Modern microelectronic devices such as backup power for computer memories, MicroElectroMechanical Systems (MEMS), medical implants, smart cards, Radio-Frequency Identification (RFID) tags and remote sensors have necessitated the development of high performance power sources at the microscale. In this context, the development of three-dimensional (3D) microbatteries forms a viable alternative to provide high volumetric energy densities to meet the demands of these devices.1 The development of nano-architectured electrodes is one of the most promising approaches to realize the 3D paradigm of microbatteries.2 Among all the potential anode materials, TiO2 nanotubes (TiO2-NTs) possess remarkable characteristics for the design of 3D Li-ion microbatteries. Self-organized nanotubular materials allow a good diffusion of Li ions in the porous structures and the 1D morphology allows an efficient charge transfer along the axis of the tube that results in a good apparent electronic conductivity of the TiO2-NTs layer when compared to a film composed of nanoparticles3, 4. Anatase TiO2 can accommodate only 0.5 Li+ per formula unit, corresponding to a theoretical capacity of 168 mAh g-1. Hence, several approaches have been investigated to improve the overall performance of TiO2-NTs for the design of high-performance Li-ion microbatteries. Doping with aliovalent ions like Niobium (Nb5+) is also a facile strategy to modify the electronic properties of titanium oxide and thereby enhance the electrochemical performance.5,6
We report the fabrication of self-supported Nb doped TiO2-NTs by anodization of Nb/Ti alloys devoid of any carbon additives or binders. An increase in the capacity of the TiO2-NTs was observed as the Nb doping concentration increased. Such a composition of 10 wt.% Nb doped TiO2-NTs (Nb10-TiO2-NTs) showed a first cycle capacity of 200 mAh.g-1 (144 µAh.cm-2) compared to pristine TiO2-NTs which gave a capacity of 115 mAh.g-1 (78 µAh.cm-2) at C/10. Galvanostatic cycling tests at various C-rates revealed the influence of Nb doping in the TiO2-NTs which is shown in Fig.1 (a) and (b). Compared to pristine TiO2-NTs, the discharge capacities of doped nanotubes are improved and almost doubled when the Nb concentration reaches 10 wt.%. Besides a good cycling behaviour at multiple C-rates, an overall capacity retention of 87 % is achieved after 100 cycles. According to Electrochemical Impedance Spectroscopy measurements, the enhanced electrochemical performance of the Nb-doped TiO2-NTs is attributed to their higher electronic conductivity.
1) B.L. Ellis, P. Knauth, T. Djenizian, Three-Dimensional self-supported metal oxides for advanced energy storage, Adv. Mater. 26 (2014) 3368-3397.
2) J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Three-Dimensional Battery Architectures, Chem. Rev., 104 (2004), 4463-4492.
3) G.F. Ortiz, I. Hanzu, T. Djenizian, P. Lavela, J.L. Tirado, P. Knauth, Alternative Li-Ion Battery Electrode Based on Self-Organized Titania Nanotubes, Chem. Mater., 21 (2009), 63-67.
4) T. Djenizian, I. Hanzu, P. Knauth, Nanostructured negative electrodes based on titania for Li-ion microbatteries , J. Mater. Chem., 21 (2011), 9925-9937.
5) Y.Wang, B. M Smarsly, I. Djerdj, Niobium Doped TiO2with Mesoporosity and Its Application for Lithium Insertion, Chem. Mater., 22 (2010), 6624–6631.
6) M. Fehse, S. Cavaliere, P. E. Lippens, I. Savych, A. Iadecola, L. Monconduit, D. J. Jones, J. Rozi`ere, F. Fischer, C. Tessierand, L. Stievano, Nb-Doped TiO2 Nanofibers for Lithium Ion Batteries, J. Phys. Chem. C, 117 (2013), 13827–13835.