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Solid State Sciences 12 (2010) 1248e1252

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Solid State Sciences

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Wet coordination method to prepare carbon-coated Li3V2(PO4)3cathode material for lithium ion batteries

Lian Wang a,*, Xiang Li a, Xianquan Jiang a, Fusheng Pan a, Feng Wu b

aChongqing Academy of science & Technology, Chongqing 401123, Chinab School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China

a r t i c l e i n f o

Article history:Received 18 January 2010Received in revised form27 February 2010Accepted 2 March 2010Available online 6 March 2010

Keywords:Lithium vanadium phosphateCarbon-coated layerWet coordinationReversible performance

* Corresponding author at: Tel.: þ86 023 6730 070E-mail address: wanglian1021@126.com (L. Wang

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.03.002

a b s t r a c t

A convenientmethod namedwet coordination is used to prepare the sample or carbon-coated Li3V2(PO4)3 inthe furnace with a flowing argon atmosphere at 600 �C for 1 h. The sample is characterized by X-raydiffraction (XRD), scanning electronmicroscope (SEM), transmission electronmicroscopy (TEM) and energydispersive analysis of X-rays (EDAX). Galvanostatic chargeedischarge between 3.3 and 4.3 V (vs. Li/Liþ)shows that the sample exhibits a high discharge capacity of 128mAhg�1with a good reversible performanceunder a current density of 95 mA g�1. It suggests that carbon-coated Li3V2(PO4)3 with good electrochemicalperformance can be obtained via this method, which is suitable for large-scale production.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Nowadays, a considerable amount of efforts has been invested tofind new cathode materials suitable for rechargeable lithiumbatteries [1e3]. Lithium vanadium phosphate, or Li3V2(PO4)3 is ofparticular interest due to its higher ionic conductivity and operatingvoltage [4e6] than that of LiFePO4. More importantly, the stableframe work can be sustained even at high charging voltage (4.8V vs.Li/Liþ), which is derived from the low Fermi level of V4þ/V3þ [1]. Inaddition, the theoretical energy density of Li3V2(PO4)3 is about500 mWh g�1[7]. All of these suggest a prospective utilization inelectric vehicles. As one kind of promising cathode materials,Li3V2(PO4)3 can be obtained with a better electrochemical perfor-mance via some solutions or resorts involving coating carbon layer[8e11], substituting [9] or doping [12,13]. Recently, Fu et al. [14]prepared Li3V2(PO4)3 by using LiF as lithium precursor. The resultsshowed that lithium vanadium phosphate can be obtained at lowersintering temperature (700 �C) than those using Li2CO3 or other Lisalts. Besides, Chang et al. [15] introduced a novel method namedrheological phase process to synthesize carbon-coated Li3V2(PO4)3more efficiently.

Up to now, powder preparation of Li3V2(PO4)3 is based mainly ontwo routes: solid-state and solegel. Solid-state including hydrogen

0; fax: þ86 023 6730 0592.).

son SAS. All rights reserved.

thermal reduction (HTR) or carbon thermal reduction (CTR)[6,16e18] needs high calcinations temperature and long reactiontime. As known to all, time-consuming or high-temperature calci-nation usually brings about abrupt particle growth [19], which isharm to electrochemical performance of the product. Solegel hasbeen extensively used due to its unique advantages. Nevertheless, itusually requires a complex or a time-consuming manipulation toobtain the precursor, which is unfavorable to industrial production.On the other hand, as one of low-temperature synthetic route, solid-state coordination has been confirmed to be a simple and effectivemethod to fabricate a number of chemical compounds such as clustercompounds, coordination compounds and certainly cathode mate-rials, involving partially substituted LiMn2O4 spinels (LiMn2�yCoyO4)[20] and LiFePO4 [21] etc. However, to the best of our knowledge,there are few works or literatures on preparing Li3V2(PO4)3 by solid-state coordination. In this work, we report an improved solid-statecoordination namely wet coordination to prepare carbon-coatedLi3V2(PO4)3 by sintering the precursor at 600 �C for only 1 h underflowingArgon. Somewhatdifferent fromthe conventional one (solid-state coordination), a small quantity of water was added into themixture to form slurry prepared for the precursor, which is namedwet coordination. Our aim is to obtain the precursor more homoge-neously than those obtained by conventional coordination. On theotherhand, different fromsolegel route, a small amountofwaterwascontrolled to drip into the mixture, which may avoid a complicatemanipulation and long dehydration process to get precursors and befavorable to industrial production. The precursor was derived from

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Fig. 2. The XRD pattern of the sample.

