Nikolai Uvarov

Composite protonic solid electrolytes in the CsHSO4-SiO2 system

Abstract Transport, thermal and structural properties of the composite solid electrolytes (1 − x )CsHSO 4 xSiO 2 (where x = 0–0.8) were investigated. The composites were prepared by mechanical mixing of components followed by heating at temperatures near CsHSO 4 melting point (483 K). The dependence of low temperature phase conductivity on x has a maximum with a value 2.5 orders of magnitude higher than that of pure CsHSO 4 and conductivity is governed by protons. Heterogeneous doping is shown to change markedly the thermodynamic parameters of the ionic component. The phase transition temperature CsHSO 4 in the composites decreases from 414 to 350 K with the increase of the content of heterogeneous additive SiO 2 from 0 to 0.7. As x raises CsHSO 4 the amorphization takes place and the relative change of ionic conductivity at phase transition diminishes, the phase transition becomes diffusive and disappears for the 0.2CsHSO 4 0.8SiO 2 composite.

Mechanochemical synthesis of LiMn2O4 cathode material for lithium batteries

Abstract The mechanochemical method was used for the synthesis of highly dispersed stoichiometric and nonstoichiometric LixMn2O4 spinel starting from different manganese (MnO2, Mn2O3, MnO) and lithium (LiOH, LiOH·H2O, Li2CO3) compounds. It is shown that the oxidation state of manganese greatly influences on the kinetics of mechanochemical reactions. On the other hand, different crystal structure and mechanical properties of initial lithium compounds result in different mechanisms of mechanochemical action on the activated mixtures. A strong effect of temperature and lithium content on the composition and lattice constant of the final products was found. Conductivity of LixMn2O4 spinels, measured by complex impedance spectroscopy, is likely caused by intergrain resistance. The activation energy of conductivity does not depend on x, the phase transition at 290±10 K, observed for stoichiometric spinel, is accompanied by a conductivity decrease. Test experiments in the electrochemical cell Li/LiPF6, EC/LiMn2O4,C demonstrate that lithium–manganese oxide obtained by the mechanochemical method is a promising cathode material for 4 V lithium batteries.

Percolation effect, thermodynamic properties of AgI and interface phases in AgI–Al2O3 composites

Abstract Properties of AgI in (1−x)AgI–xAl2O3 composites were studied by X-ray powder diffraction, electrophysical and electrochemical methods, infrared spectroscopy and differential scanning calorimetry. Frequency dependencies of conductivity and dielectric permittivity were analyzed. Two maxima were found on concentration dependence of the dielectric permittivity at x=0.2 and 0.9 in agreement with the two-threshold percolation model. As it was shown from studies of electrochemical cells, Gibbs free energy of AgI in the composites exceeds the corresponding values for pure AgI. Analysis of calorimetric data indicates the presence of three AgI phases in the composites – two crystalline ones with different temperatures of α–β transition and an amorphous phase. Their concentration was estimated from calorimetric data. The thickness of the interface amorphous AgI layer was estimated by means of a simple brick-wall model as 3±1 nm.

Nanocomposite ionic conductors in the Li2SO4Al2O3 system

Abstract Studies are carried out of morphology, crystal structure, chemical content, thermal and transport properties of Li 2 SO 4 Al 2 O 3 composites prepared in two different ways: by thermal decomposition of the precursor Li 2 SO 4 ·2Al(OH) 3 · m H 2 O at 400°C or by using conventional mixing. It is shown that, whatever the preparation technique, if grain size of alumina in composites is small enough (of order of tens nanometers) then after prolonged heating the heterogeneous system Li 2 SO 4 Al 2 O 3 reaches the metastable thermodynamic state — a nanocomposite in which lithium sulphate is in an unusual epitaxial state. The stabilization of this state is caused by a strong interface interaction that is leading to the formation of the intermediate thin layer of lithium aluminate which “glues” Li 2 SO 4 and Al 2 O 3 phases providing the contact. The phase transition temperature of Li 2 SO 4 in composites is 100 degrees lower than in the pure salt. Ionic conductivity of Li 2 SO 4 in the composites smoothly changes at the phase transition, activation energies being 0.40±0.05 and 0.66±0.01 eV for T > 470°C and T

Estimation of composites conductivity using a general mixing rule

Abstract It is proposed to use a mixing rule for the estimation of conductivity, σ , in ‘conductor–insulator’ composites (M–A) and composite solid electrolytes (MX–A). The mixing rule in its classical form σ α =(1− f )· σ M α + f · σ A α (where f is the volume fraction of A; σ M,A are conductivity of the phases M and A; −1 α α ( f )= α 1·(1− f )+ α 2· f in order to take variation of the morphology with f into account. For MX–A systems the general mixing rule has the form σ α ( f ) =(1− f − f S )· σ MX α ( f ) + f S · σ S α ( f ) + f · σ A α ( f ) , where f S is the volume fraction of high-conducting interface regions estimated in terms of the brick-wall model; σ S is the conductivity via the interface. The general mixing rules are rather simple, convenient, agree with effective medium models, qualitatively describe percolation effects and satisfactorily represent the experimental data in many composites.

