MWCNTs composite cathodes

Synthetic Metals 161 (2011) 2170–2173

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Synthesis and electrochemical properties of Li3 V2 (PO4 )3 /MWCNTs composite cathodes Yong Zhang ∗ , Yan Lv, Lizhen Wang, Aiqin Zhang, Yanhua Song, Guangyin Li Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China

a r t i c l e

i n f o

Article history: Received 8 July 2011 Received in revised form 6 August 2011 Accepted 9 August 2011 Available online 3 September 2011 Keywords: Multi-walled carbon nanotubes Lithium vanadium phosphate Physicochemical and electrochemical properties

a b s t r a c t Multi-walled carbon nanotubes (MWCNTs)-doped lithium vanadium phosphate Li3 V2 (PO4 )3 (LVP)/MWCNTsx (x = 0, 1, 3, 4, 5, 7 wt.%) cathode materials for lithium ion batteries are synthesized by a microwave assisted sol–gel method. Moreover, the influences of doped MWCNTs on the physicochemical and electrochemical properties of the as-prepared samples are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrochemical experiments. The results show that the optimal doping amount of MWCNTs is 4 wt.%. Under this condition, the LVP/MWCNTs4 wt.% sample has an monoclinic structure, the average particle size is about 0.2–4 ␮m with very fine particle size, uniform shape and loose agglomeration. When charge/discharge at 0.1 C, the sample reached the maximum discharge capacity (130 mAh g−1 ), which is comparable to its theoretical capacity. Comparing with the sample obtained by conventional solid-state route, the obtained materials display lower charge transfer resistance, higher rate capability and excellent reversibility. The above experiments demonstrate that the LVP/MWCNTs4 wt.% is a very promising cathode material which will be used in the future for lithium ion batteries. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the phosphate-containing polyanion composite materials include LiVPO4 F, LiMnPO4 and Li3 V2 (PO4 )3 (LVP) have received a lot of research attention [1]. Among them, the LiVPO4 F material exhibits a high operating voltage (4.2 V), which is rather attractive for high-voltage battery, but it suffers from bad cycle performance. LiMnPO4 also has a high flat voltage [2], but its electrochemical is inactive nearly for lithium-ion extraction [3]. As for monoclinic LVP, which can be used as a replacement of commercial layer cathode, and it features in low cost and high discharge capacity (reaches 198 mAh g−1 when charged to 4.8 V), make it an ideal material for many types of cathode materials. However, the charge voltage is so high that the electrolyte decomposition, so the sample is usually experimented in the voltage of 3.0–4.3 V and achieved the corresponding capacity of 133 mAh g−1 . Although LVP exhibits faster lithium-ion migration rate than LiFePO4 due to its open three-dimensional (3D) framework, the low electronic conductivity prevents its application of large-scale power devices in electric vehicles. Fortunately, there are a number of strategies that we can use to improve the specific capacity and cycling performance of LVP cathode material at high rates.

∗ Corresponding author. Tel.: +86 371 63556510; fax: +86 371 63556510. E-mail address: [email protected] (Y. Zhang). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.022

Coating carbon on the surface of LVP particles with a thin layer is the most useful way, especially as reported by Bhuvaneswari et al. [4]. Their experiment results indicate that the coating thickness and carbon contents play an important role in the electronic conductivity and electrochemical properties. Moreover, various carbon sources have different effects on the physicochemical and electrochemical properties [5], then further research about proper carbon source is crucial too. Recently, carbon nanotube (CNT) is taken as a novel carbon source shows exceptional electronic and mechanical properties [6], which may be considered to the high specific surface area and good conductivity around (1–4) × 102 S cm−1 along the CNT axis and 5–25 S cm−1 perpendicular to the axis, respectively [7]. At the same time, based on CNT’s capillarity and surface tension, the electrolyte can be absorbed and thus reduce the polarization of interface. And the CNT also provide a direct path for lithium-ion transport to improve its electrochemical properties [8]. Recently, there are a lot of reports about using the multi-walled carbon nanotubes (MWCNTs) as the carbon source to improve its performances in the sample of LiFePO4 or LiCoO2 [9,10], but the effects of MWCNTs doping on the electrochemical performances of LVP are relatively less. Consequently, this paper is focused on the research of LVP doped with MWCNTs on the performance of lithium ion battery. In the presented paper, microwave assisted sol–gel method was used to fabricate a series of LVP with different MWCNTs contents, using LiOH·H2 O, V2 O5 , NH4 H2 PO4 and organic acid as the raw

Y. Zhang et al. / Synthetic Metals 161 (2011) 2170–2173

2171

Table 1 The lattice parameters of LVP and LVP/MWCNTs4 wt.% composite materials. a (Å)

b (Å)

c (Å)

ß (◦ )

v (Å3 )

LVP LVP/MWCNTs4 wt.%

8.606(7) 8.620(0)

12.039(2) 12.013(5)

8.572(9) 8.532(0)

89.968(5) 89.803(4)

888.28 883.55

materials, and the LVP/MWCNTsx composite cathode materials were characterized by XRD, SEM, galvanostatic charge/discharge (GCD), cyclic voltammetry (CV) and linear polarization curve (LPC) measurements.

