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• A high W r of 4.97 J/cm3 and an η of 84.4% are obtained in BNT-based ceramics. • The W r of 2.73 J/cm3 with η of 80.6% are obtained at 140 ℃. • A superior temperature performance compared with other typical dielectric materials. • A strategy of precisely adjusting the relaxation and ferroelectricity states. Dielectric energy storage materials have attracted much attention because of their wide applications in the electronics industry. However, low energy storage density and poor thermostability limit their application. In this work, a strategy was proposed to prepare lead-free relaxor ferroelectric ceramics with ultra-high energy storage properties and superior temperature stability by precisely adjusting the relaxation and ferroelectricity states and increasing the electric field intensity. An ultra-high recoverable energy storage density (W r) of 4.97 J/cm3 and an efficiency (η) of 84.4% are simultaneously obtained for the 0.95(0.9Bi 0.5 Na 0.5 TiO 3 -0.1Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3)-0.05BaSnO 3 ceramic. Moreover, a high W r of 2.73 J/cm3 with brilliant η of 80.6% are obtained in the ceramic at 140 ℃ and there is almost no thermal degradation of energy storage properties from 25 ℃ to 140 ℃. The W r and η have decreased by 2.2% and 1.9% after 105 cycles under 200 kV/cm, respectively, which demonstrates the good cycle stability of the sample. A discharge power density of up to 99.0 MW/cm3 is obtained in the 0.95(0.9Bi 0.5 Na 0.5 TiO 3 -0.1Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3)-0.05BaSnO 3 ceramic and the good charge and discharge performance of this sample is important for practical applications. [ABSTRACT FROM AUTHOR]

• A comprehensive strategy of hybridizations of Bi3+ orbitals and relaxor was adopted. • Relaxor behavior was analyzed via Raman spectroscopy and the Curie-Weiss law. • BNT-0.40ST-0.02FN composition exhibits high W (3.36 J/cm3) and η (81%) under a low electric field of 170 kV/cm. • BNT-0.40ST-0.02FN ceramic shows superior thermal stability (25–200 °C). • The BNT-0.40ST-0.02FN ceramic possesses potential to be employed in pulsed power devices under extreme conditions. Dielectric ceramics are highly favorable for pulsed power applications owing to their ultra-fast charge-discharge speed and excellent reliability. However, their low energy density under low electric fields remains as a bottleneck for them to be employed in integrated electronic devices with shrinking dimensions. In this work, we have designed a comprehensive strategy to synthesize lead-free (Bi 1/2 Na 1/2) 1−x Sr x Ti 0.98 (Fe 1/2 Nb 1/2) 0.02 O 3 (BNT-xST-2FN, x = 0.30, 0.35, 0.40 and 0.45) ceramics via traditional solid-state method. On one hand, the hybridizations of Bi3+ orbitals and the displacement of B-site cations under the applied electric field lead to a high polarization. On the other hand, the (Fe 1/2 Nb 1/2)4+ complex-ion and SrTiO 3 were introduced to improve the relaxor behavior of the system, resulting in a high energy-storage efficiency. In this way, an excellent energy density of 3.36 J/cm3 and a high energy efficiency of 81% are simultaneously achieved in the BNT-0.40ST-0.02FN composition under a low electric field of 170 kV/cm, which is superior to other BNT-based materials under similar electric fields. Moreover, the ceramic exhibits a superior thermal stability (25–200 °C). This work provides a promising approach for designing high-performance lead-free energy storage ceramics under low electric fields. [ABSTRACT FROM AUTHOR]

• High-entropy engineering. • The activation energy. • Thermal stability. The utilization of a high entropy approach for the design of high-performance perovskite dielectric capacitors has been gaining much attention for the development of next-generation pulse power capacitors. Despite this, there are still challenges to further enhance their energy storage density and efficiency. To address this, a high-entropy strategy is proposed to significantly improve the energy storage characteristics of ceramics by increasing the configurational entropy and delaying saturation polarization. This approach has resulted in an ultra-high recoverable energy storage density (W rec) of 7.16 J/cm3 and a high efficiency (η) of 93.3 % for 0.8[0.65(Bi 0.5 Na 0.5)TiO 3 -0.35SrTiO 3)]-0.2[Ba(Zr 0.2 Ti 0.2 Sn 0.2 Hf 0.2 Nb 0.2)O 3 ] (abbreviated as 0.8BNST-0.2Ba(5 M)O) ceramic. The inclusion of Ba(5 M)O high entropy oxides has resulted in slender polarization–electric field hysteresis loops with delayed polarization saturation. The increase in atomic configuration entropy, degree of atomic disorder and lattice distortion due to the Ba(5 M)O high entropy oxides, has led to an increase in breakdown field, improved energy storage density and efficiency. Furthermore, excellent thermal stability (30–150 ℃), superior frequency stability (5–1000 Hz), and robust fatigue (100–105 cycles) endurance were achieved. The high entropy strategy has been demonstrated to be an effective method for the design of novel capacitors with superior energy storage performance. [ABSTRACT FROM AUTHOR]

