Preparation and Characterization of Tin/Carbon Composites for Lithium-Ion Cells
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Preparation and Characterization of Tin/Carbon Composites for Lithium-Ion Cells Ronald A. Guidotti1, David J. Irvin2, William R. Even, Jr.3, and Karl Gross3 1 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0614 2 Naval Air Warfare Center, Polymer Science and Engineering Branch, China Lake, CA 93555 3 Sandia National Laboratories, P.O. Box 969, Livermore, CA 94551 ABSTRACT A number of Sn/C composites were prepared for evaluation as anode materials for Li-ion cells. In one case, samples were prepared by incorporation of Sn species into organic precursors that were then pyrolyzed under an Ar/H2 cover gas to prepare the Sn/C composites. They were also prepared by decoration of various types of carbon with nanoparticles of Sn by electroless deposition using hydrazine. The carbons examined included a disordered carbon prepared in house from poly(methacrylonitrile), a mesocarbon microbead (MCMB) carbon, and a platelet graphite. The Sn/C composites were examined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) and were also analyzed for Sn content. They were then tested as anodes in three-electrode cells against Li metal using 1M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) solution. The best overall electrochemical performance was obtained with a Sn/C composite made by electroless deposition of 10% Sn onto platelet graphite. INTRODUCTION There has been increasing interest in recent years in the development of improved materials for use as anodes in lithium-ion cells. Both synthetic and natural graphites as well as disordered carbons have been studied for this purpose. Graphite has a maximum theoretical capacity of 372 mAh/g of carbon, but the disordered carbons can have capacities two or more times that. Unfortunately, the disordered carbons also exhibit high irreversible, first-cycle loss. A number of transition metals and transition-metal alloys have theoretical capacities much greater than that of graphite. Sn, for example, can alloy with Li to form Li4.4Sn, which has a theoretical capacity of 994 mAh/g of Sn. However, such materials show rapid fade with cycling due to attrition of the alloy caused by large volume changes. One way of mitigating this problem is to use the metal on a support or substrate. With combination of Sn or Sn alloys with graphite, it is possible to realize the capacities of both materials when used as lithium-ion anodes [1,2]. In this work, we report on the preparation and characterization of Sn/C composites made with platelet, spheroidal, and disordered carbon. Elemental Sn was deposited on the surfaces of the carbons by chemical reduction in both nonaqueous and aqueous solutions. In some cases, the metal salts were incorporated into the carbon precursors prior to pyrolysis. The Sn/C composites were tested over a range of current densities as lithium-ion anodes. EXPERIMENTAL Preparation of Sn/C Composites Sn/C Composites by Pyrolysis – Tin dibutyl bis(2,4-pentane-dionate) [SnBu2(acac)2] (Gelest, Inc.) was dissolved in dioxane and shaken t
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