Ž .Solid State Ionics 144 2001 31–40www.elsevier.comrlocaterssi

Structural and electrochemical properties of amorphous andcrystalline molybdenum oxide aerogels

W. Donga, A.N. Mansourb, B. Dunna,)

a UniÕersity of California, Los Angeles, Department of Materials Science and Engineering, Los Angeles, CA 90095, USAb NaÕal Surface Warfare Center, Carderock DiÕision, 9500 MacArthur BouleÕard, West Bethesda, MD 20817-5700, USA

Accepted 18 June 2001

Abstract

The structure and chemical differences among amorphous, crystalline and nanocrystalline molybdenum oxide aerogelsŽ . Ž .were determined using extended X-ray absorption fine structure EXAFS , Fourier transform infrared FTIR analysis and

Ž .powder X-ray diffraction XRD . These different forms of the same nominal material are produced by heat treatment. Theinfluence of the structural differences on electrochemical properties was examined using stepped cyclic voltammetry. Themost interesting material was the MoO aerogel heated to 3008C. The material was found to be nanocrystalline; there are no3

XRD peaks but the EXAFS were virtually identical to orthorhombic MoO . The electrochemcial response of the3

nanocrystalline material contains characteristics of both the amorphous and crystalline forms, but with better lithium capacitythan either one.q2001 Elsevier Science B.V. All rights reserved.

Keywords: Molybdenum oxide; Aerogel; Electrochemical properties; EXAFS

1. Introduction

Crystalline molybdenum trioxide has a two-di-mensional layered structure that has been shown to

w xhave interesting lithium intercalation properties 1,2 .Both its open structure and the ease of creatingoxygen vacancies in the structure make it an ideal

w xcandidate for lithium secondary batteries 3,4 andw xelectrochromic windows 5 . Lithium capacity of up

to 1.5 LirMo and discharge capacity of over 300w xmA hrg have been reported 6–8 .

Sol–gel synthesis of MoO is desirable because3

the low synthesis temperatures can lead to the forma-

) Corresponding author. Tel.:q1-310-825-1519; fax:q1-310-206-7353.

Ž .E-mail address: bdunn@ucla.edu B. Dunn .

tion of amorphous or metastable phases not availablethrough traditional synthesis methods. The low tem-perature phase can exhibit properties that are sub-stantially different from the crystalline phase. Al-though it is often assumed that lithium intercalationoccurs between the two-dimensional layers of thecrystalline MoO , previous experiments have not3

been able to show a direct correlation between thedegree of crystallinity and the Li capacity of MoO3w x9 . Even more interesting is that certain amorphousmolybdenum oxides have exhibited better intercala-tion behavior compared to that of crystalline MoO3w x10,11 . The present paper explores the structuredifferences among amorphous, crystalline, andnano-crystalline molybdenum oxide aerogels synthe-sized through the sol–gel process, where heat treat-ments are able to produce these different forms of

0167-2738r01r$ - see front matterq 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0167-2738 01 00901-8

( )W. Dong et al.rSolid State Ionics 144 2001 31–4032

the same nominal material. A combination of ex-Ž .tended X-ray absorption fine structure EXAFS ,

Ž .Fourier transform infrared FTIR analysis, and pow-Ž .der X-ray diffraction XRD provide detailed insight

concerning the chemical and structural nature ofthese materials. The influence of the structural differ-ences on electrochemical properties of these samplesis examined using stepped cyclic voltammetry. Thismethod is an effective means of identifying thecontribution of each electroactive species for systemscontaining more than one redox-active species.

2. Experimental

Molybdenum trioxide aerogels were prepared us-ing molybdenum alkoxides, acetonitrile, nitric acid,and water. Two molybdenum alkoxides were used,

Ž Ž .molybdenum isopropoxide Mo OC H , Alpha3 7 5.Aesar and molybdenum trichloride isopropoxide

Ž Ž . .MoCl OC H , Chemat Technology , both of3 3 7 2

which were dissolved in isopropanol. The iso-propanol was removed through vacuum evaporation