L. Wang et al. / Solid State Sciences 12 (2010) 1248e1252 1249

a slurry or a ground sticky mixture of LiF, NH4VO3, NH4H2PO4, acet-ylene black and C6H8O7$H2O. By this method, carbon-coatedLi3V2(PO4)3 with good electrochemical performance can be obtainedwithout any complex or additional treatments.

2. Experimental

LiF (analytical reagent), NH4VO3 (analytical reagent) andNH4H2PO4 (analytical reagent) were stoichiometrically blendedwith excessive acetylene black and C6H8O7$H2O. During adequatelyground work, through a funnel, de-ionized water was controlled todrip into the mixture till black uniform slurry obtained. pH of theslurry was regulated by diluted ammonia to about 7. The slurry wasdried at 70 �C for 3 h and ground again to obtain the precursor. Inorder to determine calcinations temperature of the dry gel mixtureor the precursor, TG/DTG (Netzsch STA449C) was performedbetween room temperature and 900 �C at a heating rate of7.5 Kmin�1 in argon atmosphere under a flowof 20mlmin�1. Then,it was put in a furnace to preheat at 300 �C for 2 h. Finally, it washeated in the furnace with a flowing argon atmosphere at 600 �Cfor 1 h.

Cathode was prepared by smearing a mixture of the sample,acetylene black and polyvinylidene fluoride on aluminum foils withthe weight ratio of 70:20:10 and punched into about 1 cm2 rounds.Utilizing 1 M LiPF6/EC-DMC (1:1 volume ratio) as the electrolyte,experimental batteries were assembled in an argon-filled glove box(MBRAUN).

XRD pattern was obtained by a Rigaku D/MAX 2400 diffractom-eter. Morphology of the sample was inspected with a scanningelectron microscope (SEM, Hitachi S-3500N) and a transmissionelectron microscope (TEM, JEM-2100) with energy dispersive anal-ysis of X-rays (EDAX). By a battery tester (LAND CT2001A), thebatteries were galvanostatically charged-discharged within4.3e3.3 V (vs. Li/Liþ) under current density of 95, 190, 285 and381 mA g�1 respectively. Cyclic voltammogram (CV) was conductedby an electrochemical work station (CHI660A).

3. Results and discussion

TG/DTGspectra for thedrygelmixture or theprecursor are shownin Fig. 1. It can be seen that an obvious derivative weight loss peak isobserved near 100 �C and the weight loss of the precursor is about6.8 wt%, which infers the loss of absorbed water. When the temper-ature is up to 200 �C, another two peaks of DTG can be attributed tocrystal water, hydrogen fluoride and ammonia escaping from theprecursor. The following step of around 35% weight loss from 200 to500 �C reflectes the decomposition of citric compounds. A strongest

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Temperature /°C

Weig

ht L

oss / % DTG

TG

Fig. 1. The TG/DTG patterns for the precursor in argon atmosphere.

peak of derivative weight loss is observed near 275 �C, which indi-cates the precursor burned acutely and citric salts broke down intoNH3, CO2 and H2O to volatilize. From 500 to 600 �C, there was noobvious weight loss in TG curve, but there was a derivative weightloss peak around 520 �C on DTG curve. During this process, lithiumoxide, vanadium oxide, phosphate and excess carbon reacted furtherto form well-crystallized Li3V2(PO4)3. In the course of subsequenttemperature-rising process, no further weight loss is observed andthe total one is about 50.7%, which implies the finishing of syntheticprocess.

3NH4VO3/3NHþ4 þ 3VO�

3 (1)

C6H8O7/C6H5O3�7 þ 3Hþ (2)

3VO�3 þ 3Hþ/1:5V2O5 þ 1:5H2O (3)

6LiFþ 2V2O5 þ 6NH4H2PO4 þ ðNH4Þ3C6H5O7

þ C/2Li3V2ðPO4Þ3þ9NH3 þ 6HFþ 7COþ 10H2O (4)

When NH4VO3 was added into water, hydrolysis equation(1) took place. Because VO3

� is an amphoteric anion, equation (3) isa reversible reaction of which extent is dominantly decided by thepH value of the solution. When C6H8O7 was added into solution,equation (2) produced acidic conditions to support the formation of

Fig. 3. The SEM picture of the sample particle.

