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7Li MAS-NMR and vibrational spectroscopic investigations of LixV2O5 (x=1.0, 1.2 and 1.4)
Solid State Ionics 167 (2004) 41 – 47 www.elsevier.com/locate/ssi
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Li MAS-NMR and vibrational spectroscopic investigations of LixV2O5 (x=1.0, 1.2 and 1.4) M. Vijayakumar a, S. Selvasekarapandian a,*, Koichi Nakamura b, Tatsuo Kanashiro b, R. Kesavamoorthy c
a
Solid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore-641 046, Tamilnadu, India b Department of Physics, Faculty of Science, Tokushima University, 2-1 Minami Josanjima-Cho, Tokushima 770-8506, Japan c Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, India Received 25 July 2003; received in revised form 25 November 2003; accepted 5 December 2003
Abstract The lithium vanadate LixV2O5 (x = 1.0, 1.2 and 1.4) has been prepared by solid state reaction method. The 7Li MAS-NMR, Laser Raman and Fourier Transform Infrared (FTIR) spectroscopic analysis reveals the y-LiV2O5 and g-LiV2O5 phase formation. The 7Li MAS-NMR spectrum shows the chemical shift around 1 and 10 ppm, which confirms the presence of g and y phases of lithium vanadate. The formation of y phase and g phase of LiV2O5 has been detected by the presence of Raman peaks at 971 and 982 cm 1. The phase transformation from y to g phases of LiV2O5 with lithium incorporation has been identified. The reduction in the oxidation state of vanadium (V5+ to V4+) has been identified, due to lithium incorporation. This reduction affects the vanadium oxygen bonding nature and leads to the phase transformations. D 2004 Elsevier B.V. All rights reserved. PACS: 61.18.Fs; 78.30 Keywords: Lithium battery; 7Li MAS-NMR spectra; FTIR analysis; Laser Raman Analysis; Ionic conductors
1. Introduction The growing desire for portable electronic devices and rechargeable power sources has fueled a strong interest in lithium batteries [1]. The development of high-performance lithium batteries is intimately linked to the availability of efficient electrode materials. The transition metal oxides are the most attractive electrode materials for batteries because of its layered structure which is useful for the intercalation [2,3]. The lithium vanadate LiV2O5 attracts a great deal of attention because of its useful properties such as high conductivity of lithium ion to gain a high cathode utility of a cell and mixed conduction to increase the charge– discharge efficiency [4,5]. Different types of synthesis procedure are reported such as sol – gel method, xerogels, solid state reaction, electro-
* Corresponding author. Tel.: +91-422-2422222x422; fax: +91-4222422387. E-mail address: [email protected] (S. Selvasekarapandian). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.008
chemical insertion of lithium ions for the preparation of lithium vanadate LiV2O5 [6– 9]. However, each procedure yields different phases of lithium vanadate such as y, q, and g-LiV2O5 depending upon the amount of lithium incorporated. Pecquenard et al. [10] reported the identification of LiV2O5 phases by EPR spectroscopy. The 7Li MAS-NMR analysis is known to be an advanced technique, which gives the information about the local Li environment in Li ion conducting samples [10]. Recently, Fujiwara et al. [11,12] reported the 7Li and 51V NMR analysis of the lithium vanadate with multiple phases prepared by chemical intercalation. The vibrational spectroscopic techniques Laser Raman and FTIR spectra are efficient tool to analyze various structures, and various phases of lithium vanadate have been identified by these methods by various researchers [13,14]. However, these studies are reported for the chemically and electrochemically prepared LiV2O5. The 7Li MAS-NMR and vibrational spectroscopic analysis of high temperature lithium vanadate is scarce. The high temperature preparation (solid state reaction) is a simple and cost-effective method. However, the lithium
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M. Vijayakumar et al. / Solid State Ionics 167 (2004) 41–47
Fig. 1. X-Ray diffraction pattern of LixV2O5 at room temperature LixV2O5 (x = 1.0, 1.2 and 1.4) {*—g-LiV2O5 and O—y-LiV2O5}.
evaporation during the synthesis is the serious draw back of this procedure and this has been overcome by addition of excess raw materials [15]. Further, the heat treatment of raw material V2O5 results in the oxygen vacancies and influences the formation of various phases in lithium vanadate. The heterogeneous phase formation of lithium vanadate during solid state synthesis has been reported previously [16]. It is essential to characterize lithium vanadate prepared by solid state reaction which contains heterogeneous phases to tailor its usage in secondary lithium battery. In this paper, the results of 7Li MAS-NMR and vibrational spectroscopic studies on lithium vanadate LixV2O5 (x = 1.0, 1.2, 1.4) prepared by solid state reaction method have been presented. The x value used in this paper represents the nominal value of lithium content in the material.