Žand replaced with acetonitrile CH CN, Aldrich3.Chemical . Nitric acid and water were then added

with vigorous stirring to form the final sol. Thepreparation of MoO aerogels has been previously3

w xdetailed 12 . The resulting aerogels were heat-treatedŽ . Žin vacuum for 10 h at 150 sample A , 300 sample

. Ž .B and 4008C sample C .Ž .FTIR measurements Nicolet, 510P were used to

establish the types of bonds present for the differentŽ .samples. Powder X-ray diffraction XRD measure-

ments were performed on a Rigaku diffractometer inreflection mode. EXAFS analysis was used to probethe oxidation state and local environment of the Mo

Table 1Ž .aSummary of local structure parameters for aerogel molybdenum oxides MoO3

2 y3 2˚ ˚ ˚Ž . Ž . Ž .Sample EXAFS temp. Dm x N R A Av. R A s 10 A R-factor

Mo RT 1.1 8.0 2.729"0.002 4.2"0.2 0.00486.0 3.148"0.002 4.3"0.2

Commercial-MoO RT 1.2 2.1"0.2 1.716"0.003 1.937 4.4"0.8 0.00103

2.6"0.2 1.950"0.002 2.7"0.51.3 2.268"0.008 4.2"1.2

Commercial-MoO LN 1.2 2.3"0.7 1.720"0.009 1.939 5.7"2.4 0.00463

2.2"0.3 1.947"0.004 0.9"0.91.5 2.262"0.016 3.7"2.5

Ž .Aerogel-MoO 1508C RT 2.4 2.2"0.6 1.731"0.008 1.951 3.7"1.5 0.01353

2.8"1.1 2.013"0.024 10.3"6.51.0 2.317"0.058 8.0"13.2

Ž .Aerogel-MoO 1508C LN 2.4 2.2"0.2 1.729"0.003 1.944 3.9"0.6 0.00233

3.2"0.3 2.018"0.010 13.4"2.50.6 2.321"0.0148 2.7"2.7

Ž .Aerogel-MoO 1508C LN 0.43 1.9"0.7 1.724"0.012 1.968 3.9"2.0 0.02723

3.3"1.3 2.021"0.038 16.3"12.20.8 2.332"0.064 5.6"12.5

Ž .Aerogel-MoO 300 C RT 0.64 2.2"0.5 1.710"0.006 1.968 5.0"1.5 0.00533

1.8"0.4 1.954"0.005 2.4"1.12.0 2.263"0.022 8.9"3.7

Ž .Aerogel-MoO 400 C RT 0.55 1.8"0.5 1.709"0.008 1.996 3.8"1.7 0.00913

2.1"0.5 1.961"0.008 4.1"1.82.1 2.278"0.023 8.3"4.0

Ž .Aerogel-MoO 400 C LN 0.56 1.8"0.6 1.705"0.008 1.998 3.7"1.8 0.00823

2.0"0.4 1.953"0.007 2.6"1.42.2 2.261"0.020 6.9"3.2

a Ž .Aerogel samples are heat-treated at 150, 300 and 4008C. A commercially obtained sample of crystalline MoO Alfa-Aesar is included3Ž .for comparison. The weighted average for the Mo–O distances i.e.,ÝN R rÝN is also listed as Av.R.i i i

( )W. Dong et al.rSolid State Ionics 144 2001 31–40 33

ion. The EXAFS experiments were conducted onbeamline X11-A at the National Synchrotron Light

Ž .Source NSLS with the electron storage ring operat-ing at an electron energy of 2.8 GeV and a current in

w xthe range 110–240 mA 13 . EXAFS spectra wereŽ .collected in the transmission mode using Si 311

double crystal monochromator with 10% detuning.The energy resolution for this monochromator at theMo K-edge energy is estimated to be 4.7 eV, whichis reasonable when compared to the Mo K-edge corehole natural line width of 4.5 eV. The X-ray intensi-ties were monitored using ionization chambers filledwith appropriate mixtures of nitrogen and argongases. A 15-mm thick Mo foil was used as aninternal reference for energy calibration and subse-quent calibration of the EXAFS amplitudes. TheEXAFS spectra were taken at room temperatureŽ .RT and near the temperature of liquid nitrogenŽ . Ž .LN . The X-ray absorption edge jumpDm x foreach sample is listed in Table 1. Fits for the oxidesamples were performed in ther-space range 1.10–

˚ w x2.09 A using the FEFFIT Code 14 . The FourierŽ . 3transform FT data were generated usingk -

weighted EXAFS spectra over thek-space range˚ y12.25–15.45 A . The fit for the Mo foil was made in

˚the r-space 1.60–3.60 A using Fourier transformgenerated withk 3-weighted EXAFS data over the

˚ y1k-space range 2.35–17.55 A . The photoelectronwave number,k, was defined by assigning the edgeenergy to the inflection point above the pre-edgepeak.

