Improvement of energy storage density with trace amounts of ZrO2 additives fabricated by wet-chemical method

Article abstract of DOI:10.1016/j.jallcom.2018.03.058

Lead-free (1-x)[0.6SrTiO₃-0.4Na_0.5Bi_0.5TiO₃]-xZrO₂ ceramics (STNBT-xZr) were synthesized in single perovskite phase. The average grain size decreased from 3.2 μm (x = 0.1 mol%) to 1.6 μm (x = 0.6 mol%) related to the addition of ZrO₂ powders prepared by microwave hydrothermal method. With the increasing of ZrO₂ content, the permittivity decreased gradually, and the breakdown strength (E_b) was remarkably improved due to composition induced disturbing of long range ferroelectric order. The increase of E_b led to the improvement of the recoverable energy storage density (W_re) from 1.80 J/cm³ (x = 0.1 mol%) to 2.84 J/cm³ (x = 0.5 mol%). However, when Zr⁴⁺ content was more than 0.5 mol%, the W_re decreased from 2.84 J/cm³ to 2.26 J/cm³ (x = 0.6 mol%) due to the reduction of the maximum polarization. The best energy storage properties were achieved in STNBT-xZr ceramic with Zr⁴⁺ content of x = 0.5 mol%, which exhibited the W_re of 2.77–2.84 J/cm³ in the range of 4–40 Hz, revealing an excellent frequency stability. All these results demonstrate the STNBT-xZr (x = 0.5 mol%) ceramics are quite promising for frequency-stable energy storage applications.

Full text of DOI:10.1016/j.jallcom.2018.03.058

Journal of Alloys and Compounds 747 (2018) 495e504
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Improvement of energy storage density with trace amounts of ZrO2
additives fabricated by wet-chemical method
Chenwei Cui, Yongping Pu*
School of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Lead-free (1-x)[0.6SrTiO3-0.4Na0.5Bi0.5TiO3]-xZrO2 ceramics (STNBT-xZr) were synthesized in single
Received 30 November 2017
Received in revised form
2 March 2018
Accepted 5 March 2018
Available online 7 March 2018
perovskite phase. The average grain size decreased from 3.2
m
m (x ¼ 0.1 mol%) to 1.6 m (x ¼ 0.6 mol%)
m
related to the addition of ZrO2 powders prepared by microwave hydrothermal method. With the
increasing of ZrO2 content, the permittivity decreased gradually, and the breakdown strength (Eb) was
remarkably improved due to composition induced disturbing of long range ferroelectric order. The in-
crease of Eb led to the improvement of the recoverable energy storage density (Wre) from 1.80 J/cm3
(x ¼ 0.1 mol%) to 2.84 J/cm3 (x ¼ 0.5 mol%). However, when Zr4þ content was more than 0.5 mol%, the Wre
decreased from 2.84 J/cm3 to 2.26 J/cm3 (x ¼ 0.6 mol%) due to the reduction of the maximum polarization.
The best energy storage properties were achieved in STNBT-xZr ceramic with Zr4þ content of x ¼ 0.5 mol
%, which exhibited the Wre of 2.77e2.84 J/cm3 in the range of 4e40 Hz, revealing an excellent frequency
stability. All these results demonstrate the STNBT-xZr (x ¼ 0.5 mol%) ceramics are quite promising for
frequency-stable energy storage applications.
Keywords:
Ceramics
Breakdown strength
Recoverable energy density
Non-polar phase
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
where P is the polarization, Pm is the maximum polarization with
respect to the maximum applied electric field, Pr is the remnant
With the increasing requirement in pulsed power technology
and power electronics [1,2], instantaneous charging can be ach-
ieved in energy storage capacitors within an extremely short time
span, developing high-performance energy storage materials is
becoming more and more imperative and important [3,4].
