Materials Science and Engineering B 121 (2005) 152–155

Low temperature synthesis of a stable MoO2 assuitable anode materials for lithium batteries

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

Received 15 December 2004; received in revised form 14 March 2005; accepted 25 March 2005

Abstract

Fissile molybdenum dioxide (MoO2) was synthesized using a rheological phase reaction as a novel method suitable for a large scale up.The oxalate precursor was initially prepared at 80◦C and was treated at different temperatures. The physical characterization was carried outby thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffractometer (XRD) and scanning electron microscope (SEM). Theresults of TG/DTA and XRD indicate that the oxalate precursor begins to yield MoO2 at 250◦C and a single phase MoO2 with monoclinesymmetry is formed at 350◦C. The electrochemical characteristics of fissile MoO2 as an anode material for lithium batteries have alsobeen studied and the morphological properties were found to play an important role in the cycling stability. The activated MoOdisplays4 V versusm e of theM©

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84 mAh g−1 capacity in the initial charge process with a capacity retention of 83.1% after 40 cycles in the range of 0.01–2.00etallic lithium at a current density of 100 mA g−1. The SEM results reveal that there is a correlation between the cycling performancoO2 powders and their morphological properties.2005 Elsevier B.V. All rights reserved.

eywords:Molybdenum oxide; Chemical synthesis; Surface properties; Negative electrode; Lithium batteries

. Introduction

The fast technological progress in the area of mobileevices puts higher demands on portable power supplies.urrently, it seems that rechargeable lithium-ion batteriesre the systems of choice for high-capacity cells. Since the

ntroduction with carbon as anode material by Sony companyn 1990, many studies has been undertaken to search for newnode materials to improve the energy density and cyclingerformance of these practical cells. Idota et al.[1] haveeported that tin-based amorphous oxides as new negativelectrode materials provide a high lithium storage capacity.ecently, some metal oxides and metal-based compositexides [2,3], and intermetallics[4,5] were also found toeliver much higher specific capacity than carbonaceousaterials for which the theoretical capacity is 372 mAh g−1

6,7]. However, there are still concerns mainly associatedith the improvement of the synthesis conditions and

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

cycling stability before these materials can be considerpossible anode candidates for lithium rechargeable batt

Auborn and Barberio[8] reported the use of MoO2powder as possible commercial anode in 1987, butresults showed a limited success because of the poor stof their electrolyte at low potential. Furthermore, sevmolybdenum oxides and composite oxides, such as Mo2+δ

[9], MnMoO4 [10], MoySnxO2 [11], V9Mo6O40 [12],Mn1−xMo2xV2(1−x)O6 [13] have been studied as anomaterials for lithium batteries. However, these compouobtained by different synthesis methods usually shoinconsistent lithium-intercalation properties.

In this paper, we introduce a new synthesis route of fiMoO2 by a rheological phase reaction method. The phcal properties of MoO2 oxides are presented as well aselectrochemical performance of the resulting electrodes

2. Experimental

The oxalate precursor was synthesized by a rheolophase reaction route. All reagents were analytical grade

921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2005.03.027

Y. Liang et al. / Materials Science and Engineering B 121 (2005) 152–155 153

used without further purification. (NH4)6Mo7O24·4H2O andC2H4O2·2H2O were fully mixed by grinding with a molarratio of 1:1.05. A proper amount of absolute alcohol wasadded to get a rheological body. The mixture was sealed in aclosed container at 80◦C for 8 h. After dried under vacuumat 80◦C, the white precursor was obtained.

The thermal stability of the oxalate precursor was exam-ined by means of thermogravimetry and differential thermalanalysis (TG/DTA) with a Netzsch STA 449 thermal analysissystem at a heating rate of 10◦C min−1 from 25 to 700◦C in aflow of argon. Identification of phases and structures was car-ried out on a Shimadzu 6000 X-ray diffractometer at a scan-ning rate of 2◦ min−1, using Cu K� radiation (λ = 1.54056A).The particle sizes and morphological features were observedby a scanning electron microscope (Hitachi SEM X-650).

The electrochemical cell consisted of a MoO2 workingelectrode and a lithium foil counter electrode. Electrodeswere prepared by mixing MoO2 powders with 15% acetyleneblack and 5% PTFE, compressing the mixture onto a nickelgauze current collector. A 1 mol L−1 solution of LiClO4dissolved in EC/DEC (1:1) was used as the electrolyte.A Celgard 2400 microporous membrane was used as aseparator. The cell was discharged and charged between2.0 and 0.01 V versus metallic lithium at a constant currentdensity of 100 mA g−1.

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Fig. 1. TG and DTA curves of the oxalate precursor.

Fig. 2. Changes in XRD patterns with the precursor treating at differenttemperatures.

lines at 26.02◦, 37.00◦ and 53.58◦. The XRD peaks assignedto a MoO2 phase obviously increased in intensity at en-suing higher temperature, and these patterns are mutuallycoincided. A single phase of MoO2 was found to be yieldedafter a heating treatment no less than 350◦C. The XRDresults of the product obtained at 400◦C is well defined asa pure phase MoO2 with monocline symmetry, space groupP21/c, with cell parametersa= 5.6034(1)A, b= 4.8544(1)A,c= 5.6190(2)A, β = 120.858(3)◦,V= 131.207(13)A3, whichis coincided with JCPDS data, card number 73-1807. Theaverage particle size is estimated to be about 650 nm accord-ing to the Sherrer formula, agreeing with the observation ofthe SEM image inFig. 3a.

ders a

. Results and discussion

The TG and DTA curves of the precursor are showig. 1. From the TG curve, the pyrolysis of the oxalate pursor proceeds with 43% mass loss from 88.5 to 332◦C. Thendothermic peaks on the DTA curve at about 106 and 17◦Correspond the loss of the adsorbed water and the decoition of surplus oxalate acid. The sharp endothermic pebout 258◦C is associated with the formation of crystallizoO2, which is confirmed by the XRD data inFig. 2.Fig. 2 illustrates the changes of XRD patterns when

xalate precursor was treated at different temperatures fn argon atmosphere. A monoclinic phase of MoO2 alreadyppeared in the sample obtained at 250◦C with diffraction

Fig. 3. SEM image of electrodes: (a) MoO2 pow

nd (b) MoO2 electrode after the initial discharge.

