S. A. Novikova

Lithium intercalation and deintercalation processes in Li4Ti5O12 and LiFePO4

We have studied the kinetics of electrochemical lithium intercalation and deintercalation processes at different currents in lithium iron phosphate and lithium titanate based composite materials containing fine carbon particles. The results demonstrate that lithium intercalation and deintercalation processes in the electrode materials are characterized by an overvoltage: 4 and 2 mV, respectively, for a cell with a lithium titanate based electrode and 4 and 24 mV for a lithium iron phosphate based cell. Li4Ti5O12 solubility in Li7Ti5O12 is 1.1% (the limit of the solid solution at Li4.03Ti5O12), and Li7Ti5O12 solubility in Li4Ti5O12 is 2.5% (the limit of the solid solution at Li6.93Ti5O12). The conductivity of the phosphate and titanate solid solutions involved in the lithium intercalation and deintercalation processes has been determined.

Cation mobility in Li1 + xHf2 − xScx(PO4)3 NASICON-type phosphates

Li1 + xTi2 − xCrx(PO4)3 NASICON-type materials have been prepared and characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. The results demonstrate that Cr3+ doping increases the ionic conductivity of LiTi2(PO4)3 within the single-phase region of the doped material, which extends to x = 0.7. From temperature-dependent ionic conductivity data, the activation energy for lithium transport through interstitial sites and the enthalpy of defect formation in LiTi2(PO4)3 are estimated at 30.0 ± 0.5 and 56 ± 1 kJ/mol, respectively.

Microstructure and ion transport in Li1 + xTi2 − xMx(PO4)3 (M = Cr, Fe, Al) NASICON-type materials

Li1 + xTi2 − xMx(PO4)3 (M = Cr, Fe, Al) NASICON-type materials have been prepared by the Pechini process and solid-state reactions and characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. We have identified the factors that determine the rate of ion transport in nanocrystalline and bulk samples at low and high temperatures. The effects of the preparation procedure and heterovalent doping on the ionic conductivity of the materials have been assessed. Heterovalent doping is shown to have a considerably stronger effect on the ionic conductivity in comparison with the microstructure of the materials.

Lithium Diffusion in Materials Based on LiFePO 4 Doped with Cobalt and Magnesium

We have studied lithium intercalation/deintercalation kinetics in magnesium- and cobalt-doped lithium iron double phosphates in a cathode material for lithium ion batteries. The results demonstrate that the incorporation of divalent cations reduces the charge and discharge capacities of the samples, the effect being stronger in the magnesium-doped materials. In addition, magnesium doping markedly increases the resistivity of the material in both the lithiated and delithiated states, whereas the resistivity of the cobalt-doped materials is considerably lower in comparison with the undoped material, which leads to an increase in the charging/discharging rate of batteries despite the marked increase in particle size. These findings can be understood in terms of different doping mechanisms: it seems likely that cobalt substitutes for iron, whereas magnesium is accommodated predominantly in the lithium site.

Catalytic activity of NASICON-type phosphates for ethanol dehydration and dehydrogenation

A1 ± xZr2 − xMx(PO4)3 (A = H3O+, Li+; M = In, Nb; x = 0, 0.1, 0.2) NASICON-type materials have been prepared and characterized by X-ray diffraction, specific surface measurements (capillary condensation of nitrogen), and impedance spectroscopy. We have assessed their catalytic performance for ethanol dehydration and dehydrogenation. The results demonstrate that, when prepared with a large specific surface area, these materials are active catalysts for ethanol conversion to hydrocarbons.