• High-entropy design strategy was adopted in this work. • The W rec of 7.6 J/cm3 and η of 90% were obtained in fabricated high-entropy ceramic. • The ceramic displayed good thermal/frequency/fatigue-resistance stability. Dielectric capacitors have gained much attention in next-generation advanced pulse power systems owing to their ultrafast charging/discharging rate. However, the early polarization saturation and large hysteresis hinder the enhancement of recoverable energy-storage density (W rec) with increasing electric field. Here, we design (Bi 0.5 Na 0.5)TiO 3 -based high-entropy dielectric capacitors to modulate polarization behavior and maximize the energy storage capacity. An ultrahigh W rec of 7.6 J/cm3, together with a high η of 90% is simultaneously obtained, showing great competitiveness among the (Bi 0.5 Na 0.5)TiO 3 -based energy storage ceramics. On the one hand, the linear-like P - E loops induced by enhanced random field, which is achieved by increasing the atomic configurational entropy, delay the polarization saturation and ensure the rapid enhancement of W rec under higher electric fields. On the other hand, the high efficiency (η) is assured due to the existence of polar nanoregions (PNRs) and the absence of relaxor ferroelectric to ferroelectric transition. Noticeably, the ceramic also exhibits excellent thermal stability (Δ W rec < 4.4%, Δ η < 10.3%, −50–200 °C), cycling stability (Δ W rec < 0.085%, Δ η < 0.12%, 1–105) and frequency stability (Δ W rec < 7.08%, Δ η < 3.79%, 1–200 Hz). This work reveals that regulating the entropy is an effective method to design high-performance dielectric capacitors. [ABSTRACT FROM AUTHOR]

Numerous electrode materials have been studied in supercapacitors for next-generation energy storage applications. As a paramagnetic metal with low resistivity, NiSe 2 has received much attention and been used extensively in many applications, including energy storage, electrocatalysts, and high temperature superconductors, etc. However, the capacitive properties of NiSe 2 are rarely investigated. In the present work, truncated cube-like NiSe 2 single crystals are synthesized by a facile hydrothermal approach and further used as electrode material in supercapacitors. The effects of different loading mass of electrode material on electrochemical capacitive behaviors are also investigated. Experimental results demonstrate that under a mass loading of 3.90 mg cm −2 , the as-prepared NiSe 2 electrode exhibits a high specific capacitance of 1044 F g −1 at 3 A g −1 (or an areal capacitance of 4.07 F cm −2 ), along with an excellent rate capability (601 F g −1 at 30 A g −1 ). Besides, the morphology change and the impedance increasement are responsible for the worse cycling performance of NiSe 2 electrode in the three-electrode system. Meanwhile, the practical electrochemical energy storage behavior of as-synthesized NiSe 2 is investigated in an asymmetric supercapacitor. The NiSe 2 //activated carbon (AC) asymmetric device possesses an outstanding cycle life (87.4% after 20,000 successive cycles), a high energy density of 44.8 Wh kg −1 at 969.7 W kg −1 and a higher power density of 17.2 kW kg −1 at 17.4 Wh kg −1 , showing attractive potential in practical applications. This work opens avenue for utilizing single crystal NiSe 2 as electrode material and providing important guidance to the further investigation of nickel selenides for advanced supercapacitors. [ABSTRACT FROM AUTHOR]

