Self-Ordered TiO₂ Nanotubes Prepared By Anodization in Fluorine-Free Electrolyte As Additive-Free Anode for Lithium-Ion Microbatteries

TiO₂-based material is widely studied as a promising anode for rechargeable lithium ion batteries due to its unique properties such as high safety, easy handling and low production cost. On top of that, the high structural stability of TiO₂ arises from the low volume expansion of less than 4% upon cycling, without Li dendrite formation or electrolyte decomposition that turns them into stable and safe anode materials. Nevertheless, the low ionic conductivity represents the main obstacle behind the low practical capacity of TiO₂ electrode. Manufacturing of TiO₂ nanostructures is one of the effective solutions to create short diffusion pathways for Li, to enhance the ionic conductivity by lattice defects and, finally, the electrochemical performance of titania. TiO₂ nanotubes represent the ideal one-dimensional nanostructure, especially when they are aligned and perfectly arranged in close packed arrays with high surface area. Ordered TiO₂ nanotubes are commonly synthesized by the anodic oxidation of titanium in fluoride ion-containing electrolytes [1]. However, the use of such an anodization bath requires complicated safety measures, due to its harmful impact on both health and the environment, which in turn is translated into a higher production cost. Despite several attempts have been reported to grow self-ordered TiO₂-NTs in fluoride-free electrolytes, only bundles of interwoven and disordered tubular morphologies were formed [2,3]. Herein, we report the successful fabrication of vertically aligned and highly ordered TiO₂-NTs by means of anodization in a fluoride-free aqueous electrolyte. The typical formation bath is composed of diluted sulfuric acid as electrolyte basis and chloride ions. Various synthesis parameters such as chloride concentration, pH, voltage, growth time, and temperature were explored to optimize the ideal fabrication conditions of the nanotubes. Structure, composition and morphology of the grown oxide tubes were investigated. The vertical alignment in an ordered porous nanostructure revealed from our results push the grown nanotubes into very promising position as candidates for lithium ion storage. The electrochemical performance of the NTs was evaluated vs. Li/Li⁺ without binder or conductive additives. The effect of the annealing temperature on the crystallinity and the lithium ion storage properties was investigated. The TiO₂-NT electrodes demonstrated a reversible capacity of 150 µAh cm^-2 after 50 charging/discharging cycles with a Coulombic efficiency close to 100% and without decomposition of the tubular structure at a current rate of 1C. The noticed cycling stability of the NT electrodes is attributed to the high structural stability of TiO₂ and the perfect contact between the oxide films and the substrate which is utilized as a current collector.

References

[1] M. Madian, A. Eychmüller, L. Giebeler, Current advances in TiO₂-based nanostructure electrodes for high performance lithium ion batteries, Batteries, 4 (2018) 7.

[2] C. Richter, E. Panaitescu, R. Willey, L. Menon, Titania nanotubes prepared by anodization in fluorine-free acids, J. Mater. Res., 22, 6 (2007) 1624- 1631.

[3] Y. L. Cheong, F. K. Yam, S.W. Ng, Z. Hassan, S. S. Ng, I. M. Low, Fabrication of titanium dioxide nanotubes in fluoride-free electrolyte via rapid breakdown anodization. J. Porous Mat., 22, (2015)1437–1444.