Influence of grain size and micro-structure on battery performance of thin film cathodes for lithium-ion batteries
R. Kohler, S. Ulrich, M. Bruns, W. Pfleging
Institute for Materials Science I (IMF-I), Karlsruhe
Institute of Technology (KIT),
Understanding and improving the electrochemical performance of cathode materials for lithium-ion batteries (LIB) is a current major focus of research and development in the areas of materials, power sources and chemistry. It is assumed that electrode materials made of nano-composited materials will improve battery lifetime and will lead to an enhancement of lithium diffusion and thus improve battery capacity and cyclability. Lithium cobalt oxide (LiCoO2) is commonly used as a cathode material and is well suited as a model electrode system for advanced LIB architectures.
Thin films of this electrode material were synthesized by non-reactive r.f. magnetron sputtering of LiCoO2 targets on silicon or stainless steel substrates. For the formation of the high temperature phase of LiCoO2 (HT-LiCoO2), which exhibits good electrochemical performance with a specific capacity of 140 mAh/g and high capacity retention, a subsequent annealing treatment is necessary. For this purpose rapid laser annealing of thin film LiCoO2 was investigated in detail. It could be shown, that the grain size could be adjusted very flexible between 20 nm and 2 Ám by either the annealing temperature or the annealing time. Laser-assited patterning with UV excimer lasers operating at 193 nm and 248 nm was used to create mirco-sized cone structures as well as micro-gratings on the thin film surfaces to increase the surface area. It was observed that laser processing led to an improvement of cycle stability in subsequent battery tests.
The effects of laser treatment on the LiCoO2 thin films were studied with Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction to determine their stoichiometry and crystallinity. The development of HT-LiCoO2 and also the formation of a Co3O4 phase were discussed. The electrochemical properties of the manufactured films were investigated via electrochemical cycling against a lithium anode.
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