Synthesis and ionic conductivity of (NH4)1–xHxHf2(PO4)3 (x = 0–1) NASICON-type materials

Methods have been proposed for the preparation of NASICON-type hafnium hydrogen phosphate- based materials. The pH of the starting solution for the hydrothermal synthesis of NH4Hf2(PO4)3 has been shown to determine whether rhombohedral or cubic NH4Hf2(PO4)3 will be obtained. The thermal decomposition of rhombohedral NH4Hf2(PO4)3 leads to the formation of the triclinic phosphate HHf2(PO4)3, whereas the decomposition of cubic NH4Hf2(PO4)3 yields a cubic phosphate with the composition (NH4)0.4H0.6Hf2(PO4)3. HHf2(PO4)3 cannot be prepared from cubic NH4Hf2(PO4)3, because the temperature of water elimination coincides with that of the elimination of the last portions of ammonia. We have studied the morphology, thermal stability, and ionic conductivity of the synthesized materials. The electrical conductivity of cubic NH4Hf2(PO4)3 has been shown to exceed that of the rhombohedral phase, and the conductivity of the hydrogen forms slightly exceeds that of individual ammonium forms of hafnium phosphate. The highest conductivity among the materials studied here is offered by cubic (NH4)0.4H0.6Hf2(PO4)3 (2.0 × 10–7 and 1.2 × 10–6 S/cm at 400 and 500°C, respectively).

Synthesis of copper and silver nanoparticles in MF-4SC and sulfonated poly(ether ether ketone) membranes and transport properties of the composites

Copper and silver nanoparticles have been produced in MF-4SC and sulfonated poly(ether ether ketone) membranes. Using transmission electron microscopy, the nanoparticles have been shown to have a bimodal size distribution. The effect of the nanoparticles on the transport properties of the membrane materials has been studied.

Cation mobility in Li 1 + x Hf 2 − x Sc x (PO 4 ) 3 NASICON-type phosphates

Li1 + xHf2 − xScx(PO4)3 (x = 0–0.3) NASICON-type mixed phosphates have been synthesized and characterized by X-ray diffraction, nuclear magnetic resonance, and impedance spectroscopy. The results demonstrate that the materials with 0 ≤ x ≤ 0.1 have a hexagonal structure, whereas in the range 0.1 < x ≤ 0.2 the materials consist of a mixture of hexagonal and orthorhombic phases. The x = 0.3 material has an orthorhombic NASICON structure. Doping with scandium leads to an increase in ionic conductivity in the range 0 < x ≤ 0.1.

Synthesis of LiFePO4 nanoplatelets as cathode materials for Li-ion batteries

Lithium iron phosphate with plateletlike morphology (length of 200 nm and thickness of 15–25 nm) was obtained using the solvothermal method. The resulting particles have the smallest dimension along the 1D channels, which are paths of Li+ ion migration. The discharge capacity of composite based on synthesized LiFePO4 and carbon was equal to 160 mAh/g at a current density of 20 mA/g and 80 mAh/g at a current density of 800 mA/g.

Gel combustion synthesis of LaxGd14–xB6Ge2O34 (x = 3 and 4) codoped with Yb3+–Er3+ and Yb3+–Tm3+ active ions

This paper reports the gel combustion synthesis of LaxGd14–xGe2B6O34 (x = 3 and 4) germanate borates codoped with Yb3+–Er3+ and Yb3+–Tm3+ active ions. The synthesized compounds are isostructural with Gd14Ge2B6O34 and crystallize in trigonal symmetry (sp. gr. P31). We have determined the unit-cell parameters of the synthesized mixed-cation germanate borates La3Gd9.74Yb0.84Tm0.42Ge2B6O34 (a = b = 9.794 Å, c = 25.7913 Å, V = 2143 Å3) and La3.16Gd10Yb0.7Er0.14Ge2B6O34 (a = b = 9.746 Å, c = 25.7450 Å, V = 2118 Å3) and assessed their thermal stability. The results demonstrate that the LaxGd14–xGe2B6O34 (x = 3 and 4) germanate borates codoped with Yb3+–Tm3+ and Yb3+–Er3+ active ions melt congruently at T = 1660 and 1700 K and crystallize with undercooling at T = 1620 and 1660 K, respectively. We have obtained an upconversion luminescence spectrum of the La3.16Gd10Yb0.7Er0.14Ge2B6O34 germanate borate. The spectrum shows two bands. The stronger band (in the green region) corresponds to two transitions: 2H11/2 → 4I15/2 (λ = 525 nm) and 4S3/2→ 4I15/2 (λ = 550 nm). The weaker band (in the red region) corresponds to the Er3+ 4F9/2→ 4I15/2 transition.