2. Experimental procedure Polycrystalline LixV2O5 (x = 1.0, 1.2, 1.4) has been prepared by conventional solid state reaction method. The stoichiometric amount raw materials Li2CO3 (Aldrich, USA) and V2O5 (Aldrich) are grinded into fine powders using mortar and pestle, and melted in porcelain crucible and the melt is cooled slowly. The detailed preparation procedure has been described elsewhere [17]. The lithium
losses during high temperature fusion have been overcome by taking excess lithium carbonate as raw material. The composition analysis have been carried out by XPS and AES analysis and found to be in good agreement with theoretical stoichiometric values [15]. X-ray diffraction patterns have been recorded using a Philips X-ray generator (Model PW 1390) with a Ni filter and CuKa radiation ˚ ) at 40 kV and 20 mA in the 2h range of (k = 1.5418 A 10j to 80j. The resultant diffraction pattern has been compared with Joint Commission for Powder Diffraction Standard (JCPDS) data of the lithium vanadate samples. The 7 Li MAS-NMR measurement has been carried out by using Bruker Avance 300 spectrometer with a 4-mm probe. The spinning speed has been maintained as 7 kHz. The MASNMR spectra at room temperature were measured at 116.672 MHz. The FTIR spectrum has been recorded in the wave number range of 400 –4000 cm 1 by using Perkin Elmer Spectrum RX1 equipment with KBr pellet method. For the Laser Raman analysis, the pellets with 0.1-cm thickness and 1.0-cm diameter of lithium vanadate have been prepared by cold-pressed method. Raman spectrometer built around a double grating monochromator SPEX model 14018 is used to record Raman spectra of these pellets in the backscattering geometry. A 30-mW 488-nm laser beam from Ar+ laser (coherent, USA) is focused on to the sample to a spot size of about 100-Am diameter. A thermoelectrically cooled photomultiplier tube ITT-FW 130 detected the scattered light after passing through the monochromator. The spectral resolution of the monochromator was 4.2 cm 1. The spectra are recorded digitally using a microprocessor-based automated data collection system with a step of 1 cm 1 and a collection time of 10 s. The commercial software PEAKFIT has been used to fit the Lorentzian line shapes along with straight base line to the Raman spectra from which the peak position and full width at half maximum (FWHM) of the Raman peak are obtained.
3. Results and discussions 3.1. X-ray diffraction analysis The lithium vanadate prepared by solid state reaction method has been subjected to the X-ray diffraction analysis.
Table 1 X-ray diffraction parameters for LixV2O5 (x = 1.0, 1.2 and 1.4) Li1.0V2O5
Li1.2V2O5
Li1.4V2O5
2h
hkl
˚) d (A
Assignment
2h
hkl
˚) d (A
Assignment
2h
hkl
˚) d (A
Assignment
16.62 17.83 26.83 27.68 29.85 31.32
002 010 103 111 310 112
5.333 4.974 3.322 3.222 2.989 2.857
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
16.62 17.83 26.81 27.66 29.83 31.31
002 010 103 111 310 112
5.333 4.974 3.325 3.221 2.995 2.856
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
16.62 17.85 26.83 27.68 29.82 31.31
002 010 103 111 310 112
5.333 4.968 3.322 3.222 2.996 2.856
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
M. Vijayakumar et al. / Solid State Ionics 167 (2004) 41–47
The XRD pattern clearly indicates the polycrystalline nature of the ingredients. The X-ray diffraction pattern recorded at the room temperature for the LixV2O5 (x = 1.0 – 1.4) has been shown in Fig. 1. The XRD patterns are compared with JCPDS values of various lithium vanadate phases. It has been found that the LixV2O5 (x = 1.0, 1.2 and 1.4) have mixed phase of yLiV2O5 [JCPDS 34-1435] and g-LiV2O5 [JCPDS 46-0102]. The major diffraction peaks of the respective to y-LiV2O5 and g-LiV2O5 phases have been indicated in Fig. 1 and tabulated in Table 1 along with d’ spacing and hkl values. Table 1 shows the co-existence of y-LiV2O5 and g-LiV2O5 in the LixV2O5 (x = 1.0, 1.2 and 1.4). Fig. 2 shows the crystal structure of the y-LiV2O5 and g-LiV2O5 [taken from Ref. [22]]. The mixed phases y-LiV2O5 and g-LiV2O5 in chemically intercalated LiV2O5 with same lithium composition have already been reported by Delmas et al. [18]. It has been found that the intensity of the major diffraction peak (16.62j) of g-LiV2O5 phase is high for the high lithium content sample Li1.4V2O5 compared with other samples in the present study. This suggests that the g-LiV2O5 phase is the major component of high lithium content sample (Li1.4V2O5). This has been proven in the MAS -NMR and vibrational spectroscopic investigations of the present study and explained in the forthcoming discussions.
Fig. 2. Crystal structure of (a) y-LiV2O5, (b)g-LiV2O5.
