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Mixer – NETL, Zhang – NCSU III.D.2 New High-Energy Nanofiber Anode Materials transfer resistance. This means the SEI structure has become more compact and less conductive by the introduction of ALD Al2O3 coating. 200 100 0 0 50 100 150 200 250 300 350 400 Z'/ohm Figure III - 101: Nyquist plots of Si/C and Al2O3-coated Si/C nanofiber anodes (ALD cycle number = 28) 66.5% at the 100th cycle. When the ALD coating cycle number is 28, the capacity exhibits the highest stability and the capacity retention at the 100th cycle increases significantly from 36.1% to 82.3%, compared to that of uncoated Si/C nanofibers. However, when the ALD coating cycle number further increases to 35, the capacity is lower than 200 mA/g and fades very quickly. These results demonstrate that Al2O3-coated Si/C composite nanofibers prepared with an ALD cycle number of 28 have the most stable cycling performance. The enhanced cycling performance is mainly due to the protective effect of conformal ALD alumina coating which could improve the mechanical integrity and prevent the side reactions between the electrode and the electrolyte. To achieve the enhanced cycling performance, it is important to select an appropriate ALD Al2O3 coating thickness, which is a critical parameter for determining the electrochemical performance of the Al2O3-coated composite nanofibers. Al2O3-coated Si/C nanofiber anode Si/C composite nanofiber anode Galvanostatic charge-discharge tests were carried out within a voltage window of 0.02 – 1.5 V to evaluate the electrochemical performance of Si/C and Al2O3- coated Si/C composite nanofiber anodes. Figure III - 102 shows galvanostatic charge-discharge curves of 1200 (B) Si/C and Al2O3-coated Si/C composite nanofibers under 1000 50 mA g-1. When the ALD coating cycle number is 35, 800 the resultant Al2O3-coated Si/C composite nanofibers exhibit no useful capacity. This means that an Al2O3 coating layer of 35 ALD cycles is too thick and blocks lithium diffusion. As shown in Figure III - 102, all other 200 uncoated and Al2O3-coated Si/C composite nanofibers show reversible capacities of greater than 900 mA/g in the first cycle. 3 2 1 0 0 200 400 600 800 Capacity (mAh g‐1) 1600 1800 Figure III - 103: Cycling performance of Si/C composite nanofiber anodes and Al2O3-coated Si/C composite nanofiber anodes Figure III - 104 compares the Columbic efficiencies of Si/C and Al2O3-coated Si/C composite nanofiber anodes during cycling. It is seen that Al2O3-coated Si/C composite nanofibers prepared with 28 ALD cycles deliver the highest Columbic efficiency of 99.9% at the 100th cycle, corresponding to a 1.5% point improvement compared to that of uncoated Si/C composite nanofibers. Similar to the enhanced cycling performance, the improved Columbic efficient can be mainly attributed to the ultrathin conformal ALD Al2O3 coating, which minimizes the side reactions between the electrode and the electrolyte. (A) First cycle 35 cycles Al2O3‐coated Si/C nanofiber anode 28 cycles Al2O3‐coated Si/C nanofiber anode 21 cycles Al2O3‐coated Si/C nanofiber anode 14 cycles Al2O3‐coated Si/C nanofiber anode 7 cycles Al2O3‐coated Si/C nanofiber anode Si/C nanofiber anode Figure III - 102: Galvanostatic charge-discharge curves of Si/C composite nanofiber anodes and Al2O3-coated Si/C composite nanofiber anodes Figure III - 103 shows the cycling performance of uncoated and Al2O3-coated Si/C composite nanofibers. It is seen that at the 100th cycle, the discharge capacities are 338.8, 378.8, 473.5, 685.1 and 827.3 mA/g, respectively, for composite nanofibers with 0, 7, 14, 21, and 28 ALD cycles. The corresponding capacity retentions are 36.1, 39.8, 47.4, 66.5 and 82.3%, respectively. When the ALD coating cycle number is 7 or 14, the cycling performance of Al2O3-coated Si/C nanofibers is comparable to that of the uncoated Si/C nanofibers. When the ALD coating cycle number is 21, the capacity fades slower than that of uncoated Si/C nanofibers and shows an increased capacity retention of FY 2013 Annual Progress Report 107 Energy Storage R&D 1000 1200 1400 600 400 Si/C nanofiber anode 7 cycles Al2O3-coated Si/C nanofiber anode 14 cycles Al2O3-coated Si/C nanofiber anode 21 cycles Al2O3-coated Si/C nanofiber anode 28 cycles Al2O3-coated Si/C nanofiber anode 35 cycles Al2O3-coated Si/C nanofiber anode 00 10 20 30 40 50 60 70 80 90 100 Cycle number Voltage (V) -Z''/ohm Discharge capacity (mAh g-1)PDF Image | Advanced Battery Development
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