High-entropy superparaelectrics with locally diverse ferroic distortion for high-capacitive energy storage.pdf
Article https://doi.org/10.1038/s41467-024-51058-6 High-entropy superparaelectrics with locally diverse ferroic distortion for high-capacitive energy storage Jianhong Duan1, Kun Wei1, Qianbiao Du1, Linzhao Ma1, Huifen Yu2, He Qi 2 , Yangchun Tan3, Gaokuo Zhong 3 & Hao Li 1 Superparaelectrics are considered promising candidate materials for achiev- ing superior energy storage capabilities. However, due to the complicated local structural design, simultaneously achieving high recoverable energy density (Wrec) and energy storage efﬁciency (η) under high electric ﬁelds remains a challenge in bulk superparaelectrics. Here, we propose utilizing entropy engineering to disrupt long-range ferroic orders into local poly- morphic distortion disorder with multiple BO6 tilt types and diverse hetero- geneous polarization conﬁgurations. This strategy reduces the switching barriers, thereby facilitating the emergence of superparaelectric behaviors with ideal polarization forms. Furthermore, it enables high polarization response, negligible remnant polarization, delayed polarization saturation, and enhanced breakdown electric ﬁelds (Eb) in high-entropy superpara- electrics. Consequently, an extraordinary Wrec of 15.48 J cm–3 and an ultrahigh η of 90.02% are achieved at a high Eb of 710 kV cm–1, surpassing the compre- hensive energy storage performance of previously reported bulk superpara- electrics. This work demonstrates that entropy engineering is a viable strategy for designing high-performance superparaelectrics. With an increasing international focus on environmental protection, efﬁcient energy storage technologies have become a focal point of societal concern1–3. Dielectric ceramic capacitors, with their ultrafast charge/discharge rate and ultrahigh power density, are extensively studied as a potential solution for energy storage4–6. However, the relatively low recoverable energy density (Wrec) and energy storage efﬁciency (η) of dielectric ceramic capacitors hinder their development towards miniaturization and integration7–9. Therefore, there is an urgent need to develop lead-free bulk ceramics with both ultrahigh Wrec and η. In recent years, numerous lead-free bulk ceramics have been developed for capacitive energy storage. For instance, high Wrec of 11.4 J cm–3 and 18.5 J cm–3 have been realized in AgNbO3 (AN)-based and NaNbO3 (NN)-based antiferroelectric (AFE) ceramics, respectively10,11. Nevertheless, their η values are capped at or below 80%, which is mainly caused by the AFE-ferroelectric (FE) phase transition. Similar low η phenomena are often observed in FEs or relaxor ferroelectrics (RFEs), such as K0.5Na0.5NbO3 (KNN)-based, Bi0.5K0.5TiO3 (BKT)-based, and BiFeO3 (BF)-based ceramics12–14. Although a high η (>90%) can be obtained in some linear dielectrics, such as CaTiO3 (CT)-based and SrTiO3 (ST)-based ceramics15–18, the relatively low intrinsic polarization leads to their Wrec usually being <7 J cm–3. In addition, although an increasein the electricﬁeld contributes to an enhancement of Wrec, it is usually accompanied by an increase in remnant polarization (Pr), hys- teresis losses, and leakage currents, all of which have negative impacts on η7,19. Therefore, the trade-off between Wrec and η has become a primary challenge in designing high-performance dielectric ceramics. Recently, superparaelectrics (SPEs) developed in RFEs have been considered as promising candidate materials for energy storage20–22. Received: 13 April 2024 Accepted: 30 July 2024 Check for updates 1College of Electrical and Information Engineering, Hunan University, Changsha 410082, China. 2Beijing Advanced Innovation Center for Materials Genome Engineering, Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China. 3Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China. e-mail: qiheustb@ustb.edu.cn; gk.zhong@siat.ac.cn; hli@hnu.edu.cn Nature Communications|        (2024) 15:6754  1 1234567890():,; 1234567890():,; The state of SPEs appears within the temperature range from Tm (the temperature corresponding to the maximum dielectric constant) to TB (the Burns temperature) and is characterized by weakly coupled polar nanoregions (PNRs). Therefore, SPEs not only maintain a high max- imum polarization (Pm) but also allow for ﬂexible polarization redir- ection with small hysteresis, leading to a higher η compared to conventional RFEs. Based on SPE engineering23,24, Bi0.5Na0.5TiO3 (BNT)- based ceramics achieved a Wrec of 7.2 J cm–3 and a η of 86% at a breakdown electric ﬁeld (Eb) of 430 kV cm–1, and further, BaTiO3- Bi0.5Na0.5TiO3-NaNbO3 (BT-BNT-NN) ternary ceramics reached a Wrec of 10.59 J cm–3 and a η of 87.6% at a relatively high Eb of 550 kV cm–1. Despite advancements in researching SPEs, the challenge of simulta- neously achieving ultrahigh energy density (Wrec ≥15 J cm–3) and minimal losses (η ≥90%) under high electric ﬁelds in bulk SPEs persists due to the lack of precise local structural design. From a thermo- dynamic perspective, when the long-range FE order in RFE materials shrinks down to nanodomains (or PNRs), the switching energy barriers also decrease. As the energy barrier diminishes to a level comparable to or lower than the thermal disturbance energy (kBT, where kB represents the Boltzmann constant), nanodomains (or PNRs) can undergo ﬂexible switching processes with minimal hysteresis, thus exhibiting ideal polarization form with SPE properties on the macro- scopic scale25,26. In this sense, it should be feasible to design ideal SPEs by disrupting long-range FE order to construct ﬂexible local polariza- tion conﬁgurations, thereby lowering the switching energy barriers. Here, we consider that entropy engineering has been demon- strated as an advanced strategy for regulating FE polarization in pie- zoelectric and energy storage dielectrics27–30. This is primarily due to the disordered component distribution leading to unmatched atomic size, mass, valence state, and electronegativity, which induce random local strain and electric ﬁelds, providing inﬁnite possibilities for tuning the local polarization conﬁgurations. Therefore, we design high- entropy SPEs with superior comprehensive energy storage performance through entropy engineering, as shown in Fig. 1. To ensure a large polarization response, Bi0.47Na0.47Ba0.06TiO3 (BNBT), known for its high polarization characteristics, is selected as the base material. The coexistence of tetragonal (T) and rhombohedral (R) phases in BNBT can reduce polarization anisotropy and promote polarization rotation, thereby lowering the switching energy barriers. Meanwhile, Sr0.7La0.2Ta0.2Ti0.75O3 (SLTT) is added to BNBT to regulate conﬁguration entropy (Sconﬁg) and create the (1 – x)BNBT-xSLTT sys- tem (abbreviated as SLTT-x). The high-entropy effect, combined with the small Pr, the low hysteresis loss of SrTiO3, and the large bandgap (Eg) of La2O3 (5.0 eV) and Ta2O5 (4.0 eV), further enhances the energy storage performance. Through entropy engineering, the long-range order is effectively disrupted, resulting in the formation of locally diverse ferroic distortions. These distortions encompass rich hetero- geneous polarization conﬁgurations of R, T, and monoclinic (M)-like phases embedded in a cubic (C) matrix, as well as in-phase tilted, anti- phase tilted, and non-tilted BO6 types. The induction of these het- erogeneous conﬁgurations leads to the eventual realization of ideal SPE behavior, while the engineered multiple BO6 tilt types effectively prevent premature polarization saturation. As expected, the high- entropy SPE (SLTT-0.30) exhibits a slender P-E loop at a large Eb of 710 kV cm–1, yielding an impressive Wrec of 15.48 J cm–3 and an ultrahigh η of 90.02%. These results signify a breakthrough in achieving superior energy storage capacities for bulk SPE ceramics. Results and discussion Induction of SPE state As shown in Supplementary Table 1, the Sconﬁg values of the SLTT-x system are 0.88 R (x = 0), 1.47 R (x = 0.20), 1.54 R (x = 0.25), 1.61 R (x = 0.30), and 1.66 R (x = 0.35), respectively. Supplementary Fig. 1 illustrates the entropy-dependent dielectric properties. When Sconﬁg reaches 1.61 (SLTT-0.30) and 1.66 R (SLTT-0.35), the Tm drops below room temperature, indicating that both samples have reached room Polarization (μC cm-2) Low Eb Large Pr Large Pm Electric field (kV cm-1) [010] [100] [001] Bi Na Ba Ti O Sr La Ta y p o rt n e g nis a e r c n I Temperature (℃) Dielectric constant (εr) Tm TB Room temperature High temperature  SPE state Temperature (℃) Dielectric constant (εr) Tm SPE state Room temperature Polarization (μC cm-2) Electric field (kV cm-1) Inducing strong disorder SPE state and ferroic distortion Superior performance Long-range ferroic orders Heterogeneous  configurations Various BO6 tilt types Diverse ferroic  distortions R-PNRs C-matrix M-PNRs T-PNRs Low Wrec and η Ehanced Eb Reduced Pr Delayed polarization saturation RFE Ultrahigh Wrec and η SPE Large atomic  differences - + - + TB Fig. 1 | Schematic diagram of using entropy engineering to achieve excellent comprehensive energy storage performance. Article https://doi.org/10.1038/s41467-024-51058-6 Nature Communications|        (2024) 15:6754  2 synergistic effects, the energy storage performance of SLTT-0.30 