The electrochemical behavior was determined fora crushed aerogel mixed with conductive carbon

Ž .powder 80:15 weight ratio, respectively . An addi-tional 5% of the mass was contributed by the poly-

Ž .vinylidene fluoride PVDF binder. The mixture wasŽ .suspended in solventN-methyl-2 pyrrolidone and

cast onto a stainless steel mesh current collector. Theresulting electrode was heat-treated at 1208C undervacuum to remove the solvent and adsorbed water.

Ž .Cyclic voltammetry CV was carried out using athree-electrode cell with the MoO as the working3

electrode and lithium foil as the counter and refer-Ž .ence electrodes. LiClO 1 M in propylene carbon-4

Ž .ate PC served as the electrolyte. The voltage wasswept at a rate of 0.1 mVrs from the open circuitŽ .OC voltage to various potentials, with the lowestbeing 1.5 V.

3. Results

The chemical composition of the aerogels wasŽdetermined through thermogravimetry TG, Dupont

.9900 thermal analysis system measurements inŽcombination with chemical analysis Texas Analyti-

.cal Laboratories . The as-prepared molybdenum ox-ide aerogels are actually hydrated molybdenum ox-ides, containing a small amount of organic from thecomplexation reaction. The calculated composition is

w xMoO P1.0 H OP0.3 CH NH 12 . Structural3 2 3 2

changes occur gradually as the amorphous aerogel isŽ .heated. Heating to 1508C sample A causes the

evaporation of physically adsorbed water and or-ganic solvents, leaving MoOP0.8 H O with a den-3 2

Ž . Ž .sity r s0.2 grcc and a surface area SA of 180o

m2rg. The rest of the water is chemisorbed andremains in the oxide structure until 3508C. There-

Ž 2 .fore sample B r s1.2 grcc; SAs80 m rg stilloŽ .retains a small amount 0-x-0.8 of chemisorbed

H O. X-ray diffraction shows that the aerogel is2

completely dehydrated and crystallized to the or-Žthorhombic phase by 4008C sample C;r s2.1o

2 .grcc; SA-10 m rg . Both samples A and B ap-pear amorphous to XRD, showing no characteristic

Ž .peaks for a crystalline material Fig. 1 .Infrared spectra for all three samples between 600

and 1100 cmy1 are shown in Fig. 2. Infrared spec-Žtroscopy of the amorphous MoO aerogel sample3

. y1A shows broad absorption bands at 700 cm andbetween 850 and 950 cmy1. The 700 cmy1 absorp-

Fig. 1. XRD of samples A, B and C. Samples A and B appearamorphous while sample C is orthorhombic. Peaks up to 2us50are indexed.

( )W. Dong et al.rSolid State Ionics 144 2001 31–4034

Fig. 2. FTIR spectra of samples A, B and C. Sample A shows the6q Ž y1.presence of valences other than Mo 700, 850–950 cm ,

with no MosO bonds. Samples B and C show very distinctŽ y1. 6qMo5O bonds 1000 cm with mostly Mo behavior.

