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