Artem M. Abakumov

Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries

Peering into cathode layered oxides The quest for better rechargeable batteries means finding ways to pack more energy into a smaller mass or volume. Lithium layered oxides are a promising class of materials that could double storage capacities. However, the design of safe and long-lasting batteries requires an understanding of the physical and chemical changes that occur during redox processes. McCalla et al. used a combination of experiments and calculations to understand the formation of O-O dimers, which are key to improving the properties of these cathode materials. Science, this issue p. 1516 A model lithium layered oxide is used to probe the enhanced charge storage capacity of this family of materials. Lithium-ion (Li-ion) batteries that rely on cationic redox reactions are the primary energy source for portable electronics. One pathway toward greater energy density is through the use of Li-rich layered oxides. The capacity of this class of materials (>270 milliampere hours per gram) has been shown to be nested in anionic redox reactions, which are thought to form peroxo-like species. However, the oxygen-oxygen (O-O) bonding pattern has not been observed in previous studies, nor has there been a satisfactory explanation for the irreversible changes that occur during first delithiation. By using Li2IrO3 as a model compound, we visualize the O-O dimers via transmission electron microscopy and neutron diffraction. Our findings establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides.

Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water

The development of water oxidation catalysts (WOCs) to replace costly noble metals in commercial electrolyzers and solar fuel cells is an unmet need that is preventing the global development of hydrogen fuel technologies. Two of the main challenges in realizing catalytic water splitting are lowering the substantial overpotential that is required to achieve practical operating current densities in the O2-evolving halfreaction at the anode, and the use of earth-abundant elements for the fabrication of inexpensive electrodes that are free from noble metals. To meet these challenges, molecular catalysts that are based upon the cubic CaMn4Ox core within photosystem II in photosynthetic organisms, which is the gold standard of catalytic efficiency, have begun to appear. Among solid-state materials, several noble-metal oxides, which include IrO2 and RuO2, are already in use in industrial electrolyzers, but are not globally scalable. Aqueous solutions of cobalt phosphate form water-oxidation catalysts under electrolysis and photolysis that are suitable for the fabrication of noncrystalline electrode materials. Nanocrystalline spinel-phase metal oxides (AM2O4, M= transition metals) that are comprised of M4O4 cubical subunits and are active water oxidation catalysts have been developed. The catalytic activity of the spinel Co3O4 has been reported for Co3O4 nanorods that are incorporated into SBA-15 silica, as well as Co3O4 nanoparticles that are adsorbed onto Ni electrodes. NiCo2O4 spinel also oxidizes water when the nanoparticles are electrophoretically deposited onto a Ni electrode. Reports that examined the effect of lithium doping on the surface of Co3O4 electrodes in solutions of KOH attributed the higher evolution rate of O2 to better electrical conductivity. However, the oxidation of water by Co3O4 was strongly dependent on crystallite size and surface area and frequently necessitates high overpotentials and alkaline conditions to accelerate the rate of reaction. In contrast, we recently reported that the catalytically inert spinel LiMn2O4 gives spinel l-MnO2, which is an active water oxidation catalyst, upon topotactic delithiation. Thus, the importance of removing the A-site lithium for catalysis by the cubic Mn4O4 core of spinels was revealed. [11]

Water electrolysis on La1-xSrxCoO3-δ perovskite electrocatalysts

Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B–O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co–O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar.

Since invention of the diamond anvil cell technique in the late 1950s for studying materials at extreme conditions, the maximum static pressure generated so far at room temperature was reported to be about 400 GPa. Here we show that use of micro-semi-balls made of nanodiamond as second-stage anvils in conventional diamond anvil cells drastically extends the achievable pressure range in static compression experiments to above 600 GPa. Micro-anvils (10–50 μm in diameter) of superhard nanodiamond (with a grain size below ∼50 nm) were synthesized in a large volume press using a newly developed technique. In our pilot experiments on rhenium and gold we have studied the equation of state of rhenium at pressures up to 640 GPa and demonstrated the feasibility and crucial necessity of the in situ ultra high-pressure measurements for accurate determination of material properties at extreme conditions.

Discovery of a Superhard Iron Tetraboride Superconductor

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

Chemistry and structure of Hg-based superconducting Cu mixed oxides

The most relevant aspects of thermodynamics, preparation, crystal structure and superconducting properties under ambient and high pressure of HgBa2Can−1CunO2n+2+δ superconductors are reviewed in this contribution. The ideas which inspired authors to search for the new superconductors among Hg-based complex cuprates are discussed as an example illustrating the benefit of the concept of intergrowth structure formation for the design of new layered materials. Recent results on solid–gas equilibriums useful for reproducible syntheses of different homologues are summarized. The crystal structures are discussed with main attention focused on features such as concentration and location of extra oxygen atoms, static atomic displacements due to partial occupation of oxygen site in a Hg-layer, substitution in a Hg position and a presence of stacking faults. It is shown that these factors have to be taken into account for a correct structure description. The relationships between Tc, doping level, amount and formal charge of extra anions and external pressure are outlined to predict possible paths for enhancement of the superconducting properties in complex cuprates.

Understanding the roles of anionic redox and oxygen release during electrochemical cycling of lithium-rich layered Li4FeSbO6.

