Effect on Performance of Composition of Li-ion carbon Anodes derived from PMAN/DVB copolymers
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ABSTRACT The effects on electrochemical performance of the nitrogen content of disordered carbons derived from polymethacryonitrile (PMAN)-divinylbenzene (DVB) copolymers were examined in galvanostatic cycling tests between 2 V and 0.01 V vs. Li/Li' in IM LiPFjethylene carbonate (EC)-dimethyl carbonate (DMC). The first-cycle reversible capacities and coulombic efficiencies increased with increase in the level of nitrogen for samples prepared at 7000C. However, the degree of fade also increased. Similar tests were performed on materials that were additionally heated at 1,000' and 1,300'C for five hours. Loss of nitrogen, oxygen, and hydrogen occurred under these conditions, with none remaining at the highest temperature in all cases but one. The pyrolysis temperature dominated the electrochemical performance for these samples, with lower reversible and irreversible capacities for the first intercalation cycle as the pyrolysis temperature was increased. Fade was reduced and coulombic efficiencies also improved with increase in temperate. The large irreversible capacities and high fade of these materials makes them unsuitable for use in Li-ion cells. INTRODUCTION A large research effort has been underway in recent years in developing suitable carbon and graphite materials for use as Li-ion anodes in ambient temperature cells using liquid organic
electrolytes, as well as polymer electrolytes. The "soft", ordered graphites typically have low first-cycle capacity losses associated with solvent/salt reduction processes [1]. These materials have capacities limited by the composition LiC, or 372 mAh/g of C. The "hard" disordered or turbostratic carbons, on the other hand, have shown to have capacities well in excess of 372 mAh/g-sometimes increased by a factor of 1.5 or more [2]. The composition of disordered carbons has an impact on its electrochemical performance. The H/C atomic ratio, for example, was found to correlate to the reversible capacity for some specific materials [3]. Incorporation of Si into the carbon lattice has also been reported to enhance the reversible capacity for select precursor materials [4]. Doping with nitrogen, however, resulted in degradation of performance, due to an increase in the irreversible capacity [5]. We have previously reported on the performance of disordered carbons derived from PMAN/DVB copolymers [6-9]. In that work, a PMAN/DVB molar ratio of 1.6:1 was used. In this paper, we present the results of electrochemical characterization of carbons derived from similar precursors, but where the PMAN/DVB ratio was varied from 0 to 3:1, to produce carbons with a wide range of nitrogen content. These materials were then galvanostatically cycled between 2 V and 0.01 V (at 0.5 mA/cm 2) to characterize their electrochemical performance.
165 Mat. Res. Soc. Symp. Proc. Vol. 575 ©2000 Materials Research Society
EXPERIMENTAL Synthesis Our standard carbon precursor is synthesized as an inverse emu
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