ceramic demonstrates excellent stability at different temperatures and frequencies. As illustrated in Supplementary Fig. 14a, b, the unipolar P-E loops maintain slim shapes at different temperatures and fre- quencies. Therefore, the SLTT-0.30 ceramic achieves superior tem- perature insensitivity over the temperature range of 25–175 °C, with Wrec ≈6.59 ± 0.51 J cm–3 and η ≈90.55 ± 2.84% (Fig. 4f). It also exhibits signiﬁcant frequency insensitivity over the frequency range of 1–500 Hz, with Wrec ≈7.43 ± 0.22 J cm–3 and η ≈87.58 ± 0.91% (Supple- mentary Fig. 14c). These superior stabilities are superior to most reported high-performance lead-free bulk ceramics32,34,55–58. Supplementary Fig. 15 shows the excellent charge-discharge properties of the SLTT-0.30 high-entropy SPE. It achieves a high dis- charge energy density (Wdis = 2.37 J cm–3) in an ultrafast time (t0.9 = 33 ns) under 320 kV cm–1. The maximum current (Imax), current density (CD), and power density (PD) reach 19.5 A, 621.7 A cm–2, and 93.3 MW cm–3 under an electric ﬁeld of 300 kV cm–1, respectively. Additionally, the charge-discharge parameters demonstrate excellent temperature stability at 240 kV cm–1. Minimal change in performance even at temperatures from 25 °C – 150 °C, with Wdis ≈1.42 ± 0.05 J cm–3, t0.9 ≈31 ± 2 ns, Imax ≈13.37 ± 0.02 A, CD ≈425.56 ± 0.78 A cm–2, and PD ≈46.83 ± 0.09 MW cm–3. Overall, the SLTT-0.30 ceramic shows great promise as a dielectric for energy storage capacitors due to its stability and impressive charge-discharge performance. In summary, a lead-free bulk SPE has been developed using entropy engineering, demonstrating exceptional comprehensive energy storage performance with a large Wrec of ~15.48 J cm–3 and an ultrahigh η of ~90.02% at 710 kV cm–1. This performance represents the best reported to date for bulk SPEs. Moreover, the high-entropy SPE exhibits remarkable temperature stability, frequency stability, and superior charge-discharge performance. The impressive performance is attributed to the induction of locally diverse ferroic distortion by entropy engineering, including various BO6 tilt types and hetero- geneous symmetries, resulting in a room temperature SPE state with simultaneous lower Pr, improved Eb, and delayed polarization satura- tion. This work provides valuable insights into the interplay between entropy engineering, SPE behavior, local structure, and energy storage performance. Methods Sample preparation The (1 – x)BNBT-xSLTT system (abbreviated as SLTT-x, x = 0, 0.20, 0.25, 0.30, 0.35) was fabricated by raw powders of Bi2O3 (purity of ≥99%), Na2CO3 (purity of 99.99%), BaCO3 (purity of 99.8%), TiO2 (purity of 99.5%), SrCO3 (purity of 99.5%), La2O3 (purity of 99.9%), and Ta2O5 (purity of 99.5%) through a conventional solid-state reaction. The corresponding raw material powders were weighed according to the chemical formulas of BNBT and SLTT, with 1 mol% of Bi2O3 and Na2CO3 added to BNBT to prevent the volatilization of Bi and Na. The BNBT and SLTT raw material powders were dispersed for 18 h by ball milling with zirconia balls in ethanol, respectively, and then dried. The dried BNBT slurry was calcined at 850 °C for 5 h, while the dried SLTT slurry was calcined at 1000 °C for 5 h. Subse- quently, the calcined powders of both materials were mixed and ball- milled for 20 h, followed by drying. The dried powder was mixed with polyvinyl alcohol (PVA) binder and pressed into pellets with a dia- meter of ~10 mm and a thickness of ~1 mm. The pellets were held at 600 °C for 3 h to volatilize PVA, then sintered at 1050–1180 °C for 2 h to obtain ceramic samples. Structural characterization The phase structures of the ceramics at room temperature were examined by X-ray powder diffraction (XRD, Philips X’Pert Pro MPD, Netherlands). The surface microstructure of the ceramics after thin- ning, polishing, and thermal etching was analyzed using ﬁeld emission scanning electron microscopy (SEM, FEI, Quanta FEG250, USA). Selected area electron diffraction (SAED), domain morphology, and lattice fringes were examined by transmission electron microscopy (TEM, JEOL, JEM-2100, Japan). The response of domain structure to electric ﬁelds was characterized using piezoresponse force micro- scopy (PFM, Asylum Research, MFP-3D-Inﬁnity, USA). The local struc- ture of well-polished ceramics was analyzed by a Raman scattering spectrometer (Renishaw, inViaTM, UK). To analyze temperature- dependent structural properties, Raman spectra were obtained using a Raman spectrometer (Horiba Jobin Yvon HR800, France) with a heating stage (Linkam, THM 600, UK) under 532 nm excitation, ran- ging from 25 °C to 250 °C, on the polished SLTT-0.30 