High energy storage performance in lead-free BiFeO₃-BaTiO₃ ferroelectric thin film fabricated by pulsed laser deposition

Dielectric capacitors have been receiving increasing attention due to their high power density and an ultra-fast charge/discharge speed, which plays an important role in micro-electronics technology, advanced pulsed power systems, hybrid electric vehicles, high frequency inverters, and so on.^1–4 However, compared with their electrochemical counterparts such as super-capacitors and Li-ion batteries, the drawback of low energy density for dielectric capacitors greatly limits their potential applications. In recent years, how to achieve dielectric capacitors with high energy density have been intensively studied.^5–9 It is well known that the recoverable energy storage density U_re and energy storage efficiency η can be calculated using the following equations:

where E is the electric field, P is the polarization, P_r is the remnant polarization, P_m is the maximum field-induced polarization and U_loss is the hysteresis loss. According to the equations, the storage energy density is determined by the values of P_m-P_r and the external field strength E.^10–12 Therefore, among the various classes of dielectric capacitors, ferroelectric capacitors have a broader potential because of their large polarization compared to the linear dielectrics. For this reason, lead-based ceramics have been extensively studied as energy storage candidates because of their strong polarization properties, for instance, the maximum field-induced polarization of PbTiO₃ is as high as 86 μC/cm².^13,14 However, since lead-containing materials are toxic and harmful to human health and the environment, nowadays, environmental demand is pushing the development toward lead-free materials, which has been spurring great efforts on the research of new lead-free system with high energy storage density.¹⁵ 

Multiferroic bismuth ferrite (BiFeO₃, abbreviated as BFO) materials, which possess high Curie temperature (T_C=850°C) and Neel temperature (T_N=370°C), have attracted increasing interests in the past several years due to their fundamental physical properties and the potential applications.^16–18 Moreover, it is reported that the spontaneous polarization of BFO can reach as high as 100 μC/cm² and it has similar electron configuration (lone 6s² electron of Bi³⁺) to Pb²⁺ ions, which means that BFO-based dielectric materials can be considered to be a potential candidate, similar to lead-based materials.^19–23 Particularly, BiFeO₃-BaTiO₃ (BF-BT) ceramics have attracted much attention due to their excellent energy storage properties compared with other lead-free ceramic systems. Liu et al.²⁴ obtained a ultrahigh recoverable energy density (2.56 J/cm³) under low electric field (16 kV/mm) in Ba(Zn_1/2Ta_2/3)O₃-modified BF-BT ceramic. But their energy density are still low due to the inferior breakdown strength in bulk ceramic forms. It is well known that thin films always have high breakdown field strength compared with their bulk ceramics counterpart, thus benefiting to higher energy storage density.^25–27 Ueda et al.²⁸ prepared BFO-0.3BT film with the coexistence of weak ferroelectricity and ferromagnetism by the pulsed-laser deposition technique. More recently, Lee et al.²⁹ reported that the local piezoelectric constant d₃₃ of 92.5 pm/V and high T_C of 405°C could be achieved in the BF-0.33BT film of 300 nm. Therefore, high energy storage performance in BF-BT films would be anticipated based on the excellent properties of ceramics and films.^30,31 However, there are still no reports concerning BF-BT film about energy storage property because the volatilization of Bi and the change of valence states between Fe²⁺ and Fe³⁺ brings about the large leakage current. In this letter, the lead-free BFO-BTO thin film is prepared by PLD processing. It is found that he prepared thin film shows a high recoverable energy density of 19 J/cm³ and an efficiency of 51% under the applied electric field of 900 kV/cm as well as good fatigue endurance.

The target of 0.15wt%MnCO₃ doping 0.68BiFeO₃-0.32BaTiO₃ (abbreviated as BFBT) used for laser ablation was prepared by the conventional solid-state reaction route. The BFBT thin film was deposited on Pt/Ti/SiO₂/Si substrates by the pulsed laser deposition (PLD) processing. A laser energy of 300 mJ and a repetition rate of 6 Hz were adopted. The distance between the target and the substrate was fixed at 5 cm. The substrate temperature was maintained at 600°C. The oxygen partial pressure was kept at 2 Pa during the laser ablation process. BFBT thin film with thickness of 200 nm were prepared by irradiating the stoichiometric target for 30 min. After deposition, the as-grown thin film was cooled down to room temperature after hold for another half an hour at 600°C.

The crystal structures were determined using a diffraction of X-rays (D8 FOCUS). The surface and cross-sectional morphologies were detected using a field-emission scanning electron microscope (SU8220). To measure the electrical properties, Au top electrodes with a diameter of 0.2mm were deposited on the film surfaces by a shadow mask using the small-ion sputtering instrument (JS-1600). Dielectric measurements were performed with a precision impedance analyzer (Agilent 4294A). The ferroelectric hysteresis loops were characterized by using a ferroelectric analyzer (TF2000).

