Improved lithium insertion/extraction properties of single-walled carbon nanotubes by high-energy ball milling
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The effects of ball milling on lithium (Li) insertion/extraction properties into/from single-walled carbon nanotubes (SWNTs) were investigated. The SWNTs were synthesized on supported catalysts by thermal chemical-vapor deposition method, purified, and mechanically ball-milled by high-energy ball milling. The purified SWNTs and the ball-milled SWNTs were electrochemically inserted/extracted with Li. The structural and chemical modifications in the ball-milled SWNTs change the insertion/extraction properties of Li ions into/from the ball-milled SWNTs. The reversible capacity (Crev) increases with increase in the ball milling time, from 616 mAh/g (Li1.7C6) for the purified SWNTs to 988 mAh/g (Li2.7C6) for the ball-milled SWNTs. The undesirable irreversible capacity (Cirr) decreases continuously with increase in the ball milling time, from 1573 mAh/g (Li4.2C6) for the purified SWNTs to 845 mAh/g (Li2.3C6) for the ball-milled SWNTs. The enhanced Crev of the ball-milled SWNTs is presumably due to a continuous decrease in the Cirr because the SWNTs develop a densely packed structure on the ball milling process. The insertion of Li ions into the ball-milled SWNTs is facilitated by various Li insertion sites formed during the ball milling process in spite of small surface area than the purified SWNTs. Lithium ions inserted into various insertion sites enhance the Crev in the ball-milled SWNTs with the large voltage hysteresis by hindrance of the extraction of Li ions from the ball-milled SWNTs. In addition, the ball-milled samples exhibit more stable cycle capacities than the purified samples during the charge/discharge cycling.
I. INTRODUCTION
Carbon nanotubes (CNTs), multi-walled carbon nanotubes (MWNTs), and single-walled carbon nanotubes (SWNTs) have received much attention as a lithium (Li) insertion host material in high energy density Li-ion rechargeable batteries. MWNTs prepared by various synthesis conditions1–3 and thermal oxidation treatments4–6 have exhibited reversible capacities of 80 to ∼640 mAh/g (Li0.2C6–Li1.7C6). Lithium reversible capacities of raw SWNTs and purified SWNTs have been reported to be 450 to ∼600 mAh/g (Li1.2C6–Li1.6C6),7,8 and to increase to 790 and 1000 mAh/g (Li2.1C6 and Li2.7C6) on either mechanical ball milling9 or chemical etching.10,11 Although the Li reversible capacity of CNTs is higher than that of graphite, 372 mAh/g (LiC6), the irreversible capacity is still very high. Therefore, the anodes using
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0291 2458
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 9, Sep 2008 Downloaded: 05 Apr 2015
CNTs that have previously been reported present a very low coulombic efficiency of 30%. High-energy ball milling techniques have long been used for the production of composite metallic powder with a fine controlled microstructure.12 Recently, this technique was used to study the effect of mechanical ball milling on Li insertion into sugar carbons, which have a microporous structure.13
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