Fig. 4. The TEM picture of the sample with the points at which analysis of EDAX were taken (a), and the corresponding EDAX spectra of the points at the sample particle (b).

L. Wang et al. / Solid State Sciences 12 (2010) 1248e12521250

V2O5. Based on the total weight loss of diffractogramms, thepossible synthetic reaction to form lithiumvanadium bronze can beinferred as equation (4).

The XRD pattern of the sample is shown in Fig. 2. There are threestrong peaks appear at 20.62�, 24.30� and 29.32� (2q), all of whichare consistent with those of monoclinic Li3V2(PO4)3. It has beenreported [22] that impurities of Li5V(PO4)2F2 or LiVPO4F can beobtained when LiF was added. Accordingly, the X-ray diffractiondatawas examined carefully, the result manifests no impure phasesor other by-products can be found. Therefore, it should be stressedthat under our experimental conditions, the pure phase ofLi3V2(PO4)3 can be obtained. In our work, citrate (C6H8O7$H2O)together with acetylene black and LiF takes key role in syntheticreaction. Citrate (C6H8O7$H2O) is used not only as the coordinationreactant to prepare a homogeneous precursor in a convenient andprompt way, but also as a carbon-bronzed net to increase contactarea of reactants during preheating process, which has a positiveimpact on the reaction rate. Moreover, as the reductant, acetyleneblackwith high specific surface area also plays a vital role, which canaccelerate the reaction rate and lower the temperature of syntheticreaction. On the other hand, LiF can be used to prevent someimpurities such as Li3PO4 [14] generated in our expected product. Allof above make it possible to synthesize pure phase of Li3V2(PO4)3 ina short time at low temperature (1 h, 600 �C). The XRD pattern canbe indexed as a monoclinic structure with a space group 14 (P21/n)and the refined lattice parameters were generated witha¼ 1.19984 nm, b¼ 0.86108 nm, c¼ 0.86038 nm, b¼ 90.50� and cell

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Potential / V vs. Li/Li+

Cu

rre

nt / m

A

Fig. 5. The cyclic voltammogram of the experimental battery at a scanning rate of0.10 mV s�1.

volume¼ 0.88873 nm3. By DebyeeScherrer equation, the size of thesample crystallites was figured out to be 60 nm.

SEM picture of the sample is shown in Fig. 3. As we know,monoclinic Li3V2(PO4)3 has been stated as a higher rate perfor-mance than LiFePO4. However, some investigations show that themorphology and surface area of obtained particles still have thenotable effect on the cycle performance of Li3V2(PO4)3. The betterperformance can be obtained with optimizing particle size orintroducing conductive additives. As for the conductive, the surfacecoating anddispersion effect of carbonhas played a beneficial role inobtaining sampleswith small and uniform particle size, as well as inenhancing their overall electronic conductivity. Due to calcining forshort time at low temperature, the sample powder with fine grains(with an average particle size of 1mm) can beobtained. It can be seenthat all particles show clear outline with smooth surface and mostare tiny grain-shaped particles. Several “big” rock-like particleswiththe dimension no less than 2 mm can be found as well. These “big”particles usually developed from agglomeration assembled bysmaller particles, which can be verified or picked out via TEM(shown in Fig. 4a). Based on Fig. 4, a coating layer can be clearlyobserved on the particle surface. Moreover, the layer is detected byEDAX at three different points: the surface, the coating layer and thesubstrate of the sample particle respectively (shown in Fig. 4a). Theresults of Fig. 4b clearly reveal that the carbon content increasesfrom surface to layer and then decreases in the core. On the otherhand, Element fluorine can not be found. It can be inferred that,under acid environment, fluorides is likely to be volatilized duringdehydration and subsequent heating processes. Additionally,carbon in the sample can not be found by XRD, which suggests thecarbon layer on the particle surface is amorphous. Consequently, itcan be deduced that the coating layer of carbon has formed on thesurface of the sample.

