Structured Carbon Nanotube/Silicon Nanoparticle Anode Architecture for High Performance Lithium-Ion Batteries
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Structured Carbon Nanotube/Silicon Nanoparticle Anode Architecture for High Performance Lithium-Ion Batteries Sharon Kotz1, Ankita Shah1, Sivasubramanian Somu1, KM Abraham1, Sanjeev Mukerjee1, and Ahmed Busnaina1 1
NSF Nanoscale Science and Engineering Center for High-rate Nanomanufacturing (CHN), Northeastern University, Boston, MA, United States ABSTRACT Silicon is emerging as a very attractive anode material for lithium ion batteries due to its low discharge potential, natural abundance, and high theoretical capacity of 4200 mAh/g, more than ten times that of graphite (372 mAh/g). This high charge capacity is the result of silicon’s ability to incorporate 4.4 lithium atoms per silicon atom; however, the incorporation of lithium also leads to a 300-400% volume expansion during charging, which can cause pulverization of the material and loss of access to the silicon. The architecture of the anode must therefore be able to adapt to this volume increase. Here we present a layered carbon nanotube and silicon nanoparticle electrode structure, fabricated using directed assembly techniques. The porous carbon nanotube layers maintain electrical connectivity through the active material and increase the surface area of the current collector. Using this architecture, we obtain an initial capacity in excess of 4000 mAh/g, as well as increased power and energy density as compared to anodes fabricated using the standard procedure of slurry casting. INTRODUCTION The performance of a battery is determined not only by the materials used in the cathode and anode, but also by how those materials are arranged within the electrode. Commercial batteries currently rely on graphitic anodes, however graphite is not the highest performing material available. With a theoretical capacity of 4200 mAh/g, silicon has more than ten times the theoretical capacity of graphite (372 mAh/g), and that, coupled with silicon’s low discharge potential, natural abundance, and low cost, make it an ideal candidate material for lithium ion battery anodes[1-15]. This high capacity is due to silicon’s ability to incorporate 4.4 lithium ions per silicon atom, which leads to a 300-400% volume increase during charging. The silicon returns to its original size when discharged, and this constant expansion and contraction leads to pulverization of the material and separation from the substrate3,4. It also leads to cracking of the Solid Electrolyte Interphase (SEI) layer, which re-exposes the silicon to the electrolyte, consequently forming an even thicker SEI layer, and over time this thickening SEI layer causes loss of access to the silicon5. The combination of these effects causes severe capacity fade with repeated cycling. It has been demonstrated that the morphology and organization of silicon on the electrode has a significant impact on the performance of the anode [1-14]. For example, the use of nanomaterials can ameliorate some of these issues, however, they are not a complete solution6,7. In addition, making a commercially viable battery requires not only a
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