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

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.