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EQCM measurements of solvent transport during Li+ intercalation in V2O5 xerogel films
Electrochimica Acta 45 (2000) 3757 – 3764 www.elsevier.nl/locate/electacta
EQCM measurements of solvent transport during Li+ intercalation in V2O5 xerogel films Eiichi Shouji 1, Daniel A. Buttry * Department of Chemistry, Uni6ersity of Wyoming, Laramie, WY82071 -3838, USA Received 3 August 1999; received in revised form 16 November 1999
Abstract The electrochemical quartz crystal microbalance technique has been used to monitor the mass changes that occur in acetonitrile electrolytes during electrochemically driven Li+ intercalation into V2O5 xerogel films prepared by aqueous sol–gel methods. The mass measurements made with the EQCM reveal that during reduction of the vanadium sites in the sol–gel oxide at low voltammetric scan rates ( 5 5 mV/s) the net compositional change corresponds to insertion of one Li+ per electron injected, with no accompanying solvent (i.e. no net mass change that can be attributed to solvent transfer into or out of the film). However, at higher scan rates ( \50 mV/s), increased mass changes during reduction are attributed to simultaneous acetonitrile transport into the V2O5 film. Based on previous descriptions of solvent swelling of V2O5 thin films in acetonitrile solutions, these results suggest the following: (1) during reduction at low scan rates the intercalation of solvated Li+ takes place simultaneously with some solvent expulsion, such that the net mass change corresponds essentially exclusively to intercalation of one Li+ per electron, (2) at higher scan rates, the rapid electromigration of Li+ into the film drives simultaneous solvent insertion, such that the net compositional change corresponds to a net gain of considerable amounts of solvent during the Li+ intercalation process. This solvent swelling leads to structural changes that allow larger amounts of solvent to transfer into the films during Li+ intercalation for some time after the high scan rate perturbation. The implications of these findings for Li+ secondary battery cathode materials are discussed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Vanadium pentoxide; Cathode; Li battery; EQCM; Quartz crystal microbalance
1. Introduction The high specific capacity of vanadium pentoxide (V2O5), coupled with its relatively attractive redox potential and ability to reversibly intercalate Li+ during reduction of the vanadium sites has made it the subject * Corresponding author. Tel.: +1-307-7666677; fax: +1307-7662807. E-mail address: [email protected] (D. A. Buttry). 1 Present address: Molecular Materials Research Center, Beckman Institute, Mail Code 139-74, California Institute of Technology, Pasadena, CA 91125, USA.
of much study for application as a cathode material in both primary and secondary Li batteries [1 – 17]. A primary goal of much of the recent work in the area has been to achieve the reversible reduction of all of the vanadium sites by two electrons each, so as to achieve the maximum possible charge storage capacity. In the crystalline material, only one of the two vanadium sites in the formula unit can be reversibly reduced at attractive voltages versus Li/Li+. We suspect this is because of irreversible structural changes that occur if more than one electron is injected per formula unit [18 – 21]. To circumvent this issue, several research groups have described the synthesis and characterization of amor-
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 4 7 1 - 0
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phous phases of V2O5 [2,6,22–27]. These amorphous materials can be produced in several ways, including sol–gel methods that give both xerogel and aerogel materials [3,13,15,16,24,28,29]. These sol–gel routes are quite simple, relying on acidification of solutions containing the VO− 3 species, and can be done under mild (e.g. room temperature) conditions. In some cases, it has been possible to achieve reversible charging and discharging corresponding to 3.3 electrons per formula unit [6]. Theoretical calculations suggest that if reversible charging of 4 electrons per formula unit could be achieved at reasonable voltages, the specific energy of a V2O5/Li battery could be as high as 1366 W h/kg [23]. This is much higher than that of many other transition metal-based oxide cathode materials [30,31], and the expectation that such specific energies might be achievable has driven much of the recent effort in the area. The structural changes that occur in crystalline V2O5 during deep charge/discharge cycles presumably arise from stresses that build up in the material. We suspect that these stresses originate from changes in coordination geometry and/or coordination number of the vanadium sites during the redox transitions. While these effects are likely to be responsible for some of the redox-induced structural changes, an additional effect that has been less well studied is the transfer of solvent during the intercalation and deintercalation processes. In fact, we are not aware of any systematic studies of this process for V2O5. The purpose of the present contribution is to report on such measurements for V2O5 xerogel films [32]. The critical measurement comprises monitoring mass changes during the redox process using the electrochemical quartz crystal microbalance (EQCM) [33–36]. A previous study has also reported on the mass changes during reduction of V2O5 xerogel films in propylene carbonate [23]. These were attributed to reversible Li+ intercalation with no simultaneous transfer of solvent. As will be seen below, similar conclusions are reached in the present study in acetonitrile for lower scan rates, while at higher scan rates, the data are consistent with net transfer of solvent into the films during reduction and out during oxidation. Taken together, the known data on this system suggest that a full understanding of the V2O5 compositional changes during charging and discharging will require consideration of solvent transport as well as Li+ intercalation and deintercalation.