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3.2. 7Li MAS-NMR analysis The Li+ ion has no field gradient associated with its own electrons, since the 1s-orbital does not participate in bonding and it is spherically symmetric. Hence, it can be used to probe the distribution of charges and micro-structural features of the lithium compounds. In the present work, 7Li with spin 3/2 has been used in the spin-echo mode systems. The 7Li MAS-NMR spectrum of LixV2O5 with x = 1.0 – 1.4 has been shown in Fig. 3a –c. The central transition spectrum has been observed between 10 and 0 ppm due to the + 1/2 ! 1/2 transition and the side bands are observed due to the rotation in MAS technique and called as rotation sideband. In the present study, the LixV2O5 (x = 1.0, 1.2 and 1.4) shows two well-defined central transition peaks. Generally, the multiple central transition peaks of Li MAS-NMR have been observed for the lithium compounds having multiple Li environments. The central transition peak observed at 1 ppm is due to the lithium ions located in the LiO6 octahedral co-ordination. The LiO6 octahedral unit is the basic skeleton of the g-LiV2O5 structure and presence of 1-ppm peak in the present studies shows the formation of g-LiV2O5 [19]. At this point, comparison is made with the g-LiV2O5 signatures in the NMR analysis reported by Stallworth et al. [20] and the present results are found to be in good agreement. The central transition peak observed at 10 ppm is due to the tetrahedral co-coordinated Li ions. The presence of the tetrahedral coordinated Li ions is due to the formation of y-LiV2O5 in the present study. Hirschinger et al. [21] has also observed the 7Li MAS-NMR peak near 10 ppm for the y-LiV2O5 prepared by high temperature fusion. Hence, the peaks at 1 and 10 ppm can be ascribed to the g LiV2O5 and y-LiV2O5, respectively. The co-existence of the g -LiV2O5 and y-LiV2O5 in the lithium domain (1 < x>1.6) has been reported previously by Cocciantelli et al. [22]. The phase diagram of the LixV2O5 for various lithium composition has been reported by Delmas et al. [18] and the present study well agrees with this phase diagram. The intensity of the peak at 1 ppm is found to be increasing with increase in lithium content, which reveals the transformation of y-LiV2O5 to g-LiV2O5 with lithium incorporation. This transformation is may be due to the production of new sites for Li ions by the distortion of VO5 pyramids of y-LiV2O5 and result in the formation of LiO6 octahedra and hence forming g-LiV2O5 with more lithium incorporation [20]. Further, the same type of y ! g irreversible transformation has also been reported by Cocciantelli et al. [22] in chemically lithium intercalated V2O5. The dipolar broadening of the central transition peaks of the present study has been found to increase with increase in lithium content. The insertion of lithium into V2O5 lattice results in the reduction of valence state of vanadium from V5+ to V4+, and hence, high lithium content samples have higher concentration of V4 + atoms [5]. The V4 + ions have
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the magnetic spin nature, which can strongly interact with Li+ ions, which will give rise to the dipolar broadening of the peaks [23,24]. Hence, the MAS-NMR peaks of the high lithium content samples have become broader than the less lithium content samples. However, varied results are reported for the dipolar broadening and chemical shift of the 7Li NMR peaks for the lithium vanadate prepared by various techniques [20 – 24], since formation of various phases and Li environments in lithium vanadate strongly depends on its preparation procedures. In the present work, the chemical shift of the 7 Li MAS-NMR peaks is relatively smaller than chemical shifts of the electrochemically prepared lithium vanadate reported by Cocciantelli et al. [23]. This may arise from the difference in the preparation methods, i.e., high temperature melting and electrochemical preparation. In the present work, high temperature melting may cause the difference in electronic states around Li nucleus and hence relatively lower chemical shift of the peaks. 3.3. Laser Raman spectroscopic analysis
Fig. 3. (a – c) 7Li MAS-NMR spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c) Li1.4V2O5.
The Raman spectra provide additional insight into the nature of the structural changes from the y phase through the g phase [25]. Hence, it has been utilized to shed more lights on the structural and phase analysis of LixV2O5 (x = x = 1.0– 1.4). The Raman spectra of the lithium vanadate are more complex than that of the pure V2O5 in the high frequency region. Since the Raman spectra of the different phases of LixV2O5 are expected to give their respective peaks in high frequency region. The pure V2O5 shows single high intense peak at 994 cm 1, which is ascribed to the Ag symmetric vibrations of the VMO3 double bond [26]. The insertion of lithium atoms may break down some of the coupling between pyramid chains and increase occupancy of the unit cell in the lithium vanadate. As a result, more vibrational modes become active in the Raman spectra of lithium vanadate. Hence, in the present study, the Raman spectroscopic results are analyzed in the high frequency region (950 – 1030 cm 1). Fig. 4 shows the Raman spectra of LixV2O5 (x = 1.0, 1.2 and 1.4) samples. The peak observed at 992 cm 1 is common peak for all the samples, which arises from the Ag symmetric vibrations of the VMO3 double bond, which is also present in pure V2O5 [27]. The low intensity peak at 982 cm 1 has been observed in the shoulder of the main intensity peak at 992 cm 1. This peak is ascribed to the Ag symmetric vibrations of the VMO double bond and known to be specific signature of the g-LiV2O5 [25]. The intensity of the peak is found to increase with increase in lithium content. This may be due to the transformation of y phases of LiV2O5 to the g-LiV2O5 as described previously. The medium intensity peaks observed at 956 and 971 cm 1 can be ascribed to the presence of y phases of LiV2O5. These two bands observed at 956 and 971 cm 1 probably contain contributions from the motion of the corner sharing oxygen
M. Vijayakumar et al. / Solid State Ionics 167 (2004) 41–47
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The pure V2O5 shows strong peak at 1020 cm 1, which ˚) is characteristic peak of the shortest VMO bond (1.58 A [28]. This characteristic peak of the vanadium pentoxide has been shifted to lower wave number 957 cm 1 for the lithium vanadate in the present study. The presence of band at 972 cm 1 on the shoulder of the characteristic band (VMO) at 957 cm 1 shows the presence of g-LiV2O5 in the samples under present investigations [25]. This band (972 cm 1) is due to the V –O stretching vibrations of VO5 square pyramids, which is the main framework of the gLiV2O5 structure [29,30]. Further, Surca et al. [31] reported that this band is due to presence of V4+ – O group modes due to Li interactions with V – O bond during lithium intercalation. In the present study, the intensity of this band finds to increase with increase in lithium content reveals the transformation of y-LiV2O5 to g-LiV2O5 upon lithium doping. The structure of the y-LiV2O5 is more similar to pure V2O5 except that there is a shifting of atomic positions by 1/2c for alternate layers. The strong band observed at 493 cm 1 for the pure V2O5 due to B2u symmetric stretching vibrations of VMO bond has been shifted to 472 cm 1 for the y-LiV2O5 [25]. The presence of medium intensity band at 472 cm 1 reveals the presence of y-LiV2O5 in the samples under
Fig. 4. Laser Raman spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c) Li1.4V2O5.
