Carbon Coated Silicon Powders As Anode Materials For Lithium Ion Batteries
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Carbon Coated Silicon Powders As Anode Materials For Lithium Ion Batteries M. Gulbinska1, F. S. Galasso1, S. L. Suib1, S. Iaconetti2, P. G. Russell2, and J. F. DiCarlo2 1 2
Dept. Chemistry, University of Connecticut, Storrs, CT 06269 Lithion Inc., 82 Mechanic St., Pawcatuck, CT 06379
ABSTRACT Novel lithium ion battery anode materials consisting of carbon-coated silicon were prepared. Chemical vapor deposition (CVD) methods were used in the syntheses of these composite materials with toluene being used as the precursor for carbon coatings on silicon powder. The temperature of carbon deposition was 950°C and the deposition time was 30 min for powdered substrates. Carbon-coated silicon powders were analyzed by HRSEM, TEM, and Raman spectroscopy. Coin cells were made and cycled to evaluate the electrochemical performance of carbon-coated silicon powders. Coated materials performed well during the initial coin cell testing. Silicon wafers (orientation ) were also used as the substrates for carbon coatings. Silicon wafers were coated for 40 seconds at 950°C. Auger spectroscopy and XPS depth profiling were used to analyze the thin films of toluene-derived carbons on silicon wafers.
INTRODUCTION Silicon-based anodes provide alternatives to carbonaceous materials used presently as anodes in lithium ion batteries. However, carbon-based anodes do not have very large energy capacities (the typical carbon-based anode discharge capacities are 290 mAh / g) [1]. Another reason for the limited value of carbons as anode materials is the slow diffusion of Li+ between graphite layers. The Li/Si ratio is high in lithium silicides (like Li4.4Si) formed in-situ in silicon-based anodes. Therefore, the theoretical capacity of silicon is exceptionally high (4000 mAh/g). In addition, lithium ions diffuse faster in Li4.4Si alloys that are formed in-situ in Si-based anodes. A major obstacle in commercializing silicon as anode material in lithium ion batteries is the poor reversibility of the alloying reaction between lithium and silicon at room temperature [2, 3]. Volume changes in silicon during the repeated charging and discharging of the anodes cause mechanical degradation of the anode. Pulverization of the silicon powder eventually leads to the loss of electrical contact between the current collector and the silicon-based anode material [4]. Thus, the excellent initial capacities (over 1000 mAh / g) offered by silicon-based anodes decrease dramatically during subsequent cycles. The addition of carbon to silicon powder improves its cycleability [5, 6] and prevents capacity fading that is observed in pure silicon anodes. The goal of this study was to obtain materials containing silicon of sub-micron particle size and siliconcarbon composite materials.
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EXPERIMENTAL DETAILS Si powder (previously ball milled) was ground in an agate mortar. About 0.5 g of sample was loaded into the reactor. The temperature was raised to 950°C. Upon reaching 950°C, argon carrier gas was passed through a bubbler filled with liquid toluene a
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