Investigations on the Energy Storage Performance of Cu Modified BaTiO₃ Ceramics

Abstract

A novel strategy was adopted to enhance the energy storage properties of materials through constructing a vacancy defect. BaTi_1−xCu_xO_3−x (abbreviated as BTCx, x = 0–0.05) ceramics were prepared. The influences of Cu doping on structure and electrical properties were systematically investigated in this study. The result reveals that the oxygen vacancies in BTCx ceramics can inhibit grain growth and improve breakdown strength. Notably, as Cu content increases, the abundance of oxygen vacancies of the BTCx ceramics intensifies the relaxor behavior and induces double hysteresis loops with high energy storage performance. The excellent energy storage density of 1.34 J/cm³ and efficiency of 90.1% were achieved for BTC3 ceramics at 180 kV/cm, which indicates that the outstanding energy storage properties of BTCx ceramics make them have broad application prospects in advanced pulse power capacitors.

1. Introduction

Compared with batteries and electrochemical super-capacitors, ceramic capacitors play a pivotal role in shipboard sewage treatment, electrical signal processing, aerospace, electric vehicles, and national defense because of their outstanding power density, long operational lifetime, excellent thermal stability, as well as ultra-fast charge–discharge rates [1,2,3]. The development of pulse power devices towards miniaturization, light-weighting, and integration has placed higher demands on the energy storage performance of ceramic capacitors [4]. Pb(Zr,Ti)O₃-based antiferroelectric (AFE) ceramics have been widely employed, owing to high energy storage densities (>5.0 J/cm³) and high efficiencies (>90%) [5,6,7]. The inherent toxicity and environmental hazards associated with lead oxide have significantly propelled the development of lead-free materials. Consequently, the development of lead-free dielectric ceramics with both high recoverable energy storage density (W_rec) and high efficiency (ƞ) has emerged as a research priority.

BaTiO₃ (BT)-based ceramics are one of the most promising materials for pulse power capacitors derived from excellent dielectric properties, adjustable structure, and low loss in the lead-free field, once it breaks through the bottlenecks to reduce long-range ordered ferroelectric domains and high residual polarization (P_r) [4,8,9]. The P_r and long-range ordered ferroelectric domains are reported to be effectively suppressed through the construction of microstructure, phase structure regulation, and heterovalent doping [10,11,12]. The defect engineering has been demonstrated as an effective strategy to disrupt long-range ordered ferroelectric domains of ceramics. For instance, Fang combined first-principles calculations with experimental verification, demonstrating that the AFE-like phenomenon and reduction of P_r can be obtained in BaTiO₃ ceramics by B-site doping of Mg²⁺ ions. This is attributed to domain wall pinning by oxygen vacancies created for charge compensation of the acceptor dopant (Mg″_Ti) defects [13]. After that, Luo engineered defect dipoles through substituting Li⁺ into the B-site of BaTiO₃ ceramics, boosting the energy storage density to 1.17 J/cm³. This ascribes that the dipole can provide restoring force and promote the rapid return of polarization to the initial low state after the removal of the external electric field, thereby forming the AFE-like loop. Meanwhile, this strategy effectively reduces P_r and avoids the intense phase transition process in BaTiO₃ ceramics, achieving the energy storage density of 1.17 J/cm³ [14]. Similar studies on heterovalent ion doping have also been reported involving Mn³⁺, Y³⁺, and Ca²⁺ ions [15,16,17]. Among various dopant ions, Cu²⁺ as another type of heterovalent ion can induce stronger local stress fields and form more stable defect dipoles because of the unique d⁹ electronic configuration. The regulation of the relaxation characteristics and energy storage performance of BaTiO₃ ceramics has become more controllable [18]. Although theoretical analyses suggest that Cu²⁺ doping is an ideal route to achieve high energy storage density and efficiency, there is currently a lack of systematic experimental studies on its underlying mechanisms in BaTiO₃. Specifically, the structural origins, microstructural evolution, and correlation mechanisms between Cu²⁺-induced AFE-like behavior and macroscopic energy storage performance remain unclear. Although B-site doping in BaTiO₃ is considered an ideal route to achieve high energy storage density and efficiency, there is a current scarcity of research specifically focusing on Cu²⁺-doped BaTiO₃ ceramics at the B-site. In particular, the intrinsic mechanism of its induced AFE-like behavior, the microstructural evolution, and their correlation with macroscopic energy storage performance have been scarcely reported.

