Preparation and study of power and thin film lithiated transition metal oxides for use in lithium ion accumulators

This experimental work deals with the synthesis of lithiated transition metal oxides for use as cathode materials in Li-ion accumulators. The materials were prepared either in powder state or as thin films. In order to study the structural, electrical and electrochemical properties of the materials, different conditions of synthesis and preparation were required, according to the state of the material. Therefore, materials are separated and presented in two major categories, powders and thin films. Materials in powder form were mainly based on the LiMn2O4. In order to improve the electrochemical characteristics of the material, Mn was substituted with either Ni, up to 100%, or with Co, Al and Ti, up to 25%. Solid state reaction was used for all materials’ synthesis in powder form. LiMn2O4 was also prepared so that a direct comparison, for materials synthesized under exactly the same conditions, could be performed and conclusions could be drawn without any compromise. In the materials that Ni substituted Mn, an excess of 20% of Li was used in the synthesis of the materials. The solid state reaction temperature was 7500C and the resulting materials were annealed and slowly cooled in an O2 atmosphere. Substitution of Mn with Ni for 25% and 40% resulted in materials that maintained the cubic structure Fd3m of LiMn2O4. For higher percentage of substitution the structure transformed to the rhombohedral R3m, as it was shown by X-ray powder diffraction measurements. Test accumulators were charged galvanostatically, with a constant current, between 3.4 – 4.4 V. 1Μ LiPF6 50:50 EC:DMC electrolyte was used in those test cells and the anode was made of Li foil. The materials with the cubic structure performed better in terms of cycle lifetime. Substitution of 25% and 40% led to materials that exhibited a second plateau, when cycled to a higher voltage. This plateau appeared at 4.75 V when charging and approximately at 4.7 V when discharging. By substituting 40% Mn with Ni, the lifetime of the material increases and cycling becomes more stable but the specific discharge capacity decreases, compared to LiMn2O4. For a higher percentage of substitution, it is verified that materials with a layered R3m structure containing Ni are extremely sensitive to the Li/Ni ratio. Therefore, they were very unstable when operating as cathodes in test accumulators and their capacity faded fast. However, materials synthesized at a very low cooling rate performed much better in terms of initial discharge capacity and lifetime, compared to similar materials cooled at a higher rate from annealing conditions to room temperature

Electrical properties of LiNbO3 (electrolyte)/Cu (anode) bi-layers

In this work specific film structures of Li–Nb–O/Li/Li–Nb–O are investigated by AC Impedance Spectroscopy measurements at different temperatures. This gives the opportunity to investigate properties of the material itself and, at the same time, to consider the influence of the grain boundaries on the ionic behavior of the polycrystalline Lithium Niobate. On the other hand, LiNbO3/Li/Cu multi-layers are studied as electrolyte/anode bi-layers and potential parts of “Li-free” microbatteries. The Li deficiency in the as deposited Li–Nb–O films is cured by forming a “sandwich” of Li–Nb–O/Li/Li–Nb–O, which after annealing becomes ionic conductor. The electrical behavior of an annealed film depends on two sources. The first is due to properties of the material itself and the second is based on the network of the grain boundaries. The average size of the grains is strongly influenced by the structure of the ohmic-contact/substrate. The electrical behavior of the electrolyte/anode interface of the “Li-free” structure LiNbO3/Li/Cu/Au is very similar to the impedance measurements of the single LiNbO3 single films. The whole multilayer structure, though, presents a third relaxation time which is consistent of a small resistance. This resistance is independent of temperature and it seems that is due to the metallic interface Li/Cu/Au

Electrical properties of LiNbO<inf>3</inf> (electrolyte)/Cu (anode) bi-layers

In this work specific film structures of Li-Nb-O/Li/Li-Nb-O are investigated by AC Impedance Spectroscopy measurements at different temperatures. This gives the opportunity to investigate properties of the material itself and, at the same time, to consider the influence of the grain boundaries on the ionic behavior of the polycrystalline Lithium Niobate. On the other hand, LiNbO3/Li/Cu multi-layers are studied as electrolyte/anode bi-layers and potential parts of "Li-free" microbatteries. The Li deficiency in the as deposited Li-Nb-O films is cured by forming a "sandwich" of Li-Nb-O/Li/Li-Nb-O, which after annealing becomes ionic conductor. The electrical behavior of an annealed film depends on two sources. The first is due to properties of the material itself and the second is based on the network of the grain boundaries. The average size of the grains is strongly influenced by the structure of the ohmic-contact/substrate. The electrical behavior of the electrolyte/anode interface of the "Li-free" structure LiNbO3/Li/Cu/Au is very similar to the impedance measurements of the single LiNbO3 single films. The whole multilayer structure, though, presents a third relaxation time which is consistent of a small resistance. This resistance is independent of temperature and it seems that is due to the metallic interface Li/Cu/Au. © 2010 Elsevier B.V. All rights reserved