12048
J. Phys. Chem. C 2010, 114, 12048–12051
Porous Spheres Assembled from Polythiophene (PTh)-Coated Ultrathin MnO2 Nanosheets with Enhanced Lithium Storage Capabilities Wei Xiao,† Jun Song Chen,† Qing Lu,‡ and Xiong Wen Lou*,† School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, Singapore, and Institute of EnVironmental Medicine, School of Public Health, Tongji Medical College, Huazhong UniVersity of Science and Technology, 13# Hangkong Road, Wuhan 430030, People’s Republic of China ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: June 5, 2010
In this work, we propose a strategy to enhance the lithium storage capabilities of metal oxide based anodes for lithium-ion batteries through nanopainting with a thin layer of conducting polymer. To demonstrate the concept, we have developed a facile, one-step aqueous/organic interfacial synthesis to prepare polythiophene (PTh)-coated ultrathin MnO2 nanosheets (≈5 nm) with a uniform mesoporous hierarchical structure. When evaluated for lithium storage capability, the MnO2/PTh nanocomposite exhibits enhanced capacity retention upon high-rate charge/discharge cycling. Introduction
Experimental Section
The demand for new lithium-ion batteries (LIBs) with features, such as high capacity, high power, and long lifetime, in electric vehicles (EVs) and consumer electronics has prompted numerous research efforts toward developing high-performance lithium storage electrode materials for next-generation LIBs.1-3 As a result, nanostructured metals/metal oxides (Si, Sn, SnO2, CoOx, FeOx, MnOx, etc.) have attracted enormous interest as potential high-capacity anode materials due to their significantly higher theoretical specific capacities than that of currently-used carbon anode materials.4-13 To date, the practical use of all of these high-capacity anode materials is still largely hindered by issues, such as poor capacity retention upon charge-discharge cycling and/or poor rate capability. These problems have been traced to a very large cyclic volume change of the active electrode materials accompanying lithium insertion/extraction, causing local stress and ultimately electrode failure. With this understanding, different strategies have been proposed to improve the capacity retention and rate capability.7,14 In particular, “nanopainting” the active particles with a layer of inorganic materials (e.g., carbon, RuO2, amorphous metal oxides) is proven very effective.6,15-18 For example, we have recently demonstrated that “breathable” SnO2@carbon coaxial hollow nanospheres exhibit largely improved cyclability and rate capability.6 However, preparation of such nanopainted composites usually involves tedious multistep procedures and harsh processes, which unavoidably restrict the general applicability. Take carbon coating as an example; it often requires a high-temperature (>500 °C) carbonization step to convert precursors to carbon materials. Herein, we propose a hypothesis that nanopainting metal oxides with a thin layer of conducting polymer can improve their lithium storage capabilities. To demonstrate the idea, we synthesize porous hierarchical spheres assembled from polythiophene (PTh)-coated ultrathin MnO2 nanosheets (ca. 5 nm) via a facile, one-step aqueous/organic interfacial synthesis.
Materials Preparation. All chemicals were purchased from Aldrich and used as received without further purification. In a typical interfacial synthesis, thiophene (2 mL) is dissolved in the organic phase consisting of 100 mL of dichloromethane (CH2Cl2). Potassium permanganate (KMnO4, 0.1 g) is dissolved in 100 mL of deionized water, and the pH value of the solution is adjusted to 2 by adding several drops of 2 M hydrochloric acid. After the aqueous and organic solutions are both cooled to 4 °C, the two solutions are mixed together to form a static organic/inorganic interface. The reaction bottle is then placed into a refrigerator with a controlled temperature at 4 °C. After the reaction, the lower organic solution is thoroughly pipetted out. The precipitate in the aqueous phase is harvested by centrifugation and washed with deionized water three times and ethanol once before room-temperature vacuum drying. Materials Characterization. Crystallographic information of all as-prepared samples was investigated with X-ray powder diffraction (XRD, Shimadzu XRD-6000, Cu KR, λ ) 1.5406 Å) at a scan rate of 1° min-1. The thermal behavior of the samples was examined by thermogravimetry analysis (TGA, Shimadzu DTG-60) from room temperature up to 600 °C at a heating rate of 5 °C min-1 in a dynamic atmosphere of air (35 mL min-1). The bonding properties of the samples were characterized with Fourier transform infrared spectroscopy (FTIR, Shimadzu FTIR-8700) using a standard potassium bromide (KBr) pellet technique. Each FTIR spectrum was collected after 32 scans at a resolution of 2 cm-1 from 400 to 4000 cm-1. The morphology of as-prepared samples was examined with field emission scanning electron microscopy (FESEM, JSM-6700F) equipped with energy-dispersive X-ray (EDX) spectroscopy and high-resolution analytical transmission electron microscopy (TEM, JEM-2010, 200 kV; HRTEM, Philips FEG CM300, 300 kV) with selected area electron diffraction (SAED) capability. Elemental compositions of prepared samples were measured with EDX microanalysis attached to FESEM. Electrochemical Measurements. The electrochemical measurements on lithium storage capability were carried out with two-electrode Swagelok-type cells (X2 Labwares) with lithium
* To whom correspondence should be addressed. E-mail:
[email protected]. † Nanyang Technological University. ‡ Huazhong University of Science and Technology.
10.1021/jp104227e 2010 American Chemical Society Published on Web 06/22/2010
Porous Spheres from PTh-Coated MnO2 Nanosheets
Figure 1. Typical FESEM (A, B) and TEM (C, D) images of the as-prepared b-MO-PTh composite. The inset of (D) shows a HRTEM image indicating a d-spacing of about 0.7 nm.
metal as the counter and reference electrodes at room temperature. The working electrode consists of 80 wt % of the active material (e.g., b-MO-PTh), 10 wt % of a conductivity agent (carbon black, Super-P-Li), and 10 wt % of binder (polyvinylidene difluoride, PVDF, Aldrich). The electrolyte is 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate and diethyl carbonate. Cell assembly was carried out in an Ar-filled glovebox with the concentrations of moisture and oxygen below 1 ppm. Cyclic voltammetry and galvanostatic charge/discharge cycling were carried out with an electrochemical workstation (CHI 660C) and a NEWARE battery tester, respectively. Results and Discussion In this organic/aqueous interfacial synthesis,16,19-22 thiophene is dissolved in the organic phase (CH2Cl2), while potassium
J. Phys. Chem. C, Vol. 114, No. 27, 2010 12049 permanganate is from the aqueous phase (see the Experimental Section and Figure S1, Supporting Information). At the liquid/ liquid interface, the reaction between MnO4- and thiophene leads to formation of ultrathin MnO2 nanosheets coated simultaneously with PTh from the chemical oxidative polymerization of thiophene. Figure 1A,B shows the field emission scanning electron microscopy (FESEM) images of the as-synthesized nanocomposite. From the panoramic morphology (Figure 1A), the sample is composed of uniform submicrospheres with diameters in the range of 300-500 nm. The magnified image (Figure 1B) clearly depicts a hierarchical structure of the submicrospheres selforganized from ultrathin nanosheets (
