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III.D.2 New High-Energy Nanofiber Anode Materials (NCSU) Bruce Mixer, NETL Project Manager Grant Recipient: North Carolina State University Xiangwu Zhang, Primary Contact North Carolina State University 2401 Research Drive Raleigh, NC 27695-8301 Phone: (919) 515-6547; Fax: (919) 515-6532 E-mail: xiangwu_zhang@ncsu.edu Start Date: September 15th, 2009 Projected End Date: November 15th, 2013 Introduction Objective: Use electrospinning technology to integrate dissimilar materials (silicon and carbon) into novel composite nanofiber anodes, which simultaneously have large energy density, high powder capability, reduced cost, and improved abuse tolerance. Progress and Current Status Achieving the DOE anode targets for advanced lithium-ion batteries will require novel material manufacturing technologies that can produce anodes with large energy density, high power capability, reduced cost, and improved abuse tolerance. In previous project years, we have used electrospinning technology (combined with carbonization) to synthesize a novel type of Si/C composite nanofiber anode (Figure III - 94), combining the advantageous properties of silicon (high storage capacity) and carbon (long cycle life). The nanofiber structure allowed the anode to withstand repeated cycles of expansion and contraction. Si/C composite nanofibers were electronically conductive and provided effective conductive pathways in electrodes. In addition, composite nanofibers formed a desirable porous electrode structure, thereby leading to fast Li-ion transport. Results demonstrated that anodes made of Si/C composite nanofibers were able to deliver high capacity and long cycle life. Silicon particles Carbon matrix Figure III - 94: Schematic of composite nanofiber anode In this project year, we utilized several novel approaches to further improve the overall performance of Si/C composite nanofiber anodes. The following are two examples: Improvement of C-rate Performance by Employing Carbon Nano Tube (CNT). In order to obtain Si/C composite nanofiber anodes with improved C-rate performance, we employed CNTs to increase the electrode conductivity. Si/CNT/C composite nanofibers were prepared by electrospinning 15 wt% Si/0.75 wt% CNT/PAN precursor. These electrospun nanofibers were firstly stabilized in air environment at 280°C for 5 h (heating rate: 5°C/min) and then carbonized at 800°C for 2 h in argon atmosphere (heating rate: 2°C/min) to form Si/CNT/C composite nanofibers. In the previous project year, we studied the charge-discharge profiles of Si/CNT/C composite nanofiber anodes. Figure III - 95 displays the impedance spectra of Si/C and Si/CNT/C composite nanofiber anodes. Both spectra show one depressed semicircle in the high and intermediate frequency range and a straight line in the low frequency range, corresponding to the migration within the surface layer, interfacial charge transfer process, and lithium diffusion in the electrode, respectively. With the addition of CNTs, the diameter of the depressed semicircle decreases, which indicates a decrease in charge transfer resistance. Figure III - 96 shows the corresponding equivalent circuit. Here, Re is the electrolyte resistance of the cell, Rsl the resistance of ions transferring through the surface layer in the high frequency range, Rct the charge transfer resistance in intermediate frequency region. Warburg impedance (W) corresponds to the diffusion process of lithium ions within the electrode in the low frequency range. Constant phase element (CPE) other than ideal capacitor is introduced due to the porous nature of the composite nanofiber anodes. The results of impedance analysis are listed in Table III - 15. The addition of CNTs does not Energy Storage R&D 104 FY 2013 Annual Progress ReportPDF Image | Advanced Battery Development
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