Moderate Fields, Maximum Potential: Achieving High Records with Temperature-Stable Energy Storage in Lead-Free BNT-Based Ceramics

Abstract

The increasing awareness of environmental concerns has prompted a surge in the exploration of lead-free, high-power ceramic capacitors. Ongoing efforts to develop lead-free dielectric ceramics with exceptional energy-storage performance (ESP) have predominantly relied on multi-component composite strategies, often accomplished under ultrahigh electric fields. However, this approach poses challenges in insulation and system downsizing due to the necessary working voltage under such conditions. Despite extensive study, bulk ceramics of (Bi_0.5Na_0.5)TiO₃ (BNT), a prominent lead-free dielectric ceramic family, have seldom achieved a recoverable energy-storage (ES) density (W_rec) exceeding 7 J cm^-3. This study introduces a novel approach to attain ceramic capacitors with high ESP under moderate electric fields by regulating permittivity based on a linear dielectric model, enhancing insulation quality, and engineering domain structures through chemical formula optimization. The incorporation of SrTiO₃ (ST) into the BNT matrix is revealed to reduce the dielectric constant, while the addition of Bi(Mg_2/3Nb_1/3)O₃ (BMN) aids in maintaining polarization. Additionally, the study elucidates the methodology to achieve high ESP at moderate electric fields ranging from 300 to 500 kV cm^-1. In our optimized composition, 0.5(Bi_0.5Na_0.4K_0.1)TiO₃-0.5(2/3ST-1/3BMN) (B-0.5SB) ceramics, we achieved a W_rec of 7.19 J cm^-3 with an efficiency of 93.8% at 460 kV cm^-1. Impressively, the B-0.5SB ceramics exhibit remarkable thermal stability between 30 and 140 °C under 365 kV cm^-1, maintaining a W_rec exceeding 5 J cm^-3. This study not only establishes the B-0.5SB ceramics as promising candidates for ES materials but also demonstrates the feasibility of optimizing ESP by modifying the dielectric constant under specific electric field conditions. Simultaneously, it provides valuable insights for the future design of ceramic capacitors with high ESP under constraints of limited electric field.

Keywords: BNT; Capacitors; Energy storage; Lead-free; Relaxor ferroelectrics.

Fig. 1

a ES density and characteristics under varying E-fields. b The theoretical correlation between W_rec and E-field. c Enhanced insulation performance and domain structure of BNKT through ST-BMN doping

Fig. 2

a XRD pattern of ceramics with varying x (0.35, 0.40, 0.45, and 0.50) in the 20°–70° range. b Enlarged views of the diffraction peaks at 39°–40.5° and 45.5°–47°. c Fitted XRD pattern for x = 0.50 ceramic powders. d Variation of lattice parameters with x for B-xSB ceramics. e Proportion variation of the R and T phases for different compositions. f Temperature evolution of XRD patterns for x = 0.50 ceramic from 30 to 100 °C at selected angles of 39.5°–40° and 46°–46.5°. g Temperature-dependent ε_r and tanδ for x = 0.35, 0.40, 0.45, and 0.50 at measurement frequencies of 0.3, 1, 10, 100, and 1000 kHz, ranging from 30 to 400 °C. The long arrow indicates the direction of increasing measurement frequency. h ε_r/ε₅₀ as a function of temperature measured at 1 kHz, where ε₅₀ was measured at 50 °C, and ε_r was measured from 30 to 400 °C. i AGS and density variations with x for B-xSB ceramics

Fig. 3

SEM images of surface morphologies for compositions a1 x = 0.35, a2 x = 0.40, a3 x = 0.45, and a4 x = 0.50 ceramics. a2–d2 Grain size distribution. a3–d3 Intensity mapping of ε_r. a4–d4 Distribution of electric potentials. a5–d5 Local E-field distribution. a6–d6 Relationship between temperature (T) and grain boundary resistance, with resistance spectra shown in the insets

Fig. 4

a Bright-field and b high-resolution TEM image of BSB-0.5 ceramic. c Redistribution of brightness based on the RGB values of b. d The polarization vector, calculated from cation displacement on the B-site, superimposed on the polarization intensity distribution, with relative polarization intensity expressed through the brightness and saturation of the background color. SAED patterns along e [110]_pc and f [111]_pc. g FORC test method. Evolution of FORC distributions for h x = 0.35 and i x = 0.50

Fig. 5

a P–E loops and b the corresponding current density–electric field (J–E) curves for x = 0.35, 0.40, 0.45, and 0.50, measured at RT, 10 Hz, and various E-fields. c W_rec and η of x = 0.35, 0.40, 0.45, and 0.50 ceramics. BDS is indicated next to the data. d P–E loops and corresponding J–E curves for x = 0.50 composition measured at RT and 10 Hz. e W_rec and η of x = 0.50 composition. f Comparison of W_rec and η for BSB-0.50 and other BNT-based bulk ceramics. g Comparison of W_rec for B-0.5SB ceramic with other bulk ceramics. h P–E loops and J–E curves of B-0.5SB ceramic measured at 10 Hz and 366 kV cm⁻¹. Temperature increases from 30 to 140 °C in steps of 10 °C. i W_loss, W_rec, and η calculated from the corresponding P–E loops of B-0.5SB. Temperature-dependent W_rec j and η (k) of B-0.5SB compared with other BNT-based bulk ceramics

Fig. 6

Pulsed overdamped discharging properties of B-0.5SB bulk ceramics, illustrating a current curves and b W_dis at RT and various electric fields. Temperature-dependent pulsed overdamped discharging properties of B-0.5SB bulk ceramics, depicting c current curves and d W_dis at 300 kV cm⁻¹ as the temperature increases from 40 to 140 °C in intervals of 20 °C. Pulsed underdamped discharging properties of B-0.5SB bulk ceramics, exhibiting e current curves, and f C_D and P_D as functions of the electric field at RT