Lithium containing layered high entropy oxide structures

Elucidation of the Transport Properties of Calcium-Doped High Entropy Rare Earth Aluminates for Solid Oxide Fuel Cell Applications

Solid oxide fuel cells (SOFCs) are paving the way to clean energy conversion, relying on efficient oxygen-ion conductors with high ionic conductivity coupled with a negligible electronic contribution. Doped rare earth aluminates are promising candidates for SOFC electrolytes due to their high ionic conductivity. However, they often suffer from p-type electronic conductivity at operating temperatures above 500 °C under oxidizing conditions caused by the incorporation of oxygen into the lattice. High entropy materials are a new class of materials conceptualized to be stable at higher temperatures due to their high configurational entropy. Introducing this concept to rare earth aluminates can be a promising approach to stabilize the lattice by shifting the stoichiometric point of the oxides to higher oxygen activities, and thereby, reducing the p-type electronic conductivity in the relevant oxygen partial pressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3 is synthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3 compounds exhibit a higher ionic conductivity than most of the corresponding Ca-doped rare earth aluminates accompanied by a reduction of the p-type electronic conductivity contribution typically observed under oxidizing conditions. In light of these findings, this study introduces high entropy aluminates as a promising candidate for SOFC electrolytes.

Porous High-Entropy Oxide Anode Materials for Li-Ion Batteries: Preparation, Characterization, and Applications

High-entropy oxides (HEOs), as a new type of single-phase solid solution with a multi-component design, have shown great potential when they are used as anodes in lithium-ion batteries due to four kinds of effects (thermodynamic high-entropy effect, the structural lattice distortion effect, the kinetic slow diffusion effect, and the electrochemical "cocktail effect"), leading to excellent cycling stability. Although the number of articles on the study of HEO materials has increased significantly, the latest research progress in porous HEO materials in the lithium-ion battery field has not been systematically summarized. This review outlines the progress made in recent years in the design, synthesis, and characterization of porous HEOs and focuses on phase transitions during the cycling process, the role of individual elements, and the lithium storage mechanisms disclosed through some advanced characterization techniques. Finally, the future outlook of HEOs in the energy storage field is presented, providing some guidance for researchers to further improve the design of porous HEOs.

Low-Temperature Synthesis of Mesoporous Half-Metallic High-Entropy Spinel Oxide Nanofibers for Photocatalytic CO2 Reduction

High-entropy oxides (HEOs) exhibit great prospects owing to their varied composition, chemical adaptability, adjustable light-absorption ability, and strong stability. In this study, we report a strategy to synthesize a series of porous high-entropy spinel oxide (HESO) nanofibers (NFs) at a low temperature of 400 °C by a sol-gel electrospinning technique. The key lies in selecting six acetylacetonate salt precursors with similar coordination abilities, maintaining a high-entropy disordered state during the transformation from stable sols to gel NFs. The as-synthesized HESO NFs of (NiCuMnCoZnFe)3O4 show a high specific surface area of 66.48 m2/g, a diverse elemental composition, a dual bandgap, half-metallicity property, and abundant defects. The diverse elements provide various synergistic catalytic sites, and oxygen vacancies act as active sites for electron-hole separation, while the half-metallicity and dual-bandgap structure offer excellent light absorption ability, thus expanding its applicability to a wide range of photocatalytic processes. As a result, the HESO NFs can efficiently convert CO2 into CH4 and CO with high yields of 8.03 and 15.89 μmol g-1 h-1, respectively, without using photosensitizers or sacrificial agents.

High-Entropy Lithium Argyrodite Solid Electrolytes Enabling Stable All-Solid-State Batteries

Superionic solid electrolytes (SEs) are essential for bulk-type solid-state battery (SSB) applications. Multicomponent SEs are recently attracting attention for their favorable charge-transport properties, however a thorough understanding of how configurational entropy (ΔSconf ) affects ionic conductivity is lacking. Here, we successfully synthesized a series of halogen-rich lithium argyrodites with the general formula Li5.5 PS4.5 Clx Br1.5-x (0≤x≤1.5). Using neutron powder diffraction and 31 P magic-angle spinning nuclear magnetic resonance spectroscopy, the S2- /Cl- /Br- occupancy on the anion sublattice was quantitatively analyzed. We show that disorder positively affects Li-ion dynamics, leading to a room-temperature ionic conductivity of 22.7 mS cm-1 (9.6 mS cm-1 in cold-pressed state) for Li5.5 PS4.5 Cl0.8 Br0.7 (ΔSconf =1.98R). To the best of our knowledge, this is the first experimental evidence that configurational entropy of the anion sublattice correlates with ion mobility. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors by tailoring the degree of compositional complexity. Moreover, the Li5.5 PS4.5 Cl0.8 Br0.7 SE allowed for stable cycling of single-crystal LiNi0.9 Co0.06 Mn0.04 O2 (s-NCM90) composite cathodes in SSB cells, emphasizing that dual-substituted lithium argyrodites hold great promise in enabling high-performance electrochemical energy storage.

