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X-ray absorption studies on lithium manganese oxide thin films deposited by atomic layer deposition

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Title: X-ray absorption studies on lithium manganese oxide thin films deposited by atomic layer deposition
Author(s): Nieminen, Heta-Elisa
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Discipline: Inorganic Chemistry
Language: English
Acceptance year: 2018
Lithium ion batteries (LIBs) are used as power sources in most of portable devices, electronic cars and many other applications today. As the industry demands for smaller devices, new focus has raised for nanostructured electrodes and all solid state batteries. LIBs are rechargeable batteries, which means that Li+-ions migrate between anode and cathode reversibly. For the battery to work, the electrodes need to be stable when Li+-ions are intercalated and removed from the structure. LiMn2O4 is a promising material for a cathode, because the three-dimensional manganese oxide network enables intercalation of additional lithium ions also beyond the 1:1 Li:Mn stoichiometry resulting in structures in the range of LixMn2O4, 0 ≤ x ≤ 2. The non-stoichiometric behavior improves the specific capacity of the battery compared to other possible lithium intercalation cathode materials like LiCoO2 and LiNiO2. Atomic layer deposition (ALD) is a gas phase thin film deposition technique where the precursors are led to react on the surface one by one. Because no chemical reaction is taking place in the gas phase, the film growth is highly controllable and the resulting films have excellent purity and high uniformity and conformity. ALD is superior thin film deposition technique even on top of 3D structures and when depositing ultra-thin films. This makes it excellent technique to deposit electrode materials for LIBs. X-ray absorption spectroscopy (XAS) measures the absorption of the material as a function of x-ray energy. With a tunable light source it is possible to study a selected electron core shell of a specific element. XAS gives then information on the electronic structure as well as the local crystal structure of the selected atom in the material. Therefore, XAS is an excellent technique for studying the intercalation and de-intercalation processes of LiMn2O4. The literature part of this thesis describes the synthesis of LiMn2O4 thin films and the behavior of the material as a working cathode in LIB. The focus is especially on XAS studies on LiMn2O4. The experimental part involves a study on ALD grown β-MnO2 and LixMn2O4, 0 ≤ x ≤ 1, thin films with XAS, X-ray diffraction (XRD), X-ray reflectivity (XRR), time-of-flight elastic recoil detection analysis (TOF-ERDA), residual stress measurements and X-ray photoelectron spectroscopy (XPS). The aim of the study was to define how and where Li+ -ions intercalate in the β-MnO2 structure. To clarify the intercalation process, K and L core shells of manganese and K core shell of oxygen were studied with XAS. Samples were prepared at 225 ˚C by using earlier known ALD processes. First β-MnO2 film was deposited and then lithium was added by pulsing LiOtBu and H2O alternately on to the β-MnO2 film. The films were annealed after the deposition, however the lithium intercalation was observed to start already during the LiOtBu – H2O pulsing sequence. For the lithium intercalation mechanism, it is proposed that in the beginning the Li+-ions penetrate only to the top part of the β-MnO2 film and a lithium deficient non-stoichiometric phase LixMn2O4, 0 < x < 0.5, is formed. When the lithium concentration exceeds x ≈ 0.5 in LixMn2O4, the phase changes from the tetragonal pyrolusite to the cubic spinel, which enables Li+-ions to migrate throughout the whole film. Annealing in air after deposition seemed to convert the phase completely to the pure cubic spinel LiMn2O4.

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