154 Y. Liang et al. / Materials Science and Engineering B 121 (2005) 152–155

An example of the microstructure developed in the elec-trodes after the initial discharge process is shown inFig. 3.The as-prepared MoO2 powders at 400◦C (Fig. 3a) showsthat the grains with average particle size about 700 nm aggre-gates and tightly stacks with a free porous state. It is differentwith coarse-grained molybdenum dioxide prepared by high-temperature reactions[14,15] or nano-sized ones obtainedthrough solution routes[16,17]. The fully lithiated particlesin Fig. 3b shows near sphere with average particle size about300 nm. It is crucial to MoO2 powders with proper particlesize as stable anode materials for lithium batteries[18]. Butthis change was caused by an electrochemical grinding, nota milling. In addition, an amorphous phase of particles withno clear fringe appeared after lithium fully intercalating, co-inciding with the XRD diagram inFig. 5.

The discharge and charge curves of fissile MoO2 recordedwith a current density of 100 mA g−1 are presented inFig. 4.The initial discharge capacity comes to about 994 mAh g−1

and the charge capacity reaches to 484 mAh g−1. There is noobvious potential plateau during the first discharge process.However, the charge and discharge curves in the followingcycle present characteristics. There are two constant poten-tial plateaus at 1.39 and 1.70 V on charge as well as 1.57 and1.28 V on discharge. Based the previous research, the inflec-tion point between these plateaus represents a transition be-tween monoclinic phase and orthogonal phase in the partiallyL hisi allyi hasete weenp ionicd tivec ace).T entw pec re,i

oOe ofc 0 and2 esec over

Fig. 5. XRD patterns of the electrode at different state: (1) 0.01 V, fullylithiated MoO2; (2) 1.36 V, LiMoO2 (#), Li0.98MoO2 (&); (3) 1.75 V,Li0.42MoO2; (4) 2.0 V, MoO2.

the electrodes, and the electrodes were subsequently rinsedin EC to remove the residual LiClO4 and finally dried undervacuum. The dried electrodes were subjected to XRD.Fig. 5presents the changes of XRD patterns in the electrodes.The major diffraction peaks at different state were identifiedwith correspondingh, k, l values. The phase of MoO2 infully lithiated particles at 0.01 V implies that the structure ofthe particles limited the lithium-ion diffusion and the innerparticles were not fully involved in the lithium-intercalationin the initial discharge process.

The cycling behavior within forty cycles is shown inFig. 6and 83.1% of the initial charge capacity was maintained after40 cycles. The charge capacity reduces slightly in the secondcycle. In fact, the observed reversible capacity of MoO2 risesslowly in subsequent cycles till the 13th cycle. From the SEMimage (Fig. 3b), a phenomenon of “electrochemical grinding”[20] took place in the electrode material. Smaller particleswere formed after the initial lithium fully intercalating. Theaverage particle size reduced, thereby increasing the exposedsurface area. That is, lithium-ion diffusion became easier aswell as better capacity utilization. Its Coulombic efficiencyascended from the first cycle to the fortieth up to 1. Thecapacity of the electrode after electrochemical grinding ap-proaches calculated theoretical capacity in following cycles.Contrarily, the effective current density became lower. Thiswould reduce the rate of electron exchange. The differente for-m cling

ixMoO2. Activated material cycle reversibly above tnflection point, but the phase transition is electrochemicrreversible. When the cell was discharged through the pransition, the capacity would decrease rapidly[19]. How-ver, from the second discharge curve, the capacity betlateaus changed little. To large particles, the rate ofiffusion 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 rutile-tyrystals actively involved in lithium-intercalation. Therefots initial Coulombic efficiency is only about 48.7%.

In order to evaluate any structural changes of M2lectrodes during Li ion extraction at different stateharge, a group of cells were stopped at 0.01, 1.39, 1.7.0 V during the initial charge process, respectively. Thells were opened in an argon-filled glove box to rec

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

ffects from two facets provide the unique cycling perance. Clearly, there is a correlation between the cy

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

Y. Liang et al. / Materials Science and Engineering B 121 (2005) 152–155 155

performance of the MoO2 powders and their morphologicalproperties.

4. Conclusions

A stable MoO2 as suitable anode materials for lithiumrechargeable batteries was prepared at low temperature, us-ing a rheological phase reaction as a novel method. The ac-tivated MoO2 displays 484 mAh g−1 capacity in the initialcharge process with a capacity retention of 83.1% after 40cycles. The results above reveal a correlation between thecycling performance of the fissile MoO2 powders and theirmorphological properties.

Acknowledgements

This work has been supported by the National NaturalScience Foundation of China (20471044). The authors grate-fully acknowledged the referee for helpful discussions andfor carefully reviewing this paper.

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