atom and the double oxygen atom in the VO5 pyramids of yLiV2O5 , which undergoes positional shift during lithium incorporation. 3.4. Fourier Transform Infrared spectroscopic analysis The Fourier Transform Infrared (FTIR) spectroscopy is proved to be a versatile tool to analyze the cathode materials, since it gives the information about the effect of intercalation atom on the host structure. Hence, the lithium vanadate samples are subjected to FTIR analysis to analyze the effect of lithium doping on the vanadium pentoxide host structure. Fig. 5 shows the FTIR spectrum of lithium vanadate in the frequency region 400 – 1500 cm 1. Modes in the region of 400 –1000 cm 1 are believed to arise from the vanadium – oxygen stretching vibrations [28]. The FTIR spectrum has been compared with previously reported chemically intercalated lithium vanadate samples and found to be in good agreement [25].
Fig. 5. FTIR spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c)Li1.4V2O5.
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Table 2 Identified Phases of Lithium vanadate with various results 7
Sample
Li MAS-NMR chemical shift
10 ppm (s) 1 ppm (m) 10 ppm (m) 1 ppm (s) 10 ppm (m) 1 ppm (s)
Li1.0V2O5 Li1.2V2O5 Li1.4V2O5
Laser Raman wave number 1
971cm (m) 982 cm 1 (w) 971 cm 1 (m) 982 cm 1 (w) 971 cm 1 (m) 982 cm 1 (w)
FTIR wave number 472 972 472 972 472 972
1
cm cm 1 cm 1 cm 1 cm 1 cm 1
(m) (m) (w) (m) (w) (m)
Phase y-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
s—strong intensity peak; m—medium intensity peak; w—weak intensity peak.
investigation. These results well match with previously explained XRD and NMR results of the lithium vanadate. The vanadium oxygen bonding is the key factor in the formation of various phases of LiV2O5. Since the formation of tetrahedral and/or octahedral arrangements of vanadium and oxygen molecules strongly depends on its valence states and bonding nature [32]. In the present study, the formation of g-LiV2O5 and y-LiV2O5 has been identified and this is due to the formation of different bonding nature and valence states of vanadium and oxygen atoms during lithium incorporation. The detailed analysis of the valence states of vanadium and oxygen in lithium vanadate by XPS and AES method has been reported elsewhere [33]. To shed further light on the vanadium and oxygen bond nature, the force constant calculations have been carried out in the present study. The theoretical wave number of vibrational modes in the IR spectra can be represented by the following relation [34], 1 f 1=2 t¯ ¼ ð1Þ 2pc l
to V2O5 lattice. Hence, insertion of lithium in V2O5 results in increase of V –O bond and facilitates the formation of various phases of LiV2O5. Further, the intense line at 828 cm 1 of V2O5, attributed to V – O –V bridges, shifted to lower wave number 804 cm 1 in the spectrum of lithium vanadate is a clear evidence of reduction of vanadium oxidation state due to the presence of lithium [40]. The reduction in vanadium oxidation state is also due to the high temperature melting of the raw materials in solid state reaction method. It has been reported that the melting of V2O5 at high temperatures will results in oxygen deficiencies [41]. These oxygen deficiencies can be compensated by the reduction in the oxidation state of vanadium (V5+ to V4+). The shift of 828-cm 1 peak also indicates that the lithium ion reduces the interaction between vanadium and oxygen atoms [42,43]. Table 2 shows the summary of the results along with the possible phases identified in LixV2O5 (x = 1.0, 1.2 and 1.4).
where t¯ is the wave number, c the velocity of light, f force constant of the bond and l the reduce mass of V –O bond which is given by the following expression,
The formation of y-LiV2O5 and g-LiV2O5 phases in LixV2O5 (x = 1.0, 1.2 and 1.4) prepared by solid state reaction method has been identified by XRD analysis. The 7 Li MAS-NMR analysis shows the transformation of y phase to g phase with lithium incorporation, which are due to the formation of LiO6 octahedra and rearrangement of the VO5 pyramids. The FTIR analysis shows the increase in the bond length of VMO bond with lithium addition, and this may be due to the formation of V4+ ions in lithium vanadate. This increase in bond length influences the transformation of y phase to g phase in lithium vanadate.
l¼
mA mo mA þ mo
ð2Þ
where mA and mo are the reduced masses of the cation and anion, respectively. The force constants are calculated for all the samples from Eq. (2). The wave number is taken as 957 cm 1 which is ascribed to VMO stretching vibration and reduced mass of this bond has been calculated to be 20.22 10 27 kg U [35]. The force constant can be related to the bond length by the equation [36], f ¼ 17=r
3
4. Conclusion
Acknowledgements
ð3Þ
The bond length of the VMO in the lithium vanadate is found to be increases with increase in lithium content. The high value of bond length (0.251 nm) has been observed for Li1.4V2O5 which is higher than the bond length (0.158 nm) of the pure V2O5 [37,38]. The insertion of lithium into V2O5 results in the transition of vanadium in V5+ state to V4+ state [5]. The ionic radius of V4+ is larger than that of V5+ [39], hence, the bond length increases with insertion of lithium in
Author MV would like to thank the Council of Scientific and Industrial Research (CSIR), Government of India for the award of Senior Research Fellowship.
