1 Silicon-Decorated Carbon Nanotubes as High Capacity Anodes for Lithium Ion Batteries Kara Evanoff a,c , Benjamin Hertzberg a , Thomas F. Fuller b,c , W. Jud Ready c , and G. Yushin a a School of Materials Science & Engineering, b School of Chemical and Biomolecular Engineering, c Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta GA USA Introduction The application of lithium-ion (Li-ion) secondary (rechargeable) batteries in electrical and hybrid electrical cars demands improvements in energy and power storage densities in addition to long cycle lifetimes. These improvements can be achieved through the development of high-capacity anodes with long cycle lifetimes. Graphitic anodes presently used for most Li-ion batteries have a theoretical reversible specific capacity of 372 mAh/g as a result of the intercalation of one lithium ion per six carbon (C) atoms (LiC 6 ). By incorporating silicon (Si), capacities greater than graphite by an order of magnitude become possible (Figure 1) [1, 2]. Fig. 1 Theoretical capacity for several Li-ion hosting materials at full lithiation. A fundamental challenge of using high-capacity materials is how to maintain long cycle life. The challenge is related to the large volume changes in anode particles upon lithium intercalation (deintercalation). During cycling, the Li x Si y alloy can expand and contract up to 4.4x the volume of unlithiated Si [3]. Without sufficient space available in the electrode for volume expansion of Si during alloy formation, significant stresses are generated, which commonly lead to mechanical degradation of the anode and the loss of electrical contact between particles and the current collector. In addition, non-uniform dilation of the lithiated material may generate critical stresses that lead to fracture of individual particles. Thus, utilizing Si in a physically non-destructive manner is crucial for stable, high-capacity performance. In this study, we explore a novel, highly structured anode architecture consisting of vertically aligned carbon nanotubes (VACNTs) decorated with nano-coatings of silicon and carbon to achieve a high-capacity with reduced anode degradation (Figure 2). Fig. 2 Schematic illustrating Li-ion intercalation (de- intercalation) into (from) a nano-structured Si layer on VACNTs. Note: the outermost C coating is not shown. Experimental VACNTs were synthesized on quartz substrates via a low-pressure chemical vapor deposition process (CVD) [4] utilizing iron (II) chloride catalyst (Alfa Aesar, 7758-94-3), high purity acetylene gas (Airgas), and water vapor. This method produces a high yield of VACNTs with measured growth rates of approximately 100 m/min. Figure 3 illustrates the vertical alignment of the CNTs both on a macroscopic and microscopic level. In contrast to other CNT synthesis methods [5, 6], catalyst pre-deposition is not required. For this study, a constant growth time was used to synthesize VACNT films of approximately 500 m in length. Low-pressure CVD was utilized to deposit nano-Si coatings of varying thickness from a high purity 5wt% silane gas in a helium precursor gas mixture (Airgas) at 500 o C. The resulting Si forms in a “beaded necklace” morphology (Figure 3). With increasing deposition time, the Si coating thickness and content increased. Preliminary x-ray diffraction (XRD) scans with a Cu Kα x-ray source indicate that the deposited Si is predominantly amorphous however a small fraction may be crystalline. An ultra-thin C coating was deposited by the decomposition of high purity propylene gas (Airgas) onto the Si-decorated VACNTs at atmospheric pressure and 700 o C for very short times. To maintain atmospheric pressure, a bubbler containing mineral oil was placed in the exhaust. Nitrogen physisorption was used to characterize the specific surface area of the Si-coated materials as a function of deposition time using Brunauer-Emmett-Teller (BET) adsorption theory. The thickness of the coating contributed to an increase in the total surface area however the simultaneous rise in mass due to the increased Si content, led to an overall decrease in the specific surface area with increased Si coating thickness. Electrochemical cells were assembled in either a pouch cell or 2032-type coin cell construction against a Li foil reference electrode (Alfa Aesar, 7439-93-2) and with a commercially blended LiPF 6 electrolyte solution (Novolyte Technologies) and a Celgard 2325 separator. The process of transferring the active materials to a thin metallic current collector will be reported. The primary considerations for a well-developed interface include good electrical conductivity, adhesion, and stability during testing. Li-ion intercalation Li-ion deintercalation Vertically aligned carbon nanotubes (VACNTs) Nano-Si coating Conductive substrate