In this work, the Ba(Ti_1−xCu_x)O_3−x (BTCx) ceramics were fabricated through doping of Cu²⁺ into the B-site of BaTiO₃ using the conventional solid-state method. The influence of the B-site doping mechanism on the energy storage properties, dielectric constant, phase structure, and microstructure is systematically investigated. The results demonstrate that Cu²⁺ doping is an effective strategy for achieving high energy storage density and efficiency.

2. Experimental Methods

2.1. Fabrication

The Ba(Ti_1−xCu_x)O_3−x (BTCx, x = 0–0.05) ceramics were synthesized via the conventional solid-state method. Initially, high-purity raw materials, BaCO₃ (99.0%), CuO (99.8%), and TiO₂ (99%), were meticulously weighed according to their precise stoichiometric compositions. All the raw materials are sourced from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). The obtained powders were milled in a ball mill for 10 h using anhydrous ethanol as the dispersion medium. Subsequently, the obtained slurry was dried for 12 h at 80 °C. Then, the dried powder was subjected to a second ball-milling process for 10 h to further ensure uniformity. The homogeneous powder was calcined in an air atmosphere at 1120 °C for 4 h. The calcined powder was mixed with an 8 wt% polyvinyl alcohol (PVA) aqueous solution as a temporary binder for granulation and then shaped into 12 mm diameter disks using uniaxial pressing. To eliminate the organic binder, the pellets undergo a binder burning process by heating at 550 °C for 4 h. Finally, the green disks underwent continuous sintering at a temperature range of 1200 to 1300 °C for 3 h. According to the different doping amounts of Cu, the samples were, respectively, named BTC0 (x = 0), BTC1 (x = 0.01), BTC2 (x = 0.02), BTC3 (x = 0.03), BTC4 (x = 0.04), and BTC5 (x = 0.05).

2.2. Characterization

Crystallographic analysis was performed via X-ray Diffraction (Rigaku D/max-2500/PC, Cu Kα1, λ = 1.5406 Å, Tokyo, Japan). The variation of oxygen vacancies of BTCx ceramics was investigated using Electron Paramagnetic Resonance (EPR) spectroscopy (Guoyi Quantum Technology Hefei Co., Ltd., Hefei, China). The micro-structure morphology was examined by Scanning Electron Microscopy (SEM; HITACHI S-4300, Tokyo, Japan) after the samples underwent thermal etching at 1150 °C for 2 h. The cell volumes were calculated using the Wincell software. The actual density of the samples was ascertained through the Archimedes’ method. The sintered pellets were first polished to create parallel surfaces. Subsequently, silver paste was applied to both sides and fired at 650 °C for 30 min to form the electrodes. The dielectric performance was analyzed by a precision impedance analyzer (4294 Agilent Inc., Bayan Lepas, Malaysia). The hysteresis loops of samples were investigated through a Precision Premier II ferroelectric material test system (Radiant Technologies, Inc., Albuquerque, NM, USA).

3. Results and Discussion

The XRD patterns of BTCx ceramics with varying Cu²⁺ contents are presented in Figure 1a. All compositions exhibit a singular perovskite phase, which indicates the successful diffusion of Cu²⁺ ions into the BaTiO₃ lattice and the formation of a homogeneous solid solution [19]. As seen in the enlarged Figure 1b, the diffraction peaks shift toward lower angles with the increase in x. This shift is ascribed to the substitution of smaller Ti⁴⁺ ions (0.605 Å) by larger Cu²⁺ ions (0.73 Å), resulting in an expansion of the unit cell volume. Moreover, the (200)/(002) peaks gradually merge into single peaks with the doping increase, indicating that the samples transform from tetragonal (BTC1) to cubic phase (BTC5). This structural evolution is driven by the generation of oxygen vacancies through Cu ions doping the B-site of BaTiO₃ ceramics. These vacancies exert a strong electrostatic pinning effect on Ti⁴⁺ in the surrounding TiO₆ octahedron, suppressing the spontaneous off-center displacement of Ti⁴⁺ and favoring the higher-symmetry cubic phase [20]. To further investigate the phase structure characteristics of the materials, Rietveld refinement was performed on a series of samples using FullProf Studio software (the GSAS ∏ propram). As shown in Figure 2, the goodness-of-fit parameter (χ²) for all samples is less than eight, indicating the reliability of the refinement results. Quantitative phase analysis at different doping concentrations reveals that the sample with BTC1 is dominated by the tetragonal perovskite phase. The samples with 0 ≤ x ≤ 0.03 depict the coexistence of a tetragonal (P4mm) phase and cubic phase (Pm3m). The content of the cubic (Pm3m) and tetragonal (P4mm) phases were 63.84% and 36.16% with BTC3 ceramics, respectively. At x = 0.05, the material exhibited a cubic phase.