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Li MAS-NMR and vibrational spectroscopic investigations of LixV2O5 (x=1.0, 1.2 and 1.4) M. Vijayakumar a, S. Selvasekarapandian a,*, Koichi Nakamura b, Tatsuo Kanashiro b, R. Kesavamoorthy c
a
Solid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore-641 046, Tamilnadu, India b Department of Physics, Faculty of Science, Tokushima University, 2-1 Minami Josanjima-Cho, Tokushima 770-8506, Japan c Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, India Received 25 July 2003; received in revised form 25 November 2003; accepted 5 December 2003
Abstract The lithium vanadate LixV2O5 (x = 1.0, 1.2 and 1.4) has been prepared by solid state reaction method. The 7Li MAS-NMR, Laser Raman and Fourier Transform Infrared (FTIR) spectroscopic analysis reveals the y-LiV2O5 and g-LiV2O5 phase formation. The 7Li MAS-NMR spectrum shows the chemical shift around 1 and 10 ppm, which confirms the presence of g and y phases of lithium vanadate. The formation of y phase and g phase of LiV2O5 has been detected by the presence of Raman peaks at 971 and 982 cm 1. The phase transformation from y to g phases of LiV2O5 with lithium incorporation has been identified. The reduction in the oxidation state of vanadium (V5+ to V4+) has been identified, due to lithium incorporation. This reduction affects the vanadium oxygen bonding nature and leads to the phase transformations. D 2004 Elsevier B.V. All rights reserved. PACS: 61.18.Fs; 78.30 Keywords: Lithium battery; 7Li MAS-NMR spectra; FTIR analysis; Laser Raman Analysis; Ionic conductors
1. Introduction The growing desire for portable electronic devices and rechargeable power sources has fueled a strong interest in lithium batteries [1]. The development of high-performance lithium batteries is intimately linked to the availability of efficient electrode materials. The transition metal oxides are the most attractive electrode materials for batteries because of its layered structure which is useful for the intercalation [2,3]. The lithium vanadate LiV2O5 attracts a great deal of attention because of its useful properties such as high conductivity of lithium ion to gain a high cathode utility of a cell and mixed conduction to increase the charge– discharge efficiency [4,5]. Different types of synthesis procedure are reported such as sol – gel method, xerogels, solid state reaction, electro-
* Corresponding author. Tel.: +91-422-2422222x422; fax: +91-4222422387. E-mail address: [email protected] (S. Selvasekarapandian). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.008
chemical insertion of lithium ions for the preparation of lithium vanadate LiV2O5 [6– 9]. However, each procedure yields different phases of lithium vanadate such as y, q, and g-LiV2O5 depending upon the amount of lithium incorporated. Pecquenard et al. [10] reported the identification of LiV2O5 phases by EPR spectroscopy. The 7Li MAS-NMR analysis is known to be an advanced technique, which gives the information about the local Li environment in Li ion conducting samples [10]. Recently, Fujiwara et al. [11,12] reported the 7Li and 51V NMR analysis of the lithium vanadate with multiple phases prepared by chemical intercalation. The vibrational spectroscopic techniques Laser Raman and FTIR spectra are efficient tool to analyze various structures, and various phases of lithium vanadate have been identified by these methods by various researchers [13,14]. However, these studies are reported for the chemically and electrochemically prepared LiV2O5. The 7Li MAS-NMR and vibrational spectroscopic analysis of high temperature lithium vanadate is scarce. The high temperature preparation (solid state reaction) is a simple and cost-effective method. However, the lithium
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Fig. 1. X-Ray diffraction pattern of LixV2O5 at room temperature LixV2O5 (x = 1.0, 1.2 and 1.4) {*—g-LiV2O5 and O—y-LiV2O5}.
evaporation during the synthesis is the serious draw back of this procedure and this has been overcome by addition of excess raw materials [15]. Further, the heat treatment of raw material V2O5 results in the oxygen vacancies and influences the formation of various phases in lithium vanadate. The heterogeneous phase formation of lithium vanadate during solid state synthesis has been reported previously [16]. It is essential to characterize lithium vanadate prepared by solid state reaction which contains heterogeneous phases to tailor its usage in secondary lithium battery. In this paper, the results of 7Li MAS-NMR and vibrational spectroscopic studies on lithium vanadate LixV2O5 (x = 1.0, 1.2, 1.4) prepared by solid state reaction method have been presented. The x value used in this paper represents the nominal value of lithium content in the material.
