Stabilizing Configurational Entropy in Spinel-type High Entropy Oxides during Discharge-Charge by Overcoming Kinetic Sluggish Diffusion.

Spinel‐type high entropy oxides (HEOs) have emerged as promising next‐generation lithium‐ion battery anodes owing to exceptional electrochemical performance. However, suppressing irreversible phase transformations caused by high‐entropy to low‐entropy state transitions during discharge–charge has remained challenging. The core issue stems from an insufficient understanding of phase evolution pathways and the key thermodynamic/kinetic driving forces, which is due to current methodological limitations in analyzing highly disordered structures. Further complicating this challenge is the elusive impact of nanosized effects on both thermodynamic and kinetic processes. This study addresses these challenges through three synergistic approaches: 1) investigating phase evolution mechanisms across different particle sizes to delineate nanosized effects; 2) resolving complex local structures by pair distribution function analyses and 7 Li magic‐angle spinning nuclear magnetic resonance spectroscopy; 3) elucidating influences of high entropy on phase evolution via DFT calculations. Comprehensive results reveal a complex phase evolution process governed by the thermodynamic‐kinetic interplay. The incomplete phase transformations of the rock‐salt‐like intermediate phase during discharge, which are attributable to high entropy‐mediated kinetic sluggish diffusion, account for the transition from high‐entropy to low‐entropy states. By shortening the solid‐state diffusion lengths, the kinetic limitations can be overcome, as demonstrated by nanosized spinel‐type HEOs achieving reversible phase transformations during discharge‐charge.

High Entropy Oxides: Structure and Properties

High entropy oxides (HEOs) are solid state inorganic compounds in which entropy, rather than enthalpy, plays a dominant role in stabilizing a single-phase structure at high temperatures. This work has been motivated in part by prior studies of multicomponent alloys in which four or more cations occupy the same crystallographic site in equal proportions, known as high entropy alloys (HEAs), which have superior mechanical properties and high radiation tolerances due in part to high configurational entropy.[1] In the case of HEOs, the first example is the rock salt (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, which has generated a great deal of interest in this class of materials.[2,3] We have recently demonstrated that HEOs prepared by mechanochemical synthesis can be prepared in pure form, and may be useful for catalysis.[4,5] It has also been shown that HEOs are of interest for high ionic conductivity and electrochemical energy storage.[6,7] In this study, we have examined the electrochemical cycling of new high entropy rock salt phases versus lithium, and found an effect of composition on the cycling performance. Samples were prepared through high energy milling of starting binary oxides, which proceed to decompose and then reform pure compounds at high temperature. We have confirmed that entropy plays a role in this transformation for these rock salt HEOs, and that the precise composition has an impact on the temperature and kinetics of pure phase formation; investigations of the synthesis and subsequent decomposition have been conducted in our laboratory using high temperature in-situ X-ray diffraction on a Panalytical diffractometer equipped with an XRK900 stage. STEM/EDS studies on quenched ex-situ samples will be presented that show how elemental segregation occurs as a function of temperature. The results of this study will be highly impactful for the growing community of researchers investigating the design and synthesis of the new class of materials, the high entropy oxides.[1] Y. Lu, et. al., Sci. Rep. 4, 6200 (2014); Y. F. Ye, et. al., Mater. Today 19 (6), 349 (2016); Zhang, Y., et. al., Nature Commun. 6, 8736 (2015); [2] C. M. Rost, et. al., Nature Commun. 6, 8485 (2015); [3] B. Jiang, et. al., Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides. Journal of the American Chemical Society 2021, 143, (11), 4193-4204; [4] H. Chen, et al., Mechanochemical Synthesis of High Entropy Oxide Materials under Ambient Conditions: Dispersion of Catalysts via Entropy Maximization, ACS Materials Lett. 2019, 1, 1, 83–88; [5] H. Chen, et. al., Entropy-stabilized metal oxide solid solutions as CO oxidation catalysts with high-temperature stability, J. Mater. Chem. A 2018, 6, 11129-11133; [6] Q. Wang, et. al., Multi-anionic and -cationic compounds: new high entropy materials for advanced Li-ion batteries, Energy Environ. Sci., 2019, 12, 2433; [7] D. Berardan, et. al., Room temperature lithium superionic conductivity in high entropy oxides, J. Mater. Chem. A, 2016, 4, 9536.

