Janina Molenda

Modification in the electronic structure of cobalt bronze LixCoO2 and the resulting electrochemical properties

Abstract The paper presents results of an analysis of the electronic conduction and thermoelectric power of the Li x CoO 2 cobalt bronze (0 x ⩽1) resulting from the electrochemical disintercalation in the Li/Li + /Li x CoO 2 cell. It has been found that the initial bronze LiCoO 2 shows both the electrical conductivity of an activated nature and high values of thermoelectric power, which is typical of the semiconductor state. Carrying the lithium ions away from the lattice of LiCoO 2 causes that the properties of the compound are changed into the metal ones, the process intensifying oppositely to the lithium content in Li x CoO 2 . It has been found, that a change in the EMF of the Li/Li + /Li x CoO 2 corresponds to the change of the Fermi level of the cathode material, and is connected with the modification of electronic structure of the cobalt bronze, which is due to the disintercalation of the bronze.

Conduction mechanism in operating a LiMn2O4 cathode

Abstract Two series of the Li x Mn 2 O 4 spinel samples were studied at low temperatures (200–300 K) on electrical, thermal (DSC) and structural (X-ray diffraction (XRD)) properties for different lithium contents. Results obtained for deintercalated spinel samples with x ∼1 revealed the existence of a broad (100 K) phase transition that can be attributed to the molecular polaron condensation, leading to the orthorhombic distortion of the initial cubic form. The differential scanning calorimetry (DSC) measurement results enable us to regard the phase transition as a form of order–disorder one. Corresponding thermoelectric power (TEP) and electrical conductivity measurements fall within such description, moreover, indicating clear inconsistency between the measured regular DC conductivity of the spinel sample and that observed for the cathode in the working lithium cell. This discrepancy points to an alternative charge transport mechanism existing in the manganese spinel cathode, and it seems to be essential for the lithium cell performance.

Structural, electrical and electrochemical properties of LiNiO2

Abstract This paper presents structural and electrical properties of LiNiO 2 cathode material. The influence of cation mixing on the structural and electrical properties were determined. It has been found that deintercalation of lithium ions from the LiNiO 2 lattice influences the properties of the compound towards metallic ones. A model of electronic structure of LiNiO 2 has been proposed.

Electrical properties of LiMn2O4−δ at temperatures 220–1100K

Abstract The work presents the results of low-temperature (220–300 K) studies of electrical conductivity and thermoelectric power for manganese spinel obtained by various methods. It has been shown that charge transport takes place through the small polaron mechanism. The results of high-temperature (300–1100 K) studies of electrical conductivity and thermoelectric power carried out in the thermodynamic equilibrium conditions as a function of temperature and oxygen pressure (10 −2 −1 atm) in the whole range of the LiMn 2 O 4− δ phase stability are also presented. It has been shown that an effective reaction related to the deviation from stoichiometry is the reaction Mn +4 +e − ⇔Mn +3 .

Transport properties and reactivity of tungsten trioxide

Abstract The paper presents the results of electrical and electrochemical measurements for tungsten trioxide, which were the basis for the proposal of a qualitative model of electronic structure of Li x WO 3− y . A close correlation has been shown between the electronic structure and reactivity of tungsten trioxide.

Structural, Transport and Electrochemical Properties of LiFePO4 Substituted in Lithium and Iron Sublattices (Al, Zr, W, Mn, Co and Ni)

LiFePO4 is considered to be one of the most promising cathode materials for lithium ion batteries for electric vehicle (EV) application. However, there are still a number of unsolved issues regarding the influence of Li and Fe-site substitution on the physicochemical properties of LiFePO4. This is a review-type article, presenting results of our group, related to the possibility of the chemical modification of phosphoolivine by introduction of cation dopants in Li and Fe sublattices. Along with a synthetic review of previous papers, a large number of new results are included. The possibility of substitution of Li+ by Al3+, Zr4+, W6+ and its influence on the physicochemical properties of LiFePO4 was investigated by means of XRD, SEM/EDS, electrical conductivity and Seebeck coefficient measurements. The range of solid solution formation in Li1−3xAlxFePO4, Li1−4xZrxFePO4 and Li1−6xWxFePO4 materials was found to be very narrow. Transport properties of the synthesized materials were found to be rather weakly dependent on the chemical composition. The battery performance of selected olivines was tested by cyclic voltammetry (CV). In the case of LiFe1−yMyPO4 (M = Mn, Co and Ni), solid solution formation was observed over a large range of y (0 < y ≤ 1). An increase of electrical conductivity for the substitution level y = 0.25 was observed. Electrons of 3d metals other than iron do not contribute to the electrical properties of LiFe1−yMyPO4, and substitution level y > 0.25 leads to considerably lower values of σ. The activated character of electrical conductivity with a rather weak temperature dependence of the Seebeck coefficient suggests a small polaron-type conduction mechanism. The electrochemical properties of LiFe1−yMyPO4 strongly depend on the Fe substitution level.

