Germanium-single-wall carbon nanotube anodes for lithium ion batteries
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Matthew J. Ganter NanoPower Research Laboratories and Golisano Institute for Sustainability, Rochester Institute of Technology, Rochester, New York 14623
Ryne P. Raffaelle Microsystems Engineering, NanoPower Research Laboratories, Golisano Institute for Sustainability, and Department of Physics, Rochester Institute of Technology, Rochester, New York 14623
Brian J. Landia) Microsystems Engineering, NanoPower Research Laboratories, Golisano Institute for Sustainability, and Department of Chemical and Biomedical Engineering, Rochester Institute of Technology, Rochester, New York 14623 (Received 2 December 2009; accepted 29 March 2010)
High-capacity thin-film germanium was coupled with free-standing single-wall carbon nanotube (SWCNT) current collectors as a novel lithium ion battery anode. A series of Ge–SWCNT compositions were fabricated and characterized by scanning electron microscopy and Raman spectroscopy. The lithium ion storage capacities of the anodes were measured to be proportional to the Ge weight loading, with a 40 wt% Ge–SWCNT electrode measuring 800 mAh/g. Full batteries comprising a Ge–SWCNT anode in concert with a LiCoO2 cathode have demonstrated a nominal voltage of 3.35 V and anode energy densities 3 the conventional graphite-based value. The higher observed energy density for Ge–SWCNT anodes has been used to calculate the relative improvement in full battery performance when capacity matched with conventional cathodes (e.g., LiCoO2, LiNiCoAlO2, and LiFePO4). The results show a >50% increase in both specific and volumetric energy densities, with values approaching 275 Wh/kg and 700 Wh/L.
I. INTRODUCTION
The demand for power generation and storage devices for applications in electric vehicles, consumer electronics, and microelectromechanical systems (MEMS) has brought lithium ion battery technology to the forefront of development to meet this growing power need.1–4 The higher energy density exhibited by lithium ion batteries places them ahead of their Ni–Cd or Ni–metal hydride competitors.1 Still, new materials are under investigation to increase reversible capacity, cycle life, and charge– discharge rates of lithium ion batteries. Alternatives to the traditional graphitic anode materials, such as silicon and germanium, are under investigation because of the significantly higher electrochemical capacity (silicon, 4200 mAh/g; germanium, 1600 mAh/g; compared with 372 mAh/g for graphite).5 However, there are a couple limitations to using these semiconductor materials, namely poor charge transport and excessive volumetric expansion upon lithiation that can lead to poor cycle life a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0184 J. Mater. Res., Vol. 25, No. 8, Aug 2010
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from active layer cracking and delamination from the current collector.6–8 The size-scale reduction of these semiconductors into the form of germanium and silicon nanowires has been the most successful strategy to date to mitigat
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