The global demand for efficient energy storage solutions is escalating due to growing environmental concerns. Dielectric ceramic capacitors, known for their fast charge/discharge rates and high power density, are promising candidates but suffer from limitations in recoverable energy density (Wrec) and energy storage efficiency (η). Current lead-free bulk ceramic options, such as AgNbO₃ (AN)-based and NaNbO₃ (NN)-based antiferroelectrics (AFEs), achieve high Wrec but limited η. Similarly, ferroelectrics (FEs) and relaxor ferroelectrics (RFEs) often exhibit this trade-off. Linear dielectrics like CaTiO₃ (CT)-based and SrTiO₃ (ST)-based ceramics offer high η but low Wrec. Increasing electric fields improves Wrec but usually worsens η due to increased remnant polarization (Pr), hysteresis losses, and leakage currents. Superparaelectrics (SPEs), developed in RFEs, offer a potential solution by combining high maximum polarization (Pm) with flexible polarization redirection and minimal hysteresis. While some SPEs have shown promise, simultaneously achieving ultrahigh Wrec (≥15 J cm⁻³) and high η (≥90%) under high electric fields in bulk materials remains challenging due to the difficulties in precise local structural design. This study proposes entropy engineering as a solution to this problem, aiming to create SPEs with superior comprehensive energy storage performance.

The literature review highlights the limitations of existing lead-free energy storage materials. Antiferroelectric materials, while achieving high energy density, suffer from low efficiency due to the AFE-FE phase transition. Similarly, ferroelectric and relaxor ferroelectric materials struggle with the trade-off between energy density and efficiency. Linear dielectrics, while offering high efficiency, have low energy density due to low intrinsic polarization. Superparaelectric materials, with their weakly coupled polar nanoregions (PNRs), have emerged as promising candidates, but achieving both high energy density and high efficiency simultaneously in bulk superparaelectrics remains a challenge. The authors highlight previous work on BNT-based ceramics and BT-BNT-NN ternary ceramics, which demonstrated improved energy storage performance but still fell short of the desired ultrahigh energy density and efficiency.

The researchers designed a high-entropy SPE system using Bi0.47Na0.47Ba0.06TiO3 (BNBT) as the base material, known for its high polarization characteristics, and Sr0.7La0.2Ta0.2Ti0.75O3 (SLTT) as an additive to regulate configuration entropy (Sconfig). They created a (1-x)BNBT-xSLTT system (SLTT-x) with varying x values. The material synthesis involved conventional solid-state reaction, including ball milling, calcining, and sintering. Characterization techniques included X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), piezoresponse force microscopy (PFM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), Raman spectroscopy, and in situ XRD. Electrical properties, including polarization-electric field (P-E) loops, dielectric properties, and charge-discharge characteristics, were measured using a ferroelectric analyzer and a charge-discharge tester. Finite element simulations were used to investigate electric field distribution and electric tree propagation. The configuration entropy, dielectric properties and unipolar P-E loops under low electric fields were also examined.

The study found that increasing Sconfig led to a decrease in Tm (temperature of maximum dielectric constant) below room temperature, indicating a transition to a room temperature SPE state. High-resolution TEM and SAED revealed the presence of locally diverse ferroic distortions with multiple BO6 tilt types (in-phase tilted, anti-phase tilted, and non-tilted) and heterogeneous polarization configurations (R, T, M-like phases in a C matrix). PFM confirmed the presence of highly dynamic PNRs, suppressing heat generation during polarization rotation and improving Eb. HAADF-STEM analysis directly visualized the heterogeneous polarization configurations and the presence of interconnected C-R-T-M-like phases, reducing polarization anisotropy and switching barriers. The SLTT-0.30 composition exhibited a slim unipolar P-E loop with a high Pm of 57.36 µC cm⁻², a low Pr of 2.52 µC cm⁻², and a high Eb of 710 kV cm⁻¹. This resulted in a Wrec of 15.48 J cm⁻³ and an η of 90.02%, exceeding previously reported values. Further analysis demonstrated excellent temperature and frequency stability of the energy storage performance. In situ Raman and XRD confirmed the structural stability over a wide temperature range. Charge-discharge tests revealed excellent performance with a high discharge energy density, fast discharge time, high maximum current, current density, and power density.

The findings demonstrate that entropy engineering effectively induces locally diverse ferroic distortions, leading to the formation of a room-temperature SPE state with desirable energy storage properties. The simultaneous reduction of Pr, enhancement of Eb, and delayed polarization saturation contribute to the superior performance. The interconnected C-R-T-M-like phases minimize polarization anisotropy and switching barriers, leading to a flattened switching pathway and low hysteresis loss. The high Eb is attributed to factors including entropy-induced lattice distortion, small grain size, dense grain boundaries, and low dielectric loss. The excellent temperature and frequency stability of the energy storage performance further highlight the potential of this material for practical applications.

This study successfully developed a high-performance lead-free bulk SPE using entropy engineering. The material achieved record-high Wrec and η values for bulk SPEs, exhibiting exceptional temperature and frequency stability along with superior charge-discharge performance. The superior properties are attributed to the creation of locally diverse ferroic distortions, resulting in a room-temperature SPE state with reduced Pr, enhanced Eb, and delayed polarization saturation. This work provides valuable insights into designing high-performance energy storage materials and opens new avenues for exploring the interplay between entropy engineering, local structure, and energy storage capabilities.

While the study demonstrates exceptional performance, future research could explore the long-term stability and reliability of the material under extreme conditions. Further investigations into the impact of synthesis parameters on the microstructure and energy storage properties could optimize the material's performance even further. The scalability of the synthesis method for industrial production also warrants further consideration.