The electrochemical properties of carbon nanotubes (CNTs), multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs), and carbon nanotubes/silicon composites have been studied for application as anode materials in Li-ion rechargeable battery. CNTs with high quality were successfully synthesized on the $Al_2O_3$ aerogel supported Fe/Mo catalysts by thermal CVD, and then effectively purified by an acidic treatment followed by the gas-phase oxidation process. The purified CNTs were chemically etched in acid solution and mechanically ball-milled by the high energy ball-milling process with an impact mode. Also, CNTs/Si composites were produced using purified CNTs and Si powder by the high energy ball-milling process. And then, the Li insertion and extraction mechanism into CNTs and CNTs/Si composites were investigated by various electrochemical test and structural and/or chemical characterization analysis.
The purified CNTs were chemically etched in an acid solution and mechanically ball-milled by the high energy ball-milling process for modifying the structural characteristics and the surface functional groups of the purified CNTs.
The reversible capacity ($C_{rev}$) of the etched MWNTs and SWNTs increased with increasing etching time, from 351 mAh/g ($Li_{0.9}C_6$) for the purified MWNTs to 681 mAh/g ($Li_{1.8}C_6$) for the etched MWNTs, and from 616 mAh/g ($Li_{1.7}C_6$) for the purified SWNTs to 878 mAh/g ($Li_{2.4}C_6$) for the etched SWNTs.
However, the undesirable irreversible capacity (Cirr) of the etched MWNTs and SWNTs also increased, from 1012 mAh/g ($Li_{2.7}C_6$) for the purified MWNTs to 1229 mAh/g ($Li_{3.3}C_6$) for the etched MWNTs, and from 1573 mAh/g ($Li_{4.2}C_6$) for the purified SWNTs to 1726 mAh/g ($Li_{4.6}C_6$) for the etched SWNTs. The purified samples presented more stable cycle capacity than the etched samples during the charge/discharge cycling. In the etched CNTs, lateral defects, opened ends, edges of graphene layers, and surface functional groups containing hydrogen and oxygen were formed on the surface of the etched CNTs with reduction in the length of the etched CNTs. These structural and chemical modifications facilitated the insertion of Li ions into the etched CNTs, and hence enhanced $C_{rev}$. However, the extraction of Li ions from the etched CNTs had a great hindrance. The large $C_{irr}$ of the etched MWNTs and SWNTs was due presumably to the formation of the SEI on the large surface area by the chemical etching.
The $C_{rev}$ of the ball-milled MWNTs and SWNTs increased with increasing ball-milling time, from 351 mAh/g ($Li_{0.9}C_6$) for the purified MWNTs to 641 mAh/g ($Li_{1.7}C_6$) for the ball-milled MWNTs, and from 616 mAh/g ($Li_{1.7}C_6$) for the purified SWNTs to 988 mAh/g ($Li_{1.7}C_6$) for the ball-milled SWNTs. The undesirable Cirr decreased continuously, from 1012 mAh/g ($Li_{1.7}C_6$) for the purified MWNTs to 518 mAh/g ($Li_{1.4}C_6$) for the ball-milled MWNTs, and from 1573 mAh/g ($Li_{4.2}C_6$) for the purified SWNTs to 845 mAh/g ($Li_{2.3}C_6$) for the ball-milled SWNTs. The decrease in Cirr of the ball-milled samples resulted in the increase in the coulombic efficiency from 25 % for the purified samples to 50 % for the ball-milled samples. And the ball-milled samples presented more stable cycle capacity than the purified samples during the charge/discharge cycling. In the ball-milled CNTs, the edges of graphene layers formed by fracture of CNTs and the surface functional groups chemically bonded to the edges of fractured graphene layers facilitated the insertion of Li ions into the ball-milled CNTs and enhanced $C_{rev}$. However, the extraction of Li ions from the ball-milled CNTs also had a great hindrance. The reduction of $C_{irr}$ in the ball-milled CNTs was due presumably to the increase in a densely packed structure of CNTs by the ball-milling process.
The CNTs and silicon composites were produced using the purified CNTs and Si powder with maximum particle size of 45㎛ by the high energy ball-milling process to overcome the large Cirr and voltage hysteresis, and the low coulombic efficiency of CNTs.
The $C_rev$ of the ball-milled CNTs/Si composites for 60 min increased to 1770 mAh/g for the ball-milled $MWNTs_0.5/Si_0.5$ composites and 1845 mAh/g for the ball-milled $SWNTs_0.5/Si_0.5$ composites. In contrast, the $C_irr$ of the ball-milled CNTs/Si composites for 60 min decreased to 469 mAh/g for the ball-milled $MWNTs_0.5/Si_0.5$ composites and 474 mAh/g for the ball-milled $SWNTs_0.5/Si_0.5$ composites. The change of $C_rev$ and $C_irr$ in the ball-milled CNTs/Si composites resulted in the increase of the coulombic efficiency from 50 % for the ball-milled CNTs to 80 %. The charge/discharge curves of the ball-milled CNTs/Si composites presented a different curve shape from those for the ball-milled CNTs and Si powder, demonstrating that the small voltage hysteresis and the large voltage plateau were observed in the charge/discharge curves for the ball-milled CNTs/Si composites. Most of Li ions were inserted and extracted into/from the ball-milled CNTs/Si composites by alloying and dealloying with Si particles well covered by CNTs which offered electron conductivity to the alloying and dealloying process of Li ions. The CNTs around Si particles in the ball-milled CNTs/Si composites prevented Si particles from electrical insulating by crumbling of Si particles, and more soft CNTs absorbed a volume expansion of LiSi compound formed during the charge/discharge process. The ball-milling process contributed to decrease in the particle size of CNTs and Si particles, and increase in the electrical contact between CNTs and Si particles in the CNTs/Si composites. These factors enhanced a cycle capacity for the ball-milled CNTs/Si composites during the charge/discharge process.