2. Experimental procedure Polycrystalline LixV2O5 (x = 1.0, 1.2, 1.4) has been prepared by conventional solid state reaction method. The stoichiometric amount raw materials Li2CO3 (Aldrich, USA) and V2O5 (Aldrich) are grinded into fine powders using mortar and pestle, and melted in porcelain crucible and the melt is cooled slowly. The detailed preparation procedure has been described elsewhere [17]. The lithium
losses during high temperature fusion have been overcome by taking excess lithium carbonate as raw material. The composition analysis have been carried out by XPS and AES analysis and found to be in good agreement with theoretical stoichiometric values [15]. X-ray diffraction patterns have been recorded using a Philips X-ray generator (Model PW 1390) with a Ni filter and CuKa radiation ˚ ) at 40 kV and 20 mA in the 2h range of (k = 1.5418 A 10j to 80j. The resultant diffraction pattern has been compared with Joint Commission for Powder Diffraction Standard (JCPDS) data of the lithium vanadate samples. The 7 Li MAS-NMR measurement has been carried out by using Bruker Avance 300 spectrometer with a 4-mm probe. The spinning speed has been maintained as 7 kHz. The MASNMR spectra at room temperature were measured at 116.672 MHz. The FTIR spectrum has been recorded in the wave number range of 400 –4000 cm 1 by using Perkin Elmer Spectrum RX1 equipment with KBr pellet method. For the Laser Raman analysis, the pellets with 0.1-cm thickness and 1.0-cm diameter of lithium vanadate have been prepared by cold-pressed method. Raman spectrometer built around a double grating monochromator SPEX model 14018 is used to record Raman spectra of these pellets in the backscattering geometry. A 30-mW 488-nm laser beam from Ar+ laser (coherent, USA) is focused on to the sample to a spot size of about 100-Am diameter. A thermoelectrically cooled photomultiplier tube ITT-FW 130 detected the scattered light after passing through the monochromator. The spectral resolution of the monochromator was 4.2 cm 1. The spectra are recorded digitally using a microprocessor-based automated data collection system with a step of 1 cm 1 and a collection time of 10 s. The commercial software PEAKFIT has been used to fit the Lorentzian line shapes along with straight base line to the Raman spectra from which the peak position and full width at half maximum (FWHM) of the Raman peak are obtained.
3. Results and discussions 3.1. X-ray diffraction analysis The lithium vanadate prepared by solid state reaction method has been subjected to the X-ray diffraction analysis.
Table 1 X-ray diffraction parameters for LixV2O5 (x = 1.0, 1.2 and 1.4) Li1.0V2O5
Li1.2V2O5
Li1.4V2O5
2h
hkl
˚) d (A
Assignment
2h
hkl
˚) d (A
Assignment
2h
hkl
˚) d (A
Assignment
16.62 17.83 26.83 27.68 29.85 31.32
002 010 103 111 310 112
5.333 4.974 3.322 3.222 2.989 2.857
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
16.62 17.83 26.81 27.66 29.83 31.31
002 010 103 111 310 112
5.333 4.974 3.325 3.221 2.995 2.856
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
16.62 17.85 26.83 27.68 29.82 31.31
002 010 103 111 310 112
5.333 4.968 3.322 3.222 2.996 2.856
g-LiV2O5 y-LiV2O5 g-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
M. Vijayakumar et al. / Solid State Ionics 167 (2004) 41–47
The XRD pattern clearly indicates the polycrystalline nature of the ingredients. The X-ray diffraction pattern recorded at the room temperature for the LixV2O5 (x = 1.0 – 1.4) has been shown in Fig. 1. The XRD patterns are compared with JCPDS values of various lithium vanadate phases. It has been found that the LixV2O5 (x = 1.0, 1.2 and 1.4) have mixed phase of yLiV2O5 [JCPDS 34-1435] and g-LiV2O5 [JCPDS 46-0102]. The major diffraction peaks of the respective to y-LiV2O5 and g-LiV2O5 phases have been indicated in Fig. 1 and tabulated in Table 1 along with d’ spacing and hkl values. Table 1 shows the co-existence of y-LiV2O5 and g-LiV2O5 in the LixV2O5 (x = 1.0, 1.2 and 1.4). Fig. 2 shows the crystal structure of the y-LiV2O5 and g-LiV2O5 [taken from Ref. [22]]. The mixed phases y-LiV2O5 and g-LiV2O5 in chemically intercalated LiV2O5 with same lithium composition have already been reported by Delmas et al. [18]. It has been found that the intensity of the major diffraction peak (16.62j) of g-LiV2O5 phase is high for the high lithium content sample Li1.4V2O5 compared with other samples in the present study. This suggests that the g-LiV2O5 phase is the major component of high lithium content sample (Li1.4V2O5). This has been proven in the MAS -NMR and vibrational spectroscopic investigations of the present study and explained in the forthcoming discussions.
Fig. 2. Crystal structure of (a) y-LiV2O5, (b)g-LiV2O5.
