68µC/cm2 at the external electric fied of 190 kV/cm. The maximum polarization of ceramic with composition x = 0.1 is 12.36 µC/cm2 at external electric field of 270 kV/cm, the highest of among all samples. When BiScO3 composition x ≥ 0.2, hystersis loops become much wider, and the maximum polarization started to decrease shown in Figure 4.

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Figure 4 P-E loops of (1-x)CaTiO3-xBiScO3 ceramics with different BiScO3 composition: (a)x=0;(b)x=0.05; (c)x=0.1; (d)x=0.2; (e)x=0.3.

3.4 Energy storage performances of lead-free CaTiO3-BiScO3 dielectric ceramics High polarization, high dielectric breakdown strength and low dielectric losses in the materials contributed to a high energy-storage density. According to the definition of recoverable energy storage density, U=∫EdP, the energy density U could be calculated by integrating P–E curves at

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backward switching. Discharged energy density and energy storage efficiency of CaTiO3-BiScO3 ceramics with different dielectric breakdown strength for different BiScO3 composition were shown in Figure 5(b). As was expected, the discharged energy density was enhanced after BiScO3 was added to CaTiO3 ceramics. The maximum energy storage density of pure CaTiO3 was 0.25 J/cm3 at the dielectric breakdown of 193 kV/cm, while the maximum energy storage density of CaTiO3-BiScO3 ceramics with x=0.1 contents was 1.55 J/cm3 at the dielectric breakdown of 270 kV/cm. From Figure 5(b), it can be clearly seen that all ceramics sample have a energy storage efficency over 80%. The energy storage efficency of (1-x) CaTiO3-x BiScO3 ceramics with x = 0.1 was 90.4%. Enhancement in energy storage density can be ascribed from the reduction in dielectric loss, increase in dielectric constant, and improvement of electric breakdown strength. Furthermore, interfacial polarization is considered to be a universal phenomenon in all heterogeneous dielectric materials.43,44 The differential polarization generated from difference between CaTiO3 and BiScO3 contribute to the energy storage density. In addition to energy storage density, power density is another crucial criterion in the evaluation of the performance of energy storage devices. According to the theory of Ragone plots,45 the maximum powder density of linear dielectric materials can be calculated based the charged energy density Uc via the equivalent series resistance (ESR) and capacitance (C) by following:

P=

Uc 2 ⋅ ESR ⋅ C

(1)

where ESR is considered as the product of capacitive reactance and dielctric loss:

ESR = tan δ × X c =

tan δ 2π fC

where tanδ is the dielectirc loss, Xc is the capacitive reactance, f is the measuremnet frequency. Thus, maximum power density can be calculated by following45,46

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(2)

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P=

π fU c tan δ

(3)

The calculated maximum power density for (1-x) CaTiO3-x BiScO3 ceramics with x = 0.1 were 1.79 MW/cm3 at frequency of 1000 Hz, eleven times higher than the power density of 0.157 MW/cm3 of pure CaTiO3.

Figure 5 (a) Charged energy density of CaTiO3-BiScO3 linear dielectric ceramics as a function of dielectric breakdown strength. (b) Discharged energy density and energy storage efficiency of CaTiO3-BiScO3 ceramics with different dielectric breakdown strength for different BiScO3 composition. (c) Comparison of energy storage density vs. dielectric breakdown strength of different lead-free ceramics. (d) Comparison of energy storage density vs. energy storage efficiency of different lead-free ceramics. Star symbols mean (1-x) CaTiO3-xBiScO3 ceramics in present work: 1a, x = 0.1; 1b, x= 0.05; 1c, x = 0.2; 1d, x = 0.3.

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Comparison of dielectric breakdown strength, energy storage density and energy storage efficiency of different lead-free ceramics were displayed in Figure 5 (c,d). The specific values and meanings of symbol number could be found in the Supporting information. From Figure 5, it could be found that most lead-free dielectric ceramics exhibited energy storage density lower than 1.5 J/cm3 and lower dielectric breakdown strength than 200 kV/cm. The 0.91BaTiO3-0.09BiYbO3 ferroelectric ceramics (shown in Figure 5 (c,d) No.2) showed an energy storage density of 0.71 J/cm3 at the dielectric breakdown strength of 93 kV/cm,

