Papers by Author: K. Świerczek

Abstract: In this paper structural, electrical, electrochemical and thermal (DSC) characterization of series of manganese spinel samples with manganese substituted to different degree (x = 0 – 0.5) with nickel are presented. The conductivity and thermoelectric power measurements were performed in wide temperature range also versus oxygen partial pressure and for deintercalated samples. Electrochemical studies of these cathode materials were conducted in Li / Li+ / LiyNixMn2−xO4 type cells. Substitution of manganese with nickel causes disappearance of the phase transition characteristic of LiMn2O4 spinel. Studies of electrical properties reveal that Ni ions do not participate in charge transport at low temperatures. In the charge curves of Li / Li+/ LiyNixMn2−xO4 cells there are two visible plateaux, separated with distinct potential jump (~0.5V), which position on Li content perfectly matches the Mn3+ content in the doped cathode material. The lower plateau is related to the Mn3+ → Mn4+ oxidation, while the next of higher voltage, of the dopant Ni2+ → Ni4+ oxidation. The schematic diagrams of relative Mn – Ni electronic levels alignment are proposed.

Abstract: The La1-xSrxCo0.2Fe0.6Ni0.2O3-d, La1-xSrxCo0.4Fe0.4Ni0.2O3-d and La1-xSrxCo0.2Fe0.4Ni0.4O3-d perovskites were synthesized with Sr contents 0.1 ≤ x ≤ 0.3 using precursors obtained by citric acid method and their structural and transport properties were measured. All obtained samples crystallize in hexagonal R-3c perovskite structure. The thermal stability was found to decrease with increasing Ni and Sr amounts, what leads to appearance of La2NiO4 like secondary phases with spinel structure and even NiO for higher Ni concentration. The DC electrical conductivity and thermoelectric power measurements at low temperatures (77 – 300K) suggest an appearance of variable hopping mechanism in this temperature range. The transport properties were found to improve with both Sr and Ni concentration increases. The moessbauer spectroscopy studies of La1-xSrxCo0.2Fe0.6Ni0.2O3-d samples revealed presence of two valence states of iron (Fe2+, Fe3+). The measured high temperature (870 – 1070K) DC electrical conductivity is relatively high for samples with high Sr and Ni amounts, what makes them good candidates for a possible application as cathode materials for Intermediate Temperature Solid Oxide Fuel Cells and points to further studies of perovskites from the (La, Sr)(Co, Fe, Ni)O3 system.