Carbon nanotube nanocomposites as anode

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energy densities, great effort has been devoted to novel anodes for lithium ion .... additives such as fluoroethylene carbonate (FEC) and vinyl carbo- nate (VC) ...

Materials Letters 114 (2014) 115–118

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Materials Letters journal homepage: www.elsevier.com/locate/matlet

CdCO3/Carbon nanotube nanocomposites as anode materials for advanced lithium-ion batteries Fan Zhang a, Ruihan Zhang a, Jinkui Feng a,n, Yitai Qian b a Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China b School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 19 August 2013 Accepted 26 September 2013 Available online 7 October 2013

Well dispersed CdCO3/carboxylated carbon nanotubes (CNTs) nanocomposites are synthesized via a facile solution method. As a novel anode material for lithium-ion batteries, the CdCO3/CNTs nanocomposites deliver an initial reversible capacity of 876 mAh g  1 and a better cycle ability. The superior electrochemical performance can be attributed to its unique hierarchy architecture, which facilitate the electron transport and accommodate the large volume change during the alloying/de-alloying reactions. & 2013 Elsevier B.V. All rights reserved.

Keywords: CdCO3 Carbon nanotubes Nanocomposites Anode materials Lithium-ion battery Energy storage and conversion

1. Introduction

2. Experimental

To develop next-generation lithium ion batteries with high energy densities, great effort has been devoted to novel anodes for lithium ion batteries. Recently, transition-metal carbonate materials are proposed to be a novel kind of high capacity anode materials via a conversion reaction [1–5]. For example, CoCO3 could deliver a initial charge capacity as high as 1073 mAh g  1 in the voltage range 0–3.0 V. However, capacity-fading was observed and a capacity of 480 mAh g  1 was noted at the 40th cycle, which owns to the large volume change during conversion process. By compositing with carbon materials such as graphene and carbon nanotubes, the cycling performance of conversion type anode could be greatly enhanced [5–9]. It will be of interest to examine other metal carbonates for their lithium storage ability involving alloying/de-alloying reactions. Cadmium can consume 3 mol Li to form the Li3Cd alloy and thereby is considered as an alloying anode material. However, there is little report on the lithium storage ability of CdCO3. Herein, we report the lithium-storage and cycling behavior of CdCO3 and CdCO3/CNTs nanocomposites prepared at ambient temperature and ambient pressure by the precipitation technique. The CdCO3/CNTs nanocomposites electrode showed a high initial reversible capacity of 876 mAh g  1 and improved cycling ability.

Synthesis of CdCO3 particles and CdCO3/CNTs nanocomposites: The carboxylated CNTs were purchased from Nanjing XFNANO materials Tech Co. Ltd. All the other chemicals were purchased from Sinopharm Chemical Reagent Corporation and used without further purification. The CdCO3 and CdCO3/CNTs nanocomposites were synthesized via an aqueous precipitation method. Firstly, 100 mmol Cd(CH3COO)2  2H2O was dissolved in 50 ml distilled water and stirred for 30 min to obtain solution A. Secondly, 200 mmol NaHCO3 was dispersed into 25 ml distilled water to get solution B. Then, solution B was slowly added into solution A with the speed of three drops per second under magnetic stirring. Some white precipitations were observed immediately and continually stirred for about 1 h. Then filtered out those precipitations and washed with 20 ml distilled water and 20 ml ethanol. Finally, the precipitations were dried at 100 1C for 2 h. For the synthesis of CdCO3/CNTs nanocomposites: The process is similar as CdCO3, only before adding solution B to A, 25 mmol carboxylated CNTs were added into solution A and stirred for 1 h. Crystallographic phases of the prepared samples were characterized with X-ray diffraction were collected by a Rigaku Dmaxrc diffractometer with Ni filtered Cu Kа radiation (V¼50 kV, I ¼100 mA) at a scanning rate of 41/min. The morphology of the as-synthesized samples was investigated by a JEOL JEM-2100 TEM. The electrodes for lithium-cycling were prepared by doctor-blade technique using a mixture of the active material, super P carbon and binder (PVDF) in the weight ratio of 70:15:15. Cu-foil was used as the current collector. Galvanostatic

n

Corresponding author. Tel.: þ 86 15066695429; fax: þ86 531 88392315. E-mail address: [email protected] (J. Feng).

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.123

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discharge–charge measurements of the cells were carried out with voltage range of 0.01  3 V at the rates of 100 mA g  1.

3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared CdCO3, CdCO3/CNTs nanocomposites and the pure CNTs. All the XRD patterns in Fig. 1(a) can be indexed to a pure trigonal structure CdCO3 (JCPDS no. 26-1079). The wide peak at 26.31 can be assigned to the amorphous MWCNTs. No other miscellaneous peaks were detected, which confirmed that the products are pure phase. Profiting from the large surface area and many surface defects of CNTs, the crystallization growth of CdCO3 is hindered. Fig. 2 shows the TEM photographs of the pure CdCO3 and CdCO3/CNTs nanocomposites. The pure CdCO3 are displayed as aggregated cubes with the particle size of 0.5–1 μm (Fig. 2a). However, it can be seen that the CdCO3/CNTs (Fig. 2b) are composed of fine CdCO3 nano-particles anchored with CNTs. The diameter of CdCO3 nano-particles bonded with the CNTs is about 20–50 nm. The three-dimensional (3D) conductive network could accommodate a large volume change, which may improve the electrochemical performance during lithium alloying/de-alloying process. The electrochemical performance of CdCO3/CNTs nanocomposite was evaluated by static-current charge/discharge cycling. For comparison, we also present the result of pure CdCO3 particles under the same electrochemical conditions. Fig. 3a and b shows the discharge/charge voltage profiles of the 1st and 100th cycles for the CdCO3/CNTs nanocomposites and pure CdCO3 electrode cycled between 0.01 and 3 V at a current density of 100 mA g  1. The first discharge and charge capacities are 1467 and 876 mAh g  1 for CdCO3/CNTs nanocomposites, and 1370 and 765 mAh g  1 for pure CdCO3. The irreversible capacity can be explained through the reduction of CdCO3 to Cd and Li2CO3 and the formation of SEI films on the highly reactive Cd nanoparticles [2,9,10,11], while the reversible capacity is due to forming and deforming of Li3Cd, CdO and some side reactions as the Li þ adsorption/desorption at the electrode surface since this composites have a large surface area [2,11,12]. The lithium storage mechanism could be concluded as following with a theoretical capacity of 779 mAh g  1, and the higher capacity of CNTs may due to the lithium intercalation/de-intercalation into CNTs [8]: CdCO3 þ 2Li-Cd þLi2CO3

