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The Capacity and Durability of Amorphous Silicon Nanotube Thin Film Anode for Lithium Ion Battery Applications Maria L A124 ECS Electrochemistry Letters, 4 (10) A124-A128 (2015) The Capacity and Durability of Amorphous Silicon Nanotube Thin Film Anode for Lithium Ion Battery Applications Maria L. Carreon, Arjun K. Thapa,∗ Jacek B. Jasinski, and Mahendra K. Sunkara∗,z Department of Chemical Engineering and Conn Center for Renewable Energy Research, University of Louisville, Louisville, Kentucky 40292, USA In this communication, we report that a silicon nanotube thin film electrode with 0.6 mg loading exhibited an initial discharge capacity of 4766 mAh g−1 and retained about 3400 mAh g−1 after 20 cycles at 100 mA g−1 rate. The silicon nanotube thin film samples with thicknesses ranging from 10–28 microns were prepared using silicon deposition on bulk produced zinc oxide nanowire films and subsequent removal of zinc oxide cores. The developed silicon nanostructures exhibit tubular geometry with both open ends. The nanotubes with thin walls are shown to accommodate large volume changes with lithiation and exhibit stable capacity retention. The presence of hydrogenated nanocrystalline silicon (nc-Si:H) is shown to be essential for the silicon nanotube thin film performance for lithium ion battery applications. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0031510eel] All rights reserved. Manuscript submitted March 13, 2015; revised manuscript received July 22, 2015. Published July 30, 2015. Silicon is a promising host for lithium due to its high theoretical by incubating in an aqueous solution. In addition, it is difficult to specific capacity ∼4200 mAh g−1 but suffers from volume expansion make thicker electrodes with the desired tap density. Another study up to 400% on full lithium insertion. The stress and strain associated reported the synthesis of a free-standing silicon nanotube mat using with such large volume changes leads to the mechanical degradation polymer nanofibers as templates prepared by electrospinning.22 Even of the electrode1–3 which further dramatically reduces the battery ca- though, this method can make long nanofibers in large numbers, the pacity and life time. Previous studies demonstrated that the tendency subsequent steps needed for the formation of the double wall SiOx-Si to fracture can be reduced or avoided by shrinking the materials size to requires several heating steps, making it less scalable. the nanometer scale.4,5 The improved performance of nanometer size Also, carbon coated silicon nanotubes synthetized by reductive materials is due to the smaller domain size compare to the critical size decomposition of SiCl4 on an alumina template followed by etching is for the fracture which is estimated to be about 300 nm for Si.6 Using another noteworthy method for the synthesis of nanotubes as anodes.23 this hypothesis, several 0-D and 1-D morphologies at nanoscale have These nanostructures showed a Coulombic efficiency of 89% since the been tried; they include nanoparticles,7 nanowires,8,9 Si thin films10,11 carbon coating stabilizes the Si-electrolyte interface and the opened and silicon/graphene12,13 composites. In the case of Si thin films, nanotube morphology offers an increased surface area available for Li the main advantage is that they are free from conductive additives ions to intercalate both in the interior and exterior of the nanotubes. and binders which simplify their characterization and use. Yet, the However, the synthesis process requires steps of huge energy needs, mechanical fracture14 limits silicon thin films for use in battery ap- such as annealing at temperatures near 1000◦C which prevents the plications. Silicon nanoparticles dispersed in conductive binders have possibility of making this process scalable as well. been investigated as anodes.15 When using traditional binders very Herein, we are proposing the use of bulk produced zinc oxide thick SEI layers grew on Si nanoparticles electrodes which results in nanowire powders as cheap templates for silicon nanotube thin films an overall decay of the material performance.16 Proper modifications synthesis. The use of this sacrificial layer combined with low tem- to the silicon nanoparticles along with appropriate designed conduct- perature silicon film deposition, results in an overall cheap, fast and ing binders can increase the cycle life performance.17,18 However the reproducible approach for the synthesis of nanotube thin films. Here, cost of these binders and the weight of inactive materials are potential the ZnO nanowire powders are produced in large quantities by at- issues for practical applications. mospheric plasma approach14 and are used to make a thin film that 1-D nanostructures such as silicon nanowires have shown up to serves as the template for producing silicon nanotubes. Then, a silicon 80% of theoretical capacity but only for a