Influence of grain size and micro-structure on battery performance of
thin film cathodes for lithium-ion batteries
R. Kohler[1], 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 µm 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.