(1)

Cd þ3Li2Li3Cd

(2)

Fig. 1. XRD patterns a: pristine MWCNTs; (b) CdCO3/CNTs; and (c) pure CdCO3.

Fig. 2. TEM images of (a) pure CdCO3 and (b) CdCO3/CNTs.

Fig. 3c compares the cyclic performance of the CdCO3 and CdCO3/CNTs electrodes. Both electrodes are cycled between 0.01 and 3 V at a current density of 100 mA g  1 for 100 cycles. From Fig. 3c, we can see that the reversible capacity of CdCO3/CNTs nanocomposites electrode remains 485 mAh g  1 after 100 cycles, which correspond to 55.3% capacity retention. As for the pure CdCO3 electrode, only 22.5% (172 mAh g  1) capacity is remained for the 100th cycle. The better capacity retention of CdCO3/CNTs nanocomposites can be explained as to the buffer function of CNTs itself, and the reduced particle size of CdCO3 by adding CNTs, which can accommodate the volume changes during the alloying/ de-alloying process. The first cycle efficiency is a little low, which may be due to the SEI film formation on the large surface area of the CNTs and the nano-CdCO3, one possible way to enhance the first cycle coulombic efficiency is adding SEI film formation additives such as fluoroethylene carbonate (FEC) and vinyl carbonate (VC), which could form a stable SEI film on the anode and prevent further reduction of the electrolyte [13]. In addition to the good capacity retention, the CdCO3/CNTs nanocomposite electrode also exhibits a good rate capability, as shown in Fig. 3d. With increase in current density, the CdCO3/ CNTs electrode can deliver a reversible capacity of 500, 390, and 310 mAh g  1 at a current density of 100, 500, and 1000 mA g  1, respectively. A reversible capacity of 450 mA g  1 can be achieved after the rate backs to 100 mAh g  1. While the pure CdCO3 could only deliver a capacity of 310, 130, 100, and 210 mAh g  1, repectively. The good electrochemical performances of the CdCO3/CNTs nanocomposite are attributed to higher electronic conductivity since the electronic conductivity of CNTs electrode

F. Zhang et al. / Materials Letters 114 (2014) 115–118

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Fig. 3. The 1st and 100th charge discharge profiles (a,b), cycle ability (c) and rate ability (d) of pure CdCO3 and CdCO3/CNTs.

significantly enhance rapid electron transfer, resulting in the significant improvement in the kinetic performance of electrochemical lithium insertion/extraction. Therefore, the CdCO3/CNTs nanocomposites electrode exhibits considerably enhanced capacity with excellent cycle stability and rate capability [2].

4. Conclusions

Fig. 4. Nyquist plots of pure CdCO3 and CdCO3/CNTs based cell.

was metal-class, which would not only greatly reduce intrinsic resistance of nanocomposites electrode. In order to gain better understanding of why CdCO3/CNTs nanocomposites exhibits such a superior electrochemical performance compared to pure CdCO3 electrode, EIS measurements were performed using CdCO3 and CdCO3–CNTs nanocomposites half cells, as shown in Fig. 4. The high frequency semicircle is corresponding to the charge-transfer resistance Rct and CPE of electrode/electrolyte interface, and the inclined line approached to about 45–501 to the real axis is corresponding to the lithiumdiffusion process. It shows that the Rct of CdCO3/CNTs electrode is 100 Ω, which are significantly lower than those of pure CdCO3 electrode (Rct ¼450 Ω). The result confirms that CNTs can

In summary, a simple method has been developed to fabricate CdCO3 and CdCO3/CNTs nanocomposites. The CdCO3 could be used as a novel high capacity anode material for lithium ion batteries with a alloying/de-alloying mechanism. The CdCO3/CNTs nanocomposites exhibit high reversible capacity, good cycle performance. The superior electrochemical performance of the nanocomposites electrode is due to its unique architecture, which is able to accommodate large volume change.

Acknowledgment This work was supported by the 973 Project of China (No. 2011CB935901), the National Natural Science Foundation of China (Grant no. 21203110) and Independent Innovation Foundation of Shandong University (IIFSDU). References [1] Xing Z, Ju ZC, Yang J, Xu HY, Qian YT. Electrochimica Acta 2013;102:51–7. [2] Sharma Y, Sharma N, Subba Rao GV, Chowdari BVR. Journal of Materials Chemistry 2009;19:5047–54. [3] Xia H, Qian YY, Fu YS, Wang X. Solid State Sciences 2013;17:61–71.

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