` `tion peak is characteristic of the bridging Mo O Mo5q w xof Mo 15 . The broad band, from 850 to 950

y1 ` 6qcm , includes different Mo O bonds for Mo ,5q 4q w xMo , and even some Mo 15–17 . Spectra of

samples B and C are very similar. However, theabsorption peaks for sample B are broader and seemto share some absorption peaks with sample A.Samples B and C have a very distinct peak at 1000cmy1 and a smaller peak at 970 cmy1, both are

6q w xindicative of the Mo5O bond for Mo 18,19 . Thebroader peaks at 890 and 640 cmy1 are those of

` ` 6q w xbridging Mo O Mo bonds of Mo 15 .Fig. 3 illustrates the XANES at the Mo K-edge of

samples A, B, and C, and that of a commerciallyobtained MoO powder. As has been shown previ-3

ously, the Mo K-edge XANES is sensitive to changesin the oxidation state and structure of the Mo ionw x20–22 . The pre-edge peak at the onset of the edgeŽ .with peak energy near 19 995 eV is due to thetransition from the 1s core states to unoccupied 4dstates. This transition is dipole forbidden in systemswith centrosymmetry and hence it is usually veryweak or absent for systems with undistorted octahe-dral coordination. The extremely strong intensity

observed here for the various oxides is due to thelarge degree of distortion in the local geometry ofthe first coordination sphere. For systems with a highdegree of distortion, this transition becomes dipoleallowed due to mixing between p and d states.

Ž .Within the uncertainty in the data"0.2 eV , thepre-edge peak energy as well as the main edgeenergy measured at half height are unchanged for theaerogels heat-treated at various temperatures and thecommercially obtained crystalline MoO sample. On3

the basis of the XANES data, it is concluded that theoxidation state of Mo in the aerogels heat-treated atvarious temperatures is similar to that of Mo incommercially obtained crystalline MoO . That is, the3

oxidation state in the aerogels isq6. It is to benoted that the main edge energy for the various

Ž .molybdenum oxides 20 001.2 eV is shifted by 8.3ŽeV relative to that of molybdenum metal 19 992.9

.eV . The intensity of the pre-edge peak however,increases on going from commercial-MoO to sam-3

ple A to sample B and then levels off with furtherŽ .heat treatment sample C . These changes are likely

due to details of the structure within the MO octa-6

hedral unit rather than to purely a change in oxida-tion state. This conclusion is supported by the factthat the pre-edge peak position is unchanged as afunction of heat treatment within the uncertainty inthe data. The XANES beyond the pre-edge peak for

Fig. 3. XANES of MoO aerogels subjected to various heat3

treatments, as compared to a commercially obtained crystallineMoO and elemental Mo.3

( )W. Dong et al.rSolid State Ionics 144 2001 31–40 35

samples B and C are almost identical and are similarto that of the commercial MoO powder. The3

XANES for sample A, however, shows a slightlydifferent behavior. This part of the spectrum is alsosensitive to local geometry beyond the first coordina-tion sphere indicating that the long range order within

˚the first 6 A for sample A differs from that for theother aerogel samples as will be shown from analysisof the Fourier transform data discussed below.

A comparison of the phase uncorrected Fouriertransform of thek 3-weighted Mo K-edge EXAFS foraerogels heat-treated at various temperatures and thecommercially obtained MoO powder, is shown in3

Fig. 4. The Fourier transforms display a number ofshells with signal above the noise level up to at least

˚ ˚6 A. The first shell, which is centered near 1.5 A,consists of single scattering contributions from a

`complex structure for the Mo O octahedral unit.6

For crystalline MoO , the first coordination sphere3Ž .consists of five distinct distances, namely, 1.671 x1 ,

˚Ž . Ž . Ž . Ž .1.734 x1 , 1.948 x2 , 2.251 x1 and 2.332 x1 Awhere the number in parentheses shows the multi-

w xplicity for each distance 23 . The second and third˚shells centered near 3.3 and 5 A arise mainly from

`Mo Mo single scattering contributions with addi-tional contributions from a large number of multiplescattering events. Again, the Fourier transforms for

Fig. 4. Room temperature EXAFS of MoO aerogels subjected to3

various heat treatments, as compared to a commercially obtainedcrystalline MoO .3

samples B and C are similar and display the first,second and third shell of atoms with no significantchange in position indicating a high degree of struc-tural order. In fact, the Fourier transforms for theaerogels heat-treated at 300 and 4008C display allthe shells observed in the Fourier transform forcommercially obtained crystalline MoO . The peak3

positions are very similar to those observed for thecommercial powder. The only major difference is inthe amplitudes of the peaks, which are significantlyhigher in the Fourier transform of commerciallyobtained crystalline MoO . Meanwhile, sample A3

shows a general shift of the maxima positions tosmaller radial distances with significantly loweroverall intensity. Distances for the aerogel heat-treated at 1508C appear to be compressed by about7% relative to that of samples B, C, and commercialMoO . On the basis of the Fourier transform data,3

the degree of order increased on going from sampleA to sample B to sample C and then to commerciallyobtained crystalline MoO .3