Although dielectric capacitors have fast charging/discharging rate
and long lifetime, compared with fuel cells, Li-ion batteries and
supercapacitors, the energy density of dielectrics is much lower
[5e7]. Thus, as the main part of capacitors, enormous types of di-
electrics have been developed and studied. In general, the recov-
erable energy storage density (Wre) can be estimated by integrating
the area between the polarization axis and the discharge curve of
the P-E hysteresis loops using equation [8]:
polarization, E is the applied electric field which leads to a response
in polarization dP. The required energy storage properties usually
meet as follows: large Pm, small Pr, and high electric breakdown
field strength (BDS) [9].
(1-x)Na0.5Bi0.5TiO3-xSrTiO3 (NBT-ST) ceramics are a solid solu-
tion system composed of ferroelectric sodium bismuth titanate and
non-ferroelectric strontium stannate. Both of them are of perov-
skite structures with an ABO3 formula. It was also determined that
(1-x)Na0.5Bi0.5TiO3-xSrTiO3 solid solutions can from a morphotropic
phase boundary (MPB) between the rhombohedral and pseudo-
cubic phases at x ¼ 0.25e0.26 https://www.sciencedirect.com/
science/article/pii/S0955221917306593 [10]. Gomah-Pettry et al.
[11] studied the structural and dielectric properties of NBT-ST
systems and found the trigonal symmetry changed to cubic
almost like a first order phase transition at x > 30. Most of the
research on the system is focused on the piezoelectric properties
[12,13]. Li et al. [14] reported that strains up to 0.2% could be ach-
ieved under a low driving fields (E < 2 kV/mm) in the 0.74NBT-
0.26ST composition resulting in excellent actuating performance of
Smax/Emax>1000 pm/V. A. R. Malathi et al. [15] studied the (1-x)NBT-
xST (x ¼ 0e1) system and found that the Pr, Pr/Pm, BDS and piezo-
Pm
Z
Wre
¼
EdP
(1)
Pr
* Corresponding author.
E-mail address: 396726140@qq.com (Y. Pu).
electric coefficient (d33) of 0.6SrTiO3-0.4Na0.5Bi0.5TiO3 sample
https://doi.org/10.1016/j.jallcom.2018.03.058
0925-8388/© 2018 Elsevier B.V. All rights reserved.

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C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
reached 0.52
m
C/cm2, 0.047
m
C/cm2, 32.807 kV/cm and 21 pC/N,
2.2. Synthesis of ZrO2 powders
respectively. In addition, Cao reported that the electrocaloric effect
(ECE) in (1-x)NBT-xST (x ¼ 0.10, 0.25, 0.26, 0.30) ceramics and found
that the maximum value of the recoverable energy density was
0.65 J/cm3 obtained at 65 kV/cm in 0.7NBT-0.3ST ceramic [16].
However, recent research on the energy storage performance of the
NBT-ST system is still scarce and their low BDS badly limits their
application in energy storage devices. In order to improve the en-
ergy storage density, various attempts have been made. Adding
metallic oxide in the matrix has drawn increasing interest because
metallic oxide can significantly enhance the BDS. For example, the
highest BDS of 21.2 kV/mm, the highest Wre (0.45 J/cm3) were ob-
tained in the glass ceramic with 2 mol% Sm2O3 [17]. Wang et al. [18]
have successfully prepared ST/ZrO composites with an enhanced
energy storage density of 1.62 J/cm3. In addition to the adjustment
of components, energy storage density can be further enhanced
through the synthesis of core-shell ceramics [19], layered ceramics
[20], film [21], nanofibers materials [22], etc. Furthermore, some
powder synthesis methods such as wet chemical method [23] and
sintering processes including liquid phase sintering [24] and spark
plasma sintering (SPS) technique [25] have been carried out.
However, these methods cannot be widely applied in actual in-
dustrial production because of their expensive cost, complex syn-
thesis processes and less production compared with conventional
solid-state method. Therefore, finding a balance between these
methods and conventional solid-state method is crucial. Therefore,
it is highly possible that higher energy storage density could be
obtained by doping and synthesis methods. Thus, understanding
how doping and synthesis methods change the structure and affect
the energy storage properties are still critical problems to solve.
