Lithium-Titanate Thin Films By Solid State Reaction As Electrode Material for Thin-Film Lithium-Ion Batteries

Monday, 27 July 2015
Hall 2 (Scottish Exhibition and Conference Centre)
N. Labyedh (imec, Belgium, Centre of Surface Chemistry and Catalysis, K.U. Leuven), B. Put (Department of Physics, KULeuven, B-3001 Leuven, Belgium, imec, Kapeldreef 75, B-3001 Heverlee, Belgium), A. Hardy (imomec, imec, Hasselt University), M. K. Van Bael (Hasselt University, imomec, imec), and P. M. Vereecken (imec, Belgium, Centre of Surface Chemistry and Catalysis, K.U. Leuven)
Lithium titanate (Li4Ti5O12) or LTO continues to attract great interest as anode for lithium ion because of its safety and the potential high power for the nanoparticle form. Its spinel structure allows a reversible three lithium ions intercalation at 1.55V vs Li+/Li which is well above the potential for the reduction of most electrolytes including solid electrolytes. The high voltage of lithium ion insertion/extraction in/from Li4Ti5O12 also prevents the formation of dendritic lithium which can happen due to overcharging for anodes which have low potentials close lithium deposition (0V vs Li+/Li). Such metallic lithium filaments  can cause a short circuit between the battery anode and cathode with potential overheating and rupture. Moreover, LTO shows a negligible volume change during battery charging and discharging. Despite all these advantages, LTO has a poor electronic conductivity that leads to a limitation in the rate capability of this electrode.

In this talk we will report on a novel and low cost fabrication method of lithium titanate thin film electrodes for thin-film lithium ion batteries. The process is based on a solid state reaction occurring between TiO2 and Li2CO3 stacked-layers upon thermal annealing. The TiO2 layer was prepared by thermal oxidation of a sputtered Ti film and the Li2CO3 layer was deposited by spin-coating. LTO thin films were prepared on both Pt and TiN current collectors.

The phase and morphology of the prepared LTO films were investigated using a X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The XRD patterns display intense characteristic diffraction LTO peaks and SEM micrographs show a close and continuous 70 nm thick LTO film.

Furthermore, the stoichiometry of the fabricated LTO films was evaluated by X-ray photoelectron spectroscopy (surface spectra and depth profile analyses) and elastic recoil detection (ERD) analyses. Films with composition the spinel Li4Ti5O12 composition were fabricated, proving that the thermal treatment causes elements intermixing leading to the formation of a Li4Ti5O12 material.

The electrochemical activity of the films was studied by cyclic voltammetry and galvanostatic charge/discharge experiments. The cyclic voltammogram reveals redox peaks around 1.55V (vs. Li+/Li) corresponding to the Li-ion insertion/extraction in/from the spinel Li4Ti5O12 upon reduction/oxidation of Ti4+/Ti3+. The lithiation and delithiation curves show extremely flat plateaus at 1.55 V (vs. Li+/Li) characteristic of Li4Ti5O12. A 100% of the LTO theoretical capacity was reached at 0.1 C.  A difference between the lithiation and delithiation capacities at the same C-rate was detected. This discrepancy will be explained by the difference in Li-ion diffusion kinetics during Li-ion insertion and extraction processes.