Enhanced energy storage properties in La(Mg_1/2Ti_1/2)O₃-modified BiFeO₃-BaTiO₃ lead-free relaxor ferroelectric ceramics within a wide temperature range

Abstract

A new ternary lead-free (0.67-x)BiFeO₃-0.33BaTiO₃-xLa(Mg_1/2Ti_1/2)O₃ ferroelectric ceramic exhibited an obvious evolution of dielectric relaxation behavior. A significantly enhanced energy-storage property was observed at room temperature, showing a good energy-storage density of 1.66 J/cm³ at 13 kV/mm and a relatively high energy-storage efficiency of 82% at x = 0.06. This was basically ascribed to the formation of a slim polarization-electric field hysteresis loop, in which a high saturated polarization P_max and a rather small remnant polarization P_r were simultaneously obtained. Particularly, its energy storage properties were found to depend weakly on frequency (0.2 Hz–100 Hz), and also to exhibit a good stability against temperature (25 °C–180 °C). The achievement of these characteristics was attributed to both a rapid response of the electric field induced reversible ergodic relaxor to long-range ferroelectric phase transition and a typical diffuse phase transformation process in the dielectric maxima.

Introduction

Dielectric capacitors were believed to provide effective technical solutions for energy-storage applications because they can offer much higher power density based on extremely high discharge speeds. Compared with linear dielectric polymer materials, nonlinear dielectrics exhibit many advantages in terms of maximum working voltage, charge/discharge rate, output power, cycling life, leakage current and ease of the fabrication [1], [2]. A normal ferroelectric material with large remnant polarization P_r should not be suitable for the energy-storage application because the charges cannot be effectively released. Relatively large energy loss from domain reorientation in ferroelectrics has restricted their practical applications. For antiferroelectric ceramics, the switching between the antiferroelectric and ferroelectric states occurs at low fields near room temperature, leading to smaller energy-storage density and temperature-sensitive energy-storage properties. In general, the energy-storage density (W) and efficiency (η) of a dielectric capacitor could be estimated according to a polarization-electric field (P-E) loop using the following formula:

$$W={\int }_{P_r}^{P_{\max }}EdP\mathrm{a}\mathrm{n}\mathrm{d}{W}_{\text{loss}}=\int \mathrm{P}\mathrm{d}\mathrm{E}$$

$$\eta =W/\left(W+W_{\text{loss}}\right)$$

where P_max is the saturated polarization, and W_loss is the area of hysteresis loops. Thereby, a large polarization difference ΔP = P_max-P_r would be definitely essential for achieving obviously enhanced energy-storage properties, apart from high dielectric breakdown strength allowing the application of an extremely high electric field. From this point of view, relaxor ferroelectrics might have large potentials against normal ferroelectrics and even antiferroelectrics for many energy-storage applications [3]. A couple of lead-based relaxor ferroelectric materials exhibited good potentials in the application of energy-storage capacitors [4], [5], [6], [7]. However, widespread applications of lead-based materials would be restricted in future due to the toxicity of lead. Endeavors to develop lead-free alternatives have been made all over the world.

In recent years, extremely large W values were reported in BiScO₃-BaTiO₃ (BT) (15 J/cm³) [8], Bi(Mg_1/2Ti_1/2)O₃-BT (37 J/cm³) [9] and (Bi_1/2Na_1/2)TiO₃ (BNT)-BT (154 J/cm³) [10] lead-free thin films, but their limited thickness has restricted the overall stored energy. Some lead-free bulk ceramics have been also investigated, such as Bi_0.5Na_0.5TiO₃ (BNT)-based and BT-based systems with W values of rarely larger than 1 J/cm³ [11], [12], [13], [14], [15]. BiFeO₃ (BF) has been recognized as a potential lead-free ferroelectric material owing to its excellent intrinsic polarization (P > 100 μC/cm²) [16], which offers the greatest scope for enhancing energy storage properties. BF-SrTiO₃ (ST) thin film was reported to have a good energy storage density of 18.6 J/cm³ at 972 kV/cm [17]. Solid solutions of (1-x)BF–xBT exhibited a high P_max at a rhombohedral to pseudocubic structural phase boundary (x = 0.33) [18], but their W values were very limited because of large energy loss from an obvious hysteresis and a large P_r value.

La(Mg_1/2Ti_1/2)O₃ (LMT) is a typical low-loss microwave dielectric material [19]. It is characterized by a distorted cubic perovskite structure and thus expected to be an appropriate end member to modify the dielectric relaxation and domain switching behavior of BF-BT binary system [20], [21]. In this work, we reported a new (0.67-x)BF-0.33BT-xLMT lead-free relaxor ferroelectric ceramic for energy storage applications. The influence of the substitution of LMT for BF on the structure, dielectric, ferroelectric and energy storage properties was explored, clarifying the mechanism of generating good energy-storage properties. An excellent energy storage property (W = ∼1.66 J/cm³, η = ∼82%) was attained in the x = 0.06 sample together with a desirable temperature stability from 25 to 180 °C and a weak frequency dependence from 0.2 Hz to 100 Hz.