Particuology 11 (2013) 401–408
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Electrochemical performance of polygonized carbon nanoﬁbers as anode materials for lithium-ion batteries Jinjin Jiang a,b , Xiaolin Tang a,b , Rui Wu a , Haoqiang Lin a , Meizhen Qu a,∗ a b
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 30 April 2012 Received in revised form 26 June 2012 Accepted 13 July 2012 Keywords: P-CNFs Anode Rate capability TVD First coulombic efﬁciency
a b s t r a c t Carbon nanoﬁbers with a polygonal cross section (P-CNFs) synthesized using a catalytic chemical vapor deposition (CCVD) technology have been investigated for potential applications in lithium batteries as anode materials. P-CNFs exhibit excellent high-rate capabilities. At a current density as high as 3.7 and 7.4 A/g, P-CNFs can still deliver a reversible capacity of 198.4 and 158.2 mAh/g, respectively. To improve their ﬁrst coulombic efﬁciency, carbon-coated P-CNFs were prepared through thermal vapor deposition (TVD) of benzene at 900 ◦ C. The electrochemical results demonstrate that appropriate amount of carbon coating can improve the ﬁrst coulombic efﬁciency, the cycling stability and the rate performance of P-CNFs. After carbon coating, P-CNFs gain a weight increase approximately by 103 wt%, with its ﬁrst coulombic efﬁciency increasing from 63.1 to 78.4%, and deliver a reversible capacity of 197.4 mAh/g at a current density of 3.7 A/g. After dozens of cycles, there is no signiﬁcant capacity degradation at both low and high current densities. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Among all the energy storage devices, rechargeable lithiumion batteries (LIBs) are the most important power sources for clean energy. LIBs have been widely used in portable electronic products, electrical vehicles (EV) and hybrid electrical vehicles (HEV) (Goodenough & Kim, 2010; Santhanam & Rambabu, 2010; Whittingham, 2004; Zhang, 2011) since it was ﬁrst commercialized in 1900s (Gao et al., 1999; Leroux et al., 1999; Ohsaki, Kanda, Aoki, Shiroki, & Suzuki, 1997). Nowadays, the anode materials of commercial LIBs are mainly graphite and graphitized carbon, due to their low cost, high safety, high electronic conductivity, and low electrochemical potential with respect to lithium metal and eco-friendliness. However, two major drawbacks, i.e. poor rate capability and low speciﬁc capacity, keep the performance of graphite far from applicable in LIBs for both high power and high energy densities. Thus to develop new high-rate, high-capacity anode materials for LIBs is of prime importance. In the last decades, the electrochemical properties of carbon nanotubes (CNTs) (Che, Lakshmi, Fisher, & Martin, 1998; Gao et al., 2000; Shen et al.,
∗ Corresponding author. Tel.: +86 28 8522 8839; fax: +86 28 8521 5069. E-mail addresses: jiangjin [email protected]
(J. Jiang), [email protected]
, [email protected]
2008; Wu et al., 1999; Yang, Wu, & Simard, 2002) and carbon nanoﬁbers (CNFs) (Camean et al., 2012; Deng & Lee, 2007; Ji & Zhang, 2009; Takeuchi, Marschilok, Lau, Leising, & Takeuchi, 2006; Zou et al., 2006) were extensively investigated to enhance the storage capabilities of lithium. Because of their high aspect ratios, common CNTs and CNFs with long cylinder do not favor rapid insertion/deinsertion of lithium ions (Wang, Wang, Chang, & Zhang, 2007; Yang, Huo, Song, & Chen, 2008; Yang et al., 2002) and exhibit poor rate performance. Studies on their rate capability have been less documented hitherto (Hu et al., 2007; Santhanam & Rambabu, 2010; Wu, Liu, Guo, & Song, 2009; Zhou, Song, Fu, Wu, & Chen, 2010). This paper investigated the rate capability of polygonized carbon nanoﬁbers (P-CNFs) synthesized using a catalytic chemical vapor deposition (CCVD) technology. The structural, morphological, and electrochemical properties of P-CNFs material were discussed (Zhou et al., 2010).
