Li-insertion Behavior in NanocrystallineTiO 2 -(MoO 3 ) z Core-Shell Materials
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Li-insertion Behavior in NanocrystallineTiO2-(MoO3)z Core-Shell Materials Gregory J. Moore1 , Dominique Guyomard and Scott H. Elder 2 Institut des Matériaux Jean Rouxel, CNRS - University of Nantes BP 32229 - 44322 Nantes Cedex 3 – France 1 Argonne National Laboratory Argonne, IL 60439, U.S.A. 2 Pacific Northwest National Laboratory Richland, WA 99352, U.S.A. ABSTRACT A fundamental study of the Li insertion behavior of a series of materials consisting of a TiO2 core having MoO3 on the surface has been carried out in order to determine the influence of the shell. These TiO2-(MoO3)z materials, where (z) denotes the fraction of coverage from a partial to a double layer, range in diameter from 40-100 Å. Calculations have been done on their theoretical lithium capacity using a maximum of Li0.5TiO2 for the core, and Li1.5MoO3 at the TiO2/MoO3 interface, and they have been compared to that found experimentally. The reversible Li-insertion capacity was shown to increase from 0.34 per Ti for the pure TiO2 sample, to 0.91 Li per transition metal when the MoO3 coverage increased to one monolayer. There was only one plateau observed in the electrochemical scans for the samples showing that they function as a single-phase material making them interesting for electrodes. The redox voltage of the TiO2/Li0.5TiO2 biphasic transformation increased 60 mV from the pure TiO2 to the sample containing one monolayer of MoO3. This effect was interpreted as due to a change in TiO2 surface charge coming from an inductive effect of Ti-O-Mo bonds.
INTRODUCTION Nanocrystalline TiO2 has shown potential as a positive electrode in a lithium cell as well as holds promise as a negative electrode material for rocking chair battery assemblies [1]. When cycled either against Li as a positive cathode, or against LiCoO2 as a negative electrode, it has demonstrated its ability to insert up to over 0.5 Li/Ti [2-7]. However, during cycling, the cyclability tends to fall off due to a phase transformation. TiO2, as a mesoscopic insertion compound, has been further investigated using zirconium as a stabilization component [8]. TiO2 has a tendency to undergo thermal induced crystalline growth, accompanied by partial phase transformation to rutile, which is an inactive phase at room temperature. The nanoarchitectured TiO2/zirconium materials proved to be resilient against these transformations, and could potentially make them amenable for battery applications. It is therefore of interest to exploit the potential of TiO2 as a Li-insertion compound, as well as exploring nanocomposite architectures as a means to stabilize and enhance its electrochemical properties. A novel type of nanoarchitectured material has recently been reported having the composition of TiO2-(MoO3)z, with a core-shell structure [9]. These materials are composed of a nanocrystalline TiO2 (anatase) core with MoO3 as a shell. These materials were synthesized by a novel surfactant induced nucleation reaction. The subscript z in TiO2-(MoO3)z indicates the fraction of shell GG10.4.1
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