Vapor-Solid Reaction Growth of Rutile TiO₂ Nanorods and Nanowires for Li-Ion-Battery Electrodes

A new synthetic method to grow O-deficient rutile TiO_2(s) nanorods (NRs) and nanowires (NWs) by a vapor-solid reaction growth method is developed. TiCl_4(g) was employed to react with commercially supplied CaTiO_3(s) (size 2-4 μm) at 973 K under atmospheric pressure to generate TiO_2(s) NRs (diameters 80-120 nm, lengths 1-4 μm). The reaction employing TiCl_4(g) and CaO_(s) at 973 K also generated CaTiO_3(s) (size 4-13 μm) as the intermediate which reacted further with TiCl_4(g) to produce NWs (diameters 80-120 nm, lengths 4-15 μm). This is the first report of 1D rutile TiO_2(s) nanostructure with such a high aspect ratio. Both of the NRs and the NWs, with compositions TiO_1.81 and TiO_1.65, respectively, were single crystals grown in the [001] direction. Their morphology was affected by the reaction temperature, the concentration of TiCl_4(g), and the particle size of CaTiO_3(s). The NRs and the NWs were investigated as anode materials for Li⁺-ion batteries. At constant current rates 1, 2, and 5 C (1 C = 170 mA g^-1) for 100 cycles, the cycling (1.0-3.0 V) performance data of the NRs were 146, 123, and 104 mA h g^-1, respectively. On the other hand, the cycling performance data of the NWs were 120, 80, and 52 mA h g^-1, respectively. This is attributed to the high Li⁺ ion diffusion rate (D_Li⁺ ) of the NRs (29.52 × 10^-15 cm² s^-1), which exceeds that of the NWs (8.61 × 10^-15 cm² s^-1). Although the [001] growth direction of the NR crystals would provide the fastest channels for the diffusion of Li⁺ ions and enhance the battery capacity, the extremely long channels in the NWs could hamper the diffusion of the Li⁺ ions. The O-deficiency in the structure would increase the conductivity of the electrode material and improve the stable cycling stability of the batteries also. The long-term cycling test at 2 C for the battery constructed from the NRs retained 121 mA h g^-1 after 200 cycles and 99.2 mA h g^-1 after 800 cycles. The device has an excellent long-term cycling stability with a loss of only 0.04% per cycle.