Structure and Electrochemical Behavior of Lithium-Manganese Spinels Doped with Chromium and Nickel

A series of compounds with the general formula LiMn_{2 - x - y} Cr _x Ni _y O₄, where x + y = 0.05, 0.5, or 1.0, is synthesized. It is shown that all these compounds are pure-phase spinels with parameter aequal to 0.8193-0.8236 nm. Doping a stoichiometric lithium-manganese spinel simultaneously with chromium and nickel makes the spinel structure stable. The initial specific capacity of a spinel depends on its doping degree. Doping LiMn₂O₄ with chromium and nickel simultaneously at an Mn : Cr : Ni ratio of 195 : 3 : 2 raises the spinel's specific capacity and reduces the cycling degradation. The change in the discharge capacity of LiMn_1.95Cr_0.03Ni_0.02O₄ electrodes cycled at 20, 0, and -14°C is determined.     

Crystal Structures of Double Vanadates LiCoVO4 and Li0.5Co1.25VO4

The crystal structures of vanadates Li_1-2xCo_1+xVO₄ with x = 0 and 0.25 have been studied by a full pattern analysis. It has been shown that in cubic spinel LiCoVO₄ (space group Fd3m), the 8a tetrahedral sites contain a majority of vanadium and a small amount of lithium; all cobalt, lithium, and a small amount of vanadium occupy the 16d octahedral sites. Li_0.5Co_1.25VO₄ crystals belong to the rhombic system (Imma space group) with unit cell parameters a = 5.939(1), b = 5.810(1), and c = 8.303(1). On substitution of lithium by cobalt according to the scheme 2Li⁺ Co²⁺ + , half of the lithium and 70% of the vacancies formed are in the 4a octahedral sites, and onethird of lithium and most of cobalt occupy the 4d octahedral sites. The 4e tetrahedral sites are completely occupied by vanadium and lithium in a ratio of 0.92/0.08. The interatomic distances in LiCoVO₄ and Li_0.5Co_1.25VO₄ are calculated, and the sizes of lithium ion transport channels are evaluated.     

Catalytic properties of manganites in the oxidation of CO and hydrocarbon

Manganites with a spinel structure MMn₂O₄ (M = Co, Cu, Zn, Mo) and M¹ _0.5M² _0.5 Mn₂O₄ (M = Co, Cu, Zn, Mg) have been synthesized and tested in the catalytic oxidation of CO, C₃H₆, and ethylbenzene. The dependence of the catalytic activity of the manganites on the nature of the cation has been established. The spinels containing transition metal ions (Cu, Co) are more active. A relation between catalytic and adsorption properties of manganites has been established. The participation of the lattice oxygen in the oxidation of CO to CO₂ has been found. The mechanism of the oxidation is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 11, pp, 2686–2669. November, 1996.     

Catalytic properties of spinel-type complex oxides in oxidation reactions

The catalytic activity of M^IM^II ₂O₃ spinel-type complex oxides (M^I = Cu, Ni, Mn, Zn, Mg, Co, M^II = Co, Cr, Al) in the oxidation of CO and ethylbenzene has been investigated. The Co-containing catalysts were more active than the Cr- and Al-containing catalysts. The nature of the cation influenced the catalytic activity. Higher activities were observed for the catalysts containing two transition elements. A correlation between the catalytic and adsorption properties was established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1730–1732, October, 1994.     

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co₃O₄, MgCo₂O₄, Co₂FeO₄, Co₂AlO₄ and CoFe₂O₄. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H₂O₂ decomposition. Studying the effect of dominant surface termination, isotropic Co₃O₄ and CoFe₂O₄ catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co³⁺ plays the major role for high activity. In H₂O₂ decomposition, Co²⁺ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction.     