Recent Advances for High-Entropy based Layered Cathodes for Sodium Ion Batteries

In recent years, layered oxides have been extensively studied as promising cathode materials for sodium ion batteries. However, layered oxides undergo complex phase transitions during charge-discharge process, which adversely affects the electrochemical performance. High-entropy layered oxides as a unique design concept can effectively improve the cycling performance of cathode materials by virtue of the 2D ion migration channels between the layers. Based on the concepts of high-entropy and layered oxides, this paper reviews the research status of high-entropy layered oxides in the field of sodium-ion batteries, focusing on the connection between high-entropy and layered oxide phase transitions during electrochemical charging and discharging. Finally, the advantages of layered cathode materials based high-entropy are summarized, and the opportunities and challenges of future high-entropy layered materials are proposed.

Surface-Stabilized High-Entropy Layered Oxyfluoride Cathode for Lithium-Ion Batteries

High-entropy materials have been demonstrated to improve the structural stability and electrochemical performance of layered cathode materials for lithium-ion batteries (LIBs). However, structural stability at the surface and electrochemical performance of these materials are less than ideal. In this study, we show that fluorine substitution can improve both issues. Here, we report a new high-entropy layered cathode material Li_1.2Ni_0.15Co_0.15Al_0.1Fe_0.15Mn_0.25O_1.7F_0.3 (HEOF1) based on the partial substitution of oxygen with fluorine in previously reported high-entropy layered oxide LiNi_0.2Co_0.2Al_0.2Fe_0.2Mn_0.2O₂. This new compound delivers a discharge capacity of 85.4 mAh g^-1 and a capacity retention of 71.5% after 100 cycles, showing significant improvement from LiNi_0.2Co_0.2Al_0.2Fe_0.2Mn_0.2O₂ (first 57 mAh g^-1 and 9.8% after 50 cycles). This improved electrochemical performance is due to suppression of the surface M₃O₄ phase formation. Although still an early stage study, our results show an approach to stabilize the surface structure and improve the electrochemical performance of high-entropy layered cathode materials.

Surface Phase Conversion in a High-Entropy Layered Oxide Cathode Material

High-entropy transition-metal oxides are potentially interesting cathode materials for lithium-ion batteries, among which high-entropy layered oxides are considered highly promising because there exist two-dimensional ion transport channels that may, in principle, enable fast ion transport. However, high-entropy layered oxides reported to date exhibit fast capacity fading in initial cycles and thus are hardly of any practical value. Here, we investigate the structural and property changes of a five-element layered oxide, LiNi_0.2Co_0.2Mn_0.2Fe_0.2Al_0.2O₂, using electrochemical and physical characterization techniques. It is revealed that the M₃O₄ phase formed at the surface of LiNi_0.2Co_0.2Mn_0.2Fe_0.2Al_0.2O₂ due to the migration of metal ions from octahedral sites of the transition-metal layer to tetrahedral 8a and octahedral sites of the lithium layer hinders the intercalation of lithium ion, which leads to the low initial Coulombic efficiency and fast decay of reversible capacity. This mechanism could be generally applicable to other high-entropy layered oxides with different elemental compositions.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn, Fe)₃O₄, ternary (Mn, Fe, Ni)₃O₄, and quinary (Mn, Fe, Ni, Cu, Zn)₃O₄ solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Compositionally complex doping for zero-strain zero-cobalt layered cathodes

The high volatility of the price of cobalt and the geopolitical limitations of cobalt mining have made the elimination of Co a pressing need for the automotive industry¹. Owing to their high energy density and low-cost advantages, high-Ni and low-Co or Co-free (zero-Co) layered cathodes have become the most promising cathodes for next-generation lithium-ion batteries^2,3. However, current high-Ni cathode materials, without exception, suffer severely from their intrinsic thermal and chemo-mechanical instabilities and insufficient cycle life. Here, by using a new compositionally complex (high-entropy) doping strategy, we successfully fabricate a high-Ni, zero-Co layered cathode that has extremely high thermal and cycling stability. Combining X-ray diffraction, transmission electron microscopy and nanotomography, we find that the cathode exhibits nearly zero volumetric change over a wide electrochemical window, resulting in greatly reduced lattice defects and local strain-induced cracks. In-situ heating experiments reveal that the thermal stability of the new cathode is significantly improved, reaching the level of the ultra-stable NMC-532. Owing to the considerably increased thermal stability and the zero volumetric change, it exhibits greatly improved capacity retention. This work, by resolving the long-standing safety and stability concerns for high-Ni, zero-Co cathode materials, offers a commercially viable cathode for safe, long-life lithium-ion batteries and a universal strategy for suppressing strain and phase transformation in intercalation electrodes.

