Keld West

Diffusion impedance in planar, cylindrical and spherical symmetry

Abstract The Laplace transformed diffusion equation is solved for finite diffusion in planar, cylindrical and spherical geometry with a Nernstian or an impermeable diffusion layer boundary condition. Analytical expressions are presented generalized as the Laplace transformed concentration to flux ratio at the electrode surface. The limiting behavior for a number of high and low frequency cases is derived. From these solutions the diffusion impedance for specific electrode reactions can be obtained.

Comparison of LiV3O8 cathode materials prepared by different methods

Lithium trivanadate, LiV{sub 3}O{sub 8}, can be prepared in a finely dispersed form by dehydration of aqueous lithium vanadate gels. Two methods of dehydration, both easily adaptable to large-scale production, are described in this work: freeze drying and spray drying. After heat-treatment of the dried gels (xerogels) to remove loosely bound water they show a high capacity for lithium insertion, approaching four additional lithium per formula unit, and good reversibility as electrode materials for high energy density lithium cells. How the heat-treatment temperature influences the crystal structure is demonstrated as well as the electrochemical properties of the vanadium oxide.

Lithium insertion in different TiO2 modifications

Abstract The insertion of Li into the three titanium dioxide modifications, anatase, rutile, and TiO2(B), is studied primarily by electrochemical techniques. At 25°C at potentials above 1.4 V versus Li/LiAsF6,PC the maximal Li uptake is 0.5 Li/Ti in anatase and TiO2(B), while rutile does not insert Li. At 120°C utilizing LiCF3SO3/PEO electrolyte an amount of 0.8, 0.5, and 0.5 Li/Ti, respectively, is inserted above 1.3 V. The different behaviour of the chemically identical, but structurally non-equivalent oxides, is discussed.

Lithium Intercalation into Layered LiMnO2

Recently Armstrong and Bruce 1 reported a layered modification of lithium manganese oxide, LiMnO 2 , isostructural with LiCoO 2 . LiMnO 2 obtained by ion exchange from α-NaMnO 2 synthesized in air is characterized by x-ray diffraction and by electrochemical insertion and extraction of lithium in a series of voltage ranges between 1.5 and 4.5 V relative to a lithium electrode. During cycling, voltage plateaus at 3.0 and 4.0 V vs. Li develop, indicating that the material is converted from its original layered structure to a spinel structure. This finding is confirmed by x-ray diffraction. Contrary to expectations based on thermodynamics, insertion of larger amounts of lithium leads to a more complete conversion. We suggest that a relatively high mobility of manganese leaves Li and Mn randomly distributed in the close-packed oxygen lattice after a deep discharge. This isotropic Mn distribution can relatively easily relax to the Mn distribution characteristic of spinels whereas the anisotropic distribution characteristic of layered structures is not reformed when excess lithium is extracted

Vanadium oxide xerogels as electrodes for lithium batteries

Vanadium oxide xerogels V2O5 · nH2O are investigated as insertion electrode materials for secondary lithium batteries. Lithium insertion into these materials is shown to be fully reversible when all the loosely bound water is removed by proper heat treatments. The disordered xerogels obtained after heat treatment at 100°C in vacuum, where the reversibly bound water is removed, are able to insert nearly 1.5 Li/V2O5. The more crystalline films obtained after heat treatment at 300°C, where most of the chemically bound water is removed, insert up to 2 Li/V2O5 at potentials above 1.5 V vs. Li, corresponding to a stoichiometric energy density of 730 Wh kg−1. Analysis of differential capacity curves show that lithium ions are accommodated at sites in the xerogel host that are different from the sites occupied in orthorhombic V2O5. It is also noted that the xerogel host is more stable towards lithium insertion, retaining its structure after repeated cycling to a depth of 2 Li/V2O5.

Vanadium oxides as electrode materials for rechargeable lithium cells

Abstract Lithium insertion has been studied in a number of vanadium oxides with special regard to their application as the active component in rechargeable lithium cells. Very high stoichiometric energy densities for lithium insertion are found for several of these materials. As these oxides are poor electronic conductors, however, the high energy densities are partially offset by the addition of conductive material necessary for practical electrodes. Cycling results are reported for the three-dimensional oxide V 6 O 13 , the layered Li 1+ x V 3 O 8 , and the non-crystalline V 2 O 5 xerogel.

All oxide solid-state lithium-ion cells

Solid-state lithium-ion cells have been prepared using thin film Li4Ti5O12 as the anode, thin film LiCoO2 as the cathode and Li0.33La0.56TiO3 as the electrolyte. The electrolyte was prepared as a relatively thick ceramic with a thickness close to 1 mm. This type of cell develops a voltage of slightly greater than 2 V and is stable to cycling. Perhaps the most interesting aspect of this cell, is that even with a relatively thick, poor quality ceramic electrolyte, this cell has been able to develop current densities as great as 40 μA/cm2.

Modeling of Porous Insertion Electrodes with Liquid Electrolyte

The dynamics of porous insertion electrodes during charge or discharge is described by a simplified mathematical model, accounting for the coupled transport in electrode and electrolyte phases. A numerical method to evaluate the response of this model to either controlled potential or controlled current is outlined, and numerical results for the discharge of a porous TiS/sub 2/ electrode in an idealized organic electrolyte are presented. It is demonstrated how electrolyte depletion is the principal limiting factor in the capacity obtained during discharge of this electrode system. This depletion is a consequence of the mobility of the ions not inserted, therefore the performance or this type of electrode is optimized by choosing electrolytes with transport number as close to unity as possible for the inserted ion. 23 refs.

Electrochemical properties of non-stoichiometric V6O13

Non-stoichiometric V6O13±0.2 (VOy) prepared by thermal decomposition of NH4VO3 in N2 atmosphere is investigated cathode material for Li batteries. The maximum lithium composition is found to be Li2y-3VOy. The emf vs composition relationship of Li/LixVO2.144 is determined in the interval 0 < x < 1.3, yielding a theoretical energy density of 890 Wh kg−1. The kinetics and reversibility of the VOy electrode is investigated by cyclic voltammetry and cycling of test cells. The lithium diffusion coefficient and the electronic conductivity of the oxide is found to decrease with increasing lithium content. The cyclability of very thin electrodes is found to be excellent, but inferior results are obtained with practical cells, presumably due to degradation of electrode structure.

V6O13 As cathode material for lithium cells

Abstract The e.m.f. vs. composition relationship of Li/LixV6O13 has been studied at 25 °C and 155 °C by cyclic voltammetry using organic and polymeric electrolytes, respectively. At both temperatures the lithium insertion reaction is found to be reversible in the composition interval: 0 ⩽, x ⩽ 8. a.c.- impedance measurements on single crystals (25 °C) show that Li+ diffusion in LixV6O13 is one dimensional and proceeds along the channels in the b axis direction. Cycling of Li/LixV6O13 cells with organic and polymer electrolyte shows that high materials utilization and good cycling performance can be achieved with both systems. It is demonstrated that LixV6O13 is sensitive to discharge below 1 V vs. Li.