Article pubs.acs.org/cm
Aqueous Solution Process for the Synthesis and Assembly of Nanostructured One-Dimensional α‑MoO3 Electrode Materials Ken Sakaushi,*,†,‡,∥ Jürgen Thomas,† Stefan Kaskel,‡ and Jürgen Eckert†,§ †
IFW Dresden, Institute for Complex Materials, Helmholtzstr. 20, D-01069 Dresden Germany TU Dresden, Department of Inorganic Chemistry, Bergstr. 66, D-01069 Dresden, Germany § TU Dresden, Institute of Materials Science, Helmholtzstr. 7, D-01069 Dresden Germany ‡
S Supporting Information *
ABSTRACT: A low-temperature aqueous solution synthesis of nanostructured one-dimensional (1D) molybdenum trioxide (MoO3) was developed. The subsequent self-assembly of the fibers to form large-scale freestanding films was achieved without any assistance of organic compounds. Indeed, the whole process, from synthesis to assembly, does not require toxic organic solvents. As an example of the application of our synthesized materials, we built two types of half-cell lithium-ion batteries: (i) the cathode made out of 1D MoO3, having the width in 50−100 nm, with the length in micro scale, and with thickness in ∼10 nm, and (ii) the anode made out of the macroscopic oxide papers consisting of 1D MoO3 and carbon materials. As a cathode material, 1D MoO3 showed a high rate capability with a stable cycle performance up to 20 A g−1 as a result of a short Li+ diffusion path along the [101] direction and less grain boundaries. As an anode material, the composite paper compound showed a first specific discharge capacity of 800 mAh g−1. These findings indicate not only an affordable, eco-efficient synthesis and assembly of nanomaterials but also show a new attractive strategy toward a possible full aqueous process for a large-scale fabrication of freestanding oxide paper compounds without any toxic organic solvent. KEYWORDS: nanostructures, aqueous solution process, self-assembly, cathode and anode, rechargeable batteries
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INTRODUCTION The recent successful preparation of one-dimensional (1D) nanomaterials shows the great improvement of properties that are difficult to achieve in bulk materials.1−5 1D materials have shown already their promising properties in numerous applications, such as lasers,2 photovoltaics6,7 and energy storage devices.8−10 From the perspective of the application, another challenge is to assemble these functional nanomaterials in macroscopic scales. Assembly techniques of 1D materials attract much interest because of their promising potential in electronic, optical, and energy storage devices. A number of techniques for the assembly of 1D materials have been proposed such as few interface-assembly methods,11−13 Langmuir−Blodgett approach14−16 and oil−water−air interface self-assembly process.17 However, all of them require the use of organic compounds. The use of organic compounds in assembled films often negatively affects performances, if we consider the application of them into electronic and optical devices. Hence, a self-assembly process for 1D oxides without assistance of © XXXX American Chemical Society
organic compounds would be highly desirable. In this report, a spontaneous and facile assembly route is described for 1D αmolybdenum trioxide (MoO3) through the evaporation of water as the dispersion medium without any organic compounds to form a large-scale freestanding film. In previous reports, MoO3 has been synthesized in a wide variety of nanostructures,18−20 and these nanomaterials have been applied in a variety of devices, such as photovoltaic21,22 and batteries.23−25 Especially, MoO3 can be used as both an anode and a cathode for lithium-ion batteries (LIBs). LIBs are of great interest because they are expected to be a green technology to achieve a sustainable storage of energy.26−29 For example, LIBs are applied to electronic vehicles and smart grid, which is an electricity supply network combined with renewable energy. The current challenge in LIBs is to increase the energy density of cells. From this point of view, MoO3 has promising Received: May 27, 2013
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dx.doi.org/10.1021/cm401697z | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
charge−discharge performance for cathodes, the electrodes made by the 1D MoO3 (50 wt %), carbon black (Super-P; 45 wt.%), and a binder (Polytetrafluoroethylene; 5 wt %), was measured at current densities of 1.0, 5.0, 10, and 20 A g−1 in the potential range 1.5−3.2 V versus Li/Li+. Normally, electrodes in commercial lithium-ion rechargeable batteries contain only 5 wt % conductive material. However, because we are investigating high rates of lithium-ion insertion in 1D MoO3, we used 45 wt % conductive material for 1D MoO3 as well as bulk MoO3 to improve electrode conductivity. The bulk MoO3 (microcrystals) was purchased from Alfa Aesar. The charge−discharge performance for anodes (= the composite oxide papers), which were made by the 1D MoO3 (80 wt %) and carbon black (20 wt %), was measured at a current density of 0.1 A g−1 in the potential range 0.05−3.2 V versus Li/Li+.
characteristics: in theory, it can provide a specific capacity of ∼1100 mAh g−1 as an anode25 by the conversion reaction30 and a specific energy of ∼750 Wh kg−1 (=300 mAh g−1 × 2.5 V vs Li/Li+)23,31 by the intercalation mechanism.32 These electrochemical properties of MoO3 are attractive compared to graphite (372 mAh g−1)33 and LiFePO4 (∼600 Wh kg−1),34 representing an anode and a cathode used in commercialized LIBs, respectively. Nanostructure is an important factor to obtain high-performance electrochemical properties;35,36 moreover, some electrochemical properties can be only obtained from nanoelectrode materials.30 Therefore, the integration of nanostructured MoO3 synthesis by an eco-efficient process and spontaneous self-assembly of nanostructured MoO3 could lead to further development of indispensable devices in a facile and cost-effective way. Previous studies on the synthesis of MoO3 from the solution phase suggest that this material is a practically suitable candidate for growing 1D morphology due to its anisotropic crystal growth.18,20 Here, we have synthesized 1D MoO3 through a low-temperature aqueous solution route by one-step. We tested this 1D MoO3 as a cathode material with the high rate capability, which was achieved by successful nanostructure control. We also fabricated oxide papers, which are macroscopic, freestanding films consisting of 1D MoO3. The low-temperature (