2. Experimental The P-CNFs used in this study were synthesized by a CCVD technology (Jiang, Qu, Zhou, Huang, & Shao, 2012). In brief, by admitting a continuous acetylene ﬂow into a N2 stream at 80 mL/min at 710 ◦ C, the catalytic decomposition of acetylene on Fe–Sn catalysts took place, and then the P-CNFs were obtained from the quartz boat after cooling down naturally under N2 ﬂow. The as-prepared
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J. Jiang et al. / Particuology 11 (2013) 401–408
P-CNFs were puriﬁed with 4.0 mol/L HCl aqueous solution, then washed with deionized water for several times, and ﬁnally dried in an oven. Carbon-coated P-CNFs were prepared by the thermal vapor deposition (TVD) (Shao, Ren, et al., 2011; Yoshio, Wang, Fukuda, Hara, & Adachi, 2000). 0.3 g P-CNFs were spread on a quartz boat placed inside a quartz tube. Benzene was injected into the quartz tube at a stable ﬂow rate at 1.2 mL/min by a peristaltic pump. The temperature of the reaction tube was maintained at 900 ◦ C. At such a high temperature, benzene vapor decomposed and the resulting carbon deposited on the P-CNFs surface. The thickness of carbon coating was controlled by the feed time of the benzene vapor. The P-CNFs coated with 103 wt% carbon after TVD of benzene for 7 min are referred to as [email protected]
The crystal structure and morphology of the samples ([email protected]
and P-CNFs) were characterized by X-ray diffraction (XRD, X’Pert MPD DY1219), scanning electron microscopy (SEM, FEI INSPECT-F) and transmission electron microscopy (TEM, JEM-100CX). The nitrogen adsorption–desorption isotherms at −196 ◦ C were recorded with a Builder SSA-4200 apparatus. The speciﬁc surface areas and porosimetries were calculated on the basis of Brunauer–Emmett–Teller (BET) theory and the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherms. Electrochemical measurements were performed at room temperature (25 ◦ C) by assembling two-electrode CR2032 coin cells in
an argon-ﬁlled glove box. For preparing working electrodes, a mixture of active materials (P-CNFs or [email protected]
), conductive carbon black (super-P) and carboxymethyl cellulose (CMC, Aldrich, MW 90,000) at a weight ratio of 80:10:10 was coated onto a Cu foil, pressed and dried under vacuum at 120 ◦ C for 12 h. Celgard 2400 (Celgard, USA) was employed as separator. Lithium foil obtained from Tianjin China Energy Lithium Co., Ltd. was used as negative electrode. The electrolyte obtained from Shenzhen Capchem Technology Co., Ltd. consisted of a solution of 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) with volume ratio of 1:1:1. The assembled cells were discharged and charged galvanostatically at various rates on a LAND CT2001A battery test system, in the voltage range of 0.01–3.0 V (vs. Li+ /Li) (Shao, Zhang, Jiang, Zhou, & Qu, 2011). 3. Results and discussion 3.1. Rate performance of P-CNFs Fig. 1 shows typical SEM images of the puriﬁed P-CNFs, and the edges connecting two walls of CNFs can be clearly observed. As shown in Fig. 1(b), adjacent walls display a V-shape. The polygonal cross section of P-CNFs in Fig. 1(c) and (d) has side length of 150–500 nm. The length of P-CNFs is up to tens of micrometer. The composition and structure of the P-CNFs were characterized in detail in our previous report (Jiang et al., 2012).
Fig. 1. SEM images of P-CNFs after treatment with HCl aqueous solution: (a) at low magniﬁcation, (b) at high magniﬁcation, (c) and (d) cross sections of the P-CNFs ((a) and (b) from Jiang et al. (2012)).
J. Jiang et al. / Particuology 11 (2013) 401–408
Fig. 3. Rate performance of the P-CNFs at different current densities.
Fig. 2. (a) The discharge/charge curves for P-CNFs at 0.074 A/g and (b) corresponding differential capacity curves.