• Grain tuning and relaxor construction strategy was adopted in this work. • The W rec of 5.5 J/cm3 and W t of 6.4 J/cm3 were achieved in 0.6BNT-0.4SBTH ceramic. • The ceramic displayed good thermal stability and frequency insensitivity. Relaxor ferroelectrics have become competitive candidates in energy storage applications owing to their relatively high maximum polarization, high breakdown strength, and relatively low remnant polarization. Here, Bi 0.5 Na 0.5 TiO 3 (BNT) with large polarization was selected as the base composition, and Sr 0.7 Bi 0.2 Ti 0.8 Hf 0.2 O 3 (SBTH), as a dopant, was added into BNT to improve its relaxor behavior and decrease the grain size. A series of (1- x)Bi 0.5 Na 0.5 TiO 3 - x Sr 0.7 Bi 0.2 Ti 0.8 Hf 0.2 O 3 (abbreviated as (1- x)BNT- x SBTH, x = 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized. The addition of SBTH resulted in the increase in structural disorder, as confirmed by the red-shift in Raman spectra, the enhancement of the relaxor characteristic and the fast response of the domains in local poling measurements. As a result, excellent energy storage properties (with the recoverable energy storage density of 5.5 J/cm3 and efficiency of 85.9 % at 410 kV/cm) were realized in the 0.6BNT-0.4SBTH sample. Moreover, the sample also exhibited excellent thermal stability (20 °C to 200 °C) and frequency insensitivity (0.5–500 Hz). These results suggest that BNT-based relaxor ferroelectric ceramics modulated by SBTH possess great potential in dielectric energy-storage capacitors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The oxygen-doped solid solution Ti 3 AlC 2-δ O δ MAX is synthesized. • The Ti 3 C 2-δ O δ T x MXene is obtained by etching the forming solid solution. • Ti 3 C 2-δ O δ T x electrodes show a capacity of 306.0 C g−1 (216.8 C g−1 for Ti 3 C 2 T x). • Improved diffusion-controlled capacitance is owed to the higher Ti active center. • Enhanced interface capacitance is owed to the improved quantum capacitance. Heteroatom doping has been proven to be an effective strategy to improve the electrochemical energy storage capacity of two-dimensional MXenes. However, the heteroatom for MXenes is mainly focused on the N atoms, while the effects and underlying mechanisms of O substituted partial C atoms of MXene have not been explored so far. Herein, comprehensive research is firstly performed to reveal the oxygen doping mechanism in Ti 3 C 2 T x MXene by the experimental characterization and the density functional theory simulation. The in situ oxygen doped Ti 3 C 2 T x nanosheets, in which oxygen atoms substitute partial carbon atoms in the titanium octahedron, are synthesized by etching the oxygen doped Ti 3 AlC 2 MAX phase, which is more facile than the complex post-doped process. The introduction of appropriate oxygen atoms can effectively improve the interlayer space, electronic conductivity, and interface charge transfer of the Ti 3 C 2 T x MXene as a supercapacitor electrode. Further, the electrochemical performances demonstrate that the oxygen doped MXene (O-Ti 3 C 2 T x -0.05) film electrode can deliver a higher capacity of 306.0 C g−1 compared to the pristine Ti 3 C 2 T x electrode (216.8 C g−1). Besides, the quantitative analysis results imply that the enhanced capacitance is mainly attributed to the diffusion-controlled capacitance and interface capacitance, which result from the higher Ti metal active center, the enhanced quantum capacitance, and the higher adsorption energy. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of ST and BMH doping in BNT on energy storage properties were studied. • By modifying composition, good energy storage properties are obtained in BNST-BMH. • Ultrahigh W rec of 5.59 J/cm3 and η of 85.3% are realized in BNST-BMH ceramic. • BNST-BMH ceramic exhibits excellent thermal stability from 20 °C to 140 °C. • A superior fatigue resistance over 105 cycles was achieved in BNST-BMH ceramic. Dielectric ceramic capacitors with high energy density are one of the great bright energy storage devices. Here, the Bi 0.5 Na 0.5 TiO 3 , 0.75Bi 0.5 Na 0.5 TiO 3 -0.25SrTiO 3 , 0.65Bi 0.5 Na 0.5 TiO 3 -0.35SrTiO 3 and 0.88(0.65Bi 0.5 Na 0.5 TiO 3 -0.35SrTiO 3)-0.12Bi(Mg 0.5 Hf 0.5)O 3 , (abbreviated as BNT, BNT-25ST, BNT-35ST and BNST-BMH, respectively) ceramics were produced by solid state sintering method. The influence of solid solution of SrTiO 3 and Bi(Mg 0.5 Hf 0.5)O 3 doping in Bi 0.5 Na 0.5 TiO 3 on crystal structure, microstructure, electrical and energy storage performance were evaluated systematically. By optimizing ceramics composition, high breakdown field and slim hysteresis loops were simultaneously realized due to enhanced band gap, refined grain size and nano-domains formed, which can be verified by the transmission electron microscope (TEM), piezoelectric force microscope (PFM) and ultraviolet–visible spectrum. Therefore, an ultrahigh recoverable energy density of 5.59 J/cm3 with brilliant efficiency of 85.3% is realized in BNST-BMH ceramic. Besides, the BNST-BMH also exhibits prominent temperature stability at 20–140 °C, superior fatigue resistance beyond 105 cycles and outstanding frequency stability at 1–200 Hz. Our work offers a novel way for designing dielectric capacitors and also proves that BNST-BMH is indeed a great hopeful dielectric energy storage material in pulse power electronics. [ABSTRACT FROM AUTHOR]