2. Experimental
2.1. Materials Acetonitrile (AN) was purchased from Aldrich Chemical Inc., carefully purified by double distillation
in the presence of P2O5 and stored over 3 A, molecular sieves under Ar. Propylene carbonate (PC) was purchased as anhydrous grade from Aldrich Chemical Inc. and purified by vacuum distillation. 18 MV water from a Millipore purification train was used. Sodium metavanadate, lithium perchlorate (99.999% grade) and ion exchange resin (Dowex 50W-X2, 50 – 100 mesh) were purchased from Aldrich Chemical Inc. and used as received.
2.2. Electrochemical measurements Cyclic voltammograms (CVs) were measured using a Wenking style potentiostat that was locally designed and built. Data are reported versus a Ag/AgCl/NaCl(sat) reference electrode, which was used in all experiments. The area of the electrochemically active region of the working electrode was 0.34 cm2, while the piezoelectrically active region was 0.28 cm2 [34]. A platinum wire (diameter: 0.2 mm, length: 60 cm coil) was used as the counter electrode. Other conditions were: 25.0 9 0.1°C, scan rate: 5 – 200 mV/s, in the potential window from −1.0 V to 1.2 V using a quiescent acetonitrile solution containing 0.5 M LiClO4, which was extensively deaerated using Ar gas prior to the experiments.
2.3. Electrochemical quartz crystal microbalance The EQCM technique has been adequately described elsewhere [33 – 36], and experimental details will not be reviewed here. The proportionality constant relating the calculated mass changes to the experimentally observed frequency changes is 56.6 Hz cm2 mg − 1 for the 1 inch diameter 5 MHz, overtone polished crystals used in this study (Valpey – Fisher, Hopkinton, Mass.). The EQCM thin film gold electrodes were deposited by thermal evaporation using an oil diffusion pumped high-vacuum system (Edwards 306A) as follows: First, quartz crystals were chemically cleaned with a 50/50 (vol%) solution of 30% H2O2/concentrated H2SO4 for 30 min., rinsed copiously with deionized water and dehydrated alcohol, and stored under vacuum. Immediately prior to evaporation, they were plasma-etched with a N2/O2 plasma. Then, a thin Cr adhesion layer (ca. 50 A, ) was deposited, followed immediately by gold (ca. 2000 A, ). A specially tailored H-cell equipped with a number 9 vacuum o-ring joint for EQCM crystal mounting was used for the electrochemical measurements. Silicone o-rings were used to define the area of contact on the front face of the crystal and to prevent solvent contact on the back face. The gold-coated EQCM crystals were placed in AN solution containing 0.5 M LiClO4 as a supporting electrolyte. There was no evidence of electrochemical or gravimetric response from chromium oxide, which can occur if Cr diffuses through the Au layer. This is a result of the very thin Cr adhesion layer used.