LVP/MWCNTs

4 wt.%

Intensity (a. u.)

Samples

LVP

2. Experimental 2.1. Preparations

20 The LVP/MWCNTsx composite cathode materials were prepared via a microwave assisted sol–gel method, using LiOH·H2 O, V2 O5 , NH4 H2 PO4 and organic acid (3:1:3:2, molar ratio) as the raw materials. Firstly, the precursors were dissolved in deionized water with constant stirring intensively. In order to avoid the MWCNTs aggregating, the MWCNTs were added until the less content of water would result. Secondly, the pastelike mixture was dried in vacuum at 120 ◦ C for 8 h to evaporate all the liquid, and then the resulting product was putted into a microwave furnace under appropriate condition. Finally, the samples were cooled to room temperature when the power was stopped, and the as-synthesized samples could be obtained after grinding. The cathode was prepared by stirring the prepared active material powder with 20 wt.% of super-P and 8 wt.% of polyvinylidene fluoride (PVDF) in N-Methyl-2-pyrrolidone (NMP) until a slurry was obtained, then coating the mixture onto an aluminum foil. Using lithium foil as an anode and the electrolyte is a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) that contained 1 mol L−1 LiPF6 by volume. The assembly of the cells was carried out in an Ar-filled glove box. 2.2. Measurements The crystal structure of LVP was identified by D8-advance with Cu K␣ radiation at the speed of 2◦ min−1 , ranging from 10◦ to 70◦ . The morphology and particle size was measured with scanning electron microscopy (SEM, model: JSM-6490LV). GCD test was carried out on NEWARE battery programcontrol test system in the range of 3.0–4.3 V vs. Li+ /Li. The other electrochemical experiments were measured on the RST 5000 electrochemical workstation, such as CV and LPC measurements. The CV was carried out at different scanning rates of 0.1, 0.2 and 0.5 mV s−1 , and the LPC was measured at the scan rate of 0.1 mV s−1 in the potential range −16 to +16 mV vs. the open circuit potential. 3. Results and discussion 3.1. XRD Fig. 1 shows the XRD patterns of LVP and LVP/MWCNTs4 wt.% cathode materials. It is observed that both of the samples fit very well with the JCPDS data (PDF: 47-0107) and literature sources [11,12]. Moreover, there was no evidence for the presence of impurity phases. Furthermore, no additional diffraction peak related-carbon is observed shows that the MWCNTs did not have a significant effect on the structure of sample. The reason might be the residual carbon is amorphous or the thickness of the residual carbon layer on the LVP/carbon powders is too thin to be detected [13]. To understand more clearly, the diffraction patterns of the two samples were refined, and Table 1 shows

30

40

50

60

2θ (°) Fig. 1. XRD patterns of LVP and LVP/MWCNTs4 wt.% cathode materials.