The lithium insertion reactions in P-CNFs were studied by using lithium metal as both the reference and counter electrode between 0.01 and 3.0 V. The galvanostatic discharge/charge curves for the ﬁrst cycle at 0.074 A/g are shown in Fig. 2(a). The ﬁrst cycle curve clearly exhibits a typical behavior of carbon without any staging mechanism, indicating that this material is not a representative graphitic carbon, otherwise it would show a clear three-stage lithium insertion as reported widely (Markovsky, Levi, & Aurbach, 1998). However, there is a plateau at 0.8 V suggesting the existence of a protective solid surface ﬁlm formed over the P-CNFs electrode due to the electrolyte being irreversibly reduced (Choi, Chung, Kim, & Sung, 2001; Fong, Vonsacken, & Dahn, 1990; Gnanaraj et al., 2001; Zhou, Zhu, Hibino, Honma, & Ichihara, 2003). After the ﬁrst cycle, there is no such behavior as expected because the ﬁlm formation is subject only to the ﬁrst cycle. To further understand the reactions occurring during the lithium insertion, the discharge/charge curves for the ﬁrst and the second cycles are differentiated with respect to the voltage and the results are shown in Fig. 2(b). It is obvious that the ﬁlm formation occurs in the ﬁrst-cycle lithium insertion at around 0.8 V, which is absent for any further cycles, indicating that the ﬁlm formation is a purely irreversible reaction ascribed to irreversibility of the ﬁrst-cycle, which contributed 260.6 mAh/g during discharge. Fig. 3 shows that in the ﬁrst-cycle lithium ion discharge capacity is 707.1 mAh/g, and the corresponding reversible capacity is
Fig. 4. SEM images of the [email protected]
: (a) at low magniﬁcation and (b) at high magniﬁcation.
J. Jiang et al. / Particuology 11 (2013) 401–408
Fig. 5. TEM images of (a) the original P-CNFs and (b) the [email protected]
446.5 mAh/g when cycled at 0.074 A/g, due to capacity degrading. After 10 cycles, the charge capacity is 376.8 mAh/g, and the capacity retention is only 84.4%. When the current density is increased to 0.185 A/g, the reversible capacity decreases a little but is still at a very high level. The P-CNFs show excellent cycling behavior when the current density is enhanced to 0.37, 0.74, 1.85, 3.7 and 7.4 A/g. The reversible capacity of the P-CNFs is still as high as 198.4 and 158.2 mAh/g even at a high current density of 3.7 and 7.4 A/g, respectively, showing the reversible capacity is stable when cycled for 10 cycles at 3.7 and 7.4 A/g. The coulombic efﬁciency increases from 96 to 98% for the ﬁrst cycle at 3.7 and 7.4 A/g to 100% for the second cycle and remains 100% for the 10th cycle. These values are extremely high compared with what has been reported in literature to date, such as the promising reversible capacity of 166 mAh/g for bambooshaped CNFs (Subramanian, Zhu, & Wei, 2006) at a high current density of 3.72 A/g, 160 mAh/g for carbon nanosprings (Wu, Liu, Guo, & Song, 2009) at 3 A/g, and 181 mA h/g for quadrangular CNTs (Zhou et al., 2010) at 1 A/g. The major difference is closely related to the structural characteristics of the CNFs and CNTs reported hitherto. As can be seen from Fig. 3, one of the most important advantages of the P-CNFs is their suitability for using the same current during both discharge and charge processes, i.e. excellent reversible capacities of 333.8, 296.4, 266.9 and 230.6 mAh/g, corresponding to current density of 0.185, 0.37, 0.74 and 1.85 A/g, respectively. However, the main problem of its high ﬁrst-cycle irreversible capacity must be solved. To reduce the ﬁrst-cycle irreversible capacity of the P-CNFs, surface modiﬁcation by TVD of benzene (Shao, Ren, et al., 2011; Yoshio et al., 2000) has been studied.
Fig. 6. (a) XRD patterns of the original P-CNFs and the [email protected]
, and (b) their corresponding enlarged (0 0 2) peaks.
