X-ray Absorption Spectroscopy Investigation of the Sub-Nanoscale Strain in Thin-Film Lithium Ion Battery Cathodes
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X-ray Absorption Spectroscopy Investigation of the Sub-Nanoscale Strain in Thin-Film Lithium Ion Battery Cathodes Faisal M. Alamgir 1, 2, Jason VanSluytman 1, Daniel Carter1, Jay Whitacre 3, Chi-Chang Kao 2, Steve Greenbaum 1, and Marten denBoer 1 1
Physics and Astronomy Department, Hunter College of the City of New York New York, NY 10021, U.S.A. 2 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA ABSTRACT LiCoO2 and LiNiO2, two important cathode materials for Li-ion batteries, were studied in their respective bulk and thin-film form. X-ray absorption spectroscopy (XAS) has been used to probe the local atomic structure and structural defects in the thin-film and bulk cathodes. Results comparing Li(Co,Ni)O2 in the bulk and thin-film forms suggests a correlation between intrinsic stress and local strain in the thin-film. This local strain is manifested by a collapse of the six-fold rotational symmetry within the metal-metal layer of the Li(Co,Ni)O2 system into a two fold one. The relationship between annealing conditions and the resulting local strain in these films is examined. INTRODUCTION Demand for localized power generation and storage has been growing over the recent past. This has driven science and technology towards miniaturization of devices such as photovoltaics and batteries. Thin-film solid-state batteries, for example, have many applications in miniaturized, long-life sensors, ID cards, aerospace technologies, etc. [1, 2]. Functionality at the fine scale, whether in the context of nanostructured solar cell materials [3], or nanocrystalline electrodes [4], or even the novel designs of devices, such as those employing 3-D architecture [5], needs to be discussed. Simultaneously, care must be taken to understand the structure and properties of low-dimensional devices. For instance, the forces are not isotropic within a two- or one-dimensional material, as they are in bulk, polycrystalline solids. This imbalance of forces causes intrinsic stress within the functional material that may lead to diminished performance or even failure of these miniaturized devices. The deposition of thin-film solid-state devices from the vapor state lends itself to the formation of structural defects not only due to the low dimensionality of the film, but also because of the rapid thermal quenching brought on by the solidification process. Many of these defects can be removed by post deposition annealing steps. Cost considerations, on the other hand, force the manufacturers to lower the process temperatures as much as possible without sacrificing functionality. In LiCoO2, which is isostructural to LiNiO2, the lowest reported temperature required to produce the high temperature (HT) cubic phase is 500 oC [6]. A more recent study has shown that
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LiCoO2 thin-film cathodes heated only up to 300 oC are viable for assembling solid-state batteries [7]. We have examined the effect of intrinsic and thermal stress in
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