To further elucidate the structure of the aerogels,the local structure parameters such as coordination

Ž . Ž . Ž 2.number N , distance R , and disorders for thefirst shell of atoms were extracted using standardnonlinear fitting procedures. Using the XRD struc-ture data for crystalline MoO , three sub-shells with3

`different Mo O distances were used to model thedistorted octahedral structure of the first coordinationsphere. Floating parameters were as follows:N , N ,1 2

R , R , R , s 2, s 2 ands 2. N was constrained to1 2 3 1 2 3 3

be equal to 6yN yN . The many body amplitude1 2

reduction factor,S2, and the inner potential wereoŽ . Ž .determined to be 1.063"0.043 andy7.2 "0.5

eV, respectively, from analysis of the first and sec-ond coordination spheres of room temperature EX-AFS data for metallic Mo. In this analysis,N and1

N for metallic Mo were constrained to the crystallo-2

graphic values, which are listed in Table 1. A sum-mary of local structure parameters is also listed inTable 1 for the aerogels heat-treated at various tem-peratures and commercial MoO powder. TheR-fac-3

tor is a measure of the goodness of the fit. Note thatfor the aerogel heat treated at 1508C, two sampleswith significantly different absorption edge jumpswere investigated in order to insure that the EXAFS

w xamplitudes are not altered by the thickness 24 orw xparticle size effects 25 . Due to the complexity of

( )W. Dong et al.rSolid State Ionics 144 2001 31–4036

Fig. 5. First cycle cyclic voltammetry for samples A, B and C.

the structure of the first coordination sphere and thecorrelation between coordination numbers and disor-ders, these results must be interpreted with extremecaution. Our results clearly show that the structure ofthe first coordination sphere for aerogels heated at300 and 4008C as well as commercially obtainedcrystalline MoO can be approximated by a distorted3

octahedral coordination, which consists of three dis-Ž . Ž .tinct distances near 1.71 x2 , 1.95 x2 , and 2.26

˚Ž .x2 A. The local structure of the first coordinationsphere of the aerogel heat-treated at 1508C, on theother hand, can be approximated by a distortedoctahedral coordination which consists of three dis-

˚Ž . Ž . Ž .tances near 1.73 x2 , 2.08 x3 , and 2.32 x1 A.CV measurements indicate that 1.1–1.5 mol

lithiumrmol MoO can be intercalated into the aero-3

Table 2Intercalation properties of MoO aerogels compared to literature3

values

MoO LirMo mA hrg3

Ž .Sample A 1508C 1.2 240Ž .Sample B 3008C 1.5 280Ž .Sample C 4008C 1.1 210w xCrystalline 26 1.2 200w xAmorphous 11 1.5 260

Ž .gels Fig. 5, Table 2 . However, not all of the initialcapacity is reversible. Approximately 72% of thecapacity is reversible for sample A while sample Bretained 82% of its initial capacity. Sample C exhib-ited the best reversibility, retaining approximately90% of the capacity. For each sample, there was nosignificant capacity change between the 2nd cycleand the 10th cycle. Sample B exhibited the bestinitial capacity and displays behavior intermediate tothat of A and C.

A series of stepwise CV experiments was per-formed in order to determine the reversibility of theintercalationrde-intercalation peaks. Stepwise CVsfor sample A show an irreversible intercalation peak

Ž .at 2.2 V and a shoulder at 2.55 V Fig. 6 . This lackof reversibility between the OCV and 2.2 V regionaccounts for the low capacity that is retained afterthe first cycle.

