Oriented single-crystalline TiO2 nanowires on titanium foil for lithium ion batteries

Abstract

Rechargeable lithium ion batteries (LIBs) are important for powering mobile or portable electronic devices and for storing energy in intermittent renewable power production systems that rely on the wind and the sun. The three key components of an LIB are the anode, the cathode and the electrolyte. During the charging and the discharging of an LIB, lithium ions insert the anode and the cathode materials, respectively. The ideal host electrode materials must have large surface-to-volume ratios and must be able to repeatedly accept and reject a large number of lithium ions without significant degradation after many charging-discharging cycles. Nanostructured electrodes provide high electrode-electrolyte interfacial area, fast lithium ion diffusion, and can accommodate large strains. Specifically, one-dimensional (1D) nanostructures such as nanorods, nanowires and nanotubes are emerging as a new class of electrode materials for LIBs because they can provide short ion diffusion lengths, improved electron transport and high surface-to-volume ratios. The use of aligned 1D nanostructured materials on a current-collecting substrate (e.g., copper foil or titanium foil) as the electrode could have additional benefits. Firstly, easy electrolyte diffusion into thicker electrodes through the open space in 1D nanostructured arrays could reduce the internal resistance for high capacity electrodes and thus improve power performance. Secondly, aligned 1D nanostructures could provide direct electrical pathways for enhanced charge transport. Finally, the direct connection of the 1D nanostructured electrode material to the current-collecting substrate eliminates the use of binders and conducting additives, which not only simplifies the battery fabrication process but also reduces the battery weight. In this presentation, we describe a simple and environmentally benign three-step hydrothermal method to grow oriented single-crystalline TiO2-B and/or anatase TiO2 nanowire arrays on titanium foil over large areas. We show that these nanowire arrays are suitable for use as the anode in lithium-ion-batteries; they exhibit specific capacities ranging from 200-250 mAh/g at 0.3 C rate (here 1 C is defined based on the theoretical capacity of anatase 168 mAh/g) and good capacity retention at high charge-discharge rates and over as many as 200 charging-discharging cycles. These promising properties are attributed to both the nanometer size of the nanowires and their oriented alignment. The comparable electrochemical performance to existing technology, improved safety, and the ability to roll titanium foils into compact three-dimensional structures without additional substrates, binders or additives suggest that these TiO2 nanowires on titanium foil are promising anode materials for large scale energy storage.