47

while the Ba0.4Sr0.6TiO3-based

ceramics (shown in Figure 5 (c,d) No.6) exhibited an energy storage density of 0.71 J/cm3 at the dielectric breakdown strength of 93 kV/cm. 48 The lead-free dielectric ceramics with high energy storage density usually suffered the lower energy storage efficiency than 90%, on the contrary, the dielectric ceramics with high energy storage efficiency usually suffered lower energy storage density than 1.2 J/cm3. The energy density obtained in 0.8(K0.5Na0.5)NbO3-0.2Sr(Sc0.5Nb0.5)O3 ceramics (shown in Figure 5 (c,d) No.13) were 2.02 J/cm3 at the dielectric breakdown strength of 295

kV/cm,

and

the

energy

storage

efficiency

was

81.4%.49

The

0.85BaTiO3-

0.15Bi(Mg2/3Nb1/3)O3 ferroelectric ceramics (shown in Figure 5 (c,d) No.7) exhibited a high energy storage efficiency of 92% at the dielectric breakdown strength of 143.5 kV/cm, but the energy storage density was only 1.13 J/cm3.18 The relaxor ferroelectric ceramics 0.85BaTiO30.15Bi(Zn2/3Nb1/3)O3 (shown in Figure 5 (c,d) No.4) displayed an even higher energy storage efficiency of 93.5%, at the dielectric breakdown strength of 131 kV/cm, but the energy storage density was only 0.79 J/cm3.50 Comparatively, the lead-free dielectric ceramics 0.9CaTiO30.1BiScO3 (shown in Figure 5 (c,d) No.1a) fabricated in present work exhibited a higher energy density of 1.55 J/cm3 and very high efficiency higher than 90 % under an electric field of 270 kV/cm, which would be showing much more potential in the design of dielectric ceramic capacitors for the energy storage applications. However, it should be noted that the energy storage density of 0.9CaTiO3-0.1BiScO3 lead-free dielectric ceramics was measured in the ceramic bulks with a thickness around 0.4 mm. It has been reported that the thinner ceramic sample usually

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displayed a higher dielectric breakdown strength because the thickness has a significant effect on the dielectric breakdown field of ceramics.51–53 Therefore, an higher energy storage density can be obtained in lead-free CaTiO3-BiScO3 dielectric ceramics by reducing the thickness of the ceramic samples. In addition, energy storage density might be increased even more if combing the industrial multi-layer ceramic capacitor technology, thus it will be arousing considerable interest or attention for the dielectric capacitors applied in energy storage fields.

3.5 First-principles calculations In order to better interpret the inner mechanism of the interaction at the atomic level, firstprinciples calculations based on density functional theory were presented to investigate the structural and electronic properties of the CaTiO3-BiScO3 materials. The superlattice structures of CaTiO3-BiScO3 before and after relaxation were shown in Figure 6. CaTiO3 has a well-known perovskite structure with PNMA symmetry and its lattice parameter is a = 5.44 Å, b = 7.63 Å and c = 5.38 Å for one formula unit per unit cell.24 The geometrized lattice parameters were listed in Table 2. The error within 2% confirmed the validation and reliability of our methods. For the convenience of computation, only one composition of (1-x) CaTiO3-x BiScO3 with x = 1/8 is considered here to investigate the structural, electronic properties. It is notable that the effects of composition and even order-disorder structure on the electric and dielectric properties still need to be investigated in the future. After full relaxation, lattice parameter of CaTiO3-BiScO3 is a = 5.570 Å, b = 7.740 Å and c = 10.918 Å. The volume of CaTiO3-BiScO3 is 470.68 Å3, larger than the volume 447.14 Å3 of CaTiO3 1x1x2 supercell, which support the XRD results shown in Figure 1. Obvious atomic displacement can be observed in the light green circle region shown in Figure 6(e). It was found that the tilting angle of oxygen octahedron for pure CaTiO3-BiScO3 is 151.1°, smaller than that of 156.8° for pure CaTiO3-BiScO3, confirming the effects of BiScO3 on the octahedron distortaion. Details of bond length and bond angle were summarized in Table 3. It can be found that the increased lattice or volume was casued by the increase of Sc-O bond length

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Page 18 of 32

compared with Ti-O bond length.After relaxation, the bond length of Ti-O5 and Ti-O3 was increased while bond length of Ti-O2 and Ti-O6 is decreased, revealing the movement of Ti ions along [001] direction. It can be concluded that the Sc subsitution induced the atomic displacement of Ti ions, which was essential for the incrased polarization. The relaxed structure of CaTiO3BiScO3 were provided in the supporting information.