For sample C, there is an irreversible intercalationŽ .peak at 2.75 V Fig. 7 . This irreversible peak has

also been previously observed in other crystallinew xMoO materials 9 . This peak appears to be part of3

an activation process. Cyclic voltammetry sweeps topotentials from OC to 2.75 V exhibited very limitedlithium capacity. However, once this peak is ac-cessed, subsequent cycles show a noticeable increasein capacity in the same region. The peak at 2.3 V is

( )W. Dong et al.rSolid State Ionics 144 2001 31–40 37

Ž . Ž . Ž . Ž . Ž .Fig. 6. Stepwise CV for sample A. Cycles range from OCVs3.2 V to 1 2.8 V, 2 2.3 V, 3 2.1 V, 4 1.9 V and 5 1.5 V.

reversible and is deintercalated at 2.45 V on thereverse sweep. This pair of peaks is shifted slightlyupon subsequent cycles but retains most of its capac-ity.

Fig. 8 shows a series of CV sweeps for sample B.ŽThe intercalation peak at 2.75 V identical to that of

.sample C was irreversible after the first cycle. TheŽintercalation peak at 2.55 V was reversible de-inter-

Ž . Ž . Ž . Ž . Ž . Ž .Fig. 7. Stepwise CV for sample C. Cycles range from OCVs3.3 V to 1 2.6 V, 2 2.5 V, 3 2.4 V, 4 2.2 V, 5 2.0 V and 6 1.5 V.

( )W. Dong et al.rSolid State Ionics 144 2001 31–4038

Ž . Ž . Ž . Ž . Ž .Fig. 8. Stepwise CV for sample B. Cycles range from OCVs3.2 V to 1 2.8 V, 2 2.3 V, 3 2.1 V, 4 2.0 V and 5 1.5 V.

.calation at 2.6 V until intercalation occurred at 2.2V. This portion of the electrochemical behavior isvery similar to that of sample A. Intercalation at thisvoltage apparently changed the structure so that both

Ž .intercalation peaks at 2.55 and 2.2 V became irre-versible. Subsequent sweeps showed no intercalationpeaks. However, capacity is then stabilized at 1.2LirMo.

4. Discussion

The XRD data show that sample C is clearlycrystalline with an orthorhombic structure. Whileboth samples A and B appear amorphous from theXRD, the EXAFS and FTIR data provide furtherinsight. From the EXAFS, it is not surprising thatsample A exhibits a local environment that is quitedifferent from that of the crystalline sample C. Incontrast, sample B is more similar to the crystallinesample C than to the amorphous sample A. Thissuggests that sample B is nanocrystalline, althoughthe crystallinity does not extend to a large enoughscale to be observed by XRD.

The FTIR data show that sample C exhibits peaks` 6qthat are indicative of bridging Mo O bonds of Mo ,

with no evidence of Mo in a lower oxidation state.This is not the case for sample A where characteris-tic peaks for Mo5q and even Mo4q are seen alongwith Mo6q. It is significant to note that these lowervalence peaks are weak and broad, signifying a lowconcentration. Thus, it is not surprising that theEXAFS data for sample A do not show evidence forMo5q and Mo4q; instead this suggests that thefraction of these lower valence species in sample Ais less than approximately 10% of the total molybde-num content. Although sample B was shown to lacklong-range order in the XRD data, its local environ-ment is very similar to that of the crystalline sampleC. Both samples B and C exhibit strong absorptionpeaks for Mo6q with no indication of the lowervalence states.

The XANES and RT-EXAFS data for sample Care consistent with the XRD and FTIR results.Namely, sample C has a high degree of structuralorder with mostly Mo6q, very similar to the com-mercial MoO powder. The XANES and RT-EX-3

AFS data also show that sample A behaves verydifferently than sample C. It is assumed that the

( )W. Dong et al.rSolid State Ionics 144 2001 31–40 39

amorphous material has a more open structure, con-tains more oxygen vacancies, and therefore a lowerMo oxidation state. The EXAFS data highlight thestructural differences between samples A and C. It isevident that both the long-range and local structureof sample A are unlike that of the crystalline sampleC. As discussed above, sample A contains Mo5q andMo4q; although at relatively low concentrations.Sample B, on the other hand, is practically identicalto sample C and very similar to the commercialMoO powder for both the XANES and RT-EXAFS3

data. Therefore, it can be safely assumed that sampleB has a local structure very similar to that of thecrystalline sample C. Although XRD detected nolong-range order for sample B, the EXAFS and FTIRdata show that the material has a local structuresimilar to the crystalline form, suggesting that sam-ple B is comprised of nanocrystalline MoO .3

The voltammetric responses are strongly depen-dent on the form of the MoO . This is best shown in3

comparing the behaviors of samples A and C. Sam-ples A and C displayed two distinctly different CV

Ž .responses Fig. 5 . The amorphous material exhibiteda strong peak at 2.2 V that is irreversible. Thecrystalline sample, on the other hand, has two peaks,at 2.75 and 2.3 V. In this case it is the low voltage2.3 V peak which is reversible. This behavior sug-gests that the Li intercalation processes are markedlydifferent in amorphous and crystalline samples.