In our former work, we found the highest energy density of
1.70 J/cm3 at ~21 kV/mm in 0.6SrTiO3-0.4Na0.5Bi0.5TiO3 ceramic
[26]. To further improve the energy storage density, in this work,
we reported energy storage properties of (1-x)[0.6SrTiO3-
0.4Na0.5Bi0.5TiO3]-xZrO2 (x ¼ 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol
%, 0.5 mol%, 0.6 mol%) ceramics. The 0.6SrTiO3-0.4Na0.5Bi0.5TiO3
and ZrO2 powders were synthesized by conventional solid-state
method (CSS) and microwave hydrothermal method (MH),
respectively. The effects of trace ZrO2 content on structure and
energy storage properties of (1-x)[0.6SrTiO3-0.4Na0.5Bi0.5TiO3]-
xZrO2 ceramics were investigated. As a result, our findings not only
expand the spectrum of room-temperature lead-free energy stor-
age materials for future applications but also may serve as a guide
for revealing other energy storage material alternatives by the
combination of CSS and wet-chemical method.
ZrO2 powders were synthesized by microwave hydrothermal
method (Fig. 1). The raw materials ZrOCl2 were dissolved in
distilled water completely. ZrOCl2 with a Zr4þ concentration of
0.05 mol/L was prepared as ZrOCl2$8H2O (purity 99%) aqueous
solution. After that, 2 mol/L NaOH was added into ZrOCl2$8H2O
prepared solution to adjust the solution pH to 13. Then, the white
precursor generated and the process was accompanied by a large
amount of heat release. After stirring for 20 min and ultrasonic
dispersing for 30 min, the reaction mixture was sealed in a sealed
vessel and heated to 220 ꢀC for 30 min in a microwave hydrother-
mal reactor. The precipitate were filtered, washed and dried, then
we obtained ZrO2 powder. The reaction equation is as follows:
ZrOCl2$8H2O þ H2Oþ2OHꢁ/Zr(OH)4Yþ2Cl-
Zr(OH)4/ZrO2þH2O
2.3. Preparation of ceramic samples
STNBT and ZrO2 powders were mixed according to the stoi-
chiometric formula and ball milled in distilled water for 24 h. After
dried, the powders were pressed into pellets of 10 mm in diameter
and 1 mm in thickness by cold isostatic pressing under a pressure of
200 Mpa. The pellets were sintered by three stage heating at
1330 ꢀC for 3 h. Among them, the three stage heating refers to the
sintering process in three stages: heating, holding and cooling, is a
common sintering method of ceramics.
2. Experimental procedure
STNBT and ZrO2 powders were synthesized by conventional
solid-state method and microwave hydrothermal method, respec-
tively. Then, STNBT-xZr (x ¼ 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%,
0.5 mol%, 0.6 mol%) ceramic samples were prepared using these
two powder precursors.
2.1. Synthesis of 0.6SrTiO3-0.4Na0.5Bi0.5TiO3 powders
Powders of analytical reagent grade, comprising Bi2O3 (99.0%),
SrCO3 (99.0%), TiO2 (99.0%), Na2CO3 (99.0%), BaCO3 (99.0%), were
used as the starting materials and mixed according to the compo-
sition, 0.6SrTiO3-0.4Na0.5Bi0.5TiO3. After ball-milled in alcohol for
24 h, the slurry was dried, then calcined in a closed environment at
1150 ꢀC for 2 h. The calcined powder was ball-milled and dried
again to obtain homogeneous powder.
Fig. 1. The flow chart of microwave-hydrothermal synthesis for ZrO2 powder.