the lattice parameters. It indicates that the two samples can be indexed as monoclinic structure with a space group of P21 /n, and no obvious difference can be found except the lattice parameters of LVP/MWCNTs4 wt.% are smaller than LVP sample. This indicates that the doped MWCNTs can suppress the crystal growth of LVP sample. 3.2. SEM Fig. 2 shows the SEM micrographs of LVP and LVP/MWCNTs4 wt.% samples obtained by microwave assisted sol–gel method. Compared with the LVP cathode material, it could be seen from Fig. 2a and b that the LVP/MWCNTs4 wt.% particles are much smaller. Meanwhile the ball-like particles are extremely homogenous. Fig. 2a and b show that the average particle size of LVP and LVP/MWCNTs4 wt.% samples are 0.3–6 ␮m and 0.2–4 ␮m, respectively. It is possible that the residual carbon prevents LVP crystal particles growth dramatically during the microwave irradiation process [14]. According to the study results of Ref. [15], it is considered that the synthesized LVP/MWCNTs4 wt.% composite material is suitable for usual electrode preparation techniques. As reported by some articles [16,17], the morphology and particle size have a notable effect on the electrochemical performances of LVP, this is the reason why LVP/MWCNTs4 wt.% shows better performances. 3.3. GCD Fig. 3 shows the initial GCD curves of LVP/MWCNTsx (x = 0, 1, 3, 4, 5, 7 wt.%) composite materials at the 0.1 C rate between 3.0 and 4.3 V. In the charge curves, there exist three plateaus around 3.59, 3.67 and 4.08 V, which correspond to a sequence of phase transition. And on the discharge curves, three plateaus around 3.58, 3.67, 4.05 V exist involving two steps, corresponding to the reduction process. As seen in Fig. 3, LVP/MWCNTsx composites have an initial discharge capacity of 117, 117, 127, 130, 123 and 108 mAh g−1 with MWCNTs doping amount of 0, 1, 3, 4, 5 and 7 wt.%, respectively. Of all the six samples, the initial discharge capacity is the highest for the LVP/MWCNTs4 wt.% sample. This may be the formation of pure monoclinic LVP coated carbon particles with smaller particles, which has relative higher specific surface area, resulting in lithium diffusion much easier and have better electrochemical performances. But there exits an interesting phenomenon that the discharge capacity decreased with the MWCNTs doping content increasing, and the MWCNTs agglomeration are observed. This phenomenon can be explained from the fact that the LVP/MWCNTs4 wt.%

2172

Y. Zhang et al. / Synthetic Metals 161 (2011) 2170–2173

Fig. 2. SEM micrographs of the as-prepared samples. LVP: (a) magnification 5 K and (a ) magnification 10 K. LVP/MWCNTs4 wt.% : (b) magnification 5 K and (b ) magnification 10 K.

150

4.0 LVP/MWCNTs 3.5

LVP/MWCNTs

1 wt.%

7 wt.%

LVP LVP/MWCNTs

3.0

LVP/MWCNTs

5 wt.% 3 wt.%

LVP/MWCNTs 2.5

0

20

40

60

80

100

120

4 wt.%

140

Discharge specific capacity (mAh/g)

+

Voltage (V vs. Li/Li )

4.5

Discharge specific capacity (mAh/g) Fig. 3. The initial GCD curves of LVP/MWCNTsx (x = 0, 1, 3, 4, 5, 7 wt.%) composite materials at the 0.1 C rate between 3.0 and 4.3 V.

sample can achieve the largest capacity only with the optimum MWCNTs-doping amount. To test the effect of different discharge rates, the LVP/MWCNTs4 wt.% composite material was discharged galvanostatically at different current densities corresponding to 0.1, 0.2, 0.5 and 1 C rates. It can be seen in Fig. 4 that the sample delivers the average discharge capacity of 130, 129, 129 and 126 mAh g−1 at 0.1, 0.2, 0.5 and 1.0 C, respectively. The electrode presents similar discharge capacity at different discharge rates, and the results confirm that LVP/MWCNTs4 wt.% composite material has better cycling performance at a high rate. The promising electrochemical performance should be attributed to the inherent monoclinic structural characteristics of LVP/MWCNTs4 wt.% and the enhanced electronic conductivity of the material as a result of carbon coating.

0.1 C

0.5 C

0.2 C

1C

120

90

60

30

0

0

5

10

15

20

Cycle number (n) Fig. 4. Cycle performances of LVP/MWCNTs4 wt.% composite material at different discharge rates.

3.4. CV CV measurements were performed to examine the electrochemical properties of the as-synthesized LVP/MWCNTs4 wt.% electrodes. To investigate the influences of scan rate on the redox behaviors of electrodes, their CV curves at different scan rates of 0.1, 0.2 and 0.5 mV s−1 from 3.0 to 4.3 V were shown in Fig. 5a. As shown in Fig. 5a, three anodic peaks occurred at 3.6, 3.7 and 4.1 V and three cathodic peaks at 3.5, 3.6 and 4.0 V. The small voltage difference between the anodic/cathodic peaks of about 0.1 V representative its good reversibility of lithium-ion extraction/insertion reactions in the LVP/MWCNTs4 wt.% composite. Fig. 5b presents the relation between anodic/cathodic peak current density (Ip ) and square root scan rates (1/2 ). As expected, the Ip − 1/2 shows a rather good

Y. Zhang et al. / Synthetic Metals 161 (2011) 2170–2173

(J0 ) was calculated by means of LPC (Fig. 6) through the follow equation in this study [18]:

a 1.6 -1

Current (mA)

2173

0.1 mV s -1 0.2 mV s -1 0.5 mV s

0.8

J0 = −

RT 1 · nF RP

where RP is the slope of polarization curve, n = 1, R = 8.314, T = 298.5 K, and F = 96,500. The calculated J0 of the LVP/MWCNTs4 wt.% is as high as 2.92 mA cm−2 , and the results are agreed well with the charge/discharge characteristic (Figs. 3 and 4), indicating that this material has better HRD.

