Highly ordered anodic TiO 2 nanotube arrays with a tube length of 9 mm are shown to provide areal capacities of 0.24 mA h cm À2 (i.e. 96 mA h g À1 ) at a charge/discharge current density of 2.5 mA cm À2 (corresponding to a rate of 5 C) and 0.46 mA h cm À2 (i.e. 184 mA h g À1 ) at 0.05 mA cm À2 , when used as 3D free-standing anodes in Li-ion microbatteries. The present nanotube electrodes, which could be cycled for 500 cycles with only 6% loss of capacity, exhibited significantly higher energy and power densities, as well as an excellent cycling stability compared to previously reported TiO 2 -based Li-ion microbattery electrodes. The influence of parameters such as ordering, geometry and crystallinity of the nanotubes on the microbattery performance was investigated. A two-step anodization process followed by annealing of the nanotubes was found to yield the best microbattery performance. It is also demonstrated that the rate capability of the electrode depends mainly on the rate of the structural rearrangements associated with the lithiation/delithiation reaction and that the areal capacity at various charge/discharge rates can be increased by increasing the tube wall thickness or the length of the nanotubes, up to 0.6 mA h cm À2 for 100 cycles.

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In the present work, we demonstrate the feasibility to form an aligned Nb(2)O(5) nanotube layer by self-organizing anodization of Nb in a NH(4)F-glycerol electrolyte. In order to achieve a nanotubular rather than a nanoporous layer, careful optimization of the anodization electrolyte is required. We show that only in a narrow window of electrolyte parameters highly aligned nanotubes of 50 nm inner diameter and several micrometres in length can be formed.

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