Applicability of Gd-doped BaZrO3, SrZrO3, BaCeO3 and SrCeO3 proton conducting perovskites as electrolytes for solid oxide fuel cells

AbstractFour proton conducting oxides of perovskite structure: BaZrO3, SrZrO3, BaCeO3 and SrCeO3 doped with 5 mol.% of gadolinium are compared in terms of crystal structure, microstructure, sinterability, water sorption ability, ionic transference number, electrical conductivity and stability towards CO2. Relations between proton conductivity, structural and chemical parameters: pseudo-cubic unit cell volume, lattice free volume, tolerance factor, crystal symmetry and electronegativity are discussed. The grain boundary resistance is shown to be the limiting factor of total proton-conductivity for the materials examined. The highest proton conductivity was observed for BaCeO3, however, it turned out to be prone to degradation in CO2-containing atmosphere and reduction at high temperatures. On the other hand, Ba and Sr zirconates are found to be more chemically stable, but exhibit low electrical conductivity. Electrical conductivity relaxation upon hydration is used to calculate proton diffusion coefficient. Selected materials were tested as electrolytes in solid oxide fuel cells.

Correlation between electronic and electrochemical properties of AxMO2-type electrode materials. Electronic criterion

Abstract An electronic model of alkaline ions intercalation into A x MO 2 cathode materials (A  Li, Na; M  Cr, Mn, Fe, Co, Ni) is presented. It is shown that the potential changes accompanying electrochemical intercalation are those of the Fermi level of the cathode material and the shape of discharge curve depends on the state density function N ( E ). It has been shown that comparison of the real metal-metal (M-M) distance with the critical distance given by Goodenough allows one to predict the properties of cathode materials.

MATERIAL PROBLEMS AND PROSPECTS OF Li-ION BATTERIES FOR VEHICLES APPLICATIONS

This paper reviews material issues of development of Li-ion batteries for vehicles application. The most important of them is safety, which is related to application of nonflammable electrolyte with large electrochemical window and possibility of forming protective SEI (solid/electrolyte interface) to prevent plating of lithium on carbon anode during fast charge of the batteries. The amount of electrical energy, which a battery is able to deliver, depend on the electromotive power of the cell as well as on its capacity — both these factors are related to the chemistry of electrode materials. Nanotechnology applied to electrode materials may be a breakthrough for Li-batteries performance due to extreme reactivity of nanoparticles in relation to lithium. The electrode-electrolyte interface phenomena are decisive for a cell lifetime. Review of physicochemical properties of intercalated transition metal compounds with layered, spinel or olivine-type structure is provided in order to correlate their microscopic electronic properties, i.e. the nature of electronic states, with the efficiency of lithium intercalation process, which is controlled by the chemical diffusion coefficient of lithium. Data concerning cell voltage and character of discharge curves for various materials are correlated with the nature of chemical bonding and electronic structure. Proposed electronic model of the intercalation process allow for prediction and design of operational properties of intercalated electrode materials. Proposed method of measuring the LixMaXb potential on the basis of the measurement of the electromotive force of the Li/Li+/LixMaXb electrochemical cell is a powerful tool of solid state physics allowing for direct observation of the Fermi level changes in such systems as a function of lithium content.

Crystallographic and electronic properties of Li1+δMn2−δO4 spinels prepared by HT synthesis

Abstract The aim of this study was to determine an influence of lithium excess in Li 1+ δ Mn 2− δ O 4 spinels (in the 0≤ δ ≤0.32 range) on their transport and structural properties. The additional lithium atoms take Mn positions causing in the lattice parameter diminution and disappearance of the phase transition observed for the stoichiometric Li 1 Mn 2 O 4 manganese spinel. Low-temperature (230–330 K) measurements of electrical conductivity and thermoelectric power indicate rather minor influence of the excess lithium on transport properties in this system.