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2.4. Synthesis of V2O5 xerogel and spin coating onto EQCM electrodes The V2O5 xerogel solution was prepared as previously described [37,38] using an ion exchange column to treat a solution of sodium metavanadate to produce the protic form, HVO3. This species readily polymerizes to produce a viscous sol–gel solution from which V2O5 can be cast by solvent evaporation. After preparation of the HVO3 solution, hydrolysis and aging were allowed to continue for at least two weeks. The resulting concentration of HVO3 was 10% (w/v). This solution was used to spin-coat xerogel films onto EQCM electrodes at a rotation rate of ca. 5000 rpm using a locally built spin-coater. Then, these electrodes were dried at room temperature for 1 h, followed by drying in an oven at 120°C for 20 h. This treatment gave uniform, adherent films. These films are xerogels, as opposed to aerogels, due to the method by which they are formed (for example, air drying as opposed to supercritical drying, respectively). By using a volumetric pipette to control the droplet volume of the V2O5 xerogel solution used for spin-coating, it was possible to make electrodes that had different thicknesses of the V2O5 layer. The thicknesses used in this study ranged from 50–200 nm, as determined using scanning electron micrographic
Fig. 1. Cyclic voltammograms (left) and EQCM frequency measurements (right) for spin coated V2O5 xerogel on EQCM gold electrode in AN solution containing 0.5 M LiClO4: scan rates are as shown. Curve (f) was done immediately following the sequence of scans (a)–(e).
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examination. These xerogel films are essentially insoluble in common organic solvents, such as acetone, acetonitrile, propylene carbonate, and chloroform. Low angle X-ray diffraction (Scintag XDS2000: CuKa, 40 kV, 30 mA, continuous scan, 1° 2 U/min, 0.01° 2 U /increment (0.60 seconds/point)) gives an interlayer spacing of ca.11.6 A, for all electrodes, consistent with previous reports for this material [37,38].
3. Results and discussion Fig. 1 shows the CVs and EQCM frequency data for a V2O5 film at a variety of scan rates. The left column shows electrochemical data and the right column shows EQCM frequency data. Curve (a) (left) shows the characteristic voltammetry of V2O5 that has been reported by several research groups [2,3,28,32]. This voltammetry is quite stable, and can be repeatedly measured with little change in the shape or magnitude of the current response. The corresponding EQCM frequency data (right) show that mass is gained during the reduction. Quantitative analysis of the total mass gain during reduction and the total reductive charge is consistent with intercalation of one Li+ per electron injected during the reduction process, as has been previously reported at such low scan rates [23]. As can be seen, the subsequent oxidation leads to mass loss, consistent with deintercalation of one Li+ per electron removed from the film during the oxidation process. Repeated scans at 5 mV/s showed essentially identical mass and current responses. Closer examination of the potential dependence of the frequency changes and reductive charges reveals that the majority of the mass gain occurs during the later parts of the reductive scans, predominantly under the second of the voltammetric features in the CV (i.e. under the wave centered at − 0.5 V). This suggests that, even though one would expect that Li+ ions must be inserting during the first reduction wave, there may be loss of mass that compensates for the Li+ insertion. This mass loss could easily be accounted for by expulsion of a small amount of solvent. In principle, the combined transport of solvent and ions can be dissected using isotopically labeled solvents [34]. However, several attempts at such measurements failed because the necessary precision could not be obtained from the measurements. Thus, these results offer only a suggestion that solvent transport may be contributing to the observed mass changes. However, more discussion of this point is presented below. Before proceeding with a discussion of the scan rate dependence, it is worthwhile to first examine the dependence of film thickness on the observed mass changes. Table 1 shows EQCM frequency data from four separate V2O5 films with different film thicknesses. The
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Table 1 Observed and calculated frequency changes for Li+ intercalation into V2O5 Sample number 1 2 3 4
DF (Hz) 1072 1263 1415 1562
Dfobs (Hz) 65 80 87 95
