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Synthetic Metals 161 (2011) 548–551

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

hort communication

ynthesis and electrochemical properties of gyroscope-like lithium ironhosphate/multiwalled carbon nanotubes composites by microwave-assistedol–gel method

ong Zhanga,b,∗, Peipei Dua, Lizhen Wanga,b, Aiqin Zhanga,b, Yanhua Songa,b, Xiaofeng Lia,b, Yan Lva

Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR ChinaHenan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou, Henan 450002, PR China

r t i c l e i n f o

rticle history:eceived 9 December 2010eceived in revised form 4 January 2011ccepted 6 January 2011

a b s t r a c t

Gyroscope-like microparticle of lithium iron phosphate/multiwalled carbon nanotubes composites issynthesized by microwave-assisted sol–gel method with ferrous oxalate, lithium carbonate, and ammo-nium dihydric phosphate as raw materials. This new composites with gyroscope-like have not beenreported in the literature. The crystal structure and surface morphology of the as-prepared particles

vailable online 31 January 2011

eywords:yroscope-like microstructureithium iron phosphate/multiwalledarbon nanotubes compositeslectrochemical properties

are characterized by X-ray diffraction and transmission electron microscopy tests, and their electro-chemical properties are investigated by cyclic voltammetry and galvanostatic charge/discharge tests.The results demonstrated that the composites have an olivine structure and superior electrochemi-cal performances in terms of discharge capacity, cycling stability and rate capability. These favorableelectrochemical properties should be attributed to its special gyroscope-like microstructure.

© 2011 Elsevier B.V. All rights reserved.

icrowave-assisted sol–gel method

. Introduction

Lithium-ion batteries are considered as the most competitiveower sources due to their high energy density and superior powerapability [1,2]. However, demands are now increasing for the bat-eries with higher energy, such as the small-size electronic devicesnd the power sources for electrical vehicles [3–6]. Much attentionas been paid to cathode materials. In recent years, lithium ironhosphate (LiFePO4) with an olivine-type structure was first pro-osed by Goodenough and co-workers [7]. It is considered to behe most promising candidates for cathode materials because of itson-toxicity, high security, rich resources, low cost and environ-ental friendship. What is more, it has high theoretical capacity

170 mAh g−1) and very flatly charge/discharge potential plateaut 3.45 V (vs. Li/Li+), compared with other cathode materials. How-ver, LiFePO4 still shows some disadvantages such as the lowlectrical conductivity and lithium ion diffusion coefficient, whichbstructs its commercial production.

In view of the above-mentioned problems, many researchroups have attempted to improve the electrochemical perfor-ances of LiFePO4 by carbon coating [8,9], cation doping [10,11],

nd particles-sized controlling [12]. Although each method can

∗ Corresponding author. Tel.: +86 371 63556510; fax: +86 371 63556510.E-mail address: zy@zzuli.edu.cn (Y. Zhang).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.01.008

increase the electronic conductivity or lithium ion diffusion coeffi-cient, some problems still exist: (i) the density of LiFePO4 will lowerwith an increase of carbon amount; furthermore, the reversiblecapacity of LiFePO4 will decrease with thicker carbon coating [13];(ii) the theoretical capacity of LiFePO4 will reduce with dopingcations [14]; (iii) the particle-size controlling can also reduce thetap density of LiFePO4 [15]. Multi-wall carbon nanotubes (MWC-NTs) are considered to be the ideal raw materials for variousapplications due to its unique one-dimensional structure and out-standing mechanical characteristics. In particular, MWCNTs candramatically enhance the electrical conductivity of the matrix. Jinet al. [16] reported that compared with pure LiFePO4, the electri-cal conductivity of LiFePO4/MWCNT composite was improved from5.86 × 10−9 S cm−1 to 1.08 × 10−1 S cm−1. Liu et al. [17] indicatedthat LiFePO4 with carbon nanotube doping showed better electro-chemical performance compared with carbon black. Xu et al. addedMWCNTs in the hydrothermal synthesis of LiFePO4 in attempt toincrease the conductivity of the final product [18]. These compos-ite materials indicated that MWCNTs are coated onto the surface ofLiFePO4 to improve its electrochemical performance. However, theas-prepared gyroscope-like LiFePO4/MWCNTs composites show

that the MWCNTs are in the LiFePO4 matrix. To our knowledge,up to now, the new type of gyroscope-like LiFePO4/MWCNTs com-posites has not been reported in the literature.

Here, we report a simple synthesis method to prepare anovel gyroscope-like LiFePO4/MWCNT composite using ferrous

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Y. Zhang et al. / Syntheti

xalate, lithium carbonate, and ammonium dihydric phosphates the starting materials. Furthermore, the structure, morphology,nd electrochemical properties of the synthesized materials werenvestigated in detail.

