Enhanced Energy Storage Performance in La-Doped CaBi4Ti4O15 Films Through the Formation of a Weakly Coupled Relaxor

Abstract

Relaxor ferroelectric film capacitors exhibit high power density with ultra-fast charge and discharge rates, making them highly advantageous for consumer electronics and advanced pulse power supplies. The Aurivillius-phase bismuth layered ferroelectric films can effectively achieve a high breakdown electric field due to their unique insulating layer ((Bi₂O₂)²⁺ layer)). However, designing and fabricating Aurivillius-phase bismuth layer relaxor ferroelectric films with optimal energy storage characteristics is challenging due to their inherently stable ferroelectric properties. In this work, lead-free CaBi_4-xLa_xTi₄O₁₅ films were synthesized using the sol-gel technique and a weakly coupled relaxor design. On one hand, the introduction of La³⁺ ions weaken the dipole-dipole interactions, thereby enhancing the relaxor behavior. Alternatively, the expansion of grain size is restricted to enhance the number of grain boundaries, which possess improved insulating properties. This leads to a higher breakdown electric field. The results indicate that CaBi_4-xLa_xTi₄O₁₅ (x = 1.0) films exhibit excellent recoverable energy storage density (70 J/cm³) and high energy efficiency (73%). Moreover, the film exhibited good temperature stability and frequency stability. This study not only identifies a promising material for dielectric film capacitors but also demonstrates that the energy storage capabilities of Aurivillius-phase bismuth layer ferroelectric films can be effectively modulated through a design incorporating weakly coupled relaxor characteristics.

Keywords: PNRs; breakdown strength; energy storage; weakly coupled relaxor.

Figure 1

(a) The crystal structure diagram of CaBi₄Ti₄O₁₅ film. (b) XRD patterns of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films. (c) Raman spectra of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films.

Figure 2

(a₁–d₁) Surface SEM images of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films, and the inset shows the corresponding cross-sectional SEM images of CBLT-0.0, CBLT-0.8, CBLT-1.00, and CBLT-1.2 films, respectively. (a₂–a₅) EDS mapping images of specific elements (Ca, Bi, Ti, and O) of CBLT-0.0 film. (b₂–b₆) EDS mapping images of specific elements (Ca, Bi, La, Ti, and O) of CBLT-0.8 film. (c₂–c₆) EDS mapping images of specific elements (Ca, Bi, La, Ti, and O) of CBLT-1.0 film. (d₂–d₆) EDS mapping images of specific elements (Ca, Bi, La, Ti, and O) of CBLT-1.2 film.

Figure 3

(a) Ca 2p, (b) Bi 4f, (c) Ti 2p and (d) O 1s XPS spectra of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films.

Figure 4

Temperature-dependent ε_r and tanδ measured at different frequencies of (a) CBLT-0.0, (b) CBLT-0.8, (c) CBLT-1.0, and (d) CBLT-1.2 films. The inset displays fitted curves that correspond to the modified Curie–Weiss law.

Figure 5

Phase PFM images, the evolution behavior of domains under ± 50 V and after a duration of 15 min for (a–c) CBLT-0.0 film and (d–f) CBLT-1.0 film.

Figure 6

(a) P-E loops of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films at 1040 kV/cm. (b) variations in the P_max and P_r values of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films at 1040 kV/cm. (c) P-E loops of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films were measured under E_b. (d) The Weibull distribution of E_b for CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films. (e) J–E curves of leakage current for CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films. (f) the W_rec and ƞ of CBLT-x (x = 0.0, 0.8, 1.0, and 1.2) films. (g) W_rec/E comparison of CBLT-1.0 film with other energy storage film systems. (h) The discharge current curve of CBLT-1.0 film under ambient conditions. (i) The W_dis of CBLT-1.0 film at different electric fields.

Figure 7

(a) Temperature-dependent P-E loops of CBLT-1.0 film at 1500 kV/cm. (b) The values of W_rec and ƞ were measured at various measured temperatures. (c) The variation of W_rec/E in the CBLT-1.0 film at different temperatures. (d) Frequency-dependent P-E loops of CBLT-1.0 film at 1500 kV/cm. (e) The values of W_rec and ƞ were measured at various measured frequencies. (f) The variation of W_rec/E in the CBLT-1.0 film at different frequencies.