N/B Co-Doping Ultrathin Nanotube-Constructed Urchin Shaped Titania Enabling Superior Rate Capability for Advanced Lithium Ion Batteries
Nanostructured titania materials are intensively investigated as anodes of rechargeable lithium-ion batteries (LIBs) because of their better safety and superior rate capability compared with commercial graphite.1 Among the various polymorphs of titania (anatase, rutile, brookite and TiO2-B), TiO2-B has the highest capacity with promising high rate capabilities due to its unique open crystal structure with channels parallel to the b-axis for fast Li+ diffusion.2-3 TiO2-B can accommodate 1 Li+ per Ti, giving a theoretical capacity of 335 mA h g-1. For bulk anatase titania, only 0.5 Li+ per formula unit can be accommodated. However, the reduction of the particle size to nanoscale allows Li insertion beyond limit. In addition, it has been reported that the appropriate element doping would promote the interstitial diffusion of lithium ions in nanosized TiO2, enhancing rate capability.4 In this case, nanostructured titania with TiO2-B/anatase crystal phases and appropriate element doping is an ideal host for the fast insertion/extraction of lithium ions.
Herein, we report on the facile and large-scale fabrication of N/B co-doping ultrathin nanotube-constructed urchin shaped titania spheres (NT-T), and the evaluation of their electrochemical properties. The urchin shaped microspheres consisting of numerous tiny nanotubes exhibit a high capacity and superb rate capability, due to the ultrathin tubular morphology, TiO2-B/anatase phases and N/B co-doping.
Typically, the ultrathin nanotube-constructed urchin shaped titania spheres were prepared by a modified hydrothermal route, followed by an ion-exchange process and further heat treatment. The morphology and structure of the product were measured by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical tests were carried out in 2032 coin type cells. The working electrodes were prepared by mixing the active material, super P and polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone in a mass ratio of 8:1:1, and then coated onto a Cu foil. A lithium foil of 1 mm thick was used as the counter and reference electrode, Celgard 2300 as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as the electrolyte. Galvanostatic charge-discharge measurements were carried out on a Land Battery Measurement System (Land, China) at various current densities with a cutoff voltage of 1–3 V at room temperature.
Results and Discussion
The morphology and structure of the NT-T product are presented in Fig. 1a-d. The NT-T shows an urchin-like morphology, as observed in Fig. 1a and 1b. More details in Fig. 1c reveal that each individual urchin-like microsphere consists of numerous tiny nanotubes. The outer diameters of the tiny nanotubes are around 8 nm and the inner diameters are 2-3 nm. The clear lattice spacing of 0.62 nm in Fig. 1d corresponds to the (001) space of TiO2-B (JCPDF no. 74-1940), which is vertical to the radial direction of the nanotube. The SAED pattern in the inset further confirms the co-exsistence of anatase and TiO2-B in the NT-T product.
The lithium-storage performance of the as-prepared NT-T product was evaluated using lithium half-cells. Fig. 1e demonstrates the rate capability of the as-prepared NT-T electrode at different current densities. Reversible discharge capacities of 279, 237, 202, 178 and 160 mA h g-1 can be obtained at 0.03, 0.5, 3, 6 and 12 A g-1, respectively. Fig. 1f shows the cycling performance of the NT-T electrode at 6 A g-1. The capacity of 180 mA h g-1 is steadily retained even after 100 cycles, indicating a high reversibility of this electrode. For comparison, controllable samples of nanobelt shaped TiO2-B (NB-T) and nanorod shaped anatase TiO2 (NR-T) were also evaluated under the same conditions. The NB-T and NR-T electrodes deliver much poorer rate and cycling performances than the NT-T electrode. It is believed that our strategy would be efficient to enhance the rate capability of TiO2-based electrode materials by reducing the materials dimension, hybridizing TiO2-B/anatase crystal phases, and co-doping with N and B.
Figure captions: Figure 1. (a) SEM, (b,c) TEM and (d) HR-TEM images of the NT-T product, (e) Rate performance and (f) cycling performance of the NT-T, NB-T and NR-T electrodes. Insets in (d) show the SAED pattern of the NT-T product.
1 J. B. Goodenough, K. S. Park, J. Am. Chem. Soc. 2013, 135, 1167.
2 A. G. Dylla, G. Henkelman, K. J. Stevenson, Acc. Chem. Res. 2013, 46, 1104.
3 C. J. Chen, X. L. Hu, Y. Jiang, Z. Yang, P. Hu, Y. H. Huang, Chem. Eur. J. 2014, 20, 1383.