Multiphase composition design is a strategy to optimize the microstructures and properties of ceramic materials through mutual inhibition of grain growth, complementary property improvement, or even mutually reinforcing effects. More interesting phenomena can be expected if chemical interactions between the constituent phases exist. In this study, spark plasma sintering was used to prepare fully dense dual-phase (Zr, Hf, Ta)B₂-(Zr, Hf, Ta)C ceramics from self-synthesized equimolar medium-entropy diboride and carbide powders. The obtained ceramics were composed of two distinct solid solution phases including the Zr-rich diboride phase and the Ta-rich carbide phase, indicating metal element exchange occurred between the starting equimolar medium-entropy diboride and carbide during sintering. Owing to the mutual grain-boundary pinning effect, fine-grained dual-phase ceramics were obtained. The chemical driving force originating from the metal element exchange during the sintering process is considered to promote the densification process of the ceramics. The metal element exchange between the medium-entropy diboride and carbide phase significantly increased Young’s modulus of the dual-phase ceramics. The dual-phase medium-entropy 50 vol.% (Zr, Hf, Ta)B₂-50 vol.% (Zr, Hf, Ta)C ceramics with the smallest grain size exhibited the highest hardness of 22.4 ± 0.2 GPa. It is inferred that optimized comprehensive properties or performance of dual-phase high-entropy or medium-entropy ceramics of diborides and carbides can be achieved by adjusting both the volume content and the metal element composition of the corresponding starting powders of diborides and carbides.

The coordinated development of new energy vehicles and the energy storage industry has become essential for reducing carbon emissions. The cathode material is the key material that determines the energy density and cost of a power battery, but currently developed and applied cathode materials cannot meet the requirements for high specific capacity, low cost, safety, and good stability. High-entropy materials (HEMs) are a new type of single-phase material composed of multiple principal elements in equimolar or near-equimolar ratios. The interaction between multiple elements can play an important role in improving the comprehensive properties of the material, which is expected to solve the limitations of battery materials in practical applications. Therefore, this review provides a comprehensive overview of the current development status and modification strategies of power batteries (lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs)), proposes a high-entropy design strategy, and analyses the structure–activity relationship between the high-entropy effects and battery performance. Finally, future research topics related to high-entropy cathode materials, including computational guide design, specific synthesis methods, high-entropy electrochemistry, and high-throughput databases, are proposed. This review aims to provide practical guidance for the development of high-entropy cathode materials for next-generation power batteries.

Rare-earth aluminates (REAlO₃) are potential thermal barrier coating (TBC) materials, but the relatively high thermal conductivity (k₀, ~13.6 W·m⁻¹·K⁻¹) and low fracture toughness (K_IC, ~1.9 MPa·m^1/2) limit their application. This work proposed a strategy to improve their properties through the synergistic effects of high-entropy engineering and particulate toughening. High-entropy (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)AlO₃ (HEAO)-based particulate composites with different contents of high-entropy (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZO) were designed and successfully prepared by solid-state sintering. The high-entropy feature of both the matrix and secondary phases causes the strong phonon scattering and the incorporation of the HEZO secondary phase, remarkedly inhibiting the grain growth of the HEAO phase. As a result, HEAO–xHEZO (x = 0, 5%, 10%, 25%, and 50% in volume) ceramic composites show low thermal conductivity and high fracture toughness. Compared to the most commonly applied TBC material—yttria stabilized-zirconia (YSZ), the HEAO–25%HEZO particulate composite has a lower thermal conductivity of 0.96–1.17 W·m⁻¹·K⁻¹ (298–1273 K), enhanced fracture toughness of 3.94±0.35 MPa·m^1/2, and comparable linear coefficient of thermal expansion (CTE) of 10.5×10⁻⁶ K⁻¹. It is believed that the proposed strategy should be revelatory for the design of new coating materials including TBCs and environmental barrier coatings (EBCs).

It is generally reported that the grain growth in high-entropy ceramics at high temperatures is relatively slower than that in the corresponding single-component ceramics owing to the so-called sluggish diffusion effect. In this study, we report a fast grain growth phenomenon in the high-entropy ceramics (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)MgAl₁₁O₁₉ (HEMA) prepared by a conventional solid-state reaction method. The results demonstrate that the grain sizes of the as-sintered HEMA ceramics are larger than those of the corresponding five single-component ceramics prepared by the same pressureless sintering process, and the grain growth rate of HEMA ceramics is obviously higher than those of the five single-component ceramics during the subsequent heat treatment. Such fast grain growth phenomenon indicates that the sluggish diffusion effect cannot dominate the grain growth behavior of the current high-entropy ceramics. The X-ray photoelectron spectroscopy (XPS) analysis reveals that there are more oxygen vacancies (O_V) in the high-entropy ceramics than those in the single-component ceramics owing to the variable valance states of Eu ion. The high-temperature electrical conductivities of the HEMA ceramics support this analysis. It is considered that the high concentration of O_V and its high mobility in HEMA ceramics contribute to the accelerated migration and diffusion of cations and consequently increase the grain growth rate. Based on this study, it is believed that multiple intrinsic factors for the high-entropy ceramic system will simultaneously determine the grain growth behavior at high temperatures.

The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O₂ phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr₃C₂ and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo₂C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS.

High-entropy oxides (HEOs) are a new class of emerging materials with fascinating properties (such as structural stability, tensile strength, and corrosion resistance). High-entropy oxide coated Ni-rich cathode materials have great potential to improve the electrochemical performance. Here, we present a facile self-ball milling method to obtain (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEO) coated LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811). The HEO coating endows NCM811 with a stable surface, reduces the contact with the external environment (air and electrolyte), and inhibits side reactions between cathode and electrolyte. These favorable effects, especially when the coating amount is 5 wt%, result in a significant reduction of the battery polarization and an increase in the capacity retention from 57.3% (NCM811) to 74.2% (5HEO-NCM811) after 300 cycles at 1 C (1 C = 200 mA·h·g^-1). Moreover, the morphology and spectroscopy analysis after the cycles confirmed the inhibitory effect of the HEO coating on electrolyte decomposition, which is important for the cycle life. Surprisingly, HEO coating reduces the viscosity of slurry by 37%-38% and significantly improves the flowability of the slurry with high solid content. This strategy confirms the feasibility of HEO-modified Ni-rich cathode materials and provides a new idea for the design of high-performance cathode materials for Li-ion batteries.