Preparation, characterization and lithium-intercalation performance of different morphological...

Materials Chemistry and Physics 93 (2005) 395–398

Preparation, characterization and lithium-intercalation performance ofdifferent morphological molybdenum dioxide

Yongguang Liang, Shuijin Yang, Zonghui Yi, Jutang Sun∗, Yunhong ZhouDepartment of Chemistry, Wuhan University, Wuhan 430072, PR China

Received 15 November 2004; received in revised form 19 March 2005; accepted 25 March 2005

Abstract

Molybdenum dioxide (MoO2) powders with various morphologies were synthesized using a hydrothermal reaction and a rheological phasereaction as two novel routes. The physical characterization was carried out by TG/DTA, XRD and SEM. The electrochemical characteristicsof as-prepared MoO2 powders as anode materials for lithium batteries has also been studied and the morphological properties were found toplay an important role in their lithium-intercalation activity and cycling stability. The fissile MoO2 displays 484 mAh g−1 capacity in the initialcharge process with a capacity retention of 98.9% after 20 cycles, while the porous spherical MoO2 provides 769 mAh g−1 high capacity inthe first charge process and the charge capacity decreases rapidly in subsequent cycles in the range of 0.01–2.00 V versus metallic lithium ata ce of theM©

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current density of 100 mA g−1. The SEM and XRD results reveal that there is a correlation between the electrochemical performanoO2 powders and their morphological properties.2005 Elsevier B.V. All rights reserved.

eywords:Oxides; Chemical synthesis; Surface properties; Lithium-intercalation

. Introduction

The fast technological progress in the area of portableevices puts higher demands on high capacity rechargeable

ithium-ion batteries. Since the introduction of carbon as an-de material by Sony Company in 1990, research has beenndertaken to search for new anode materials to improve thenergy density and cycling performance of these practicalells. Idota et al.[1] have reported that tin-based amorphousxides as new negative electrode materials provide a high

ithium storage capacity. Recently, some metal oxides andetal-based composite oxides[2,3], and intermetallics[4,5]ere also found to deliver much higher specific capacity thanarbonaceous materials for which the theoretical capacity is72 mAh g−1 [6,7]. However, there are still concerns mainlyssociated with the improvement of the synthesis conditionsnd cycling stability before these materials can be consid-red as possible anode candidates for lithium rechargeableatteries.

∗ Corresponding author. Tel.: +86 27 87218494; fax: +86 27 68754067.E-mail address:jtsun@whu.edu.cn (J. Sun).

Auborn and Barberio[8] reported the use of MoO2powder as possible commercial anode in 1987, butresults showed a limited success because of thestability of their electrolyte at low potential. Furthmore, several molybdenum oxides and composite oxsuch as MoO2+δ [9], MnMoO4 [10], MoySnxO2 [11],V9Mo6O40 [12], Mn1−xMo2xV2(1−x)O6 [13] have been studied as anode materials for lithium batteries. Howethese compounds obtained by different synthesis mods usually showed inconsistent lithium-intercalation perties. It is meaningful to investigate the electrochecal behaviors, especially the lithium-intercalation permance in these materials with different physical proties.

In this paper, we introduce two novel routes tosyntheses of porous spherical MoO2 and fissile MoO2by a hydrothermal reaction and a rheological phasetion, respectively. The physical properties of MoO2 pow-ders with different morphologies were presented asas the electrochemical performance of the resulting erodes.

254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2005.03.034

396 Y. Liang et al. / Materials Chemistry and Physics 93 (2005) 395–398

2. Experimental

2.1. Materials preparation

The MoO2·xH2O precursor was synthesized through hy-drothermal reaction method. 1.77 g (NH4)6Mo7O24·4H2Owas fully dissolved in 20.0 mL distilled water and the stirredsolution was slowly dropped with 10.0 mL N2H4·H2O. Themixture was sealed in a closed container at 130◦C for 12 h,then cooled and filtrated. The filter cake was washed succes-sively with distilled water and absolute alcohol. After driedunder vacuum at 100◦C, a brown precursor was yielded.

The oxalate precursor was prepared by a rheological phasereaction route. (NH4)6Mo7O24·4H2O and C2H4O2·2H2Owere fully mixed by grinding with a molar ratio of 1:1.05. Aproper amount of absolute alcohol was added to get a rheo-logical body. The mixture was sealed in a closed containerat 80◦C for 8 h. After dried under vacuum at 80◦C, a whiteprecursor was obtained.