Figure 6 Fully relaxed superlattice structure of CaTiO3 (a) and CaTiO3-BiScO3 (b); [001] view of relaxed superlattice CaTiO3-BiScO3 before relaxation (c) and after full relaxation (d); (e) the atomic displacement after relaxation is shown in the light green circle; detail scheme of (f) tilting angle and (g) bonding in oxygen octahedron. Table 2 Lattice parameters of CaTiO3 and CaTiO3-BiScO3 before and after relaxation. CaTiO3

CaTiO3-BiScO3

Before relaxation

After relaxation

Error

Before relaxation

After relaxation

Error

a, Å

5.44

5.50

1.14%

5.44

5.57

2.39%

b, Å

7.63

7.68

0.69%

7.64

7.74

1.32%

c, Å

5.38

5.41

0.48%

10.76

10.92

1.47%

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V, Å3

223.57

228.49

447.14

470.68

Table 3 Atomic bond lengths and bond angles in oxygen octahedron of CaTiO3-BiScO3 before and after relaxation, together with the percentage difference. Before relaxation

After relaxation

Percentage difference

Sc-O1

1.950

2.075

6.4%

Sc-O2

1.957

2.096

7.1%

Sc-O3

1.957

2.118

8.2%

Sc-O4

1.950

2.077

6.5%

Sc-O5

1.957

2.115

8.1%

Sc-O6

1.957

2.080

6.3%

Ti-O1

1.950

1.915

-1.8%

Ti-O2

1.957

1.917

-2.0%

Ti-O3

1.957

2.136

9.1%

Ti-O4

1.950

1.937

-0.7%

Ti-O5

1.957

2.167

10.8%

Ti-O6

1.957

1.894

-3.2%

O1-Sc-O2

90.39

92.57

2.4%

O1-Sc-O3

90.73

90.11

-0.7%

O2-Sc-O3

90.59

90.91

0.4%

O1-Ti-O2

90.39

91.61

1.3%

O1-Ti-O3

90.73

86.63

-4.5%

O2-Ti-O3

90.59

90.30

-0.3%

Bond length (Å)

Bond angle(°)

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Figure 7 exhibited the difference charge density of lead-free CaTiO3-BiScO3 dielectric crystal lattice. It could be observed that the electron was transfer from the Ti and Sc atom to O atoms. Hybridization between Ti and O and hybridization between Sc and O were also clearly seen in the difference charge density. The Born effective charges and static dielectric tensors are calculated through a linear response method using density functional perturbation theory(DFPT). The Born effective charges control the amplitude of the long-range Coulombic interactions between nuclei and the splitting between longitudinal optic (LO) and transverse optic (TO) phonon modes.54–56 The Born effective charge tensors were defined as the derivative of the polarization with respect to atomic displacement at zero macroscopic field, or as the derivative of the force on an atom with respect to the macroscopic field at zero atomic displacement. Generally, these dynamical effective charges are larger in magnitude than the nominal charges, especially for Ti and for O in the direction of the Ti-O bonds.57 DFPT results of CaTiO3-BiScO3 The Born effective charges of Ti

ions

shown

in

the

square

regions

in

Figure

7

were

ZTi* , xx = 6.78e, ZTi* , yy = 6.78e, ZTi* ,zz = 6.78e , and the Born effective charges of the corresponding O ion in the direction of the Ti-O bonds were Z O* , xx = −3.09e, Z O* , yy = −3.16e,

ZO*,zz = −2.57e . The Born effective charges of Sc ions shown in the square regions in Figure 7 *

*

*

were Z Sc , xx = 4.93e, Z Sc , yy = 5.05e, Z Sc,zz = 5.24e , and the Born effective charges of the corresponding O ion in the direction of the Sc-O bonds were Z O* , xx = −2.59e, Z O* , yy = −3.24e,

ZO*,zz = −2.80e . These anomalously large effective charges appear to be a general feature of ABO3 perovskites and essential for the large polarization and phonon instability.58 The calculated static

dielectric

tensor

of

CaTiO3-BiScO3

crystal

lattice

was

κ xx = 142.72, κ xx = 95.47, κ xx = 71.08 ,which was larger than the static dielectric tensor of

