Li Uptake in Carbon Nanotube Systems: A First Principles Investigation
- PDF / 734,049 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 73 Downloads / 192 Views
LI UPTAKE IN CARBON NANOTUBE SYSTEMS: A FIRST PRINCIPLES INVESTIGATION Vincent Meunier, Jeremy Kephart, Christopher Roland, and Jerry Bernholc Department of Physics, North Carolina State University Raleigh, NC 27695-8202, USA ABSTRACT Carbon nanotube systems can substantially increase their capacity for Li ion uptake, provided that the nanotube interiors become accessible to the ions. We examine theoretically, with ab initio simulations, the ability of Li ions to enter a nanotube interior. While our calculations show that it is quite unlikely for the ions to pass through pristine nanotubes, they are much more likely to enter via large-sized topological defects consisting of at least 9- or more membered rings. It is unlikely that such defects are formed spontaneously, but it may be possible to induce such topological defects by violent non-equilibrium means such as ball milling, chemical means and/or ion bombardment. Indeed, recent experiments on ball milled nanotube samples do report an important increase in the Li ion uptake. INTRODUCTION Over the past few decades, developments in secondary Li-ion battery technology have focused on replacing metallic Li electrodes with Li-carbon guest-host compounds, in order to enhance both safety and cycle efficiency concerns. In Li-carbon systems, the use of a solid electrode host prevents the reduction of Li ions to dangerous levels of metallic Li during the charge-discharge process [1]. This is the main reason why current rechargeable battery technology is dominated by the use of graphitic carbon anodes. However, the use of graphite as a host material has a practical tradeoff, namely in the reduction of the energy density when compared to that of the Li metal. This drawback is chiefly due to the limited Li uptake in graphite. At best the Li/C ratio for graphite is 1:6. In terms of battery power, this implies a reduction of the specific capacity of the anode from 3860 mAh/g for metallic Li to 372 mAh/g for Li-graphite intercalation systems [1]. Thanks to their unique structure, it is believed that carbon nanotubes may provide a good alternative to graphite for improving Li capacity for ion battery systems. Carbon nanotubes are typically formed in carbon arc systems, by means of laser vaporization, or chemical vapor deposition [2]. Typically, the nanotubes arrange themselves into “nanotube ropes” consisting of a large number of nanotubes situated in a hexagonal lattice, all held together by the van der Waals interaction. Hence, in terms of intercalation, these ropes may well have two channels available for Li uptake: the interstitial channels between the individual nanotubes in the ropes, and the nanotube interiors – provided that the latter region is accessible to the ions. Theoretically, if both of these channels were available, this would dramatically increase the Li/C ratio to 1:2, thereby increasing the specific capacity of the battery to 1118 mAh/g [3]. Various experimental groups have demonstrated the successful use of carbon nanotubes as a host material for Li intercalat
Data Loading...
