Enhanced Energy Storage Performance and Efficiency in Bi0.5(Na0.8K0.2)0.5TiO3-Bi0.2Sr0.7TiO3 Relaxor Ferroelectric Ceramics via Domain Engineering

Abstract

Dielectric materials are highly desired for pulsed power capacitors due to their ultra-fast charge-discharge rate and excellent fatigue behavior. Nevertheless, the low energy storage density caused by the low breakdown strength has been the main challenge for practical applications. Herein, we report the electric energy storage properties of (1 - x) Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-xBi_0.2Sr_0.7TiO₃ (BNKT-BST; x = 0.15-0.50) relaxor ferroelectric ceramics that are enhanced via a domain engineering method. A rhombohedral-tetragonal phase, the formation of highly dynamic PNRs, and a dense microstructure are confirmed from XRD, Raman vibrational spectra, and microscopic investigations. The relative dielectric permittivity (2664 at 1 kHz) and loss factor (0.058) were gradually improved with BST (x = 0.45). The incorporation of BST into BNKT can disturb the long-range ferroelectric order, lowering the dielectric maximum temperature T_m and inducing the formation of highly dynamic polar nano-regions. In addition, the T_m shifts toward a high temperature with frequency and a diffuse phase transition, indicating relaxor ferroelectric characteristics of BNKT-BST ceramics, which is confirmed by the modified Curie-Weiss law. The rhombohedral-tetragonal phase, fine grain size, and lowered T_m with relaxor properties synergistically contribute to a high P_max and low P_r, improving the breakdown strength with BST and resulting in a high recoverable energy density W_rec of 0.81 J/cm³ and a high energy efficiency η of 86.95% at 90 kV/cm for x = 0.45.

Keywords: dielectric; domain engineering; energy storage; lead-free ceramic capacitors; relaxor ferroelectric.

Figure 1

Schematic diagram of (a) recoverable energy density and hysteresis loss from P-E hysteresis loop of a dielectric material. (b) Domain evolution and formation of FE to RFE transition with the substitution of BST into BNKT, resulting in enhanced W_rec and η.

Figure 2

(a) XRD patterns of the (1 − x)BNKT-xBST ceramics for x = 0.15–0.50, where (b) 2θ = 39–59°. (c) Raman spectra of BNKT-BST ceramics along with spectral deconvolution.

Figure 3

FESEM images of (1 − x) BNKT-xBST ceramics for (a) x = 0.15, (b) x = 0.30, (c) x = 0.40, (d) x = 0.45, and (e) x = 0.50. The inset of (a–e) shows the average grain size versus counts (grain size distribution histogram). (f) The variation in grain size with composition (x).

Figure 4

(a) Frequency variation of relative dielectric permittivity and loss factor of BNKT-BST ceramics for x = 0.15–0.50. (b) Composition vs. relative dielectric permittivity and loss factor. (c,d) Temperature variation of relative dielectric permittivity and loss factor of BNKT-BST for x = 0.15 and 0.45 (The left and right sides of the arrows with circles enclosed by curves indicate relative dielectric permittivity and loss factor, respectively). The inset of (c,d) shows the $log\left(T-T_m\right)$ versus $log\left[\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\epsilon }_r$}\right.\right)-\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\epsilon }_r^m$}\right.\right)\right]$ of BNKT-BST for x = 0.15 and 0.45, respectively, at 1 MHz.

Figure 5

RT P-E and I-E curves of BNKT-BST ceramics for (a) x = 0.15, (b) x = 0.30, (c) x = 0.40, (d) x = 0.45, and (e) x = 0.50. (f) Composition (x) versus polarization and electric field.

Figure 6

P-E loops of BNKT-BST ceramics measured at E_BD and 10 Hz for (a) x = 0.15, (b) x = 0.30, (c) x = 0.40, (d) x = 0.45, and (e) x = 0.50. (f) Composition (x) versus W_rec, W_loss, and η.

Figure 7

Fatigue behavior of BNKT-BST for x = 0.45 composition measured up to 10⁶ electric cycles.