V. S. Pervov

Metastable states in inorganic systems

Changes in inorganic systems associated with the formation of metastable states and their relaxation are considered using supramolecular theory of eutectics. After analysis of the existing theories and concepts describing possible transition mechanisms of nonequilibrium amorphous structures to a crystalline state, the conclusion that there is poor correspondence of experimental data to those theories is made. Owing to the complexity of relaxation processes, it was proposed to divide the objects of investigation into groups considering the different interaction potentials and size parameters of substructures. Inorganic binary eutectic systems were classified as a special group. The peculiarities of the relaxation processes in quenched non-autonomous phases are shown in a few examples with consideration of characteristic temperatures and time.

LiCoO2- and LiMn2O4-based composite cathode materials

We have prepared composite cathode materials based on two electrochemically active compounds, LiCoO2 and LiMn2O4, and investigated their properties. The results indicate that the discharge capacities of all the materials studied exceed the additivity rule values calculated from the discharge capacities of the starting materials. The effect of heat treatment on the physicochemical properties of the composites is analyzed.

Supramolecular model of eutectics: Functional materials based on nonautonomous phases

New data in favor of the supramolecular theory of eutectics are given. It is shown that, on the basis of metastable states appearing in eutectic compositions, new materials having a functional purpose can be obtained. The concept that explains the pseudo-single-phase character and defect structure of such states allows these specific features to be used for obtaining a composite cathode material for rechargeable lithium-ion batteries. On the basis of postulates of the theory and a series of experimental data, a technique for the synthesis of such a material is developed; the efficiency of its application is shown.

Surface-modified cathode materials based on an LiCoO2-LiMn2O4 composite

A process has been developed for surface modification of an equimolar LiCoO2-LiMn2O4 composite with a mixture of alumina (Al2O3) and boehmite (AlO(OH)). The alumina: boehmite ratio and pH value have been optimized in order to achieve adequate surface adhesion between the coating and powder and good homogeneity of the nanocoatings. The effects of ultrasonic processing, coating conditions, and thermal annealing have been examined. The results demonstrate that the nanocoatings considerably improve the cyclability of the composite in an extended voltage range (up to 4.5 V) in comparison with the unmodified material and slightly increase its average discharge voltage.

Mixing enthalpies of oxides in the PbO-Y2O3 and PbO-MoO3 systems

The enthalpies of mixing of oxides in the binary systems PbO-Y2O3 and PbO-MoO3 have been determined for near-eutectic compositions by solution calorimetry. The results, combined with earlier solution calorimetry data for these systems, provide clear evidence for self-organization of the interacting substructures in the melt (formation of nonautonomous phases similar to supramolecular ensembles).

Nonadditivity of Properties and the Possibility of Formation of Ensembles of Incommensurate Structures in the GaSe–Ga2Se3 System

The GaSe–Ga2Se3 system is studied by x-ray diffraction, electron microscopy, and differential scanning calorimetry. The results demonstrate that the nonadditivity of heat capacity observed near the eutectic composition after quenching is similar to the nonadditivity of properties in misfit layer compounds, which contain incommensurate structural components. The origin of this similarity is discussed in terms of the self-organization of interacting incommensurate elements in the melt and the formation of unstable suprastructural ensembles.

On Some Problems of Inorganic Supramolecular Chemistry

In this study, some features that distinguish inorganic supramolecular host-guest objects from traditional architectures are considered. Crystalline inorganic supramolecular structures are the basis for the development of new functional materials. Here, the possible changes in the mechanism of crystalline inorganic supramolecular structure self-organization at high interaction potentials are discussed. The cases of changes in the host structures and corresponding changes in the charge states under guest intercalation, as well as their impact on phase stability and stoichiometry are considered. It was demonstrated that the deviation from the geometrical and topological complementarity conditions may be due to the additional energy gain from forming inorganic supramolecular structures. It has been assumed that molecular recognition principles can be employed for the development of physicochemical analysis and interpretation of metastable states in inorganic crystalline alloys.

Surface modification of cathode materials for lithium ion batteries

We have studied the effect of surface modification with aluminum hydroxide and alumina-boehmite mixtures on the electrochemical performance of an equimolar LiCoO2/LiMn2O4 composite cathode material. A process has been developed for the deposition of aluminum hydroxide nanocoatings using ultrasonic processing. Al(OH)3 nanocoatings, as well as alumina-boehmite nanocoatings, considerably improve the cyclability of the composite in an extended voltage range (up to 4.5 V) in comparison with the unmodified material, with an insignificant reduction in specific capacity in the first cycles. Both types of coatings markedly improve the cyclability of the composite at high current rates in comparison with the unmodified composite.

On the Additivity of Properties and Structural Stability of Misfit Layer Compounds

Differential scanning calorimetry and temperature-dependent magnetic susceptibility measurements demonstrate that the misfit layer compounds (SnS)1.15(TaS2) and (PbS)1.14(TaS2) undergo a few phase transitions between 2 and 700 K. The transitions are interpreted as due to the incommensurability of the suprastructure components. The heat capacity data indicate that the properties of the misfit layer compounds do not obey the additivity rule.