Structural characterization of carbons obtained from polyparaphenylenes prepared by the Kovacic and Yamamoto methods
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Structural characterization of carbons obtained from polyparaphenylenes prepared by the Kovacic and Yamamoto methods M. Endo, C. Kim, T. Hiraoka, T. Karaki, and K. Nishimura Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan
M. J. Matthews and S. D. M. Brown Department of Physics, MIT, Cambridge, Massachusetts 02139
M. S. Dresselhaus Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts 02139 (Received 24 November 1997; accepted 18 February 1998)
The structure of polyparaphenylene (PPP)-based carbons prepared by the Kovacic and Yamamoto methods heat-treated at 650–3000 ±C have been characterized comparatively by using x-ray diffraction, SEM, TEM, and Raman spectroscopy. Both kinds of carbons indicate not typical but poor graphitizing behavior, especially for the case of PPP Yamamoto samples, and much less for PPP Kovacic samples, by heat treatment up to 3000 ±C. The Kovacic-based samples heat-treated at 600–2400 ±C have a more developed layer structure than that of Yamamoto-based samples. In contrast, for HTT’s (heat-treatment temperature) more than about 2400 ±C, PPP Yamamoto-based carbons exhibit a more developed crystallite structure than PPP Kovacic-based carbons. At a given HTT, PPP Kovacic-based carbons have a much more quinoid-like structure and graphene-type structure than PPP Yamamoto-based carbons, as indicated by the carbon yield and Raman scattering measurements. It is suggested that the detailed structure of the starting polymers influences the texture as well as the microstructure of resultant carbons even though both are obtained from the same kinds of precursors. These microstructures also largely influence the anode performance when these carbons are used in Li ion batteries.
I. INTRODUCTION
The performance of Li ion batteries depends strongly on the crystallinity of the carbon materials used for the anode electrode.1–3 Especially polyparaphenylene (PPP) Kovacic-based carbons heat-treated at temperatures as low as 700 ±C have been shown to possess a superior lithium storage capacity (more than 1100 mAhyg), which is three times higher than that of well-ordered graphite-based, first-stage intercalation compounds (GIC, LiC6 , 372 mAhyg).1,2,4 This is also four times higher than carbonized PPP obtained by the Yamamoto method (PPP-Yamamoto based carbon) heat-treated at the same temperature (280 mAhyg).4 In order to enhance the cell capacities for use as an anode in Li ion batteries, different types of low temperature forms of graphitizing as well as nongraphitizing carbons have been investigated extensively,5 such as mesocarbon microbeads6 and phenolic resin.7 A high excess of Li storage capacity, higher than LiC6 , in the range of 550–700 mAhyg and corresponding to LiC3 has been reported.8 However, the lithium storage mechanisms in such kinds of disordered carbonaceous systems are not yet well understood, and these mechanisms might be J. Mater. Res., Vol. 13, No. 7, Jul 1998
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