Chapter Fifteen - Metabolomic Studies of Patient Material by High-Resolution Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Recent status and challenging perspective of high entropy oxides for chemical catalysis

In situ flow magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and synchrotron-based pair distribution function (PDF) analyses were applied to study water's interactions with the Brønsted acidic site and the surrounding framework in the SAPO-34 catalyst at temperatures up to 300 °C for NMR spectroscopy and 700 °C for PDF. 29 Si enrichment of the sample enabled detailed NMR spectroscopy investigations of the T-atom generating the Brønsted site. By NMR spectroscopy, we observed dehydration above 100 °C and a coalescence of Si peaks due to local framework adjustments. Towards 300 °C, the NMR spectroscopy data indicated highly mobile acidic protons. In situ total X-ray scattering measurements analyzed by PDF showed clear changes in the Al local environment in the 250-300 °C region, as the Al-O bond lengths showed a sudden change. This fell within the same temperature range as the increased Brønsted proton mobility. We suggest that the active site in this catalyst under industrial conditions comprises not only the Brønsted proton but also SiO4 . To the best of our knowledge, this is the first work proposing a structural model of a SAPO catalyst by atomic PDF analysis. The combination of synchrotron PDF analysis with in situ NMR spectroscopy is promising in revealing the dynamic features of a working catalyst.

The burgeoning sector of electric vehicles has significantly spurred the exploration of high-energy density and long-term cycling life electrode materials for advanced lithium-ion batteries (LIBs). Presently, the constrained capacity of commercialized graphite (LiC6: 372 mAh g–1) and Li4Ti5O12 (Li7Ti5O12: 175 mAh g–1) anodes falls short of meeting the heightened requirements for energy density in such applications. Transition metal oxides (TMOs, MxOy) with conversion reaction have been widely scrutinized as next-generation LIB anode materials owing to their resonable reversible capacity in comparison to conventional graphite anode. Unfortunately, the crystal structures of TMOs typically undergo severe degradation during rapid conversion/alloying reactions over continuous lithiation/delithiation process. This phenomenon leads to rapid capacity decay, poor reversibility, and consequently, a significant hindrance to their applicability as LIB anodes. In recent developments, high-entropy oxides (HEOs), analogous to high-entropy metallic alloys (HEMAs), have garnered considerable interest as an emerging class of solid solutions, involving a multitude of metalic cations in an equimolar ratio. Intriguingly, the high configurational entropy (Sconfig) present plays a crucial role in stabilizing their single-phase crystal structures. Pioneering this concept, Rost et al. introduced entropy-stabilization into TMOs, successfully creating the rocksalt HEO, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O with the value of Sconfig ≥ 1.6R. Leveraging substantial compositional flexibility by modifying the stoichiometry and incorporating diverse cationic species, HEOs possess unforeseen and distinctive physicochemical properties. Consequently, they have been applied across diverse domains, encompassing catalysts, thermoelectrics, superionic conductors, and battery electrodes.The initial investigation into HEOs as LIB anode materials featured the rocksalt-type composition (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O. Sarkar et al. demonstrated its remarkably reversible Li-storage properties, showcasing stable cycling performance with reversible capacities ranging from 500 to 700 mAh g−1 at current density of 200 mA g−1 even after 300 cycles. Subsequently, Patra et al. introduced the single-phase spinel HEO (Fe0.2Cu0.2Ni0.2Cr0.2Mn0.2)3O4 as a LIB anode, achieving a specific capacity of 640 mAh g−1 at current density of 500 mA g−1 over 400 cycles. Impressively, the specific capacity remained at 596 mAh g−1 at a high current density of 2.0 A g−1 after 1200 cycles, retaining 86.2%. In contrast to conventional TMOs, entropy-stabilized HEOs possess the ability to maintain partial stability even in a fully lithiated state, acting as a matrix to accommodate conversion reactions and greatly improve reversible cycling stability. Nevertheless, rocksalt HEOs encounter challenges stemming from inadequate active components, thereby affecting the reversible capacity during cycling. In contrast, the spinel structure of HEOs facilitates ionic diffusion through three-dimensional pathways. Furthermore, the induction of oxygen vacancies by multivalent metallic cations at Wyckoff positions (tetrahedral and octahedral) in the spinel serves to augment ionic conduction. Therefore, the imperative further development of new spinel HEO anodes necessitates the rational design of active metallic cation components within HEOs. Concerning the enhancement of Li-storage properties in HEO anodes for LIBs, a comprehensive understanding of their Li storage mechanisms during Li-insertion/extraction is critically important.Herein, we present the development of a high-performance conversion-type anode comprising new multi-component HEOs with 5, 6, 7, and 8 cations for LIBs, synthesized through rapid techniques, specifically solution combustion synthesis (SCS). Additionally, we characterize their in-depth structural information using synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). With an increase in the number of cations, the HEO anodes exhibit improved Li-storage properties, including enhanced cycling performance and rate capability. To further evaluate the electrochemical performance of HEOs synthesized by different methods, we selected a specific composition based on its electrochemical performance and synthesized it using solvothermal synthesis. Subsequently, we assessed the electrochemical performance of these samples. Furthermore, we investigated the Li-storage mechanism of HEOs during lithiation/delithiation through ex-situ analytical techniques, encompassing XRD and XAS. The results collectively indicate that the highly reversible conversion reaction in cycled HEO anodes contributes to their outstanding Li-storage characteristics, facilitating stable cycling retention and fast rate capability. The findings of this study delve deeply into the highly reversible Li-storage in HEOs through conversion reactions, with potential implications extending to a broader class of HEO anodes, suggesting the promise of advanced LIBs exhibiting exceptional electrochemical performance.

