Giant energy density and high efficiency achieved in bismuth ferrite-based film capacitors via domain engineering

Abstract

Developing high-performance film dielectrics for capacitive energy storage has been a great challenge for modern electrical devices. Despite good results obtained in lead titanate-based dielectrics, lead-free alternatives are strongly desirable due to environmental concerns. Here we demonstrate that giant energy densities of ~70 J cm^-3, together with high efficiency as well as excellent cycling and thermal stability, can be achieved in lead-free bismuth ferrite-strontium titanate solid-solution films through domain engineering. It is revealed that the incorporation of strontium titanate transforms the ferroelectric micro-domains of bismuth ferrite into highly-dynamic polar nano-regions, resulting in a ferroelectric to relaxor-ferroelectric transition with concurrently improved energy density and efficiency. Additionally, the introduction of strontium titanate greatly improves the electrical insulation and breakdown strength of the films by suppressing the formation of oxygen vacancies. This work opens up a feasible and propagable route, i.e., domain engineering, to systematically develop new lead-free dielectrics for energy storage.

Fig. 1

Schematic illustrations. a A typical P–E loop of a dielectric and an illustration of (discharged) energy density U_e, hysteresis loss U_loss and efficiency η. The red arrows indicate the charging and discharging processes. b Schematic of domain evolution and FE-to-RFE transition induced by the incorporation of STO into BFO, leading to concurrently improved U_e and η. The blue outlines indicate the ferroelectric domains and the red arrows denote the spontaneous polarization directions

Fig. 2

Microstructure, dielectric, and ferroelectric properties. a XRD pattern of the BFSTO film with x = 0.45; the insets shows the phi scans of its {101} planes corresponding to the Nb:STO substrate. b A high-resolution TEM image of the interface of BFSTO film with x = 0.45 and Nb:STO substrate (scale bar: 5 nm); the inset is the SAED pattern at the interface zone. c Frequency-dependent dielectric permittivity and loss tangent and d bipolar P–E loops of the BFSTO films

Fig. 3

Energy storage performance. a Discharged energy density and b efficiency of the BFSTO films as a function of the applied electric field. c Comparisons of energy density and efficiency between BFSTO and representative dielectric systems^–,–, showing that the BFSTO films possess the most outstanding energy storage performance. d Energy density and efficiency for x = 0.60 and 0.75 at an electric field of 2.5 MV cm⁻¹ over 1 × 10⁷ charging-discharging cycles. e Temperature-dependent energy storage performance for x = 0.60 and 0.75 at an electric field of 1.5 MV cm⁻¹

Fig. 4

Evolution of FORC distribution and domain configuration. a–d FORC distribution p(α,β) and e–h HAADF-STEM images of the atomic-scale ferroelectric domain structure of the BFSTO films (scale bars: 10 nm). The yellow dashed lines in e–h mark the domains with spontaneous polarization and the red arrows denote the Fe/Ti ion displacement (δ_Fe/Ti) directions. The insets of e–h are magnified images of selected areas to show the δ_Fe/Ti, where the yellow and red circles denote the Bi/Sr and Fe/Ti ion columns, respectively

Fig. 5

Breakdown strength and electric conduction. a Two-parameter Weibull Distribution analysis of dielectric breakdown strengths, b room-temperature leakage current densities as a function of biased electric field, c XPS fitting for Fe valences and d percentage of Fe²⁺ out of all Fe ions in the BFSTO films