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CaTiO3 crystal lattice, κ xx = 6.92, κ xx = 37.78, κ xx = 6.85 , revealing the enhanced polarization. Details of Born effective charge tensors for each ions and the static dielectric tensor of CaTiO3BiScO3 could be found in the supporting information. Furthermore, the band structure and the projected density of states (PDOS) were calculated by projecting the electron wave functions onto spherical harmonics centered on each type of atom,59 as shown in Figure 8. Fermi level was referenced at zero energy, indicated by the dotted line. For the CaTiO3 as shown in Figure 8(a), the maximum of the valence band and the minimum of the conduction band are located at the same G point, revealing the direct band gap characteristics with the value of about 2.37 eV. It is clear that the band gap can be underestimated due to the choice of the exchange correlation functional, but the shape is essentially correct. The band gap of CaTiO3-BiScO3 is 2.59 eV, larger than that of the pure CaTiO3. From Figure 8(b), new band originated from the hybridization between O-p orbital and Bi-s orbital can be observed near Fermi level. Comparing with pure CaTiO3, the maximum of the valence band of CaTiO3-BiScO3 was moved down and the minimum of the conduction band was moved up, resulting in the increased band gap. The introduction of Bi atom plays key roles in tuning the upward valence band. From Figure 8(e), it is clear to see that the upper valence bands are dominantly consisted of O-2p states, together with smaller contributions from Ti 3d and Bi 6s states, revealing that valence electrons are transferred from Ti and Bi sites to O sites. Unlike Ca atom, partially covalent characteristic was formed due to the interaction between O 2p and Bi 6p orbital as seen in high conduction band located at 4~6eV. Strong hybridization between Ti 3d states and O 2p states contribute to the first conduction band located at 0~4eV, while hybridizations between Sc 3d states and O 2p states and Bi 6p and O 2p states contribute to high conduction band located at 4~6eV. Thus, the interactions of O 2p-to-Ti 3d, O 2p-to-Sc 3d and O 2p -to- Bi 6p states would be microscopic origin of the enhancement in polarization and dielectric properties at the electronic level.

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Figure 7 Difference charge density of lead-free CaTiO3-BiScO3 dielectric crystal lattice. The charge density is plotted through a (010) from 0 (Orange) to 1 (Red) e Å−3, with the corresponding density isosurfaces shown as the sphere-like shade. Oxygen atoms gain electrons, while Ti and Sc atoms lose electrons.

Figure 8 Band structure of CaTiO3 (a) and CaTiO3-BiScO3 (b), together with the band gaps (c); Density of states of CaTiO3 (d) and CaTiO3-BiScO3 (e). The Fermi level is located at zero energy, indicated by the dotted line.

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4. CONCLUSIONS In summary, (1-x) CaTiO3-xBiScO3 ceramics with enhanced polarization and energy storage density were fabricated. The introduction of BiScO3 into the CaTiO3 crystal lattice resulted in the lattice expansion. With the composition of BiScO3 increasing, the dielectric constant firstly increased and then decreased after the composition is higher 0.1, while the dielectric loss decreased firstly and increased. For the composition x = 0.1, the polarization was increased to 12.36 µC/cm2 from 2.68 µC/cm2 of pure CaTiO3. The reduction in dielectric loss, increase in dielectric constant, and improvement of electric breakdown strength given rise to the enhancement in energy storage density of CaTiO3-BiScO3. The energy density of 0.9 CaTiO3-0.1 BiScO3 was 1.55 J/cm3 with the energy storage efficiency of 90.4 % at the breakdown strength of 270 kV/cm, and the power density was 1.79 MW/cm3. First-principles calculations revealed that Sc subsitution of Ti-site induced the atomic displacement of Ti ions in the whole crystal lattice, and lattice expansion was confirmed by the bond angles and lenghths. New band originated from hybridization between O 2p and Bi 6p was observed near Fermi level. Strong hybridization between O 2p and Ti 3d was observed in both valence band and conduction band, and the hybridization between O 2p and Sc 3d at high conduction band enlarged the band gap, which was the essential for the enhancement of polarization and dielectric properties. Future optimization of the grain, defects, interfaces between dielectric and electrode, and combining the industrial multilayer ceramic capacitor technology will further lead to higher energy storage density.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXXX. Grain size distribution, relaxed geometry structure, Comparison of energy storage performance, Static dielectric tensor, Born effective charge tensors.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Bingcheng Luo: 0000-0003-0918-6326 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work was supported by Ministry of Sciences and Technology of China through National Basic Research Program of China (973 Program 2015CB654604), National Natural Science Foundation of China for Creative Research Groups (Grant No.51221291), National Natural Science Foundation of China (Grant No. 51272123), and also supported by CBMI Construction Co., Ltd.

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Enhanced Energy-Storage Density and High Efficiency of Lead-Free

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