K. Petrov

Synthesizing nanocrystalline LiMn2O4 by a combustion route

Nanocrystalline samples of lithium manganese oxide with cubic spinel structure have been prepared by combustion of reaction mixtures containing Li(I) and Mn(II) nitrates that operate as oxidisers, and sucrose that acts as fuel. The samples were characterised by X-ray diffraction, transmission electron microscopy, thermal analysis, and impedance and electrochemical measurements. The effect of the fuel content on the purity and morphology of the products was analysed. The samples as prepared showed small amounts of Mn2O3 and Mn3O4 as impurities, depending on the amount of sucrose used in the synthesis. Annealing at 700 °C led to single-phase cubic spinels. In these phases, the smallest average particle size (ca. 30 nm) corresponded to the sample obtained with a hyperstoichiometric amount of fuel. This sample showed the Li1.05Mn1.95O4 composition as deduced from the thermal and electrochemical data. No variation in conductivity associated with the cubic⇔orthorhombic phase transition was observed. The electrochemical behaviour as positive electrode showed good cyclability at high current densities (reversible capacity of 73 mAh g−1 at 2.46 mA cm−2).

Nanosize LiNiyMn2 −yO4(0 < y≤ 0.5) spinels synthesized by a sucrose-aided combustion method. Characterization and electrochemical performance

Nanosize crystalline cathode materials of LiNiyMn2 − yO4 (0 < y ≤ 0.5) composition and spinel-type structure have been obtained by a single-step sucrose-aided self-combustion method. The as-prepared samples contained some amorphous organic impurities that were removed after a short period of heating at 500 °C. The pure single-phase spinels have been characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and nitrogen sorption isotherms. The samples consist of particles (ca. 24 nm size) that are aggregated in clusters (ca. 1 µm size) in which mesopores (10–80 nm size) appear among the particles. Additional heating at 800° and 1000 °C produces a slight increase in the cubic lattice parameter and a pronounced increase in particle size (>100 nm). Electrical conductivity decreases as the Ni content increases in accordance with an electron hopping mechanism between Mn3+ and Mn4+ ions. The 500 °C- and 800 °C-heated LiNi0.5Mn1.5O4 samples show good electrochemical behaviour at 4.7 V as cathode materials. The capacity (132.7 mA h g−1) found is close to the nominal capacity (146.7 mA h g−1) and remains constant for current densities in the range C/24–2C (where C = 2.6 mA cm−2). At higher current densities (2C–10C) the capacity decreases progressively. The cyclability at the C current density is ca. 99.7% for both samples.

Infrared spectra of Cu(II)-Co(II) mixed hydroxide nitrates

Abstract The infrared spectra from 4000 to 50cm −1 of copper(II)-cobalt(II) hydroxide nitrate mixed crystals, Cu x Co 2 − x (OH) 3 NO 3 (0.0 x 3 polarization, decreasing metal-oxygen force constants and weakening of the hydrogen bonds upon substitution of cobalt for copper. The spectroscopic data are in good agreement with the recently reported structural disorder in these phases.

Structural study of coals by means of directly-prepared potassium-coal adducts☆

Abstract Directly-prepared potassium adducts (PA) of different rank coals and of an anthracite are studied by X-ray diffraction and e.s.r.-spectroscopy. The reactivity of the PAs towards butyl iodide and methanol are also examined. Complete disruption of the lamellar coal structure (i.e. disappearance of the 002 line in the X-ray diffraction patterns) is observed after reaction with potassium. E.s.r.-spectroscopic parameters are not substantially changed by the potassium except for coals with C, 85–90 wt%, for which the concentration of paramagnetic centres is significantly increased. These coals are also capable of significant butylation and reduction (thus becoming soluble). A correlation between the number of unpaired electrons and the number of alkyl groups introduced is established. Anthracite is not capable of butylation or reduction in this manner. With all coals (including anthracite) the layer stacking indicated by the 002 diffraction line is partially recovered after the treatment of the PA with butyl iodide or alcohol. Also, the diffraction intensities in the γ-band region are increased depending on the degree of the butylation or reduction occurred. The results are interpreted with respect to the similarity of PA of different rank coals with the intercalation compounds of graphite and with the ionic adducts of polycyclic aromatic hydrocarbons with alkali metals.

