TS2. Coatings for Fuel Cells & Batteries
C. Ziebert, J. Fischer, N. Thiel, J. Pröll, R. Kohler, M. Rinke, S. Ulrich, W. Pfleging
Institute for Applied Materials, Applied Materials Physics (IAM-AWP), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
Li-Mn-O thin film cathodes for Li-ion batteries have been deposited onto Si and gold coated stainless steel substrates by non-reactive r.f. magnetron sputtering at a constant power of 200 W and various working gas pressures ranging from 0.25 to 25 Pa. The composition, crystal structure and thin film morphology were examined using inductive coupled plasma optical emission spectroscopy (ICP-OES), inert gas fusion analysis (IGFA), X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Intrinsic stress and film density were determined and battery tests have been performed in Swagelok cells with Li-Mn-O film as cathode, Li metal as anode and standard EC:DMC (1:1) electrolyte containing 1 mol LiPF6.
Using ICP-OES and IGFA a pressure-dependent variation of the stoichiometry from LiMn0.9O1.9 at 0.25 Pa to LiMn2.3O3.3 at 25 Pa was found. As slight Li loss and an increase in the O content upon furnace annealing were expected based on our previous study on LiCoO2, we focused on a gas pressure of 10 Pa, because the related composition LiMn1.9O3.13 offered the best conditions for the formation of the desired LiMn2O4 spinel phase. By XRD and Raman spectroscopy it was revealed that during furnace annealing at 600 °C for 3 hours in argon/oxygen atmosphere (Ar:O2=4.5:5) of 10 Pa the layered orthorhombic LiMnO2 phase developed. Thus laser annealing using a high power diode laser with a maximum laser output power of 50 W and a wavelength of 940 nm was performed. It was shown that at 600 °C the spinel phase could be adjusted by an appropriate choice of laser annealing time. After 10 s the inactive monoclinic Li2MnO3 rock salt phase formed, which could be transformed into the spinel phase using annealing times up to 100 s.
To investigate the effect of different surface morphologies on the battery performance, selected Li-Mn-O films were patterned by laser ablation with a UV laser of wavelength λ = 248 nm. For the systematic investigation of topography, erosion rate and roughness parameters an ablation array was generated by systematically varying the number of lasers pulses and the laser fluence and were studied by SEM. The formation of flat and smooth ablation profiles was observed at laser fluences above 0.5 J/cm2 independent of the process gas (He, O2). The battery performance of the as grown, the structured and the annealed thin film cathodes was studied and it was revealed that the discharge capacity strongly depends on the crystal structure, the morphology and the surface structure of the thin films, which offers further opportunities for the optimisation of the performance of future 3D thin film batteries.