Παρασκευή και μελέτη κόνεων και λεπτών υμενίων λιθιωμένων οξειδίων μεταβατικών μετάλλων για συσσωρευτές ιόντων λιθίου

Abstract

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.