2.2. Physical characterization

The thermal stability of the precursors was examined bymeans of thermogravimetry and differential thermal analysis(TG/DTA) with a Netzsch STA 449 thermal analysis systema ◦ −1 ◦o rriedo ningrp d bya

2

e desw eb ickelgs Cel-g aratorT othw ubse-q .0 and

Fig. 2. XRD patterns of the resultant samples.

0.01 V versus metallic lithium at a constant current densityof 100 mA g−1.

3. Results and discussion

The TG and DTA curves of the precursors are presented inFig. 1. It indicates that the decomposition of the MoO2·xH2O(x∼ 0.75) (Fig. 1a) proceeds one step to give amorphousMoO2 with successive loss of the adsorbed water from 79.1 to336◦C, which is different with the results reported by Man-thiram and Tsang[9]. The endothermic peak on the DTAcurve at about 171◦C is due to the loss of the adsorbed wa-ter. The sharp exothermic peak located at about 372◦C inthe DTA curves is attributed to the formation of crystallizedMoO2 [14]. As shown inFig. 1b, the pyrolysis of the oxalateprecursor proceeds with 43% mass loss from 88.5 to 332◦C.The endothermic peaks on the DTA curve at about 106 and178◦C correspond the loss of the adsorbed water and the de-composition of surplus oxalate acid. The sharp endothermicpeak located at about 258◦C is associated with the formationof crystallized MoO2, which is confirmed by the XRD data.

Fig. 2 illustrates the XRD patterns of the resultantswhen the precursors treated at 400◦C for 6 h in ar-gon atmosphere. The patterns are assigned to a sin-gle phase of MoO. MoO -H (obtained by a hydrother-m ithM TheX rep upP

rs: (a)

t a heating rate of 10C min from 25 to 700 C in a flowf argon. Identification of phases and structures was caut on a Shimadzu 6000 X-ray diffractometer at a scanate of 2◦ min−1, using Cu K� radiation (λ = 1.54056A). Thearticle sizes and morphological features were observescanning electron microscope (Hitachi SEM X-650).

.3. Electrochemical measurement

The electrochemical cell consisted of a MoO2 workinglectrode and a lithium foil counter electrode. Electroere prepared by mixing MoO2 powders with 15% acetylenlack and 5% PTFE, compressing the mixture onto a nauze current collector. A 1 mol L−1 solution of LiClO4 dis-olved in EC/DEC (1:1) was used as the electrolyte. Aard 2400 microporous membrane was used as a sephe cell was assembled in an Ar-filled glove box with bater and oxygen concentrations less than 5 ppm. Suently, the cell was discharged and charged between 2

Fig. 1. TG and DTA curves of the precurso

.

2 2al reaction) displays lower intensity comparing woO2-R (prepared by a rheological phase reaction).RD results of MoO2-H were well defined as a puhase MoO2 with monocline symmetry, space gro21/n, with cell parametersa= 5.6062(2)A, b= 4.8596(1)A,

MoO2·xH2O precursor and (b) oxalate precursor.

Y. Liang et al. / Materials Chemistry and Physics 93 (2005) 395–398 397

Fig. 3. SEM images of MoO2 powders: (a) MoO2-H and (b) MoO2-R.

Fig. 4. The discharge and charge curves of MoO2/Li test cells.

c= 5.5371(3)A, β = 119.400(2)◦, V= 131.4244(2)A3, co-inciding with JCPDS 32–0671. The data of MoO2-Rwere identified as monoclinic MoO2 (P21/c) with cell pa-rametersa= 5.6034(1)A, b= 4.8544(1)A, c= 5.6190(2)A,β = 120.858(3)◦, V= 131.207(3)A3, agreeing with JCPDS73-1807.

Fig. 3shows the SEM images of the resulting MoO2 pow-ders. As seen inFig. 3a, spherical particles with average par-ticle size of 400 nm aggregated and piled up loosely with aporous state. The presence of tight-stacked particles inFig. 3bshows fissile with diameter about 700 nm and thickness lessthan 100 nm estimated from SEM data.

The discharge–charge profiles of spherical MoO2 and fis-sile MoO2 recorded with a current density of 100 mA g−1

are presented inFig. 4. The initial discharge curves of spher-ical MoO2-H and fissile MoO2-R do not show too much

Fig. 6. Cycle performance of the obtained MoO2 samples.

difference in except of the capacity, which the former is1426 mAh g−1, the latter 996 mAh g−1. The initial chargecapacities are 769 and 484 mAh g−1, respectively. How-ever, the charge and discharge curves in the following cy-cle present characteristics. There are no distinct plateaus inMoO2-H but two constant potential plateaus in MoO2-R at1.39 and 1.70 V on charge as well as 1.57 and 1.28 V on dis-charge during charge and discharge process. Clearly, differ-ent lithium-intercalation reaction took place during the pro-cess.

