Otávio José Bandeira Jovino Marques

High entropy oxides such as (MgCoNiCuZn)O have shown promising results as conversion anodes for Li-ion batteries with high gravimetric capacity and excellent cycle stability. This material and a number of derivatives are initially prepared in a rocksalt structure which is immediately reduced to a heterogeneous mixture of metallic nanoparticles and metal oxides, some of which appear to be redox active. It is this heterogeneous mixture which appears to inhibit metallic nanoparticle growth, thereby slowing capacity fading with cycling. In order to better understand the role of each component and how composition affects the performance, we have successfully prepared the rocksalt structure as a 4-metal "medium" entropy oxide by removing each of the 5 metal components in turn, as well as a 6-metal high entropy oxide by adding Fe. Here we present our in situ x-ray absorption spectroscopy and x-ray pair distribution function measurements on these materials designed to probe the contributions of the individual metal components and overall structure to the electrochemical performance of this anode material. The electrochemical capacity is governed not only by the redox active species but also by the capacity of the metallic nanoparticles to be reduced by additional Li ions on the surface or formation of Li-metal alloys. This research was supported in part by Illinois Institute of Technology's Duchossois Leadership Program. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

High-entropy oxides (HEOs) are single phase solid solutions where five or more metals share the same sublattice, giving rise to unexpected features in various fields of applications. Recently, HEOs have emerged as an alternative conversion electrode anode material for next-generation Li-ion batteries, where the combination of several different elements in a single solid solution can synergistically act to overcome some of its main drawbacks, improving performance. Due to their chemical complexity, x-ray absorption spectroscopy (XAS) emerges as an appropriate technique to study the electronic (x-ray absorption near edge structure, XANES) and local structure (extended x-ray absorption fine structure, EXAFS) of these compounds as a function of cycling. This work aims to highlight the capabilities of XAS as an element-specific probe to understand a material’s structure at the atomistic level through EXAFS modeling of (MgFeCoNiCuZn)O high-entropy system and how to extract valuable information about the bond distance, number of near neighbors, and local disorder, which are crucial to a full understanding of the electrochemical reaction mechanisms of such battery electrodes.

Activated carbons play an essential role in developing new electrodes for renewable energy devices due to their electrochemical and physical properties. They have been the subject of much research due to their prominent surface areas, porosity, light weight, and excellent conductivity. The performance of electric double-layer capacitors (EDLCs) is highly related to the morphology of porous carbon electrodes, where high surface area and pore size distribution are proportional to capacitance to a significant extent. In this work, we designed and synthesized several activated carbons based on lignin for both supercapacitors and Li-S batteries. Our most favorable synthesized carbon material had a very high specific surface area (1832 m²·g⁻¹) and excellent pore diameter (3.6 nm), delivering a specific capacitance of 131 F·g⁻¹ in our EDLC for the initial cycle. This translates to an energy density of the supercapacitor cell at 55.6 Wh·kg⁻¹. Using this material for Li-S cells, composited with a nickel-rich phosphide and sulfur, showed good retention of soluble lithium polysulfide intermediates by maintaining a specific capacity of 545 mA·h·g⁻¹ for more than 180 cycles at 0.2 C.

Lithium-sulfur (Li-S) batteries can provide at least three times higher energy density than lithium-ion (Li-Ion) batteries. However, Li-S batteries suffer from a phenomenon called the polysulfide shuttle (PSS) that prevents the commercialization of these batteries. The PSS has several undesirable effects, such as depletion of active materials from the cathode, deleterious reactions between the lithium anode and electrolyte soluble lithium polysulfides, resulting in unfavorable coulombic efficiency, and poor cycle life of the battery. In this study, a new sulfur cathode composed of graphitic nitride as the polysulfide absorbing material and reduced graphene oxide as the conductive carbon host has been synthesized to rectify the problems associated with the PSS effect. This composite cathode design effectively retains lithium polysulfide intermediates within the cathode structure. The S@RGO/GN cathode displayed excellent capacity retention compared to similar RGO-based sulfur cathodes published by other groups by delivering an initial specific capacity of 1415 mA h g⁻¹ at 0.2 C. In addition, the long-term cycling stability was outstanding (capacity decay at the rate of only 0.2% per cycle after 150 cycles).

High-entropy oxides (HEO) enable stabilization of numerous transition metals in a single solid solution phase, and provide a new platform for electrode design, overcoming the rapid degradation of simple metal oxide conversion anodes for Li-ion batteries (LIBs). Their promising electrochemical performance with high capacity and long cycling stability was first demonstrated for the (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O composition. In this study, we use advanced characterization methodologies to assess individual contribution of each metal in this composition to the electrochemical reaction mechanism on charge and discharge. The change in local structure at the endpoints of the initial conversion reaction and through three hundred cycles is analyzed by ex situ extended x-ray absorption fine structure (EXAFS) for Co, Ni, Cu, and Zn K-edges. The number of near neighbors and bond distances extracted from the EXAFS elucidates the electrochemical activity of each atomic species and describes a new disordered state consisting of very small, isolated metallic nanoparticles in an oxide matrix, capable of storing remarkable amounts of Li for hundreds of cycles.

The magnetic properties of the double perovskites Sm ₂ Mn _{1+ x} Co _{1− x} O ₆ ( x = 0, 0.05, 0.12 and 0.26) were investigated. It was found that the Curie temperature, the lattice parameters and the net magnetic moments increased for increasing amounts of Co. An irreversible behavior was observed by measuring the magnetization after cooling the sample with and without applied magnetic fields ( H ). The temperature below which the irreversibility was observed is H dependent and the data were nicely fit to de Almeida–Thouless lines. The ac magnetic susceptibility was measured for frequencies f in the range 0.03–10 kHz yielding for the shifting in the freezing temperature per decade of f . The spin-dynamics were found to follow a power-law with a product of the critical exponents of about 4.99. The overall results are understood within a framework where the variation in the bonding angle associated to the super-exchange interactions are taken into consideration.