few cycles.9 Low dura- layer is deposited using plasma enhanced chemical vapor deposition bility with cycling has been attributed to several processes such as: (PECVD) on the ZnO nanowire thin film followed by the removal the formation of metal silicides from the substrate-silicon reactions,19 of the ZnO nanowires in the same reactor just by switching gases. the detachment of nanowires from the current collector due to mis- The use of plasma allows a low temperature operation route making matched volume changes between the substrate and the nanostructure it suitable for producing nanotube thin films on thin metal foils. The during lithiation and delithiation20 and the low yield of synthesized study here investigated the cyclability of silicon nanotube thin films nanowires.17 produced using ZnO nanowire film as sacrificial layer without the use Silicon nanotubes are attractive nanostructures for lithium ion bat- of any additional binders and conducting polymers. Even further, the tery applications due to the hollowness that offers additional space for open-ended morphology is expected to offer an increased surface area the expansion during lithiation/delithiation process preventing the me- for intercalation. Another advantage of this approach is that one can chanical pulverization of silicon. For silicon nanotubes synthesis, the increase the thickness and packing density of the nanotube films for use of zinc oxide nanorod arrays as templates for the development of increased loading (g/cm2) and tap density as required. Potentially, this closed capped silicon nanotube arrays as anodes has been reported.21 approach offers a scalable approach for synthesizing silicon nanotubes The closed tip vertical silicon nanotube arrays with diameters around films with desired loading for lithium ion batteries applications. 80 nm size have a direct electrical contact back to the underlying substrate. Even though, the resulting silicon tube arrays showed good performance, the synthesis requires deposition of ZnO nanorod array Experimental on the substrate. This is accomplished by the deposition of a seed- ing layer of Zn by metalorganic chemical vapor deposition followed Silicon nanotube thin films were prepared using PECVD on ZnO nanowire thin films at temperatures around 400 ◦C or lower. This is lower than other methods used for synthesis of silicon nanotubes22 ◦ ∗Electrochemical Society Active Member. (around 490 C). The synthesis process for Si nanotube thin films zE-mail: mahendra@louisville.edu involved three main steps: 1) ZnO nanowire films were prepared on Downloaded on 2015-11-04 to IP 64.49.95.110 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Electrochemistry Letters, 4 (10) A124-A128 (2015) A125 Figure 1. Schematic of silicon nanotubes synthesis. a) Three step synthesis of silicon nanowtubes: i) Substrate covered with ZnO nanowires ii) Si nanowires covered with a thin Si film. iii) Silicon nanotubes formation by ZnO nanowires removal using a mixture of 2%H2 in Ar. b) SEM cross section image of a silicon nanotube thin film c) SEM cross section magnified image. Figure 2. SEM images of silicon nanotubes synthesis. a) ZnO nanowires used stainless steel substrates by dropcasting ZnO nanowire powder dis- for the synthesis of the silicon nanotubes. b) ZnO nanowires covered with Si persion in acetone. ZnO nanowire powders were produced in another film. c) Si nanotubes after ZnO nanowires removal for 2 hours d) Silicon nanotubes after ZnO nanowires removal by using a mixture of 2% H2 in Ar reactor. The ZnO nanowires with an average diameter of 50 nm were ◦ for 5 hours at 600 C. used as the sacrificial layer. 2) The covered substrates were placed inside our custom-made reactor. Si deposition on ZnO nanowire thin ◦ films was done using a mixture of 2% SiH4/H2 at 400 C, 70W for about 30 minutes. 3) Finally, the selective removal of the ZnO nanowires reference electrode. The cyclic voltammetry measurement was carried was done using a mixture of 10% H /Ar and using just thermal oven out at the voltage range of 2.2 – 0.002 V with a scan speed of 1 mV 2 − at 600◦C in-situ for over five (5) hours. Note, both duration and the s 1 using eDAQ e-corder potentiostat. required temperature can be drastically reduced using plasma acti- vation and pressure in this step. For example, our previous studies Results and Discussion have shown that atmospheric plasma based processes can reduce time scales from days to minutes.14 However, for studies conducted here, The SEM images of the resulting silicon nanotubes from the start- this is achieved just using thermal process. This process step allowed ing ZnO nanowires are shown in Figure 2. The thickness of the nan- the formation of silicon nanotubes opened from both sides. Figure 1a otubes wall is approximately ∼30 nm with silicon deposition for 30 illustrates the overall 3-step procedure followed for the synthesis of minutes.
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