Browsing by Author "Pehto, Erkka"

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Korkeaentropiset metalliseokset ja niiden potentiaalinen käyttö ydinvoimalaitosmateriaaleina
(2016-05-08) Pehto, Erkka
Kemiantekniikan korkeakoulu | Bachelor's thesis
Lithium nickel manganese cobalt oxide as a positive electrode material in lithium ion batteries
(2019-07-31) Pehto, Erkka
Kemian tekniikan korkeakoulu | Master's thesis
Li-ion batteries are the most popular rechargeable batteries due to their high power and energy density, long cycle life, safety and environmental friendliness. Lithium nickel manganese cobalt oxide, LiNixMnyCo1-x-yO2 is an increasingly widely used positive electrode material in Li-ion batteries. Its advantages over other positive electrode materials are its high energy density and long cycle life. It was originally designed to replace lithium cobalt oxide, LiCoO2 in order to decrease the usage of cobalt, which is currently an expensive and often unethically produced material. Lithium nickel manganese cobalt oxide is widely studied and developed into better characteristics. In the literature part of this work, the electrochemistry of Li-ion battery cells is introduced, after which the characteristics of lithium nickel manganese cobalt oxide as the positive material in Li-ion batteries are described. The characteristics include composition, crystal structure and electrochemical performance. Effects of structural modifications, doping, coatings and additives on the electrochemical performance are discussed. In the experimental part, four LiNi0.6Mn0.2Co0.2O2 materials were investigated as the positive electrode material in Li-ion battery cells. Differences between the materials were the secondary particle size of the nickel manganese cobalt oxide precursor and temperature of the lithiation process. Half cells using lithium as the negative electrode and full cells using graphite on the negative electrode were investigated. Structural characterization was performed using X-ray diffraction and scanning electron microscopy. Electrochemical characterization was performed using cyclic voltammetry, electrochemical cycling and electrochemical impedance spectroscopy. Higher lithiation temperature was found to lead to an increase in the secondary particle size and to a higher deviation from the stoichiometric layered lithium-transition metal oxide crystal structure. The material with the lowest deviation from the layered crystal structure had the highest cycle life, and both of the materials with the higher lithiation temperature had faster lithium ion diffusion. All the materials had very similar electrochemical performances. High calendering pressure enhanced the rate capability of the electrodes by increasing electron transfer between the active material particles. The material with the smallest secondary particle size had the lowest discharge capacities at all charging rates. The materials with a larger particle size were found to most likely have a better contact with the current collector on the positive electrode. Charge transfer resistances were growing faster in cells with a 70 % state of charge than in cells with a 30 % state of charge.