The kinetic and thermodynamic analysis of Li + in multi-walled carbon nanotubes
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The kinetic and thermodynamic analysis of Li+ in multi-walled carbon nanotubes Kezhi Lin, Yanhui Xu, Xiaolin Wang1 1.Department of Chemical Engineering, Tsinghua University Beijing, 100084, P. R. China Abstract: The multi-walled carbon nanotubes (MWNTs) prepared by CVD were used in electrochemical intercalation reaction with Li+. The results of the first several charge-discharge cycles showed that the reversible capacity of the MWNTs was larger than the theoretical capacity of graphitized carbon. The equivalent circuit of the Li+ intercalated into the MWNTs was simulated The diffusion coefficient calculated based on the results of the impedance was in the range between 1 ~ 4×10-10 cm2s-1. The diffusion coefficient calculated based on the technique of the chronoamperometery was 3.5×10-10 cm2s-1. The diffusion coefficients of the Li+ reacted with MWNTs was in the range of that of the most carbon materials. Using EMF method the partial molar entropy ( ∆S x ) and partial molar enthalpy ( ∆H x ) of the Li+ intercalated into the MWNTs were calculated as a function of x in LixC6 at different temperatures. The values of ∆S x were in the range of 150 ~ 550 J mol-1K-1. The values of ∆H x at 35 ºC were in the range of 40~110 kJ mol-1. Both of the ∆S x and ∆H x were lower and the changes were smaller than the values of some other carbonaceous materials. By analyzing the results conclusion is obtained that Li+ exist in MWNTs with different types which include adsorption and intercalation types. INTRODUCTION Much attention has been focused on the preparation and application of the carbon nanotubes (MWNTs) since they were discovered by Iijima in 1991 [1]. There are several methods to prepare MWNTs, comprising the chemical vapor deposition (CVD) [2], arc discharge (AD) [3] and laser ablation (LA) [4]. Because of their structure properties with nanometer-order size and pseudo-graphite layers, MWNTs have been expected to be applied for the electrochemical storage of hydrogen [5] and lithium [6]. With the development of the CVD technique it is possible to produce MWNTs on a large scale. The ratio of the length to the radii of the MWNTs produced by CVD was much larger than of the MWNTs produced by AD. There are some literatures which report the Li+ intercalation into the carbon nanotubes. For the Li+ intercalated into the carbon nanotubes it has been suggested to occur in the interlayer of the MWNTs. However, it has also been proposed that micro-textural irregularities in the tube walls cause the trapping of lithium ion at the edges of pseudo-graphitic aromatic layers of MWNTs, and are fully responsible for the irreversible sorption phenomenon [7]. In this article the electrochemical behavior of MWNTs in 1M LiPF6/EC+DMC solution will be investigated in details.
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EXPERIMENT The MWNTs produced by CVD were obtained from Shenzhen nanotechnology Port Co Lt. China. The TEM and the Raman spectrum were obtained by using corresponding technique. The tested cells were employed in measuring the electrochemical properties. Composi
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