Table 1. Rietveld refined structural parameters along with lattice parameter and cell volume for BTCx ceramics.

Sample	Lattice Parameters (Å)			Volume (Å3)	T Phase (%)	C Phase (%)	R Factors (%)
x	a	b	c				Rwp	Rp	χ2
BTC0	3.9985	3.9985	4.0039	64.34	91.62	8.38	7.67	5.45	6.53
BTC1	3.9991	3.9991	4.0064	64.42	74.54	25.46	6.36	6.21	5.16
BTC2	4.0014	4.0014	4.0085	64.51	60.48	39.52	5.73	5.77	7.12
BTC3	4.0101	4.0101	4.0101	64.59	36.16	63.84	4.93	6.67	5.23
BTC4	4.0124	4.0124	4.0124	64.65	24.45	75.55	7.21	6.95	7.12
BTC5	4.0136	4.0136	4.0136	64.69	13.57	86.43	6.17	7.16	6.21

lists the refined lattice parameters, agreement factors, and unit cell volumes of the BTCx ceramics at various contents. With an increase in x, the difference between the lattice parameters a and c becomes more pronounced, further confirming the cubic symmetry of the samples. Meanwhile, the unit cell volume shows a gradual increasing trend.

In order to explore and determine the site occupancy and local coordination environment of Cu²⁺ ions in the BaTiO₃ lattice, Figure 3 presents the EPR spectra of BTCx (x = 0.01–0.04) ceramics. All samples show typical EPR signal, indicating that Cu ions exist in the form of unpaired electrons. As increasing x, the EPR spectra exhibit fine splitting, revealing superimposed signals of two different defect clusters (DC1 and DC2). Generally, the negative charge introduced by ${\mathrm{Cu}}_{\mathrm{Ti}}^{″}$ requires compensation by oxygen vacancies (Vö) to maintain charge neutrality. The two defect clusters are ${{(\mathrm{Cu}}_{\mathrm{Ti}}^{″}{\text{-}V}_{ö})}^{″}$ (DC1) and ${{(V}_{\text{ö}}{\text{-}\mathrm{Cu}}_{\mathrm{Ti}}^{″}{\text{-}V}_{\text{ö}})}^{·}$ (DC2), respectively. The similar intensities of the DC1 and DC2 signals are characteristic of defects formed by Cu²⁺ substitution at the B-site. This is consistent with the analysis in Figure 1. Moreover, the increasing intensity of the DC2 signal with increasing x confirms a higher concentration of defect dipoles. This defective dipole can interact with ferroelectric domains under the action of an electric field. The opposite internal electric field will be formed after removing the external electric field, resulting in an extended relaxation time. This phenomenon contributes to the enhanced relaxation characteristics and the constriction of the hysteresis loop [21].

Figure 4 delineates SEM images of BTCx ceramics with different doping amounts. The average grain sizes of each component are 3.47 μm, 2.94 μm, 2.23 μm, 1.38 μm, 1.01 μm, and 0.87 μm, respectively. The average grain size of the sample gradually decreases with the increase in x, which indicates that Cu²⁺ ion doping has a significant inhibitory effect on the grain growth of BaTiO₃ ceramics. According to the description of the lattice strain energy (ΔG_strain) of BTCx ceramics [22],

$$\Delta {\mathrm{G}}_{\mathrm{strain}}=4\mathbf{\pi }{\mathrm{MN}}_{\mathrm{A}}\left[{\displaystyle \frac{{\mathrm{r}}_0}{2{\left({\mathrm{r}}_{\mathrm{d}}{-\mathrm{r}}_0\right)}^2}}+{\displaystyle \frac{1}{3}}{\left({\mathrm{r}}_{\mathrm{d}}{-\mathrm{r}}_0\right)}^3\right]$$

(1)

where r₀, r_d, N_A, and M represent the optimal radius of the lattice site, the ionic radius of the dopant, Avogadro’s constant, and Young’s modulus, respectively. The substitution of smaller Ti⁴⁺ (0.605 Å) ions with larger Cu²⁺ (0.73 Å) ions increases the lattice strain energy (ΔG_strain), which impedes grain boundary migration, and thus restricts grain growth. The average grain size and relative density of BTCx ceramics are illustrated in Figure 5. The relative density of the samples increases from 91.7% (BTC0) to a maximum value of 97.6% (BTC3). High densification reduces surface defects and mitigates charge concentration, which is beneficial for achieving a higher breakdown strength. As x increases further, the relative density decreases to 92.8% (BTC5), which is associated with domain switching and domain wall motion.

