Electrochemical Characterization of Phosphorus Encapsulated in Drilled Carbon Nanotubes as Anode Material for Lithium Ion Batteries

Abstract

Lithium ion batteries (LIBs) are one of attractive candidates for portable electronic devices as well as electric vehicles. For their applications, high energy density is the most important in order to help reduce the cost per unit of energy capacity. In the last decade, various electrode materials have been proposed to replace graphite and lithium cobalt oxides in conventional LIBs to materials with higher energy density. For example, silicon, tin and phosphorus as anode materials have a huge theoretical capacity of about 4200, 1000 and 2600 mAh/g, respectively.[1] However, enormous volumetric change up to 300% in LIBs is known to be a main reason of rapid capacity fading upon Li+ insertion and extraction reaction (i.e.charge and discharge, respectively), which hinder practical applications for LIBs. Up to now, several composite materials have been demonstrated as an electrode for LIBs to circumvent the problem, even though their reversible capacities still showed low value of 200-800 mAh/g in addition to the low capacities after cycling.[2] On the other hand, phosphorus encapsulated in single walled carbon nanotubes (P/SWCNTs) showed a high reversible capacity of 2000 mAh/g at the 1st cycle and a good capacity retention of 50% of the 1st cycle still delivered after 10 cycles.[3] The improved performance is thought to be due to suppressing volumetric expansion of phosphorus. However, it has been reported that there was still the expansion and the capacity fading of 50%, which might be attributed to blocking the Li+ insertion and extraction in and from SWCNTs. Therefore, it is needed to form an effective path of Li+ diffusion to inner carbon nanotubes. In this study, chemically-drilled carbon nanotubes were prepared as starting materials, using an impregnation method[4] and an air-oxidation treatment[4]in order to form the path and insert a large amount of phosphorus. The macro and micro morphologies were evaluated in TEM, SEM, EDX, XRD and Raman spectroscopy before and after electrochemical cycling in an Li based organic electrolyte. Multi walled carbon nanotube (MWCNT) was used in this study because its tensile strength is 10 times higher than that of SWCNTs, in order to reduction of structural expansion during Li+ insertion reaction on phosphorus in the inner tubes. Pristine MWCNT was refluxed in concentrated nitric acid at 110 oC for 24 h to remove amorphous carbon and attach oxygen functional groups on the side walls. The functionalized MWCNT was washed repeatedly with distilled water and then dried over night. To partially cut the side walls of MWCNT, the functionalized MWCNT was impregnated with Co nanoparticles. After the impregnation, the sample was thermally treated at 300 oC under an air environment. Then, the sample was refluxed in concentrated nitric acid at 110 oC for 1 h, followed by washing and drying to obtain drilled-MWCNT with low metal contents. To achieve encapsulation of phosphorus in the drilled-MWCNT (P@DMWCNT), phosphorus powder and DMWCNT powder were placed in a glass tube under vacuum followed by thermal treatment at 500 oC. The excess phosphorus on the outer walls of DMWCNT was removed by re-thermal treatment at 200 oC.[3] After structural analyses of the obtained samples, the electrochemical anode performance for LIBs was observed in a three electrode cell using 1 mol/L LiPF6dissolved in a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1. The anodes as working electrodes were prepared using P@DMWCNT and a binder without any carbon additives. Counter and reference electrodes employed were lithium metals. In this presentation, I will discuss the structural analysis of phosphorus encapsulated in drilled-carbon nanotubes upon Li+insertion and extraction processes.