CV curve obtained at a scanning rate of 0.10 mV s�1 is shown inFig. 5. There are three pairs of oxidativeereductive peaks near 3.60,3.68 and 4.10 V (vs. Li/Liþ) as well as 3.56, 3.64 and 4.03 V (vs. Li/Liþ)during positive and negative scanning. It corresponds to the lithiumextraction and insertion as the stoichiometric ranges: x¼ 0.0e0.50,0.50e1.0 and 1.0e2.0 in Li3�xV2(PO4)3 [23] respectively. It’s impor-tant to note that, the reported scanning rate of Li3V2(PO4)3 is usuallylower than 0.05mV s�1. In our cyclic voltammogram, the separationof redox peaks (DEP) is no more than 70 mV at a scanning rate of0.10mV s�1, promising ahigh rate performance. As a result, thewell-defined peaks and smaller value of potential interval show the fineelectrode reaction reversibility. The first chargeedischarge profileunder the current density of 95 mA g�1 between the cut-off voltageof 3.3e4.3 V (vs. Li/Liþ) is shown in the inset of Fig. 6. Three pairs of

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Fig. 6. The cyclic behavior of the experimental batteries, and the first chargeedischargeprofile (inset) under the current density of 95 mA g�1.

L. Wang et al. / Solid State Sciences 12 (2010) 1248e1252 1251

charge (3.61, 3.68 and 4.09 V) and discharge plateaus (3.56, 3.65 and4.03 V) indicate the characteristic of electrochemical reactions fromLi3V2(PO4)3 to LiV2(PO4)3 [10], which is in good agreement with theresults of the cyclic voltammogram. The difference between thosecharge and discharge plateaus is 50, 30 and 60mV respectively. Thelow polarization in charge and discharge processes shows thatelectron and ion transport are easy. Under 95 mA g�1, a dischargecapacity of 128 mAh g�1 is obtained, which corresponds closely tothe theoretical value (132 mAh g�1) [9].

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tial / V

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i/L

i+

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Fig. 7. The rate performance of the experimental battery (a), and the discharge profilesof the experimental battery under different current densities during Round 1 (b).

The cyclic behavior of the sample or the experimental batteriesperformed between the cut-off voltage of 3.3e4.3 V (vs. Li/Liþ) underthe current density of 95 mA g�1 is shown in Fig. 6. The initialdischarge capacity of the batteries is nearly 130mAh g�1and it keepsso fairly stable that a discharge capacity of more than 125 mAh g�1

can still be held after 50 galvanostatical cycles. The batteries delivera good cyclic performance with the first columbic efficiency of 97.3%and 96.2% capacity retention. Fig. 7a presents the rate performance ofthe experimental batteries under different current densities between3.3 and 4.3 V (vs. Li/Liþ). The initial discharge under 95mAg�1 showsa high capacity of 128mAh g�1 and retains almost unchanged by the5th round cycling test. Under a high current density of 381 mA g�1,a capacityof about100mAhg�1 can still beheldafter 5 rounds,whichexhibits a high reversible performance. The discharge profiles duringround1 are shown in Fig. 7b. According to the formula (C¼nF/3.6M),there are about 1.94, 1.74, 1.67 and 1.50 mol lithium inserted into thesample under current density of 95, 190, 285 and 381 mA g�1

respectively. It can be deduced that, in this paper, the sample withgood electrochemical performance is derived from its small particlesize and carbon-coated layer.

4. Conclusion

A carbon-coated layer has been successfully generated on thesurface of Li3V2(PO4)3 via a convenient and efficient method namelywet coordination. Electrochemical tests show that the sampleexhibits high reversible capacity of 128 mAh g�1 under currentdensity of 95 mA g�1, which is very close to theoretical capacity of132 mAh g�1 between 3.3 and 4.3V (vs. Li/Liþ). Besides, it exhibitsa high reversible performance either. It suggests that, by ourmethod,carbon-coated Li3V2(PO4)3 with good performance can be synthe-sized very conveniently without any other complex or additionaltreatments, which proposes a promising method suitable for indus-trial production of carbon-coated Li3V2(PO4)3 with goodperformance.

Acknowledgments

This work was financially supported by the National 973 Programof China (Contract No. 2009CB220100), the National 863 Program ofChina (Contract No. 2008AA11A104) and the Key Science & Tech-nology Brainstorm Project of ChongQing (CSTC2009AB4162).

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