Li-rich oxides continue to be of immense interest as potential next generation Li-ion battery positive electrodes, and yet the role of oxygen during cycling is still poorly understood. Here, the complex electrochemical behavior of Li4FeSbO6 materials is studied thoroughly with a variety of methods. Herein, we show that oxygen release occurs at a distinct voltage plateau from the peroxo/superoxo formation making this material ideal for revealing new aspects of oxygen redox processes in Li-rich oxides. Moreover, we directly demonstrate the limited reversibility of the oxygenated species (O2(n-); n = 1, 2, 3) for the first time. We also find that during charge to 4.2 V iron is oxidized from +3 to an unusual +4 state with the concomitant formation of oxygenated species. Upon further charge to 5.0 V, an oxygen release process associated with the reduction of iron +4 to +3 is present, indicative of the reductive coupling mechanism between oxygen and metals previously reported. Thus, in full state of charge, lithium removal is fully compensated by oxygen only, as the iron and antimony are both very close to their pristine states. Besides, this charging step results in complex phase transformations that are ultimately destructive to the crystallinity of the material. Such findings again demonstrate the vital importance of fully understanding the behavior of oxygen in such systems. The consequences of these new aspects of the electrochemical behavior of lithium-rich oxides are discussed in detail.

Perovskite-like Mn2O3: a path to new manganites.

Among complex oxides, perovskite-based manganites play a special role in science and technology. They demonstrate colossal magnetoresistance, and can be employed as memory and resistive switching elements or multiferroics. The perovskite structure ABO3 has two different cation sites: B-sites that are octahedrally coordinated by oxygen, and cuboctahedrally-coordinated (often heavily distorted) Asites. The magnetic and transport properties of perovskite manganites are largely determined by the Mn O Mn interactions in the perovskite framework of corner-sharing MnO6 octahedra. Although the A cations do not directly participate in these interactions, they control the Mn valence and the geometry of the Mn O Mn bonds. Complex phenomena, such as charge and orbital ordering, often accompany chemical substitutions on the A-site. Requirements on formal charge and ionic radius are usually different for cations adopting theA or B positions and prevent A/B mixing. Small and often highly charged transition-metal B-cations are unfavorable for the large 12coordinated A-site. Partial filling of the A-position with transition metals is, nevertheless, possible in a unique class of A-site ordered perovskites AA’3B4O12 (where A= alkali, alkali-earth, rare-earth, Pb, or Bi cations, A’=Cu or Mn, and B= transition metals, Ga, Ge, Sb, or Sn). A key ingredient of such compounds is the A’ cation that should be prone to a first-order Jahn–Teller effect (Cu or Mn). An oxygen environment suitable for such transition-metal cations at the A’ position is created by the aaa octahedral tilt system (in Glazer s notation) with a notably large magnitude of the tilt (for example, in CaCu3Ti4O12 the Ti O Ti bond angle is only 140.78). The tilt creates a square-planar anion coordination, favorable for Jahn–Teller-active A’ cations. The ap= ffiffiffi

Preparation, Structure, and Electrochemistry of Layered Polyanionic Hydroxysulfates: LiMSO4OH (M = Fe, Co, Mn) Electrodes for Li-Ion Batteries

The Li-ion rechargeable battery, due to its high energy density, has driven remarkable advances in portable electronics. Moving toward more sustainable electrodes could make this technology even more attractive to large-volume applications. We present here a new family of 3d-metal hydroxysulfates of general formula LiMSO4OH (M = Fe, Co, and Mn) among which (i) LiFeSO4OH reversibly releases 0.7 Li(+) at an average potential of 3.6 V vs Li(+)/Li(0), slightly higher than the potential of currently lauded LiFePO4 (3.45 V) electrode material, and (ii) LiCoSO4OH shows a redox activity at 4.7 V vs Li(+)/Li(0). Besides, these compounds can be easily made at temperatures near 200 °C via a synthesis process that enlists a new intermediate phase of composition M3(SO4)2(OH)2 (M = Fe, Co, Mn, and Ni), related to the mineral caminite. Structurally, we found that LiFeSO4OH is a layered phase unlike the previously reported 3.2 V tavorite LiFeSO4OH. This work should provide an impetus to experimentalists for designing better electrolytes to fully tap the capacity of high-voltage Co-based hydroxysulfates, and to theorists for providing a means to predict the electrochemical redox activity of two polymorphs.

Design of new electrode materials for Li-ion and Na-ion batteries from the bloedite mineral Na2Mg(SO4)2·4H2O

Mineralogy offers a large database to search for Li- or Na-based compounds having suitable structural features for acting as electrode materials, LiFePO4 being one example. Here we further explore this avenue and report on the electrochemical properties of the bloedite type compounds Na2M(SO4)2·4H2O (M = Mg, Fe, Co, Ni, Zn) and their dehydrated phases Na2M(SO4)2 (M = Fe, Co), whose structures have been solved via complementary synchrotron X-ray diffraction, neutron powder diffraction and transmission electron microscopy. Among these compounds, the hydrated and anhydrous iron-based phases show electrochemical activity with the reversible release/uptake of 1 Na+ or 1 Li+ at high voltages of ∼3.3 V vs. Na+/Na0 and ∼3.6 V vs. Li+/Li0, respectively. Although the reversible capacities remain lower than 100 mA h g−1, we hope this work will stress further the importance of mineralogy as a source of inspiration for designing eco-efficient electrode materials.