Characterization of polyparaphenylene (PPP)-based carbons
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Characterization of polyparaphenylene (PPP)-based carbons M. J. Matthews Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. Endo, Y. Sasabe, T. Takahashi, and K. Takeuchi Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan (Received 18 December 1995; accepted 5 September 1996)
Polyparaphenylene (PPP)-based carbons heat-treated at temperatures sTHT d from 600 ±C up to 3000 ±C have been characterized both structurally and in terms of their physical properties. Special attention is given to PPP heat-treated at 700 ±C (denoted by PPP-700), since samples heat-treated to this temperature were observed to have exceptionally high lithium affinities when electrochemically doped with Li. At low THT below 700 ±C, it is found that the local structure of PPP-based samples can be characterized mostly in terms of a disordered polymer. As a result of heat treatment to high temperature, PPP-based carbon shows graphitization behavior with regard to x-ray diffraction d002 (graphite c-axis d-spacing) development and to the increase of the Raman IGyID intensity ratio (where IG and ID are the integrated intensities of the 1580 cm21 and 1360 cm21 Raman modes, respectively), as is found in so-called graphitizing carbons. However, development of the c-axis crystallite size sLc d is restricted to very small values, in the range of so-called nongraphitizing carbons, while the a-axis crystallite size sLa d ˚ for heat treatments near 3000 ±C. These structural attains values up to roughly 120 A properties of PPP-based carbons are consistent with the observed electrical characteristics and their dependence on THT . Low temperature magnetic susceptibility measurements were analyzed, along with Raman spectra, allowing for the characterization of disorder in terms of localized spin states for several heat-treated PPP samples. By interpreting the results of these various characterization techniques, we are able to present an insightful perspective on the nature of PPP-based carbons and the role of PPP-700 as an effective lithium host material for secondary battery applications.
I. INTRODUCTION
To enhance the capabilities of many present lap-top computers and other electronic products, it would be highly beneficial to increase the power storage capacity of rechargeable batteries, such as so-called lithium ion batteries. Much effort has been expended and a large number of disordered and well-ordered graphite materials have been tested for use as carbon anodes in high capacity lithium ion batteries.1–6 Polyparaphenylene (PPP)-based carbons, heat-treated at temperatures as low as 700 ±C, have been shown to yield a superior negative electrode material, with energy storage capacities as high as 1170 A-hykg. This extremely high Li storage capacity in PPP-based, low heat
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