Effect of the Thermal Treatment on the Particle Size and Electrochemical Response of LiCr0.2Mn1.8 O 4 Spinel

Spinel of composition LiCr 0 . 2 Mn 1 . 8 O 4 has been synthesized by a combustion-aided procedure. The particles of the sample are single crystals of ca. 10 nm size as deduced from X-ray diffraction and transmission electron microscopy. Thermal treatment of the as-prepared sample at increasing temperature (400, 600, 700, 750, 900, 1000, and 1100°C) for different times (1, 3, or 24 h) leads to an increase of the particle size, which attains a value as high as 1560 nm after heating at 1100°C for 1 h. Electrochemical response of the heated samples as positive electrodes in a lithium cell has been studied at three rates (C/24, C, and 3C; where C = 2.9 mA). At C/24 the discharge capacity measured at 4 and 4.8 V is not appreciably affected by the particle size. At C and 3C the discharge capacity measured at those voltages decreases as the particle size increases; a linear dependence has been found for particle sizes above 50 nm. On cycling we have found that the capacity fade also depends on the particle size; the larger the particles the less the capacity fade at the mentioned voltages.

Comparison of the Structure and Chemical Composition of Crystalline and Amorphous Electroless Ni-W-P Coatings

Electroless deposition was applied to prepare Ni-W-P coatings of two types: crystalline, with low P and W, and amorphous with high W content. Their morphology was studied by atomic force microscopy. Polycrystalline Ni-W-P alloys consist of grains of stacked plates (lamellas), as it was revealed by transmission electron microscopy. The coatings exhibit a (100) texture. X-ray diffraction and electron diffraction analysis demonstrated the unit cell parameter of the crystalline phase in Ni-W-P is practically equal to that of pure Ni. This implies W and P are localized along the grain boundaries. The Warren/Averbach method was applied to determine microstrain and size of coherent scattering domains. The crystalline structure promotes the surface oxidation of Ni-W-P to higher extent in comparison with the amorphous structure. X-ray photoelectron spectroscopy analysis demonstrated the presence of oxygen and carbon in the bulk of the crystalline Ni-W-P coatings in larger quantities than in the amorphous. In the crystalline coatings in addition to oxygen and carbon, scanning Auger electron spectroscopy showed the presence of nitrogen. It is supposed that all these elements come from ligand residues adsorbed at the grain boundaries during coating growth. Nanoindentation tests indicated that amorphous Ni-W-P samples display more uniform surface mechanical properties at the nanometer scale than the crystalline ones.

Effect of composition on the lattice parameters and thermal behaviour of nickel(II)–cobalt(II) hydroxide nitrate solid solutions

A series of coprecipitated nickel(II)–cobalt(II) hydroxide nitrate samples with different cationic compositions has been studied by means of X-ray diffraction, infrared spectroscopy and thermal analysis. A continuous series of solid solutions NixCo2–x(OH)3NO3(0⩽x⩽2) is formed between the end members Ni2(OH)3NO3 and Co2(OH)3NO3. The observed changes in the IR spectra have been interpreted in terms of increased polarization of the NO3– group and increased strength of the hydrogen bonds when Ni2+ is gradually substituted for Co2+. The thermal stability of the solid solutions as well as the phase composition of the solid products of thermal decomposition depend upon the initial Ni : Co ratio in the samples.

Neutron diffraction study on the spinel Cu0.72Co2.28O4

Abstract A neutron powder diffraction refinement of the positional and thermal structure parameters of Cu0.72Co2.28O4 spinel has been carried out. It has been found that the spinel is inverse, with inversion parameter approximately equal to one half of the copper content value.

Neutron diffraction study of the cationic distribution in CuxCo3−xO4 (0<x⩽1.0) spinels prepared by thermal decomposition of layered hydroxide nitrate precursors

Abstract A neutron powder diffraction refinement of the structure parameters of six samples of copper cobalt oxide spinels Cu x Co 3− x O 4 (0 x ⩽ 1.0), prepared by thermal decomposition of layered hydroxide nitrate precursors has been carried out. It was found that the Cu 2+ ions are statistically distributed over the tetrahedral ( A ) and the octahedral ( B ) lattice sites. This particular cationic distribution is responsible for the observed cubic lattice symmetry and the dependences of the unit cell parameters, the oxygen parameter and the tetrahedral and octahedral bond lengths on the copper content, x , in the formula unit. The results can be regarded as the first direct confirmation of the suggestion that the cationic distribution in the spinels is predetermined by the structure of the precursors and the mechanism of their thermal decomposition.

Preparation and X-ray diffraction characterization of two modifications of the cobalt hydroxide nitrate Co(OH)NO3 · H2O

Abstract Samples of cobalt hydroxide nitrate, Co(OH)NO3 · H2O, have been prepared by thermal treatment of Co(NO3)2 · 6H2O melts at different temperatures, in the presence of soft hydrolyzing agents, such as urea or ammonium hydrogen carbonate amide. The results of chemical analyses and the values of the unit cell parameters, refined from X-ray powder diffraction data, have shown that the investigated compound crystallizes in two different monoclinic modifications, whose relative amount varies depending on the temperature of preparation.