C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
497
2.4. Characterization techniques
The phase composition of the samples was determined by X-ray
powder diffraction (Rigaku Corporation, D/max2200 PC, CuK ). The
a
morphology of the samples was studied by scanning electron mi-
croscopy (Rigaku Corporation, S4800). The samples used for SEM
measurement were treated through the processes of polishing and
thermal etching. To measure the electrical properties, the silver
paste was coated on both sides of the sintered pellets and fired at
600 ꢀC for 20 min to form electrodes. Raman spectroscopy
(Renishaw-invia, Renishaw, U. K.) was used to further confirm the
phase evolution of these ceramics in the region of
100e1000 cmꢁ1 at room temperature. The room temperature X-ray
photoelectron spectroscopy (XPS) studied by (S/N: 10001, Prevac,
Poland) spectra were taken with AlK
a
(h
n
¼ 51,486.6 eV) radiation
and a hemispherical energy analyzer. The temperature-dependent
dielectric properties were measured using an LCR meter (3532-
50, Hioki, Ueda, Japan) and an Agilent 4294 A impedance analyzer
(ꢁ180 ꢀCe300 ꢀC). The polarization-electric field (P-E) hysteresis
loops were measured using a ferroelectric tester (TF Analyzer,
2000) under room temperature (Radiant Technologies, Premier II).
Fig. 3. Raman spectra of STNBT-xZr ceramics. The fitting curves of x ¼ 0.1 mol% sample
are shown with the dot curves.
the band centered at 253 cmꢁ1 is dominated by B-O vibration. The
modes at 558 cmꢁ1 are associated with the vibration of BO6-octa-
hedra. The broad bands at 804 cmꢁ1 are likely to be the A(LO) and
E(LO) overlapping bands [28e32]. Moreover, all peaks in Zr4þ
doped STNBT structures spectra are fully matched with that of
undoped [26] structures.
3. Results and discussion
Fig. 2(aec) shows the XRD patterns of the ceramics with
different ZrO2 contents, and (d) the lattice parameters and cell
volume of the ceramics with different ZrO2 contents. All composi-
tions studied exhibited a single phase pseudo-cubic perovskite
structure. It suggests that Zr4þ cation has diffused into the STNBT
lattices to form a homogenous perovskite solid solution. Fig. 2(b)
and (c) shows a magnified (111) and (200) peak. With increasing
ZrO2 contents, all diffraction peaks shift toward lower angles. The
lattice parameters and cell volume increases with increasing x, as
shown in Fig. 2(d). It is known that the Zr4þ and Ti4þ ionic radii are
0.072 and 0.0605 nm, respectively [27]. Zr4þ ion can be alternative
to Ti4þ ion, which leads to the variation of the diffraction peaks and
the increasing lattice parameters.
Fig. 3 shows the Raman spectra of the STNBT-xZr ceramics
recorded at room temperature in the wavenumber range of
100e1000 cmꢁ1. It can be seen that the modes of the STNBT-xZr
ceramics are mainly four bands centered at 100-200 cmꢁ1, 200-
400 cmꢁ1, 450-700 cmꢁ1, and 750-900 cmꢁ1 [28,29]. In order to
describe the changes of Raman spectra in detail, we have fitted the
Raman spectra of x ¼ 0.1 mol% sample with the Gaussian-line
shape. Among them, the mode at 150 cmꢁ1 is assigned to A-sym-
metry, which is associated with Na, Bi and Sr cations vibration and
High resolution XPS spectra in the electron binding energy re-
gions corresponding to Ti 2p and Zr 3 d core excitations provide
additional information about the structure and electronic proper-
ties in selected STNBT-xZr samples, as shown in Fig. 4. The XPS
results indicate that the binding energies of the Ti 2p doublet with
2p3/2 and 2p1/2 are 458.07 eV and 464.06 eV, respectively, which are
in good agreement with the literature values [33,34], as shown in
Fig. 4(a). The separation of the Ti 2p3/2 and Ti 2p1/2 peaks (the spin-
orbit splitting) is 5.99 eV, indicating that titanium mainly exists as
Ti4þ. The Zr 3 d core level spectra is shown in Fig. 4(b), doublet
peaks were observed related to Zr 3d5/2 and Zr 3d3/2, aligned at
181.30 eV and 184.16 eV, respectively. The doublet peaks were de-
tached by 2.86 eV, the degree of spin-orbit splitting constant of Zr
3 d, and our results agree well with the split seen for ZrO2 [35,36].