3.2. Structural comparison between P-CNFs and [email protected]
SEM images of the [email protected]
are shown in Fig. 4. In comparison with Fig. 1, where the original P-CNFs have smooth surface and clear edges, the surface of [email protected]
is coarse and its prism structure is covered with carbon coating. The microstructures of the original P-CNFs and [email protected]
are compared further. Fig. 5(a) depicts the TEM image of the original P-CNFs, showing clearly that the graphitization degree of edges is better than that of walls (Qian, Chen, Wu, Cao, & Chen, 2006). Fig. 5(b) is the TEM image of the [email protected]
, in which the edges of the P-CNFs cannot be observed, just as the SEM results, and the graphitization degree of the deposited carbon on the surface of the P-CNFs is low. As observed from Fig. 6(a), diffraction peaks at ca. 26◦ and 45◦ (2) correspond to (0 0 2) and (1 0 1) crystal planes of graphite materials, respectively. Similarly, the characteristic peaks recorded at ca. 40◦ and 43◦ agree well with the (1 1 1) crystal plane of Fe3 Sn (JCPDS Card No. 65-7052), (1 0 2) and (1 1 0) crystal planes of Fe5 Sn3 (JCPDS Card No. 65-9135), respectively. It is clear that XRD patterns of both samples show the same high relative intensity of a graphite diffraction peak of (0 0 2) plane and board peak proﬁle, which indicates the disordered nature of the samples. In Fig. 6(b), it is noted that the (0 0 2) peak of the [email protected]
splits slightly into two peaks and the distance between adjacent (0 0 2) planes d002 at 24.816◦ is 0.0159 nm wider than (0 0 2) distance of the original P-CNFs at 25.989◦ (listed in Table 1). Furthermore, it is observed
J. Jiang et al. / Particuology 11 (2013) 401–408
Fig. 7. (a) Nitrogen adsorption–desorption isotherms and (b) cumulative pore volume vs. pore diameter curves of the original P-CNFs and the [email protected]
that (0 0 2) peak half-width of the [email protected]
is wider than that of the original P-CNFs, indicating that the graphitization degree of this carbon-coated P-CNFs is lower than that of the original PCNFs. To examine the speciﬁc surface area and the pore volume, nitrogen adsorption–desorption isotherms were measured. Fig. 7(a) shows the different adsorption-desorption curves of the original PCNFs and [email protected]
, revealing that their BET speciﬁc surface areas are about 156.7 and 8.3 m2 /g, respectively. The high surface area of the original P-CNFs results in more irreversible lithium through building a solid electrolyte interphase (SEI) layer during the ﬁrst cycle of discharge, while the speciﬁc surface area of the [email protected]
signiﬁcantly reduces compared with that of the original P-CNFs. The diagram of cumulative pore volume vs. pore size is shown in Fig. 7(b). The dotted lines set demarcation lines for mesopores between 2 and 50 nm, demonstrating that these samples consist of micropores and mesopores. When employed as anode materials for LIBs, the mesoporous structure of [email protected]
is beneﬁcial to increasing Li-uptake capacity (Shao, Zhang, et al., 2011). Table 1 Distance between adjacent (0 0 2) planes of the original P-CNFs and the [email protected]
by TVD for 7 min
2 (◦ ) d002 (nm)
Fig. 8. (a) The discharge/charge curves for the [email protected]
at 0.074 A/g and (b) the corresponding differential capacity curves.
3.3. Electrochemical performance of [email protected]
Fig. 8(a) presents the discharge/charge curves of the [email protected]
electrode at constant current density of 0.074 A/g. The three-stage lithium insertion phenomenon also does not exist. The sloping discharge/charge plateaus between 0.5 and 1 V become shorter in comparison with the characteristics of the original P-CNFs (cf. Fig. 2(a)), indicating that the consumption of Li+ in the formation of solid electrolyte interphase (SEI) layer declines. This [email protected]
electrode exhibits initial discharge/charge capacities of 474.4 and 371.9 mAh/g, with ﬁrst-cycle irreversible capacity being 102.5 mAh/g, showing that after 7-min carbon coating by TVD of benzene, both reversible and irreversible capacities for the P-CNFs decrease. Their corresponding differential discharge/charge capacity curves are illustrated in Fig. 8(b). The voltage plateau of the [email protected]
electrode is detected at ca. 0.8 V in the ﬁrst cycle, which is absent as the cycle number increases, similar to the characteristics of the original P-CNFs. For a better understanding of the superiority of the [email protected]
over P-CNFs as anode materials in high power LIBs, the rate capabilities of [email protected]
and P-CNFs electrodes are compared. Fig. 9(a) exhibits that the charge capacity of the original P-CNFs electrode drops markedly at 0.074 A/g, while the rate performance of [email protected]
electrode stabilizes at the same current density and retains 371.8 mAh/g after 10 cycles, scarcely any loss per cycle. The [email protected]
electrode delivers charge capacities of 336.6, 309.6, 285.2
J. Jiang et al. / Particuology 11 (2013) 401–408
Fig. 10. Relationships between deposited carbon amount and (a) irreversible capacity and (b) coulombic efﬁciency of [email protected]
electrodes at the ﬁrst cycle. Fig. 9. (a) Rate capabilities of the original P-CNFs and the [email protected]
electrodes at different current densities and (b) rate capability plots for P-CNFs after carboncoated by TVD for 7, 15 and 23 min.