In order to evaluate any structural changes of MoO2electrodes during Li ion extraction, a group of cells werestopped at different states during the initial charge pro-cess, respectively. These cells were opened in an argon-filled glove box to recover the electrodes, and the elec-trodes were subsequently rinsed in EC to remove the resid-ual LiClO4 and finally dried under vacuum. The dried elec-trodes were subjected to XRD.Fig. 5 presents the changesof XRD patterns in the electrodes. The major diffractionpeaks at different states were identified with correspondingh, k, l values. The MoO2 electrodes of both samples pre-sented amorphous phase at 0.01 V, which is coincided withsubsequent SEM images inFig. 7. The spherical MoO2-H remains amorphous phase whereas fissile MoO2-R rep-resents a transition of phase during the initial charge pro-cess.

earch[ i

F ctrodeL

ig. 5. XRD patterns of the electrodes at different states: (a) MoO2-H eleiMoO2 (#), Li0.98MoO2 (&); (3) 1.75 V, Li0.42MoO2; (4) 2.0 V, MoO2.

Based on the XRD results above and the previous res15], the possible reaction of these MoO2 electrodes with L

s and (b) MoO2-R electrodes (1) 0.01 V, fully lithiated MoO2; (2) 1.36 V,

398 Y. Liang et al. / Materials Chemistry and Physics 93 (2005) 395–398

Fig. 7. SEM image of MoO2 electrodes after the initial discharge: (a) MoO2-H and (b) MoO2-R.

during the initial charge process were presented as below:

MoO2-H :

Li3.8MoO2 → Li2.3MoO2 + 1.5Li+ + 1.5e

(0.01–1.46 V);

Li2.3MoO2 → MoO2 + 2.3Li+ + 2.3e (1.46–2.00 V).

MoO2-R :

Li2.4MoO2 → LiMoO2(Li0.98MoO2) + 1.4Li+ + 1.4e

(0.01–1.36 V);

LiMoO2 → Li0.42MoO2 + 0.58Li+ + 0.58e

(1.36–1.75 V);

Li0.42MoO2 → MoO2 + 0.42Li+ + 0.42e (1.75–2.00 V).

As seen inFigs. 4 and 5b, the inflection point betweenthese plateaus represents a transition of phase in the partiallyLixMoO2 [8,15]. When the cell was discharged through theinflection point, the capacity would decrease rapidly. How-ever, in the second discharge curve of MoO2-R test cell, thecapacity between plateaus changed little. The cycling behav-iors within 20 cycles are shown inFig. 6. The reversible ca-p sb ityr d re-v lest

p eds se int on-t -f tivec ace).T entw ile-t isb Mi nd-

ing” [16] took place in the MoO2-R electrode. Smaller par-ticles shows near sphere with average particle size about300 nm. It is crucial to MoO2 powders with proper parti-cle size as stable anode materials for lithium batteries[17].The average particle size reduced, thereby increasing theexposed surface area. So the effective current density be-came lower. At same time, lithium-ion diffusion becameeasier as well as better capacity utilization. Its coulom-bic efficiency ascended from the first cycle to the forti-eth up to 1. The different effects from two facets providethe unique cycling performance of fissile MoO2-R. Clearly,there is a correlation between the cycling performance ofMoO2 powders and their morphological properties and par-ticle sizes.

4. Conclusions

The spherical and fissile MoO2 powders were preparedthrough two novel methods and presented different lithium-intercalation performance. Morphological properties of as-prepared MoO2 play an important role in their lithium inter-calating activity and cycling stability because of the changesof structure. Proper morphology, structure and particle sizeare very suitable for fissile MoO2 to be employed as anodematerials for rechargeable lithium batteries.

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acity of MoO2-H diminished rapidly in the following cycleut the process in fissile MoO2-R was stable with a capacetention of 98.9% after 20 cycles. In fact, the observeersible capacity of MoO2 rises slowly in subsequent cycill the 13th cycle.

As mentioned above, the physic properties of MoO2-Hrovide a full Li ionic diffusion, so that the crystalliztructure changes irreversible leading amorphous phahe following cycles and a rapid fall of capacity. On crast, to large particles of MoO2-R, the rate of ionic difusion through the particle is slow relative to the effecurrent density (the rate of charge transfer at the surfhis would result in a radial lithium concentration gradiith the particle, with only the outer layers of the rut

ype crystals actively involved in lithium intercalation. Iteneficial for MoO2-R to keep its structure. From the SE

mage (Fig. 7b), a phenomenon of “electrochemical gri

cknowledgement

This work was supported by the National Natural Scieoundation of China (No. 20471044).

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