The peak binding energy indicates the sub-oxidation states (1þ, 2þ,
3þ) of zirconium which is very difficult to extract from the metal-
oxide peak [37]. There is no obvious change in the binding energies
of Ti with the increase of Zr4þ doping content. As a consequence of
the structural changes caused by Zr4þ inside the STNBT lattice,
Fig. 2. X-ray diffraction patterns of STNBT-xZr samples at room temperature (a) Full range, (b) the magnified view of (111) peak, (c) the magnified view of (200) peak, (d) the
variation in lattice parameter and cell volume.

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C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
Fig. 4. The x-ray photoelectron (XPS) spectra of (a) Ti 2p, (b) Zr 3 d, from the x ¼ 0.1 mol% and x ¼ 0.5 mol% sample.
which can stabilize the charge of Ti and suppress the oxygen
dissociation when sintered at high temperature [38].
indicates that the temperature for the maximum permittivity (Tm)
peak is broadened. The Tm values at 10 kHz move obviously lower,
with the values for the specimens with x ¼ 0.1 mol%, 0.2 mol%,
0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol% being 9.98 ꢀC, 6.54 ꢀC,
4.88 ꢀC, ꢁ5.12 ꢀC, ꢁ10.07 ꢀC and ꢁ13.26 ꢀC, respectively, as shown
in Fig. 7(b). The Tm value is 15 ꢀC for the STNBT ceramic [26]. As
reported in literature [40], Zr4þ ions can diffuse into BaTiO3 lattice
and slightly influence its Curie temperature, which is confirmed
clearly in our experiments. Moreover, the maximum permittivity
(εm) decreased from 4079 at x ¼ 0.1 mol% to 3400 at x ¼ 0.6 mol%. It
can be seen that the addition of ZrO2 obviously leads to a decrease
in permittivity of STNBT-xZr ceramics, a similar phenomenon also
appeared in CaCu3Ti4O12 (CCTO) ceramics doped with 0.5 wt% of
ZrO2 [41]. ZrO2 doping has been shown to both reduce the
permittivity, which is due to that ZrO2 has a lower permittivity and
depresses the permittivity of the ceramics [42].
In order to visualize the evolution of the microstructures, we
acquired polished surfaces images of STNBT-xZr ceramics in Fig. 5.
Few cracks and pores can be observed in all samples and well
dense microstructure is obtained. The average grain size of the
samples, calculated by Image Pro Plus software was found to
decrease with increasing x. The average grain size of the samples
are approximately 3.2, 2.5, 2.0, 1.7, 1.4 and 1.6
m
m for x ¼ 0.1 mol%,
0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol%, respectively.
Moreover, the reason behind the non-homogeneous grain
morphology may be attributed to three stage heating as well as
high calcination and sintering temperature in solid state synthesis
method which results in abnormal grain growth [39]. Interest-
ingly, the relatively smaller grains efficiently reduce the porosity
of the STNBT-xZr ceramics and improve its BDS. As expected, we
obtain fine-grained STNBT-xZr ceramics with ZrO2 contents,
especially for x ¼ 0.5 mol% sample. The corresponding EDS
elemental map (in Fig. 6) confirms that Zr4þ cation has diffused
into the STNBT lattices, consistent with the results obtained by the
structural analysis. In the case of a small amount of ZrO2 additives,
each element is evenly distributed.
For a standard ferroelectric material, the permittivity above
Curie temperature (Tc) follows the Curie-Weiss law described by
the following relation:
C
ε ¼
(2)
T ꢁ TCW
The temperature dependence of permittivity of the STNBT-xZr
ceramics at 10 kHz from ꢁ180 ꢀC to 300 ꢀC is shown in Fig. 7(a).
With increasing ZrO2 content, the dielectric characterization
where C refers to the Curie-Weiss constant and TCW to the Curie-
Weiss temperature. The parameter △Tm, which is often used to
Fig. 5. SEM images of the polished and thermally etched surfaces of STNBT-xZr ceramics (a) x ¼ 0.1 mol%, (b) 0.2 mol%, (c) 0.3 mol%, (d) 0.4 mol%, (e) 0.5 mol%, (f) 0.6 mol%.