and 240 mAh/g at current densities of 0.185, 0.37, 0.74 and 1.85 A/g, respectively, superior over the original P-CNFs electrode. When the current density is increased to 3.7 A/g, the charge capacities of the two anode materials become close. When cycled at 7.4 A/g, degradation of the speciﬁc capacity of the [email protected]
is more serious than that of the original P-CNFs, retaining 135.7 mAh/g. After cycling at various current densities, the charge capacity of the [email protected]
electrode can recover to its initial value when the current density returns to 0.074 A/g, indicating outstanding stability and invertibility. To gain more insight into the electrochemical performance of [email protected]
, their discharge/charge characteristics with different amounts of carbon deposited were tested. Table 2 lists some of these results. As we know, the lowest degradation of capacity with increasing current density indicates the best capability. Fig. 9(b) shows the variation in the capacity as a function of the current density. The [email protected]
by 7-min TVD of benzene exhibit the best rate capability. Increasing the deposited carbon amount results in the degradation of the speciﬁc capacity and poor rate capability. Based on the data listed in Table 2, Fig. 10(a) and (b) shows the relationship between the deposited carbon amount and the irreversible capacity and coulombic efﬁciency at the ﬁrst cycle, respectively. From Fig. 10(a), it can be observed that the irreversible capacity becomes smaller as the deposited carbon amount increases. Some previous studies (Disma, Aymard, Dupont,
& Tarascon, 1996; Fong et al., 1990; Shao, Ren, et al., 2011) have indicated that the irreversible capacity of carbonaceous anode materials is closely related to their speciﬁc surface areas. In this study, the trend appears to be similar because the speciﬁc surface area of [email protected]
decreases with the increase of deposited carbon on the surface of P-CNFs, as illustrated in Table 2, and then the irreversible capacity decreases accordingly. But the speciﬁc surface areas of [email protected]
by TVD for 15 and 23 min are close, and their irreversible capacities are almost the same. From Fig. 10(b), it can be seen that as the deposited carbon amount increases, the coulombic efﬁciency increases. In addition to the signiﬁcantly enhanced coulombic efﬁciency at the ﬁrst cycle, the [email protected]
exhibit excellent cycle performance with no degradation of rate capacity. The electrode was Table 2 Relationship between deposited carbon amount and electrochemical properties of P-CNFs coated by TVD of benzene. Samples
, coated for 7 min
Deposited carbon amount (wt%) BET surface area (m2 /g) First coulombic efﬁciency (%) Discharge capacity at the ﬁrst cycle (mAh/g) Charge capacity at the ﬁrst cycle (mAh/g) Irreversible capacity at the ﬁrst cycle (mAh/g)
0 156.7 63.1 707.1
103 8.3 78.4 474.4
172 5.6 84.1 384.4
299 6.1 86.2 441.5
J. Jiang et al. / Particuology 11 (2013) 401–408
Fig. 11. Cycling performance and the corresponding coulombic efﬁciency of the [email protected]
at (a) 0.074 A/g and (b) 3.7 A/g.
ﬁrstly cycled at a low current density of 0.074 A/g for ﬁve cycles to stabilize the electrode before the subsequent cycles at a high current density of 3.7 A/g. After 30 and 100 cycles at a current density of 0.074 A/g (Fig. 11(a)) and 3.7 A/g (Fig. 11(b)), respectively, no signiﬁcant capacity loss is found. Besides, the coulombic efﬁciencies are always stable at 99–100% after the 10th cycle at 0.074 A/g and almost retain 100% after the ﬁrst cycle at 3.7 A/g. The results imply that [email protected]
possess a robust structure and are very effective in improving the cycling stability at a current density of 0.074 A/g. 4. Conclusions The CCVD produced P-CNFs have been demonstrated as a kind of high-rate Li storage material for LIBs. The reversible capacity of the P-CNFs is as high as 198.4 and 158.2 mAh/g even at high current densities of 3.7 and 7.4 A/g, respectively. Furthermore, carbon-coated P-CNFs were prepared by TVD of benzene. It is found that the carbon-coated P-CNFs with an appropriate amount of carbon deposited exhibit an increased coulombic efﬁciency, improved cycling stability and rate performance compared to the original P-CNFs. Acknowledgment This work was carried out with ﬁnancial support from the Ministry of Science and Technology of China (2011CB932604).
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