[Display omitted] • 0.9NBST-0.1BMS lead-free ceramics with W rec = 6.68 J/cm3 and η = 89.1 % were designed and fabricated. • Small grain size and high activation energy of grain boundary leads to high breakdown field (405 kV/cm). • The energy storage potential (W rec /E b) is up to 0.01649 μC/cm2. • Excellent fatigue endurance over 105 electrical cycles (the W rec variation is less than 1.5%). Ferroelectric ceramics, as a potential candidate for high-power energy storage capacitors, lies in their excellent recoverable energy storage density (W rec) and outstanding efficiency (η) in practical applications. Herein, a new type of lead-free ceramics (1- x)(Na 0.5 Bi 0.5) 0.65 Sr 0.35 TiO 3 - x BiMg 0.5 Sn 0.5 O 3 or (1- x)NBST- x BMS was prepared with the aim of enhancing the breakdown strength (E b) and reducing the energy storage loss through grain refinement. It was found that E b of 0.9NBST-0.1BMS reaches 405 kV/cm due to the reduction in the grain size of ceramic and thus the extremely high ratio of grain boundary resistance to grain resistance. Besides, a remarkable energy-storage performance was obtained, that is, W rec and η are ∼ 6.68 J/cm3 and 89.1% at 405 kV/cm, respectively, along with excellent stability in terms of frequency, temperature, and fatigue endurance. The outstanding energy-storage performance is resulted from modulating the grain size via doping the moderate content of Bi3+ and Mg2+/Sn4+, which is beneficial to increase the breakdown field by increasing resistivity under high electric field while increasing the grain boundary activation energy and promote the formation of a relaxor state at the same time. More importantly, energy storage potential (defined as W rec /E b) is up to 0.01649 μC/cm2, being the highest value reported so far for BNT-based ceramics in energy-storage application. Our results pave the way for practical applications of NBST-based ferroelectric capacitors with excellent energy storage performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Utilize an average ionic polarizability design strategy to balance W rec and η. • The high-polarization gene is maintained and the polar order is disrupted. • Achieve excellent energy-storage properties both in low-field and moderate-field. • The existence of dynamic PNRs is confirmed by PFM measurements. • The temperature-dependent Raman spectra reveal the variation of local structure. Electrostatic capacitors are promising candidates in the application of pulse-power systems due to their high power density and fast charge-discharge rate. However, the insufficient recoverable energy density (W rec) and energy efficiency (η) under low/moderate electric field is still the bottleneck for their applications in miniaturized equipment and portable devices. In this work, we utilize an average ionic polarizability design strategy to enhance W rec by sustaining the high-polarization gene of the system and simultaneously improve η by disrupting the polar order in (1- x)[0.7Bi 0.5 Na 0.5 TiO 3 -0.3Sr 0.7 Bi 0.2 TiO 3 ]- x Sr(Zn 1/3 (Nb 0.85 Ta 0.15) 2/3)O 3 ceramics. As a result, excellent energy-storage properties have been achieved both in low-field (W rec ∼ 2.7 J/cm3 and high η ∼ 93% at 200 kV/cm) and moderate-field (W rec ∼ 5.2 J/cm3 and η ∼ 91% at 330 kV/cm) for x = 0.12 sample. Additionally, the x = 0.12 sample exhibits good thermal stability (−50 ∼ 200 ℃), frequency stability (0.5 ∼ 500 Hz) and cycle stability (105 cycles). Hence, the current work may provide some novel perspectives for improving the W rec and η of dielectric capacitors under low/moderate electric fields. [ABSTRACT FROM AUTHOR]

[Display omitted] • Moderate Bi3+ content is helpful to enhance activation energy and breakdown field. • Through introducing A-site defect, the slim loop and high Δ P are received in SBT2. • Ultrahigh W rec of 4.77 J/cm3 and η of 85.7% were obtained in Sr 0.7 Bi 0.2 TiO 3 ceramic. • Sr 0.7 Bi 0.2 TiO 3 ceramic shows good temperature stability at 20–160 °C. • An outstanding fatigue resistance after 105 cycles was achieved in Sr 0.7 Bi 0.2 TiO 3. The application of dielectric capacitors with high energy density is very important to solve the increasingly serious energy crisis. However, the further improvement of energy density is seriously confining by their poor breakdown fields. In this work, outstanding energy storage performance is achieved in Sr 0.7 Bi x TiO 3 (x = 0.1, 0.2, 0.3 and 0.4) ceramics via A-site defect and grain size tuning. It was found that the moderate Bi3+ content is helpful to reduce sintering temperature and refine grain size, which lead to higher activation energy and breakdown field. Besides, moderate Bi3+ doping in A-site enhances polarity owing to the hybridizations between 6p orbital of Bi and 2p orbital of O. Thus, a high recoverable energy density of 4.77 J/cm3 with prominent efficiency of 85.7% at 570 kV/cm is realized in Sr 0.7 Bi 0.2 TiO 3 ceramic. Moreover, the Sr 0.7 Bi 0.2 TiO 3 ceramic also displays superior thermal stability from 20 °C to 160 °C and excellent fatigue endurance over 105 cycles. Our results in this work offer a better understanding and design methodology for developing novel high-performance dielectric capacitors. [ABSTRACT FROM AUTHOR]