ceramic samples. Additionally, temperature-dependent X-ray diffraction (XRD) of SLTT- 0.30 ceramic was obtained using CuKα radiation at temperatures ranging from 25 °C to 250 °C with an XRD instrument (X’pert PRO, PANalytical, Netherlands). To analyze polarization vectors, ampli- tudes, and angles, high-angle annular dark-ﬁeld (HAADF) atomic-scale images of the SLTT-0.30 ceramic were obtained using atomic- resolution STEM (aberration-corrected Titan Themis 3300), and cus- tom MATLAB scripts were employed for analysis. Electrical performance measurement For energy storage measurements, the samples were thinned and polished to ~0.05 ± 0.01 mm thickness, and gold electrodes with a diameter of 1.5 mm were prepared on their surfaces using ion sput- tering. The ferroelectric analyzer (Aix ACCT, TF analyzer 1000, Germany) was utilized to measure the unipolar P-E loops at room temperature and 10 Hz frequency. Additionally, the same equipment was employed to measure unipolar P-E loops at different temperatures and frequencies to calculate the temperature-dependent and frequency-dependent energy storage performance. The dielectric properties were tested by a dielectric analysis instrument (Tongguo Technology, HCT1821, China). The charge-discharge performance of the ceramics were investigated using a charge-discharge tester (Tongguo Technology, CFD-003, China). 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Acknowledgements This work was supported by the National Natural Science Foundation of China (No.52102129, H.L.), the Hunan Provincial Natural Science Foun- dation of China (No.2023JJ30138, H.L.), the Science and Technology Innovation Program of Hunan Province (2023RC3094, H.L.), and the Shenzhen Science and Technology Program (Grant No. RCYX20200714114733204 and JCYJ20200109115219157, G.K.Z.). Author contributions This work was conceived and designed by J.H.D., H.Q., G.K.Z., and H.L. Sample fabrication was performed by J.H.D. and K.W., who also con- ducted energy storage, dielectric, and charge-discharge performance tests, as well as analyzed relevant data. Finite element simulations of breakdown characteristics were conducted by H.F.Y. SEM and Raman spectroscopy were performed by H.L. and L.Z.M. UV-Vis data was col- lected and processed by H.L. PFM images were captured and processed by G.K.Z. HAADF-STEM imaging was carried out by H.Q. and processed accordingly. Temperature-dependent Raman and XRD data were col- lected by H.Q. and analyzed by J.H.D. XRD testing and corresponding data analysis were carried out by Y.C.T. The manuscript was drafted by J.H.D. and revised by H.L., Q.B.D., H.Q., G.K.Z., and Y.C.T. All authors participated in data analysis and discussions. Competing interests The authors declare no competing interests Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-024-51058-6. Correspondence and requests for materials should be addressed to He Qi, Gaokuo Zhong or Hao Li. Peer review information Nature Communications thanks Dibakar Das, Jigong Hao and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review ﬁle is available. 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Title: High-entropy superparaelectrics with locally diverse ferroic distortion for high-capacitive energy storage ​
Authors: Jianhong Duan, Kun Wei, Qianbiao Du, Linzhao Ma, Huifen Yu, He Qi, Yangchun Tan, Gaokuo Zhong, Hao Li ​

Publisher: Nature Communications
Date: 2024
Abstract: Superparaelectrics are considered promising candidate materials for achieving superior energy storage capabilities. ​ However, due to the complicated local structural design, simultaneously achieving high recoverable energy density (Wrec) and energy storage efficiency (η) under high electric fields remains a challenge in bulk superparaelectrics. ​ Here, we propose utilizing entropy engineering to disrupt long-range ferroic orders into local polymorphic distortion disorder with multiple BO6 tilt types and diverse heterogeneous polarization configurations. ​ This strategy reduces the switching barriers, thereby facilitating the emergence of superparaelectric behaviors with ideal polarization forms. ​ Furthermore, it enables high polarization response, negligible remnant polarization, delayed polarization saturation, and enhanced breakdown electric fields (Eb) in high-entropy superparaelectrics. ​ Consequently, an extraordinary Wrec of 15.48 J cm–3 and an ultrahigh η of 90.02% are achieved at a high Eb of 710 kV cm–1, surpassing the comprehensive energy storage performance of previously reported bulk superparaelectrics. ​ This work demonstrates that entropy engineering is a viable strategy for designing high-performance superparaelectrics.  