0.0

-0.8

4. Conclusions -1.6 3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

+

Voltage V vs. Li /Li)

b

16

anodic peak current density

-4

Ip E A

8

0

Acknowledgements

-8

-16 0.009

LVP/MWCNTsx (x = 0, 1, 3, 4, 5, 7 wt.%) composite cathode materials have been prepared by a microwave assisted sol–gel method. When the MWCNTs-doping content was 4 wt.%, the best electrochemical properties of LVP/MWCNTs4 wt.% sample were achieved. This due to the formation of pure monoclinic LVP coated carbon particles with smaller particles, which has relative higher specific surface area, resulting in lithium diffusion much easier. The doping of MWCNTs can demonstrate that the as-prepared sample is capable of HRD and would be used in the future for lithium ion batteries.

cathodic peak current density 0.012

0.015 1/2

υ

0.018

V/s

0.021

0.024

1/2

Fig. 5. (a) CV curves of LVP/MWCNTs4 wt.% recorded at different scan rates of 0.1, 0.2 and 0.5 mV s−1 ; (b) the anodic/cathodic peak current density (Ip ) as a function of the square root scan rates (1/2 ).

This work is supported by the National Natural Science Foundation of China (Grant No. 21001097), the Basic and Frontier Technology Research Program of Henan Province, China (Grant No. 102300410107), the Project for Outstanding Young Teachers in Higher Education Institutions of Henan Province (Grant No. Henan Higher Education [2009]844), the Key Projects of Science and Technology in Zhengzhou City (Grant No. 0910SGYG23259), and the Key Projects of Science and Technology in Jinshui District, Zhengzhou City (Grant No. [2009]35-35). References

Fig. 6. LPC curve of LVP/MWCNTs4 wt.% composite material.

linear relationship, indicating LVP/MWCNTs4 wt.% is a reversible material. 3.5. Exchange current density To test the high rate dischargeability (HRD) of the LVP/MWCNTs4 wt.% electrode, the exchange current density

[1] T. Jiang, W. Pan, J. Wang, X. Bie, F. Du, Y. Wei, et al., Electrochim. Acta 55 (2010) 3864. [2] Y. Zhang, C.S. Sun, Z. Zhou, Electrochem. Commun. 11 (2009) 1183. [3] D. Arcon, A. Zorko, P. Cevc, R. Dominko, M. Bele, J. Jamnik, et al., J. Phys. Chem. Solids 65 (2004) 1773. [4] M.S. Bhuvaneswari, N.N. Bramnik, D. Ensling, H. Ehrenberg, W. Jaegermann, J. Power Sources 180 (2008) 553. [5] X. Zhou, Y. Liu, Y. Guo, Electrochim. Acta 54 (2009) 2253. [6] Y. Liu, C. Cao, J. Li, Electrochim. Acta 55 (2010) 3921. [7] J. Xu, G. Chen, X. Li, Mater. Chem. Phys. 118 (2009) 9. [8] Z. Bakenov, I. Taniguchi, J. Power Sources 195 (2010) 7445. [9] Y. Feng, Mater. Chem. Phys. 121 (2010) 302. [10] T. Jiang, C. Wang, G. Chen, H. Chen, Y. Wei, X. Li, Solid State Ionics 180 (2009) 708. [11] X. Zhou, Y. Liu, Y. Guo, Solid State Commun. 146 (2008) 261. [12] P. Fu, Y. Zhao, X. An, Y. Dong, X. Hou, Electrochim. Acta 52 (2007) 5281. [13] Y. Li, Z. Zhou, X. Gao, J. Yan, Electrochim. Acta 52 (2007) 4922. [14] X. Hu, Y. Huai, Z. Lin, J. Suo, Z. Deng, J. Electrochem. Soc. 154 (2007) A1026. [15] N. Kalaiselvi, C.-H. Doh, C.-W. Park, S.-I. Moon, M.-S. Yun, Electrochem. Commun. 6 (2004) 1110. [16] P. Fu, Y. Zhao, Y. Dong, X. An, G. Shen, J. Power Sources 162 (2006) 651. [17] X.J. Zhu, Y.X. Liu, L.M. Geng, L.B. Chen, J. Power Sources 184 (2008) 578. [18] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, et al., Electrochim. Acta 54 (2009) 3206.