. Experimental

.1. Preparation of materials

MWCNTs (purity >95%, Shenzhen Nanotech Port Co. Ltd., China)ere purified to open caps and the surface impurities were

emoved, including amorphous carbon and metal catalysts, whichould seriously impede the electrochemical properties of MWC-Ts. Firstly, the raw MWCNTs were placed in a mixed solution ofoncentrated nitric acid and sulfuric acid (1:3 v/v) under ultrasonictirring for 5 min; the preconditioning procedure was repeatedhrice, followed with ultrasonic treatment for 2 h, then centrifugallyeparated, washed, and dried at room temperature.

LiFePO4 was synthesized by microwave-assisted sol–gelethod using a stoichiometric amount of ferrous oxalate, lithium

arbonate, and ammonium dihydric phosphate as raw materials.he materials were first dissolved in distilled water with citric acids a chelating agent in order to prevent the oxidation of Fe2+ to Fe3+,nd then the purified MWCNTs were added into the solution andagnetically stirred at 35 ◦C until the gel formation. The obtained

recursor was pressed into small pellets and placed into an aluminarucible, then covered with carbon conductive additive (Super P)nd calcined in air with a 400 W microwave for 5–20 min.

.2. Measurements

The characterizations of the as-prepared powder were carriedut by means of XRD (D8 Advance) and TEM (JEM-2010). The cath-

Fig. 2. TEM images of (a) pristine MWCNTs, (b) purified

Fig. 1. XRD patterns of (a) JCPDS(#40-1499) LiFePO4, (b) pristine MWCNTs, (c) puri-fied MWCNTs, and (d) LiFePO4/MWCNTs composites.

ode was fabricated by blending the prepared powders, Super P,and polyvinylidene fluoride with a weight ratio of 80:15:5 in N-methylpyrrolidinon, and then the viscous slurry was coated onto anAl foil and dried at 120 ◦C for 24 h. CR2016 coin cell consisted of thecathode, Li foil as the anode, 1 mol L−1 LiPF6 in the mixture of ethy-lene carbonate, dimethyl carbonate, and ethylene methyl carbonate(1:1:1, by volume) as the electrolyte. The cell was cycled galvano-

statically between 2.7 and 4.2 V at various current densities usingCT2001A battery testers (Wuhan Land Electronic Co. Ltd., China).Cyclic voltammetry (CV) measurements were conducted using anelectrochemical workstation (CHI660B, Shanghai Chenhua Instru-

MWCNTs, and (c) LiFePO4/MWCNTs composites.

550 Y. Zhang et al. / Synthetic Metals 161 (2011) 548–551

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ent Co. Ltd., China) at a scan rate of 0.1 mV s−1 in the range from.7 to 4.2 V vs. Li/Li+.

. Results and discussion

Fig. 1 shows the XRD patterns of JCPDS standard LiFePO4,aw MWCNTs, purified MWCNTs, and LiFePO4/MWCNTs compos-tes. In Fig. 1(b) and (c), the XRD peaks appearing at (0 0 2),1 0 0) and (1 0 1) can be attributed to the presence of a hexago-al structure [19]. Compared to raw MWCNTs (Fig. 1(b)), carboniffraction peaks in purified MWCNTs (Fig. 1(c)) decrease signif-

cantly, which can be concluded that MWCNTs were destroyeduring the chemical purification step [20]. Fig. 1(d) shows the XRDattern of LiFePO4/MWCNTs composites, which is consistent withhe standard LiFePO4 pattern (Fig. 1(a)). The lattice parametersf LiFePO4/MWCNTs composites, calculated by XRD data (Fig. 1),re a = 6.019 A, b = 10.347 A, and c = 4.704 A, which are close to thealues reported in the previous literature [21]. There are no anyecondary phases such as Fe2P, Li3Fe2(PO4)3, LiFe(P2O7), Li3PO4,e2O3, and Fe3P observed except olivine phase in LiFePO4/MWCNTsomposites, which indicate that LiFePO4 is single phase. In addition,here is no obvious carbon diffraction peak in the composite, whichuggests that the carbon presence is possible to an amorphous formnd does not affect the structure of LiFePO4 due to its low content22]. The crystal size was calculated based on the (1 3 1) diffractioneaks by the Scherrer’s equation: D = k�/ˇcos � [23] and the averagearticle size was around 32 nm.

