Lithiated MoO3 Nanobelts with Greatly Improved Performance for Lithium Batteries

Recently, nanostructured materials have attracted great interest in the field of lithium-ion batteries, essentially because of their substantial advantages, such as short transport path lengths for both electrons and Li ions, a large amount of contact surface area between the electrode and electrolyte, and large flexibility and toughness for accommodating strain introduced by Li insertion/extraction. Among the transition-metal oxides, nanostructured MoO3 has been extensively investigated as a key material for fundamental research and technological applications in optical devices, smart windows, catalysts, sensors, lubricants, and electrochemical storage. There are two basic polytypes of MoO3: orthorhombic MoO3 (a-type) being a thermodynamically stable phase, and the metastable monoclinic MoO3 (b-type) with a ReO3-type structure. The most important structural characteristic of a-MoO3 is its structural anisotropy, which can be considered as a layered structure parallel to (010) (See the inset of Fig. S1, Supporting Information). Each layer is composed of two sub-layers, each of which is formed by corner-sharing octahedra along [001] and [100]; the two sub-layers stack together by sharing the edges of the octahedra along [001]. An alternate stack of these layered sheets along [010] would lead to the formation of a-MoO3, where a van der Waals interaction would be the major binding force between the piled sheets. One might take advantage of the intrinsic structural anisotropy of a-MoO3 for tuning its properties by interlayer structural modification, annealing, and lithiation. In this Communication, we report the electroactivity of a-MoO3 nanobelts after lithiation that show superior performance to nonlithiated a-MoO3 nanobelts. An X-ray diffraction (XRD) measurement was performed using a D/MAX-III X-ray diffractometer. Fourier-transformed infrared (FTIR) absorption spectra were recorded using the 60-SXB IR spectrometer. Raman spectra were taken using a Renishaw RM-1000 laser Raman microscopy system. Scanning electron microscopy (SEM) images were collected with a JSM-5610 and FES-EM LEO 1530. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) were recorded by using a JEOL JEM2010 FEF microscope. The electrochemical properties were studied with a multichannel battery testing system. Batteries were fabricated using a lithium pellet as the negative electrode; 1 M solution of LiPF6 in ethylene carbon (EC)/dimethyl carbonate (DMC) as the electrolyte; and a pellet made of the nanobelts, acetylene black and PTFE in a 10:7:1 ratio as the positive electrode. The fabrication of a single nanobelt-based device has been described in detail elsewhere. XRD measurement was first used to study the phase and lattice modification of the nanobelts before and after lithiation (Fig. 1A). The diffraction peaks of the XRD pattern for both samples can be readily indexed to be orthorhombic with lattice constants of a = 3.962 A, b = 13.85 A, c = 3.697 A (International Centre for Diffraction Data (ICDD) No. 050508). No peaks of any other phases were detected, indicating the high purity of the MoO3 nanobelts. For the non-lithiated MoO3 nanobelts, the stronger intensities of (020), (040), and (060) peaks than those for the bulk MoO3 (Fig. S1, Supporting Information) indicates the anisotropic growth of the nanostructure as well as the preferred orientation of the nanobelts on the substrate. Importantly, in comparison to the nonlithiated sample, there is a small shift of the (020) peak toward a lower diffraction angle for the lithiated sample. This is direct evidence of an expanded b-plane interlayer distance for 0.065 A after lithiation, possibly due to the introduction of Li interstitials between the layers. The morphology and microstructure of the products were observed by using SEM and TEM. Before lithiation (Fig. 1B), C O M M U N IC A TI O N