High-Entropy Sn0.8(Co0.2Mg0.2Mn0.2Ni0.2Zn0.2)2.2O4 Conversion-Alloying Anode Material for Li-Ion Cells: Altered Lithium Storage Mechanism, Activation of Mg, and Origins of the Improved Cycling Stability

Benefits emerging from applying high-entropy ceramics in Li-ion technology are already well-documented in a growing number of papers. However, an intriguing question may be formulated: how can the multicomponent solid solution-type material ensure stable electrochemical performance? Utilizing an example of nonequimolar Sn-based Sn_0.8(Co_0.2Mg_0.2Mn_0.2Ni_0.2Zn_0.2)_2.2O₄ high-entropy spinel oxide, we provide a comprehensive model explaining the observed very good cyclability. The material exhibits a high specific capacity above 600 mAh g^-1 under a specific current of 50 mA g^-1 and excellent capacity retention near 100% after 500 cycles under 200 mA g^-1. The stability originates from the conversion-alloying reversible reactivity of the amorphous matrix, which forms during the first lithiation from the initial high-entropy structure, and preserves the high level of cation disorder at the atomic scale. In the altered Li-storage mechanism in relation to the simple oxides, the unwanted aggregated metallic grains are not exsolved from the anode and therefore do not form highly lithiated phases characterized by large volumetric changes. Also, the electrochemical activity of Mg from the oxide matrix can be clearly observed. Because the studied compound was prepared by a conventional solid-state route, implementation of the presented approach is facile and appears usable for any oxide anode material containing a high-entropy mixture of elements.

Acoustic Emission Monitoring of High-Entropy Oxyfluoride Rock-Salt Cathodes during Battery Operation

High-entropy materials with tailorable properties are receiving increasing interest for energy applications. Among them, (disordered) rock-salt oxyfluorides hold promise as next-generation cathodes for use in secondary batteries. Here, we study the degradation behavior of a high-entropy oxyfluoride cathode material in lithium cells in situ via acoustic emission (AE) monitoring. The AE signals allow acoustic events to be correlated with different processes occurring during battery operation. The initial cycle proved to be the most acoustically active due to significant chemo-mechanical degradation and gas evolution, depending on the voltage window. Irrespective of the cutoff voltage on charge, the formation and propagation of cracks in the electrode was found to be the primary source of acoustic activity. Taken together, the findings help advance our understanding of the conditions that affect the cycling performance and provide a foundation for future investigations on the topic.

Operando acoustic emission monitoring of degradation processes in lithium-ion batteries with a high-entropy oxide anode

In recent years, high-entropy oxides are receiving increasing attention for electrochemical energy-storage applications. Among them, the rocksalt (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O (HEO) has been shown to be a promising high-capacity anode material. Because high-entropy oxides constitute a new class of electrode materials, systematic understanding of their behavior during ion insertion and extraction is yet to be established. Here, we probe the conversion-type HEO material in lithium half-cells by acoustic emission (AE) monitoring. Especially the clustering of AE signals allows for correlations of acoustic events with various processes. The initial cycle was found to be the most acoustically active because of solid-electrolyte interphase formation and chemo-mechanical degradation. In the subsequent cycles, AE was mainly detected during delithiation, a finding we attribute to the progressive crack formation and propagation. Overall, the data confirm that the AE technology as a non-destructive operando technique holds promise for gaining insight into the degradation processes occurring in battery cells during cycling.

Sol-Gel Synthesis of Ceria-Zirconia-Based High-Entropy Oxides as High-Promotion Catalysts for the Synthesis of 1,2-Diketones from Aldehyde

Efficient Lewis-acid-catalyzed direct conversion of aldehydes to 1,2-diketones in the liquid phase was enabled by using newly designed and developed ceria-zirconia-based high-entropy oxides (HEOs) as the actual catalysts. The synergistic effect of various cations incorporated in the same oxide structure (framework) was partially responsible for the efficiency of multicationic materials compared to the corresponding single-cation oxide forms. Furthermore, a clear, linear relationship between the Lewis acidity and the catalytic activity of the HEOs was observed. Due to the developed strategy, exclusively diketone-selective, recyclable, versatile heterogeneous catalytic transformation of aldehydes can be realized under mild reaction conditions.