High entropy oxides (HEOs), in which multication occupation of a single crystallographic site plays an important role in the properties, have become relevant in energy storage [1,2], catalysis [3.4], and many more areas. In a subset of these compounds the entropy, rather than enthalpy, plays a dominant role in stabilizing a single-phase structure at high temperatures. In other cases, the multication occupation merely contributes to stability and properties, but the entropy remains dominant in the stability. The field originated with high entropy metal alloys (HEAs)[5], and then expanded to oxides, borides, sulfides, and more. In the case of HEOs, the first example is the rock salt (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, which has generated a great deal of interest in this class of materials.[6,7] It has been shown that HEOs are of interest for high ionic conductivity and electrochemical energy storage. We have examined the electrochemical performance of new high entropy elecrolytes and found an effect of composition on the cycling performance. Samples were prepared through sol-gel routes and high energy milling of starting binary oxides. We have investigated the synthesis using high temperature in-situ X-ray diffraction on a Panalytical diffractometer equipped with an XRK900 stage. STEM/EDS studies on ex-situ samples will be presented that show elemental distribution, with Raman and EIS measurements providing information on ionic diffusion. The results of this study will be highly impactful for the growing community of researchers investigating the design and synthesis of the new class of materials, the high entropy oxides.[1] Q. Wang, et. al., Multi-anionic and -cationic compounds: new high entropy materials for advanced Li-ion batteries, Energy Environ. Sci., 2019, 12, 2433; [2] D. Berardan, et. al., Room temperature lithium superionic conductivity in high entropy oxides, J. Mater. Chem. A, 2016, 4, 9536.; [3] H. Chen, et al., Mechanochemical Synthesis of High Entropy Oxide Materials under Ambient Conditions: Dispersion of Catalysts via Entropy Maximization, ACS Materials Lett. 2019, 1, 1, 83–88; [4] H. Chen, et. al., Entropy-stabilized metal oxide solid solutions as CO oxidation catalysts with high-temperature stability, J. Mater. Chem. A 2018, 6, 11129-11133; [5] Y. Lu, et. al., Sci. Rep. 4, 6200 (2014); Y. F. Ye, et. al., Mater. Today 19 (6), 349 (2016); Zhang, Y., et. al., Nature Commun. 6, 8736 (2015); [6] C. M. Rost, et. al., Nature Commun. 6, 8485 (2015); [7] B. Jiang, et. al., Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides. Journal of the American Chemical Society 2021, 143, (11), 4193-4204

Micro-structural and magnetic analysis of spinel high entropy oxides synthesized by two-step pressureless sintering provides insight into high entropy ceramics

High entropy spinel oxides (CrFeMnNiCox)3O4 (x = 2, 3, 4) nanoparticles as anode material towards electrochemical properties

SummaryAlthough 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.

Thermodynamics of high entropy oxides

A glassy carbon phosphonitride material with bulk chemical composition roughly approximating C3N3P was synthesized through a high-pressure, high-temperature process using a pure P(CN)3 molecular precursor. The resulting material (hereafter referred to as “HPHT-C3N3P”) was characterized using a variety of techniques, including X-ray scattering, pair distribution function analysis, 31P, 13C, 15N magic-angle spinning nuclear magnetic resonance spectroscopies; X-ray photoelectron spectroscopy, and Raman and IR spectroscopies. The measurements indicate that HPHT-C3N3P lacks long-range structural order with a local structure predominantly composed of a sp2, s-triazine-like network in which phosphorus atoms substitute for bridging nitrogen sites found in related C3N4 materials. The HPHT-C3N3P sample exhibits semiconducting properties, with electrical transport dominated by variable-range hopping. The high phosphorus content of HPHT-C3N3P (approaching 13 at. %) is associated with a major decrease in the optical a...

The distribution of sodium ions in aluminosilicate glasses: a high-field Na-23 MAS and 3Q MAS NMR study

High entropy oxides are a class of materials distinguished by the use of configurational entropy to drive material synthesis. These materials are being examined for their exciting physiochemical properties and hold promise in numerous fields, such as chemical sensing, electronics, and catalysis. Patterning and integration of high entropy materials into devices and platforms can be difficult due to their thermal sensitivity and incompatibility with many conventional thermally-based processing techniques. In this work, we present a laser-based technique, laser-induced thermal voxels, that combines the synthesis and patterning of high entropy oxides into a single process step, thereby allowing patterning of high entropy materials directly onto substrates. As a proof-of-concept, we target the synthesis and patterning of a well-characterized rock salt-phase high entropy oxide, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, as well as a spinel-phase high entropy oxide, (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)Cr2O4. We show through electron microscopy and x-ray analysis that the materials created are atomically homogenous and are primarily of the rock salt or spinel phase. These findings show the efficacy of laser induced thermal voxel processing for the synthesis and patterning of high entropy materials and enable new routes for integration of high entropy materials within microscale platform and devices.