C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
499
Fig. 6. EDS mapping images on polished and thermally etched surfaces of STNBT- 0.5 mol%Zr ceramics.
show the degree of deviation from the Curie-Weiss law, is defined
as follows:
Fig. 8 for different compositions. The fitting parameters are
summarized in Table 1 as the Tm, TB, TCW and
Tm (the x ¼ 0
sample [26] as the comparison baseline). As can be seen from the
Table 1, the values of Tm are nonzero, this suggests the appear-
ance of a kind of relaxation near Tm upon doping which is
responsible for flattening out the ε-T peaks and the departure from
the classical Curie-Weiss behavior. It is also evident that, as a
consequence of the peak broadening, TCW becomes greater than
Tm for all samples.
D
D
DTm ¼ TB ꢁ Tm
(3)
where TB denotes the temperature from which the permittivity
starts to deviate from the Curie-Weiss law. To demonstrate the
behavior of STNBT-xZr ceramics, the reciprocal of as a function of
temperature at a constant frequency of 10 kHz has been plotted in
Fig. 7. (a) Temperature dependence of permittivity of STNBT-xZr ceramics measured at 10 kHz and (b) Tm and εm versus x value of STNBT-xZr samples.

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C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
Fig. 8. Color online 104/ε vs temperature plot showing the deviation from the classical Curie-Weiss behavior with increasing x. (For interpretation of the references to colour in this
figure legend, the reader is referred to the Web version of this article.)
Fig. 9(a) shows P-E loops of the STNBT-xZr ceramics, which were
measured at 10 Hz and room temperature under an electric field of
20 kV/mm. All samples show slim and pinched P-E loops. Thus the
maximum polarization (Pm), remnant polarization (Pr) and coercive
Fig. 10 shows the P-I-E loops of STNBT-xZr with the application
of 10 Hz at RT until they went through dielectric breakdown. The
dielectric breakdown strength of STNBT-xZr ceramics significantly
increases when addition of ZrO2. The maximum breakdown electric
field is up to 28.50 kV/mm for x ¼ 0.5 mol% sample, this can be
attributed to the reduced grain size with increasing x. When
x ¼ 0.1 mol%, the specimen exhibits both relatively larger Pr
field (Ec) decrease from 28.70
m m m
C/cm2 to 22.22 C/cm2, 5.45 C/cm2
to 3.64
m
C/cm2 and 2.42 to 0.44 kV/mm, respectively, with ZrO2
addition from x ¼ 0.1 mol% to x ¼ 0.6 mol%, which indicates
composition induced disturbing of long range ferroelectric order in
Fig. 9(b).
(5.89
two peaks ( E1) located in the first and third quadrants,
m
C/cm2) and Ec (2.66 kV/mm), accompanied by a I-E loop with
Table 1
The relational Tm, TB, TCW and DTm at 10 kHz values of the Curie-Weiss law for STNBT-xZr ceramics.
Composition
x ¼ 0
x ¼ 0.1%
x ¼ 0.2%
x ¼ 0.3%
x ¼ 0.4%
x ¼ 0.5%
x ¼ 0.6%
Tm (ꢀC)
TB (ꢀC)
15
9.98
6.54
4.88
ꢁ5.12
210.44
86.22
ꢁ10.07
192.29
55.07
ꢁ13.26
164.53
48.90
130
ꢁ20
115
161.54
61.24
151.66
190.86
58.27
184.32
194.10
85.95
189.22
TCW (ꢀC)
D
Tm (ꢀC)
215.56
202.36
177.79

C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
501
Fig. 9. (a) P-E loops and (b) Pr, Pm and Ec of STNBT-xZr ceramics at 20 kV/mm, 10 Hz, RT.