Construction of three-dimensional reduced graphene oxide wrapped nZVI doped with Al2O3 as the ternary Fenton-like catalyst: Optimization, characterization and catalytic mechanism

The rational design and synthesis of novel nanocomposites as effective heterogeneous catalysts is meaningful for the advances in Fenton-like technology. Herein, multiple variants of three-dimensional reduced graphene oxide wrapped nZVI doped with Al₂O₃ (3D-RGO@nZVI/Al₂O₃) were prepared by three different self-assembly methods. The composites were characterized by field emission scanning electron microscopy, nitrogen adsorption/desorption isotherms, Raman spectrum analysis, X-ray diffraction, and X-ray photoelectron spectroscopy. A series of experiments on chloramphenicol degradation at different pH values were employed to evaluate the catalytic properties of the prepared catalysts. With the systematical investigation of their morphologies, chemical components and catalytic performance, the optimal 3D-RGO@nZVI/Al₂O₃ catalyst was synthesized, which was favorable for inducing the Fenton-like reaction by activation of dissolved oxygen (DO) within a wide pH range. The anchored nZVI particles were the main active sites for catalytic oxidation, and doped Al³⁺ played a major role in buffering the pH of CAP solution. Electron spin resonance spectroscopy revealed the existence of the superoxide radicals (·O₂^-) and singlet oxygen (¹O₂), which provides a new insight into the reaction mechanism of reactive oxygen species in the Fenton-like system. This work is an essential effort to explore the promoting effect of synthesis methods on the catalytic behavior of catalysts, and to further study the Fenton-like reaction triggered by DO activation.

High-Entropy Metal-Organic Frameworks for Highly Reversible Sodium Storage

Prussian blue analogues (PBAs) are reported to be efficient sodium storage materials because of the unique advantages of their metal-organic framework structure. However, the issues of low specific capacity and poor reversibility, caused by phase transitions during charge/discharge cycling, have thus far limited the applicability of these materials. Herein, a new approach is presented to substantially improve the electrochemical properties of PBAs by introducing high entropy into the crystal structure. To achieve this, five different metal species are introduced, sharing the same nitrogen-coordinated site, thereby increasing the configurational entropy of the system beyond 1.5R. By careful selection of the elements, high-entropy PBA (HE-PBA) presents a quasi-zero-strain reaction mechanism, resulting in increased cycling stability and rate capability. The key to such improvement lies in the high entropy and associated effects as well as the presence of several active redox centers. The gassing behavior of PBAs is also reported. Evolution of dimeric cyanogen due to oxidation of the cyanide ligands is detected, which can be attributed to the structural degradation of HE-PBA during battery operation. By optimizing the electrochemical window, a Coulombic efficiency of nearly 100% is retained after cycling for more than 3000 cycles.

High Entropy and Low Symmetry: Triclinic High-Entropy Molybdates

Metal molybdates constitute a promising class of materials with a wide application range. Here, we report, to our knowledge for the first time, on the preparation and characterization of medium-entropy and high-entropy metal molybdates, synthesized by an oxalate-based coprecipitation approach. The high-entropy molybdate crystallizes in a triclinic structure, thus rendering it as high-entropy material with the lowest symmetry reported so far. This is noteworthy because high-entropy materials usually tend to crystallize into highly symmetrical structures. It is expected that application of the high-entropy concept to metal molybdates alters the material's characteristics and adds the features of high-entropy systems, that is, tailorable composition and properties. The phase purity and solid solution nature of the molybdates were confirmed by XRD, Raman spectroscopy, TEM, XPS, and ICP-OES.

Magnetic properties of high entropy oxides

High entropy oxides (HEOs) are single phase solid solutions consisting of five or more elements in equiatomic or near-equiatomic proportions incorporated into the cationic sub-lattice(s). The uniqueness of the HEOs lies in their extreme chemical complexity enveloped in a single crystallographic structure, which in many cases results in novel functionalities. From the local structure perspective, HEOs consist of an unusually large number of different metal-oxygen-metal couples. Consequently, magnetic correlations in HEOs that inherently depend on the coordination geometry, valence, spin state and type of the metal cations that are hybridized with the bridging oxygen, are naturally affected by an extreme diversity of neighboring ionic configurations. In these conditions, a complex magneto-electronic free-energy landscape in HEOs can be expected, potentially leading to stabilization of unconventional spin-electronic states. This Frontier article provides an overview of the unique magnetic features stemming from the extreme chemical disorder in HEOs along with the possible opportunities for further research and exploration of potential functionalities.