Browsing by Author "Karppinen, Maarit, Academy Professor"

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Studies on stability and oxygen and water absorption characteristics of YBaCo4O7+delta and LiFePO4
(2012) Räsänen, Samuli
School of Chemical Technology | Doctoral dissertation (article-based)
Systematic engineering of functional oxide materials aims to either improve their functional properties or remove serious shortcomings. In this work various chemical and elevated-pressure techniques were employed as oxygen-engineering tools for first controlling the oxygen content of the perovskite cobalt oxide, SrCoO3-δ , and then, more particularly, for evaluating the oxygen storage capability of the new oxygen storage material candidate, YBaCo4O7+δ . The oxygen nonstoichiometry range in YBaCo4O7+δ was found to be between 0 ≤ δ ≤ 1.5. Effects of cation substitution on the oxygen storing capacity and thermal stability of YBaCo4O7+δ were also studied. The YBaCo4O7+δ phase has a problem of decomposition at around 600 °C in oxygen containing atmospheres, however, cobalt-site substitutions with Al and Ga were found to enhance the phase stability without any major effect on the oxygen storing capacity. For example a 20 % Ga-substituted material showed no phase decomposition in an oxygen containing atmosphere across a temperature range from 25 to 1000 °C. High-sensitivity in-situ thermogravimetric measurements were carried out under controlled humidity conditions to investigate the water absorption/desorption characteristics of LiFePO4 - a material used as a positive electrode material in Li-ion batteries. Plain LiFePO4 powder and composite-electrode powders with polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), acryle and carboxymethyl cellulose (CMC) binder were studied. It was found that the amount of absorbed water had a linear dependence on the humidity level of surrounding atmosphere. A major part of the absorbed water is uptaken in the first few minutes of exposure to humid air. It was also found that the binders (except CMC) or the conductive carbon in the composite electrode protect the material against water absorption to some extent. Partial substitution of Mn-to-Fe increases the amount of absorbed water but at the same time improves the reversibility of the water absorption/desorption process. This finding is due to the change from a mixed physi-chemisorption to pure physisorption process.