The temperature dependence of the dielectric constant for BTCx ceramics at various frequencies is presented in Figure 6. All samples show a single dielectric peak, and the peak value decreases slightly with increasing frequency. This is attributed to the fact that the rotation direction of the dipole in the matrix cannot be consistent with the direction of the electric field at high frequencies, resulting in frequency dispersion. With the increase in Cu content, the dielectric peak gradually broadens, indicating an enhanced relaxation behavior. This is attributed to the local lattice distortion caused by doping, which will form a random electric field and thereby generate polar nano-regions (PNRs) [16]. At the same frequency, the dielectric peak decreases progressively as the component addition goes up, which is associated with the change in the phase structure of ceramics. According to the Maxwell–Wagner (MW) mechanism of grain boundaries [23,24], when Cu²⁺ ions move from the central symmetry position of the crystal along the c-axis into the Ti site of the tesquare phase, they will generate permanent electric dipoles, thereby resulting in a higher dielectric constant. However, in the cubic phase structure, the Cu²⁺ ion coordinates with six oxygen atoms to form an octahedron with extremely high symmetry and reduces its dielectric constant. Furthermore, the dielectric loss of the samples is below 0.1, which indicates that higher breakdown strength can be achieved. The Curie temperature (Tc) of the samples slightly decreases with increasing x. This is attributed to oxygen vacancies created by the B-site substitution of Cu²⁺. These vacancies act as pinning centers for local dipoles, whose polarization fails to relax upon removal of the electric field, thereby disrupting the long-range ferroelectric order. As a result, the transition from the ferroelectric to the paraelectric state occurs at a lower thermal energy, resulting in the reduction in Tc. The dielectric relaxation behavior of BTCx ceramics above Curie temperature can be evaluated by the Curie–Weiss law [25]:

$${\epsilon }_r=\frac{C}{\left(T-T_0\right)} (T>T_0)$$

(2)

where ${\mathbf{\epsilon }}_{\mathrm{r}}$ is the dielectric constant at temperature T, C is the Curie constant, and T₀ is the Curie temperature. Inverse dielectric constant plots as function of temperature at 1 kHz are shown in Figure 7. ΔT_m usually represents the degree of deviation from the Curie–Weiss law, which is defined as follows:

$$\Delta T_m=T_{C-W}-T_m$$

(3)

where T_m and T_C−W represent the temperature corresponding to the maximum permittivity and the temperature at which the permittivity begins to deviate from the Curie–Weiss law, respectively. The deviation of Curie–Weiss behavior gradually increases with the increase in x, indicating that the ergodic-state relaxation of the sample exists in a wide temperature range, and also revealing the existence of active polar nano-regions in the sample. This can also prove that the sample has strong relaxation behavior. To further substantiate the relaxor behavior of BTCx ceramics above Tc, the relationship between the dielectric constant and temperature of BTCx ceramics at 1 MHz was fitted using the modified Curie–Weiss law: [17]:

$$\frac{1}{\mathbf{\epsilon }}-\frac{1}{{\mathbf{\epsilon }}_{\mathrm{m}}}=\frac{{\left(\mathrm{T}-{\mathrm{T}}_{\mathrm{m}}\right)}^{\mathbf{\gamma }}}{\mathrm{C}}$$

(4)

where ε_m, T_m, and C are the maximum permittivity, the temperature corresponding to the dielectric peak, and the Curie constant, respectively. The diffusion coefficient (γ) typically ranges from 1 to 2, where a larger value signifies a stronger degree of relaxation. Figure 6 presents the γ values for the ceramics from BTC0 to BTC5, which are 1.21, 1.29, 1.48, 1.57, 1.69, and 1.76, respectively, which demonstrate that the relaxor behavior of ceramics can be significantly enhanced through the incorporation of Cu²⁺ ions. The mismatch in ionic valence and radii within BaTiO₃ ceramics facilitates the nucleation and growth of nano-domains, creating chemical disorder and intensifying local random fields and stress within the host lattice. These effects break down the long-range ferroelectric order, leading to the formation of PNRs, which is conducive to achieving higher energy storage performance.