Fig. 2 shows TEM images of raw MWCNTs, purified MWCNTs,nd LiFePO4/MWCNTs composites. As shown in Fig. 2(a), pristineWCNTs are closed at the cap and have some black spots whichay be catalyst and amorphous carbon [24]. However, the caps are

pened for purified MWCNTs (Fig. 2(b)) by the oxidation treatment.he opening of the tubes may be beneficial to insertion/extractionf Li+ into their inner core and favorable for increasing their capacitynd removing the impurities completely. The nature of gyroscope-ike LiFePO4/MWCNT particle, as analyzed by TEM, is shown inig. 2(c). MWCNTs are hollow fiber and were inserted in the cen-er of LiFePO4, thus forming a conductive network, which not onlyrovide pathways for electron transference but also lead to inter-article electronic connection. On the other hand, the existence ofWCNTs also provides mechanical strength to the solid matrix.

herefore, this special structure of LiFePO /MWCNTs composites is

4ikely to improve the electrochemical properties of LiFePO4 elec-rodes.

Fig. 3 shows the typical CV curves of LiFePO4/MWCNTs cathodesn the first three cycles. The profiles of all CV curves are almost

Fig. 4. (a) Initial charge/discharge curves and (b) cycling performance curves ofLiFePO4/MWCNTs composites at different rates.

reduplicate except the first cycle represents a good reversibil-ity of the composite electrode [10]. The curves of gyroscope-likemicroparticle LiFePO4/MWCNTs do not present an obvious polar-ization with a potential separation of only 0.24 V between the anodeand cathode peaks, which is smaller than that of pure LiFePO4and LiFePO4/MWCNTs composite reported by Jin et al. [16], andthe redox peaks profile are more symmetric and sharp than thelatter. This phenomenon is most likely due to the formation of con-ducting network for the composite with the special gyroscope-likemicrostructure. However, the CV curves of the composites showthe other two pairs of peaks at 2.98, 2.82, 2.86 and 2.78 V, whichare still unknown at present.

Fig. 4 shows the initial charge/discharge curves and cyclingperformance curves of LiFePO4/MWCNTs composites at differentdischarge rates between 2.7 and 4.2 V. It can be seen from Fig. 4(a)that the battery exhibits an excellent discharge performance. Theinitial discharge capacities of the LiFePO4/MWCNTs compositesare about 153.3, 149.6, 140.7, 135.3 and 130.1 mAh g−1 at 0.1, 0.2,0.5, 0.8 and 1 C rate, respectively, which is higher than previouslyreported in literature [16,17]. Moreover, the sample also exhibitsa good cycling property as shown in Fig. 4(b). One possible reasonis attributed to the conductive network in the internal and sur-face of LiFePO4 resulting from the LiFePO4/MWCNTs particle withgyroscope-like structure. On the other hand, for the opened MWC-NTs, Li+ can be inserted into the inner core of the tubes [25], which

is favorable for improving their electrochemical properties. This isconsistent with the CV results in Fig. 3. At the same time, the methodis different and better than carbon-coating and size-controlling toimprove the electrochemical properties of LiFePO4 [26,27].

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[[23] F. Gao, Z. Tang, J. Xue, Electrochim. Acta 53 (2007) 1939.[24] I.W. Chiang, B.E. Brinson, A.Y. Huang, P.A. Willis, M.J. Bronikowski, J.L. Margrave,

et al., J. Phys. Chem. B 105 (2001) 8297.[25] Z.-h. Yang, H.-q. Wu, Solid State Ionics 143 (2001) 173.[26] S.H. Ju, Y.C. Kang, Mater. Chem. Phys. 107 (2008) 328.

Y. Zhang et al. / Syntheti

. Conclusions

Gyroscope-like LiFePO4/MWCNTs composites with good elec-rochemical performance were successfully synthesized by

icrowave-assisted sol–gel method. The sample has well-rystallize structure and no any impurities. In particular, theyroscope-like morphology and excellent electrochemical proper-ies of LiFePO4/MWCNTs in this work are different to those obtainedrom other works, which may be attributed to its special struc-ure. And the initial discharge capacities of the composite are about53.3, 149.6, 140.7, 135.3 and 130.1 mAh g−1 at 0.1, 0.2, 0.5, 0.8 andC rate, respectively.

cknowledgements

This work is supported by the National Natural Science Foun-ation of China (Grant No. 21001097), the Basic and Frontierechnology Research Program of Henan Province, China (Granto. 102300410107), the Project for Outstanding Young Teachers inigher Education Institutions of Henan Province (Grant No. Henanigher Education [2009]844), the Key Projects of Science and Tech-ology in Zhengzhou City (Grant No. 0910SGYG23259), and the Keyrojects of Science and Technology in Jinshui District, Zhengzhouity (Grant No. [2009]35-35).

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