The reversible peak at 2.3 V observed for sampleC is most likely related to the long range layeredstructure of the crystalline material. This is becausethere is no 2.3 V peak observed for sample B, whichhas short range but no long range order. Both sam-ples A and B show a featureless but very capacitiverange between 2.3 and 1.5 V after the first cycle.This is consistent with the presence of disorder in thestructure. It has been shown that the amorphousaerogel does not exhibit much of a layered structure,

w xeven on the nanometer scale 12 . The disorder meansthat there will be numerous sites having a largeenergy spread available to the Li.

The first cycle irreversibility of both the crys-talline and amorphous MoO observed in this study3

w xwas also previously documented 9,11,26,27 . Sug-gestions for the cause of this irreversibility includestructural change, Mo ion migration, and decomposi-

w x Ž .tion of H O 9,26 . In the crystalline sample C , the2

activation effect of the peak at 2.75 V is indicative ofa structural change. Prior work has shown that theinterlayer distance almost doubles in crystallineMoO with only a relatively small amount of Li3

w xintercalation 9 . The intercalation can effectivelydisrupt the structure enough to cause local structuralchange. Moreover, since almost all the water hasbeen driven off by 4008C, it is very unlikely for theirreversibility to be caused by the decomposition ofwater. The cause for irreversibility in the amorphousmaterial is less obvious. The disappearance of the2.2 V peak can be due to the decomposition of wateras well as structural change since it is known that inamorphous MoO , dehydration and structural3

w xchanges are very often coupled 11 . Mo ion migra-tion to Li sites during the first cycle would notexplain the activation process of the irreversible peakin sample C nor the large capacitive behavior be-tween 2.3 and 1.5 V after the first cycle in samplesA and B.

Sample B was extremely interesting in that itexhibited higher Li capacity compared to either the150 or the 4008C samples. In addition, the CVresponse for the 3008C sample displayed character-istics from both the amorphous and the crystallinematerials, a reversible shoulder at 2.75 V and anirreversible peak at 2.2 V. This further supports thesupposition that sample B is indeed nanocrystallinewith long-range disorder. The ability to intercalate inboth the crystalline and amorphous sites can be thereason for the higher level of intercalation.

5. Conclusion

Using the sol–gel process, molybdenum oxideaerogels have been produced which are crystalline,nanocrystalline or amorphous depending upon theheat treatment temperature. Through the use of XRD,FTIR, and EXAFS, it has been shown that MoO3

aerogels heat treated to 3008C are crystalline at thenanoscale while maintaining long range disorder.Stepwise CV was used to characterize the reversibleand irreversible electrochemical responses for thedifferent materials. The first cycle irreversibility inthe crystalline material is most likely due to localstructural change. The cause for the first cycle irre-versibility for the amorphous material is not as clear

( )W. Dong et al.rSolid State Ionics 144 2001 31–4040

as both structural change and the decomposition ofwater are likely to contribute. The nanocrystallinesample exhibits electrochemical characteristics simi-lar to both the crystalline and amorphous samplesand, interestingly, intercalates greater amounts oflithium than either form.

Acknowledgements

The authors acknowledge financial support byONR. ANM also acknowledges financial support bythe Carderock Division of the Naval Surface WarfareCenter’s In-house Laboratory Independent ResearchProgram sponsored by ONR administered under Pro-gram Element 0601152N. The support of the USDOE under Contract no. DE-AS05-80-ER-10742 forits role in the development and operation of beamline X-11A at NSLS is also acknowledged. TheNSLS is supported by the US DOE under Contractno. DE-AC02-76CH00016.

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