respectively. It can be seen that E1 appears across E ¼ 0. These
Pmax
current peaks around E ¼ 0 can be explained with the fact that the
effect produced during previous loading can be recovered partially
during unloading and partially by reversing the electric field. With
further increasing ZrO2, both Pr and Ec were depressed, accounting
for slim P-E loops, followed by I-E loops with less obvious E1
current peaks. The transformation of I-E loops illustrates disturbed
ferroelectric order and reduced polar phase by ZrO2 content. With
the substitution of Ti with Zr, the paraelectric-like behavior in the
STNBT-xZr ceramics is more dominant because of the decrease of
Tm with increasing x. As expected, the x ¼ 0.6 mol% sample exhibits
Z
Wst
¼
EdP
(4)
0
The energy storage efficiency
materials with large Pm, low Pr, and high breakdown electric field
would be supposed to possess obviously enhanced energy storage
properties. Fig. 11(b) presents the Wre and
h
equals to Wre/Wst. Thus, the
h
of STNBT-xZr ceramics
values of
under the critical electric field (Eb). The Eb, Wre, Wst and
h
STNBT-xZr ceramics are reported in Table 2. When ZrO2 content
increases from x ¼ 0.1 mol% to x ¼ 0.5 mol%, the Wre increases from
both reduced Pr (3.93 m
C/cm2) and Ec (0.60 kV/mm).
2.07 J/cm3 to 2.84 J/cm3 with the
h increases from 59.31% to 71.54%.
Fig. 11(a) is a schematic diagram for the calculation of energy
storage properties. As can be seen from Fig. 11(a), the recoverable
energy storage density (Wre) can be estimated by equation (1)
already mentioned in the previous section, the stored energy
density (Wst) caused by the domain reorientation is obtained by
integrating the area between the polarization axis and the charge
curve [43,44] by the following relation:
This results from the enhancement of the breakdown electric field,
which is directly related to Wre [45]. However, with further
increasing ZrO2 content to x ¼ 0.6 mol%, the Wre slightly decreases
to 2.26 J/cm3 with the
h slightly decreases to 68.07%, due to sharp
decline of the Pm as shown in Fig. 10(f). A comparison of ferro-
electric and energy storage properties among several lead-free
ferroelectric systems was made in Table 3 [8,16,18,27,46e48]. It
Fig. 10. The P-E loops and corresponding (IeE) curves of ST-NBT-1000xZr samples ceramics at different x under critical electric field.

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C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
Fig. 11. (a) Schematic diagram for the calculation of energy storage properties of ceramics, (b) Zr4þ content dependent recoverable energy storage density Wre and energy storage
efficiency
h under critical electric field.
pulsed power energy storage capacitors.
Table 2
The Eb, Wre, Wst and
h values of STNBT-xZr ceramics at 10 Hz, RT.
The frequency stability of energy storage properties for
x ¼ 0.5 mol% ceramic is discussed. Fig. 12(a) reveals P-E loops of
x ¼ 0.5 mol% at 28.5 kV/mm under different frequency. The Pr
slightly reduces and the P-E loop becomes more pinched with
increasing frequency, due to the fact that the domain reversal
cannot follow the electric field at higher frequency [49], which
facilitates energy storage density. Meanwhile, the inset depicts the
reduction of Pm exerting an adverse impact on it. The above two
opposite effects lead to relatively stable Wre in the range of 4e40 Hz
Composition x ¼ 0.1% x ¼ 0.2% x ¼ 0.3% x ¼ 0.4% x ¼ 0.5% x ¼ 0.6%
Eb (kV/mm)
Wre (J/cm3)
Wst (J/cm3)
22.05
1.80
3.03
24.72
2.17
4.04
27.00
2.35
4.18
28.00
2.61
4.23
28.50
2.84
3.97
27.50
2.26
3.32
h
(%)
59.41
53.71
56.22
61.70
71.54
68.07
Table 3
as shown in Fig. 12(b). Wre keeps around 2.77e2.84 J/cm3, and
h
Comparison of energy storage properties between STNBT-xZr (x ¼ 0.5 mol%) ce-
ramics and other lead-free ceramics.