Composite solid electrolytes MeNO3-Al2O3 (Me = Li, Na, K)

Abstract Conductivities of nitrate composites (1 − x ) MeNO 3 - xAl 2 O 3 , where M = Li , Na and K , were investigated in comparison with conductivities of the individual salts. The composites were prepared by mechanical treatment in planetary mills followed by sintering. Conductivity isotherms exhibit maxima at x = 0.40−0.60, with the conductivity values around 10 −2 S/cm at 500 K, these are 10 3 –10 5 times higher than those of the pure salt. Temperature dependences of conductivity are not linear in Arrhenius coordinates. A possible reason for the drastic change of transport properties of nitrates in composites under study may be the stabilization of amorphous high-conducting phases of nitrates which are formed on MeNO 3 -Al 2 O 3 contacts as a result of the interface interaction between the components.

Composite Solid Electrolytes

Studies of composite ionic conductors are overviewed. Mechanisms of defect formation at ionic crystal surfaces and at interphase boundaries in the composites are discussed; the Stern model that allows calculating surface potential has been involved. Methods for the calculating of the composite’s electrical conductance and other physicochemical characteristics are suggested. Thermodynamic stability of nanocomposites and the genesis of their morphology during sintering are analyzed. General regularities of changes in ionic-salt properties over wide range of the “ionic salt-oxide” systems, as well as size effects are discussed.

High ionic conductivity and unusual thermodynamic properties of silver iodide in AgI-Al2O3 nanocomposites

Abstract Composites (1 − x ) AgI - xAl 2 O 3 were obtained using mechanical treatment of pure AgI with highly dispersed alumina (specific area 270 m 2 /g) in planetary ball mill followed by sintering at 570–870 K. As the concentration of alumina rises, properties of the composites change: both temperature of the superionic phase transition and the heat of the transformation decreases. Average grain size of AgI diminishes with x and at x > 0.6 becomes less than 100 nm, hence, the composites may be attributed to nanocomposites. The phase transition, being very sharp in AgI, becomes diffusive for x > 0.5, the absolute values of the conductivity of both phases approaches each other. All the facts mentioned may be explained by strong interface interaction between Al 2 O 3 and AgI in the nanocomposites leading to a stabilization of a metastable high-disordered state of AgI at low temperatures. Properties of the composites are shown to be sensitive to agents adsorbed on the alumina surface. Silver iodide occurring in a metastable state within the composite may be easily transformed to stable β-AgI by treatment with water vapour. It is likely that water molecules substitute for Ag + ions weakly bonded to surface oxygen atoms of alumina, destroying the AgI-Al 2 O 3 interfaces.

Conductivity of long-chain silver carboxylates and their thermal decomposition products

Abstract We have measured ac conductivities and dielectric properties for silver stearate and for a silver soap mixture, which is predominantly silver behenate [AgO2C22H43]2. Excess fatty acid affects conductivity, thermal expansion and dielectric parameters of these samples. Conductivity increases accompanied by conductivity maxima occur at temperatures close to the acid melting points (around 65°C) for samples containing excess fatty acid. At temperatures higher than 65°C, relatively high conductivity of these samples is almost entirely the result of the acid liquid phase. Conductivity of all the samples under study was independent of Cu2+ content and seems limited by the extent of excess fatty acid removal. Silver stearate samples free from excess fatty acid show phase transitions at 124°C and at 153°C. The former is irreversible and leads to decreased conductivity, the latter is accompanied by the thermal decomposition of the silver carboxylates and an increase in conductivity. The conductivity increase and maximum at 65°C, which appear after decomposition of acid-free silver carboxylate, suggest that free fatty acid is one of the decomposition products. IR spectroscopy confirms that the free fatty acid is indeed the product of this reaction. This acid formation greatly alters conductivity, and the resulting protons may function as charge carriers.

Determination of the influence of the NaF rate on the charge carriers parameters in some fluoride glasses containing the NaF alkali fluoride

Abstract The bulk impedance parameters of various fluoride glasses series containing a large extent in NaF rate have been determined from an analysis of ac conductivity data measured in a wide temperature range. The charge carriers concentration in each glass studied has been evaluated using the Almond-West formalism and shown to be temperature independent. Transport properties in these materials are due to a hopping mechanism of the fraction of F − ions mobile at long range (3–5%). The variation of different conductivity parameters as a function of the NaF content has been determined. When the NaF rate increases, a conductivity decrease is observed, due mainly to a decrease of the charge carriers mobility.