C. Ziebert1,*, B. Ketterer1, M. Rinke1,
C. Adelhelm1, S. Ulrich1, K.-H. Zum Gahr1,
S. Indris2, T. Schimmel3
1 Institute
for Materials Research I, Karlsruhe Institute of
Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
2
Institute for Nanotechnology,
Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz
1, 76344 Eggenstein-Leopoldshafen, Germany
3 Institute of Applied Physics, Karlsruhe
Institute of Technology, Wolfgang-Gaede-Strasse 1,
76131 Karlsruhe, Germany
Although lithium-ion batteries are widely-used in
mobile communication and portable electronic equipment, there is a strong need
for further development and optimization of their components, especially of the
cathode. Because nanocrystalline thin film cathode materials can provide both
high power and high energy density there is an increasing interest for the
research in this field.
At first LiCoO2 thin film cathodes have been deposited onto Si and stainless steel substrates by RF magnetron
sputtering from a ceramic LiCoO2 target under a wide variation of
the working gas pressure from 0.15 to 25 Pa. The composition, crystal structure, morphology and topography were studied
using inductive coupled plasma optical emission spectroscopy (ICP-OES), carrier
gas hot extraction (CGHE), X-ray diffraction (XRD), Raman spectroscopy (RS),
atomic force microscopy (AFM) and scanning electron microscopy (SEM). As thin
film properties the intrinsic stress, the electric conductivity and the density
were determined by wafer bending, four-probe method and X-Ray reflectivity respectively.
As deposited films at 0.15 Pa as
well as in the range between 5 Pa and 10 Pa working gas pressure showed a
nanocrystalline metastable rocksalt structure with an
unordered cation arrangement and were nearly stoichiometric.
Heat treatment of the films deposited at 10 Pa Argon gas pressure at 600 °C
leads to the formation of the hexagonal high temperature phase HT-LiCoO2
with a layered structure. The battery performance of the as grown and the
annealed thin film cathodes was studied and it was revealed that the discharge
capacity strongly depends on the crystal structure of the films. The highest
capacity was found for the films deposited at 10 Pa.
To look for a further
optimisation of the capacity at second the substrate bias was varied from
0 to -80 V at a constant pressure of 10 Pa. By additional ion bombardment of the growing film it was possible to
obtain LiCoO2 thin films with controlled texture and morphology. Depending on the substrate
bias (003), (104) and (110) orientations of the hexagonal lattice were
observed. The highest capacity was reached for (104) oriented films, which were
deposited at -15 V substrate bias. Films with such an orientation are expected
to have a superior intercalation rate, due to their orientation of the lithium
diffusion plane being directed towards the electrolyte solution.
Oral presentation preferred
* Corresponding author. tel: +49 (0) 7247 82 2919; fax: +49 (0) 7247 82
4567;
E-mail address: Carlos.Ziebert@kit.edu.de