The XRD patterns of the prepared BFBT thin film and target are shown in Fig. 1(a), in which the characteristic peaks indicate that both film and target have a perovskite structure. The preferential orientation of (100) pc can be observed in the thin film and the single (110) pc peak suggests that the BFBT thin film and target are characterized as cubic phase structure.³² A surface morphology SEM image of the BFBT thin film is displayed in Fig. 1(b). The film shows highly dense microstructure without any visible pores. Fig. 1(c) shows the cross-sectional SEM images of BFBT thin film, it can be seen that the thin film average thickness is about 200 nm and the grains size are around 80 nm. The highly dense, uniform microstructure and small grain size reveal that the thin film has a good quality.

The room-temperature frequency dependence of dielectric constant ε_r and loss tanδ are measured in the range of 100 Hz–100 kHz, as shown in Fig. 2(a). It can be observed that the dielectric constant ε_r and loss tanδ are 145 and 0.02 at 1 kHz, respectively. As the frequency increases, ε_r gradually decreased. Fig. 2(b) shows the room-temperature leakage current density dependence of electric field for the prepared BFBT thin film. The leakage current curves are asymmetrical, which may be due to the different barriers between different electrodes and films.^33,34 Under positive bias voltage, the curves do not coincide due to the existence of interface barrier polarization, however, the coincidence occurred under negative bias voltage. As seen, leakage current increases with the increasing of electric field, leakage current increases from 1×10^-8 to 1×10^-4 A/cm² when E increases from 0 to 300 kV/cm. The low leakage current also proved that high quality BFBT thin film was be prepared.³⁵ 

Fig. 3(a) shows the room-temperature P-E hysteresis loops of the BFBT thin film under different applied electric fields at 1 kHz. It is seen that all the P-E loops show slim shapes with large P_m and the BFBT thin film can hold 900 kV/cm electric field. As shown in the Fig. 3(a), the hysteresis loop exhibited asymmetrical to some extent. This phenomenon may be due to the intrinsic defects of the film and the different work functions between the top electrode and down electrode, which is common in some thin films.^34,36 In order to reduce the influence of the hysteresis loop offset caused by the electrode-film interface barrier on the energy storage properties, the unipolar hysteresis loop was tested as shown in Fig. 3(b). Under an applied low electric field of only 900 kV/cm, the film has high field-induced polarization value of 86 μC/cm². And the recoverable energy density (U_re) reaches up to 19 J/cm³ and energy efficiency (η) is 51% at room temperature. Fig. 3(c) gives the electric field dependence of U, U_re, U_loss, and η. It is seen that U, U_re, and U_loss gradually increase to 37 J/cm³, 19 J/cm³ and 18.00 J/cm³, respectively. In addition, η decreases from 92% to 51% with electric field strength increasing from 100 kV/cm to 900 kV/cm. It can be observed that the BFBT thin film exhibits a high energy density and efficiency simultaneously, which means that BFBT thin film can be considered as a promising candidate for lead-free ferroelectric thin film capacitors in energy storage applications. In addition, a comparison of energy storage performances of representative materials are plotted, as shown in Fig. 4. It can be seen that BFBT thin film has a relatively higher energy storage density than most lead-free materials, and is even comparable to some lead-based materials.

It is well known that fatigue resistance is another important parameter for ferroelectric capacitors in energy storage applications. Fig. 5(a) shows the P-E hysteresis loops before and after 1×10⁷ cycles with an applied electric field of 600 kV/cm at 1 kHz, it can be observed that there is no significant change in the loops. The corresponding P_m and P_r shown in the inset of Fig. 5(a) show a slight increase from 46.7 to 47.5 μC/cm² and 20.5 to 23.6 μC/cm², respectively. It should be noted that the asymmetrical hysteresis loop is related to the electrode materials. Therefore, applying the opposite bias voltage will cause the hysteresis loop to bias another direction, thus resulting in different hysteresis loops in Fig. 3 and Fig. 5.³⁴ Fig. 5(b) shows a slight decrease in U_re and η from 12.73 to 11.19 J/cm³ and 63.9% to 54.8%. All results indicate that the BFBT thin film has outstanding fatigue endurance, which is necessary for energy storage applications.

In summary, BFBT thin film was deposited on Pt/Ti/SiO₂/Si substrates by PLD processing, and then we investigated the energy storage properties of the BFBT thin film. It is found that the BFBT thin film shows outstanding energy storage properties with a recoverable energy density of 19 J/cm³ and an energy efficiency of 51% under an applied electric field of 900 kV/cm at room temperature. In addition, the film exhibits outstanding fatigue endurance after 1×10⁷ cycles. All results suggest that BFBT thin film can be considered as lead-free candidates in energy storage applications.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51772192 and 11574334), the Science and Technology Commission of Shanghai Municipality (No. 17070502700), and the Youth Innovation Promotion Association, Chinese Academy of Sciences (2016231).