Browsing by Author "Karppinen, Maarit, Prof., Aalto University, Department of Chemistry, Finland"

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Electrode Material Solutions for Large-Scale Lithium-Ion Batteries
(2015) Jalkanen, Kirsi
School of Chemical Technology | Doctoral dissertation (article-based)
Lithium-ion (Li-ion) batteries have become the power source of large-scale applications, such as electric or hybrid-electric vehicles and energy storage systems, in addition to their conventional use in small consumer electronics. However, the traditional electrode materials do not meet the requirements of safety and lifetime, which are emphasized in the large battery packs. Furthermore, for transportation purposes, a high energy density is needed. The aim of this thesis was to study different electrode materials from these perspectives. The lifetime of commercial Li-ion cells was studied under different cycling temperatures. Especially after prolonged cycling at elevated temperature, the graphite negative electrode was found to be not only the main source of aging but also a safety risk as dendritic Li depositions, a potential source of short-circuits, were observed. Possible reasons promoting Li plating could be the extensive passivation layer growth on graphite and the consequential cell drying and formation of gaseous components. A new alternative to the graphite negative electrode is Li4Ti5O12 which however decreases the cell voltage thus sacrificing the energy density. This sets a demand for novel, safe, high-voltage positive electrodes of which mixed Li(Fe1-yMy)PO4 (M = Co, Ni) materials were investigated in this work. A beneficial, mutual influence of the Fe and M occupying the same lattice site was observed as e.g. shift of redox potentials and changes in the delithiation/ lithiation reaction mechanisms. Especially the local environment of Fe3+ was affected by the substitution. An optimal composition for the Co substitution is around Li(Fe0.5Co0.5)PO4. In the case of the Ni substitution, on the other hand, the Ni2+/Ni3+ redox couple could not be reversibly activated, presumably due to lack of electrical conductivity or possible structural changes. Knowledge of the heat generation in a Li-ion cell is needed for accurate design of cooling systems and thus avoiding the undesirable cell temperature increase. The reversible part of heat generation depends on the entropy change in the electrode materials during the cell reaction, affected mainly by the arrangement of Li in the electrode lattices. In this work, the entropy change behavior was studied for LiFePO4, Li(Fe0.33Mn0.67)PO4, graphite, and Li4Ti5O12 electrode materials and their combinations. Specific materials and states of charge were determined to be unfavorable due to extra heat generation. Furthermore, the effect of Mn substituent was observed in the entropy change behavior of Li(Fe0.33Mn0.67)PO4. The impact of reversible heat generation was demonstrated in practice as a cooling effect during the discharge of a commercial Li-ion cell.
Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties
(2015) Niemelä, Janne-Petteri
School of Chemical Technology | Doctoral dissertation (article-based)
The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive post-deposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 ⁰C showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the as-deposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phonon-boundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.