Breakdown strength (E_b), as a critical parameter for BTCx energy storage ceramics, is used to assess the electric field of ceramics in practical operating range and is also a prerequisite for achieving excellent energy storage performance. The E_b, which exhibits significant dispersion due to the influence of numerous factors, is analyzed by the Weibull distribution [26]:

$${\mathrm{X}}_{\mathrm{i}}=\ln \left({\mathrm{E}}_{\mathrm{i}}\right)$$

(5)

$${\mathrm{Y}}_{\mathrm{i}}=\ln \left(-\ln \left(1-\frac{\mathrm{i}}{1+\mathrm{n}}\right)\right)$$

(6)

where X_i and Y_i represent the transformed data points for the Weibull distribution plot. The parameter β denotes the slope of the resulting fitted straight line. The relationship between X_i and Y_i can be plotted, and the slopes of each fitted straight line were obtained to determine the β. The higher β value signifies a smaller dispersion of the breakdown strength data, implying a more reliable fitting result. Figure 8a illustrates the β values for BTCx ceramics with various compositions. The β of BTCx ceramics increases with the increase in x from 10.1 to 15.6, indicating that the breakdown strength possesses high reliability. The E_b of each composition was determined by fitting the intercept of the straight line, as presented in Figure 8b. The E_b progressively increases with increasing x from 198 kV/cm to 291 kV/cm, which is related to the reduction in grain sizes and the improvement in density. The E_b is inversely correlated with grain size. The smaller grain sizes are typically associated with a greater volume of grain boundaries, whose much higher resistance compared to the grains helps to inhibit leakage current and thus improve the overall breakdown strength [27]. It is worth noting that E_b slightly decreases as x further increases, which is associated with the decrease in the density of BTCx ceramics.

The room temperature P-E loops for BTCx ceramics at 180 kV/cm are presented in Figure 9a–f. All samples display well-saturated loops, and the P-E loop gradually becomes thinner with increasing x, signifying a transition from normal to relaxor ferroelectric. The P-E loops of BTCx progressively develop double hysteresis characteristics. This is ascribed to the aging effect induced by the substitution of acceptor Cu²⁺ ions onto the B-site. Based on the symmetry-conforming short-range order theory [15], the distribution of oxygen vacancies aligns with the host lattice symmetry, which results in the formation of defect dipoles. These defect dipoles generate an internal restoring force that compels the PNRs to revert to their initial state upon removal of the electric field. This mechanism leads to the formation of the double P-E loops.

Figure 10a plots the P_r and P_max of BTCx ceramics. Both P_r and P_max decrease with increasing x, which is attributed to a “softening” effect in the ceramics. However, the excellent energy storage density requires large P_max and small P_r, and acceptor doping causes P_r to decrease more rapidly than P_max. The W_rec and η are presented in Figure 10b. The W_rec gradually increases from 0.84 J/cm³ (BTC0) to a peak value of 1.34 J/cm³ (BTC3), then decreases to 0.73 J/cm³ (BTC5). Concurrently, the η values for these compositions are 70.1%, 90.4%, and 76.9%, respectively. The enhancement of W_rec and η is associated with the increased oxygen vacancy concentration induced by Cu ion B-site doping. Based on the aging phenomenon in acceptor-doped materials [28], the formation of oxygen vacancies leads to their accumulation or redistribution as charged defects under a high electric field. The resulting synergistic effect modifies the shape of the P-E loops. As a result, for materials predominantly governed by charge compensation mechanisms and acceptor doping, the energy storage efficiency of BTCx ceramics can be improved to approximately 90%. With further increasing x, the excessive oxygen vacancies trap more electrons. After the electric field is removed, these electrons cannot migrate rapidly enough, causing a reduction in energy storage properties.

4. Conclusions

BaTi_1–xCu_xO_3–x (x = 0–0.05) ceramics were synthesized via the conventional solid-state method. The influence of B-site acceptor doping with Cu²⁺ ion on the phase structure, microstructure, dielectric properties, and energy storage characteristics was systematically studied. All samples maintained a single perovskite structure and transitioned from tetragonal (BTC0) to cubic phase (BTC5). The substitution of Cu on the B-site was found to promote microstructural densification and strengthen relaxor behavior, facilitating the enhancement of breakdown strength. AFE-like double loops were obtained, which originate from the acceptor-induced hysteresis aging phenomena through acceptor doping, resulting in the improvement in the energy storage density and efficiency. As a result, a superior energy storage density of 1.34 J/cm³ coupled with high efficiency of 90.1% was obtained for the BTC3 ceramic at 180 kV/cm. These findings provide a feasible strategy for improving the energy storage performance of BaTiO₃-based ceramics.