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3.2. 7Li MAS-NMR analysis The Li+ ion has no field gradient associated with its own electrons, since the 1s-orbital does not participate in bonding and it is spherically symmetric. Hence, it can be used to probe the distribution of charges and micro-structural features of the lithium compounds. In the present work, 7Li with spin 3/2 has been used in the spin-echo mode systems. The 7Li MAS-NMR spectrum of LixV2O5 with x = 1.0 – 1.4 has been shown in Fig. 3a –c. The central transition spectrum has been observed between 10 and 0 ppm due to the + 1/2 ! 1/2 transition and the side bands are observed due to the rotation in MAS technique and called as rotation sideband. In the present study, the LixV2O5 (x = 1.0, 1.2 and 1.4) shows two well-defined central transition peaks. Generally, the multiple central transition peaks of Li MAS-NMR have been observed for the lithium compounds having multiple Li environments. The central transition peak observed at 1 ppm is due to the lithium ions located in the LiO6 octahedral co-ordination. The LiO6 octahedral unit is the basic skeleton of the g-LiV2O5 structure and presence of 1-ppm peak in the present studies shows the formation of g-LiV2O5 [19]. At this point, comparison is made with the g-LiV2O5 signatures in the NMR analysis reported by Stallworth et al. [20] and the present results are found to be in good agreement. The central transition peak observed at 10 ppm is due to the tetrahedral co-coordinated Li ions. The presence of the tetrahedral coordinated Li ions is due to the formation of y-LiV2O5 in the present study. Hirschinger et al. [21] has also observed the 7Li MAS-NMR peak near 10 ppm for the y-LiV2O5 prepared by high temperature fusion. Hence, the peaks at 1 and 10 ppm can be ascribed to the g LiV2O5 and y-LiV2O5, respectively. The co-existence of the g -LiV2O5 and y-LiV2O5 in the lithium domain (1 < x>1.6) has been reported previously by Cocciantelli et al. [22]. The phase diagram of the LixV2O5 for various lithium composition has been reported by Delmas et al. [18] and the present study well agrees with this phase diagram. The intensity of the peak at 1 ppm is found to be increasing with increase in lithium content, which reveals the transformation of y-LiV2O5 to g-LiV2O5 with lithium incorporation. This transformation is may be due to the production of new sites for Li ions by the distortion of VO5 pyramids of y-LiV2O5 and result in the formation of LiO6 octahedra and hence forming g-LiV2O5 with more lithium incorporation [20]. Further, the same type of y ! g irreversible transformation has also been reported by Cocciantelli et al. [22] in chemically lithium intercalated V2O5. The dipolar broadening of the central transition peaks of the present study has been found to increase with increase in lithium content. The insertion of lithium into V2O5 lattice results in the reduction of valence state of vanadium from V5+ to V4+, and hence, high lithium content samples have higher concentration of V4 + atoms [5]. The V4 + ions have
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the magnetic spin nature, which can strongly interact with Li+ ions, which will give rise to the dipolar broadening of the peaks [23,24]. Hence, the MAS-NMR peaks of the high lithium content samples have become broader than the less lithium content samples. However, varied results are reported for the dipolar broadening and chemical shift of the 7Li NMR peaks for the lithium vanadate prepared by various techniques [20 – 24], since formation of various phases and Li environments in lithium vanadate strongly depends on its preparation procedures. In the present work, the chemical shift of the 7 Li MAS-NMR peaks is relatively smaller than chemical shifts of the electrochemically prepared lithium vanadate reported by Cocciantelli et al. [23]. This may arise from the difference in the preparation methods, i.e., high temperature melting and electrochemical preparation. In the present work, high temperature melting may cause the difference in electronic states around Li nucleus and hence relatively lower chemical shift of the peaks. 3.3. Laser Raman spectroscopic analysis
Fig. 3. (a – c) 7Li MAS-NMR spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c) Li1.4V2O5.
The Raman spectra provide additional insight into the nature of the structural changes from the y phase through the g phase [25]. Hence, it has been utilized to shed more lights on the structural and phase analysis of LixV2O5 (x = x = 1.0– 1.4). The Raman spectra of the lithium vanadate are more complex than that of the pure V2O5 in the high frequency region. Since the Raman spectra of the different phases of LixV2O5 are expected to give their respective peaks in high frequency region. The pure V2O5 shows single high intense peak at 994 cm 1, which is ascribed to the Ag symmetric vibrations of the VMO3 double bond [26]. The insertion of lithium atoms may break down some of the coupling between pyramid chains and increase occupancy of the unit cell in the lithium vanadate. As a result, more vibrational modes become active in the Raman spectra of lithium vanadate. Hence, in the present study, the Raman spectroscopic results are analyzed in the high frequency region (950 – 1030 cm 1). Fig. 4 shows the Raman spectra of LixV2O5 (x = 1.0, 1.2 and 1.4) samples. The peak observed at 992 cm 1 is common peak for all the samples, which arises from the Ag symmetric vibrations of the VMO3 double bond, which is also present in pure V2O5 [27]. The low intensity peak at 982 cm 1 has been observed in the shoulder of the main intensity peak at 992 cm 1. This peak is ascribed to the Ag symmetric vibrations of the VMO double bond and known to be specific signature of the g-LiV2O5 [25]. The intensity of the peak is found to increase with increase in lithium content. This may be due to the transformation of y phases of LiV2O5 to the g-LiV2O5 as described previously. The medium intensity peaks observed at 956 and 971 cm 1 can be ascribed to the presence of y phases of LiV2O5. These two bands observed at 956 and 971 cm 1 probably contain contributions from the motion of the corner sharing oxygen
M. Vijayakumar et al. / Solid State Ionics 167 (2004) 41–47
45
The pure V2O5 shows strong peak at 1020 cm 1, which ˚) is characteristic peak of the shortest VMO bond (1.58 A [28]. This characteristic peak of the vanadium pentoxide has been shifted to lower wave number 957 cm 1 for the lithium vanadate in the present study. The presence of band at 972 cm 1 on the shoulder of the characteristic band (VMO) at 957 cm 1 shows the presence of g-LiV2O5 in the samples under present investigations [25]. This band (972 cm 1) is due to the V –O stretching vibrations of VO5 square pyramids, which is the main framework of the gLiV2O5 structure [29,30]. Further, Surca et al. [31] reported that this band is due to presence of V4+ – O group modes due to Li interactions with V – O bond during lithium intercalation. In the present study, the intensity of this band finds to increase with increase in lithium content reveals the transformation of y-LiV2O5 to g-LiV2O5 upon lithium doping. The structure of the y-LiV2O5 is more similar to pure V2O5 except that there is a shifting of atomic positions by 1/2c for alternate layers. The strong band observed at 493 cm 1 for the pure V2O5 due to B2u symmetric stretching vibrations of VMO bond has been shifted to 472 cm 1 for the y-LiV2O5 [25]. The presence of medium intensity band at 472 cm 1 reveals the presence of y-LiV2O5 in the samples under
Fig. 4. Laser Raman spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c) Li1.4V2O5.