Electrochemical properties of LiMn<Subscript>2−<Emphasis Type="Italic">y</Emphasis></Subscript>Me<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>4</Subscript> (Me = Cr,Co, Ni) spinels as cathodic materials for lithium-ion batteries

A number of spinel phases with a general formula LiMn_2?y Me _y O (Me = Cr, Co, Ni) was synthesized using the melt-impregnation and sol-gel methods. All synthesized materials were subjected to electrochemical testing of their suitability as cathodes in lithium-ion batteries. Cyclic voltammograms were used in the testing. The cathode materials prepared using the melt-impregnation method showed the highest initial discharge capacity (up to 120 mA h/g) and stable operation during the cycling. The partial substitution of chromium and cobalt atoms for manganese gives positive effect: the spinel structure is stabilized during the cycling. The double doping of the Li-Mn-O system with small amounts of Co and Ni results in the stabilizing of the discharge capacity. An overstoichiometry excess of lithium in Co- or Cr-doped spinels also favors the increasing of the discharge capacity and slows down its decaying during the cycling.     

Magnetic properties of ferromagnetic diluted Zn x Cd1–x Cr2Se4 spinels are studied by Green's functions,mean field theory and high temperature series expansions theories

The field induced reorientation of the magnetization of ferromagnetic (or antiferromagnetic) structure is treated within the framework of many-body Green's function theory by considering all components of the magnetization. The mean field theory is used to calculate the nearest neighbour and the next-neighbour super-exchange J₁(Cr–Cr) and J₂(Cr–(Zn(Cd)–Se)–Cr), respectively, for the Zn_1–x Cd _x Cr₂Se₄ in the range 0 < x < 1. The intraplanar and the interplanar interactions are deduced. The high temperature series expansions (HTSEs) are derived for the magnetic susceptibility and the two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation (RPA). The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The obtained results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The obtained values are comparable to those of 3D Heisenberg model.     

Improvement of stability of out-layer MgAl2O4 spinel for a Ni/MgAl2O4/Al2O3 catalyst in dry reforming of methane

Summary The stability of Ni/-Al₂O₃ catalyst in dry reforming of methane was found to be improved by the addition of MgO into the catalyst, probably due to the formation of out-layer MgAl₂O₄ spinels, which can effectively suppress the phase transformation to form NiAl₂O₄ spinel phases, stabilize the tiny Ni crystallites and suppress carbon deposition in dry reforming of methane.     

“From Heteronuclear Complex to Composite Oxide” Approach in the Synthesis of Nanosized Ferrites MIIFeIII 2O4 (M = Mn,Co, Ni) with Spinel Structure and Their Catalytic Properties

A study was carried out on the preparation of ferrites M^IIFe₂O₄ (M^II = Mn, Co, Ni) by the thermal decomposition of the corresponding ₃-oxoacetates [Fe₂M^IIO(CH₃COO)₆(H₂O)₃]·2H₂O. Nanocrystallites of M^IIFe₂O₄ were formed under mild conditions at 300-400 °C in this reaction. An increase in the temperature leads to an increase in particle size. Nickel ferrite showed high catalytic activity in the reduction of nitrogen oxides (NO _x ) by hydrocarbons.     

Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe2–xMnxO4 (x = 0.0–2.0) spinel nanocrystallites

The replacement of cations at the B-sites in the spinel ferrite ZnFe₂O₄ by Mn³⁺ ions brings in several interesting changes, the most striking among them being a transformation from the spinel cubic structure to a body-centered tetragonal one. Concomitantly, there are variations in the nanocrystallite sizes and also in the lattice parameters. These are examined through high-precision X-ray diffraction measurements and transmission electron microscopic analysis. A more interesting aspect is the success of positron annihilation spectroscopy comprising of the measurements of positron lifetime and coincidence Doppler broadening measurements in understanding the effects of cation replacement and the resultant generation of vacancy-type defects. There are definite changes in the positron lifetimes and intensities which show positron trapping in trivacancy-type defect clusters and the nanocrystallite surfaces. The presence of ortho-positronium atoms within the extended intercrystallite region is also identified, although in small concentrations. The cubic to tetragonal transformation is indicated through definite decrease in the values of the positron lifetimes. We also performed a model analysis to predict the expected effect of substitution on the positron lifetime in the bulk of the sample and the experimentally obtained positron lifetimes significantly differed, indirectly hinting at the possibility of a structural transformation. Finally, Mössbauer spectroscopic studies have indicated a ferromagnetic nature for one of the samples, i.e. the one with Mn³⁺ doping concentration x = 0.4, which incidentally had the lowest crystallite size ~10 nm.