Boundary layers and thin films of ionic conductors: Electronic structure, electrochemical potentials, defect formation and degradation mechanisms

Prof. Dr. W. Jaegermann (Div. Surface Science)
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Electrical fatigue of ion and electron conducting ceramic materials for high performance batteries and micro-batteries (in particular the cathodes for intercalation batteries) leads to a loss of the capacity and the battery voltage during repeated loading and unloading. Irreversible behavior of the intercalation reaction and the transport of mobile ions are responsible and lead to phase transformations and/or decomposition reactions in the volume and at interphase boundaries. The attainable reversible cycles are dependent on inherent factors (materials and material combinations, element structure) and external factors (current density, potential ranges, stoichiometry variation, temperature). Some information on the structural changes in the volume is available. There is no systematic knowledge, however, on the changes of the electronic structure and electro-chemical potentials caused by the charge transferand the resulting defect formation mechanisms and their following agglomeration or reaction.

Oxide cathode materials for intercalation batteries LixMO2 (M=Ni, CO, Mn) (in the first project phase in particular LiCoO2 and its substitutional compounds in the cation lattice) are investigated. Changes in the electronic structure and the correlated electro-chemical potentials, in connection with the intercalation and deintercalation reactions as well as the defect formation and decomposition reactions in the volume and at the internal and external boundary surfaces, are to be investigated. Thin film batteries of different morphology are to be produced and investigated using a number of coupled spectroscopic or structure-sensitive methods and electro-chemical measurements. From such measurements the relationships between oxidation level of the constituents and the electronic structure and battery voltages as a function of current densities and state of charge are to be determined. Defect formation processes and changes of the chemical composition are to be correlated with changes of the capacity. Results are used to employ strategies for an optimisation of the systems, which are to be examined in the following stages with respect to their applicability