atom and the double oxygen atom in the VO5 pyramids of yLiV2O5 , which undergoes positional shift during lithium incorporation. 3.4. Fourier Transform Infrared spectroscopic analysis The Fourier Transform Infrared (FTIR) spectroscopy is proved to be a versatile tool to analyze the cathode materials, since it gives the information about the effect of intercalation atom on the host structure. Hence, the lithium vanadate samples are subjected to FTIR analysis to analyze the effect of lithium doping on the vanadium pentoxide host structure. Fig. 5 shows the FTIR spectrum of lithium vanadate in the frequency region 400 – 1500 cm 1. Modes in the region of 400 –1000 cm 1 are believed to arise from the vanadium – oxygen stretching vibrations [28]. The FTIR spectrum has been compared with previously reported chemically intercalated lithium vanadate samples and found to be in good agreement [25].
Fig. 5. FTIR spectrum at room temperature for (a) Li1.0V2O5, (b) Li1.2V2O5, (c)Li1.4V2O5.
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Table 2 Identified Phases of Lithium vanadate with various results 7
Sample
Li MAS-NMR chemical shift
10 ppm (s) 1 ppm (m) 10 ppm (m) 1 ppm (s) 10 ppm (m) 1 ppm (s)
Li1.0V2O5 Li1.2V2O5 Li1.4V2O5
Laser Raman wave number 1
971cm (m) 982 cm 1 (w) 971 cm 1 (m) 982 cm 1 (w) 971 cm 1 (m) 982 cm 1 (w)
FTIR wave number 472 972 472 972 472 972
1
cm cm 1 cm 1 cm 1 cm 1 cm 1
(m) (m) (w) (m) (w) (m)
Phase y-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5 y-LiV2O5 g-LiV2O5
s—strong intensity peak; m—medium intensity peak; w—weak intensity peak.
investigation. These results well match with previously explained XRD and NMR results of the lithium vanadate. The vanadium oxygen bonding is the key factor in the formation of various phases of LiV2O5. Since the formation of tetrahedral and/or octahedral arrangements of vanadium and oxygen molecules strongly depends on its valence states and bonding nature [32]. In the present study, the formation of g-LiV2O5 and y-LiV2O5 has been identified and this is due to the formation of different bonding nature and valence states of vanadium and oxygen atoms during lithium incorporation. The detailed analysis of the valence states of vanadium and oxygen in lithium vanadate by XPS and AES method has been reported elsewhere [33]. To shed further light on the vanadium and oxygen bond nature, the force constant calculations have been carried out in the present study. The theoretical wave number of vibrational modes in the IR spectra can be represented by the following relation [34], 1 f 1=2 t¯ ¼ ð1Þ 2pc l
to V2O5 lattice. Hence, insertion of lithium in V2O5 results in increase of V –O bond and facilitates the formation of various phases of LiV2O5. Further, the intense line at 828 cm 1 of V2O5, attributed to V – O –V bridges, shifted to lower wave number 804 cm 1 in the spectrum of lithium vanadate is a clear evidence of reduction of vanadium oxidation state due to the presence of lithium [40]. The reduction in vanadium oxidation state is also due to the high temperature melting of the raw materials in solid state reaction method. It has been reported that the melting of V2O5 at high temperatures will results in oxygen deficiencies [41]. These oxygen deficiencies can be compensated by the reduction in the oxidation state of vanadium (V5+ to V4+). The shift of 828-cm 1 peak also indicates that the lithium ion reduces the interaction between vanadium and oxygen atoms [42,43]. Table 2 shows the summary of the results along with the possible phases identified in LixV2O5 (x = 1.0, 1.2 and 1.4).
where t¯ is the wave number, c the velocity of light, f force constant of the bond and l the reduce mass of V –O bond which is given by the following expression,
The formation of y-LiV2O5 and g-LiV2O5 phases in LixV2O5 (x = 1.0, 1.2 and 1.4) prepared by solid state reaction method has been identified by XRD analysis. The 7 Li MAS-NMR analysis shows the transformation of y phase to g phase with lithium incorporation, which are due to the formation of LiO6 octahedra and rearrangement of the VO5 pyramids. The FTIR analysis shows the increase in the bond length of VMO bond with lithium addition, and this may be due to the formation of V4+ ions in lithium vanadate. This increase in bond length influences the transformation of y phase to g phase in lithium vanadate.
l¼
mA mo mA þ mo
ð2Þ
where mA and mo are the reduced masses of the cation and anion, respectively. The force constants are calculated for all the samples from Eq. (2). The wave number is taken as 957 cm 1 which is ascribed to VMO stretching vibration and reduced mass of this bond has been calculated to be 20.22 10 27 kg U [35]. The force constant can be related to the bond length by the equation [36], f ¼ 17=r
3
4. Conclusion
Acknowledgements
ð3Þ
The bond length of the VMO in the lithium vanadate is found to be increases with increase in lithium content. The high value of bond length (0.251 nm) has been observed for Li1.4V2O5 which is higher than the bond length (0.158 nm) of the pure V2O5 [37,38]. The insertion of lithium into V2O5 results in the transition of vanadium in V5+ state to V4+ state [5]. The ionic radius of V4+ is larger than that of V5+ [39], hence, the bond length increases with insertion of lithium in
Author MV would like to thank the Council of Scientific and Industrial Research (CSIR), Government of India for the award of Senior Research Fellowship.
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