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III.D.2 New High-Energy Nanofiber Anode Materials Mixer – NETL, Zhang – NCSU Figure III - 99 compares the cycling performance of Si/C and Si/CNT/C composite nanofiber anodes under two different current densities. It is seen that under 50 mA/g and at the 30th cycle, the discharge capacities are 775 and 837 mA/g, respectively, for Si/C and Si/CNT/C nanofiber anodes. The corresponding capacity retentions are 81.6 and 84.5%, respectively. At a current density of 300 mA/g, after 30 cycles, the discharge capacities are 492 and 710 mA/g, respectively, for Si/C and Si/CNT/C nanofiber anodes. The corresponding capacity retentions are 64.9 and 83.3%, respectively. Results indicate that at both current densities, the Si/CNT/C nanofiber anode have greater capacity retention than the Si/C nanofiber anode. 1000 800 600 400 200 00 10 20 30 Cycle number Figure III - 99: Cycling performance comparison of Si/C and Si/CNT/C composite nanofiber anodes under different current densities The comparison of rate capacities of Si/C and Si/CNT/C nanofiber anodes is shown in Figure III - 100. The discharge capacity of the Si/C nanofiber anode decreases rapidly with increase in current density. However, the capacity decrease of the Si/CNT/C nanofiber anode is much slower. Even at a high current density of 300 mA g-1, a relatively high capacity value of 602 mA/g is still achieved with the Si/CNT/C nanofiber anode. The capacity of Si/CNT/C nanofiber anode almost rebounds back to the original value when a low current density of 50 mA/g was applied again after 20 cycles. This demonstrates the excellent stability of the Si/CNT/C nanofiber anode under various charge/discharge conditions. The enhanced C-rate performance is mainly due to the use of well-dispersed CNTs. Because of their high aspect ratio and high conductivity, the dispersed CNTs can effectively form a conducting pathway throughout the composite nanofibers even at low concentrations. 1000 800 600 400 200 0 0 5 10 15 20 25 Cycle number Figure III - 100: Rate capabilities of Si/C and Si/CNT/C composite nanofiber anodes The results demonstrated that the electrochemical performance of Si/CNT/C nanofibers, especially the capacity retention and rate capability, has been improved significantly, which can be ascribed to the enhanced electrical conductivity of composite nanofibers. The unique morphology and structure of the one-dimensional conductive nanofibrous composite are beneficial to improve the electrochemical behavior of Si/CNT/C composite nanofibers. The incorporation of CNTs further enhances the electrochemical efficiency. These factors lead to Si/CNT/C composite nanofiber anode with high reversible capacity, good cycling performance, and excellent rate capability. Improvement of Cycling Performance by Employing ALD Alumina Coating. In order to improve the cycling performance of Si/C composite nanofiber anodes, atomic layer deposition (ALD) was employed to coat the fiber surface. ALD is a unique technique for the deposition of conformal and homogenous thin films. In this study, we coated Si/C composite nanofibers with Al2O3 by the ALD method. The purpose was to control the stability of the solid electrolyte interphase (SEI) by employing the Al2O3 coating layer to prevent side reactions between the electrode and the electrolyte. The ALD process of alumina was performed on the surface of Si/C nanofiber electrodes using tri-methyl aluminum (TMA) and H2O as precursors at 120°C. Figure III - 101 displays the impedance spectra of Si/C and ALD Al2O3-coated Si/C composite nanofiber anodes prepared with an ALD cycle number of 28. 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. The ionic conduction of the SEI film is a result of the migration of solvated Li ions through the micro-pores of SEI since the dried SEI itself is neither electronically nor ionically conductive. Hence, higher interfacial resistance corresponds to a more compact and more stable SEI. As shown in Figure III - 101, the addition of ALD Al2O3 coating results in an increased semicircle diameter, i.e., increased charge Energy Storage R&D 106 FY 2013 Annual Progress Report 50 mA g-1 100 mA g-1 200 mA g-1 300 mA g-1 50 mA g-1 Si/CNT/C composite nanofiber anodes Si/C composite nanofiber anodes Si/CNT/C nanofiber anode under 50 mA g-1 Si/CNT/C nanofiber anode under 300 mA g-1 Si/C nanofiber anode under 50 mA g-1 Si/C nanofiber anode under 300 mA g-1 Discharge capacity (mAh g-1) Discharge capacity (mAh g-1)PDF Image | Advanced Battery Development
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