slightly increases with increasing frequence. Futhermore, the op-
timum energy storage properties at 28.5 kV/mm is obtained for the
Composition
Eb (kV/mm)
Wre (J/cm3)
h
(%)
Ref.
x ¼ 0.5 mol% sample with Wre reaching 2.84 J/cm3 and
h reaching
NBTBT-SZ-NN
NBT-BT-BF
NBT-BT-KNN
BF-BT-LMT
Ba0.3Sr0.7TiO3
NBT-ST
11
8
5.6
13
23
6.5
28.9
28.5
0.95
1.27
0.59
1.66
1.13
0.65
1.62
2.84
~66
~79.7
/
~82
~86.8
~73.6
/
[27]
[46]
[47]
[8]
[48]
[16]
[18]
This study
71.54% at 10 Hz.
To further evaluate the feasibility for practical energy storage
applications, P-E loops for the STNBT-xZr (x ¼ 0.5 mol%) ceramics
measured at different electric fields and frequency of 10 Hz are
shown in Fig. 13(a). It is found that Pmax increases constantly with
the growing amplitude of electric field while Pr nearly remains
unchanged, suggesting that the energy storage density can be
further enhanced by improving dielectric breakdown strength of
ST-Zr
STNBT-Zr
~71.54
NBT: Na0.5Bi0.5TiO3; ST: SrTiO3; BT: BaTiO3; KNN: K0.5Na0.5NbO3; BF:BiFeO3.
LMT: La(Mg0.5Ti0.5)O3;SZ: SrZrO3; Ba0.85Ca0.15Ti0.85Zr0.1Sn0.05O3:BCTZS; NN:
NaNbO3.
the ceramics. Fig. 13 (b) presents the Wre and
h of different com-
positions at various electric fields. Thus the Wre increases from
0.03 J/cm3 to 2.84 J/cm3 under the electric field increases from 2 kV/
can be seen that the STNBT-xZr (x ¼ 0.5 mol%) possessed a much
mm to 28.5 kV/mm. Meanwhile, the
as well.
h increase from 50% to 71.54%
higher Wre value together with an acceptable
h value for advanced
Fig. 12. (a) The P-E loops of STNBT-5 mol%Zr ceramic at several frequency at room temperature, (b) Wre and
h values of STNBT-5 mol%Zr ceramics varying with the magnitude of the
applied frequency.

C. Cui, Y. Pu / Journal of Alloys and Compounds 747 (2018) 495e504
503
Fig. 13. (a) P-E loops measured under different electric fields at 10 Hz for STNBT-5 mol%Zr samples, (b) Wre and
the applied electric field.
h values of STNBT-5 mol%Zr samples varying with the magnitude of
anomalies near the MPB region of Na0.5Bi0.5TiO3-SrTiO3 solid solution, RSC
Adv. 5 (2015) 50644e50654.
4. Conclusions
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The structure, dielectric and energy-storage properties of
STNBT-xZr (x ¼ 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%,
0.6 mol%) ceramics were comprehensively investigated. STNBT and
ZrO2 powders were synthesized by solid-state reaction method and
microwave hydrothermal method, respectively. All samples
exhibited pseudo-cubic phase and Zr4þ cation had diffused into the
STNBT lattices. The average grain size of the ceramics showed an
evident effect on grain size reduction with increasing ZrO2. The
€
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highest Wre of 2.84 J/cm3 and
h of 71.54% under a Eb of 28.5 kV/mm
was achieved in STNBT-xZr with x ¼ 0.5 mol% addition. In addition,
the x ¼ 0.5 mol% ceramic exhibited a good frequency stability for
the energy storage application within the measurement frequency
range of 4e40 Hz, make it potential for high power capacitor
applications.
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Acknowledgments
This research was supported by the National Natural Science
Foundation of China (51372144,51641207), the Key Program of
Innovative Research Team of Shaanxi Province (2014KCT-06) and
National Undergraduate Training Programs for Innovation and
Entrepreneurship (201710708009).
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