amount of V2O5 on each crystal was determined by measuring the total frequency change for the quartz crystal (in air and after drying at 120°C for 20 h) that occurred as a result of the spin-coating deposition of the xerogel film. This frequency change is indicated by DF, and is given in the second column of Table 1. As can be seen, this frequency change shows that the thickest film is roughly 1.5 times more massive than the thinnest film. Absolute film thickness measurements were complicated by lack of information on the refractive index and density of this xerogel material, and are not reported here. However, crude calculations suggest that these films have thicknesses in the 50–100 nm range. Table 1 also shows three additional frequency-related quantities, Dfobs, DfDF and DfQ. Respectively, these correspond to the frequency change observed during the reduction of the film at a scan rate of 5 mV/s (presumably due to Li+ intercalation, as discussed above), the frequency change expected to occur during this reduction based on the DF value (i.e. the total mass of V2O5 deposited during the spin-coating process) assuming one Li+ intercalated per electron injected and an extent of reduction of two electrons per V2O5 unit, and the frequency change expected to occur based on the electrochemical charge for the reduction and again assuming intercalation of one Li+ per electron. The calculation of DfDF additionally requires the molar mass of the vanadium pentoxide unit. It is known [13] that the actual formula for this material, when prepared as described in the experimental section, is V2O5·1.6H2O, so this value was used in the calculation. Quantitative analysis of EQCM frequency changes requires rigid film behavior on the part of the xerogel [34]. Previous studies [23] of spin-coated V2O5 xerogels in propylene carbonate solutions employed acoustic impedance measurements that verified this behavior. Further, the constant ratio of DF to Dfobs over a factor of 1.5 in film mass (Table 1) is also indicative of rigid film behavior, since one expects the mass of intercalated Li+ to scale linearly with the total film mass [34]. Thus, in the present study, rigid film behavior is assumed and the Sauerbrey equation is used to calculate mass changes from frequency changes, with the proportionality constant given in the Experimental Section 2.
DfDF (Hz) 72 85 95 105
DfQ (Hz) 63 78 89 97
DF/Dfobs 16.5 15.8 16.3 16.4
The data in the table show excellent agreement between the observed frequency change (mass uptake during reduction) and those predicted based on the assumptions given above. Specifically, the excellent agreement between Dfobs and DfQ clearly shows that during the reduction process at low scan rates one Li+ is intercalated per electron injected into the film with no net accompanying solvent transport, as previously reported for this material in propylene carbonate supporting electrolytes [23]. The good agreement between DfDF and the other two values suggests either that the extent of reduction represented by the voltammograms in Fig. 1 corresponds to one electron per vanadium site (the assumption used in calculating DfDF from DF, vide supra) or that the extent of reduction is greater than 1 e−/V site, but that the film is not 100% electroactive. These data do not allow these two possibilities to be distinguished. The present results indicate that net solvent transfer does not occur during the Li+ intercalation process at low scan rates in acetonitrile solutions. Note that this does not imply that bare, unsolvated Li+ intercalates during reduction, only that the total amount of solvent within the film before and after reduction remains the same. One way for this to occur would be for intercalation of solvated Li+ to cause expulsion of solvent molecules from within the V2O5 interlayer regions such that the net composition (and probably also the total volume) of the system remains constant. Another would be for unsolvated Li+ ions to intercalate after desolvation at the point of entry into the V2O5 interlayer region. In both cases, the behavior would seem to be driven by a tendency for the V2O5 material to resist increasing its free volume. This tendency of redox-active thin film systems to resist creation of additional free volume during redox-induced compositional changes has been previously discussed for organic polymers [35]. This behavior is not unexpected for the present case of a structurally rigid transition metal oxide lattice. Returning now to the scan rate dependence of the redox-induced compositional changes, one can see the voltammetric and EQCM frequency responses for higher scan rates in Fig. 1(b) – (e). Curve (f) shows the data for a slow (5 mV/s) scan rate experiment performed immediately after the sequence of scans (b) – (e).
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Fig. 2. Plots of cathodic (open symbols) and total charges (filled symbols) vs. scan rate for the four films listed in Table 1, all at 5 mV/s (see text for details). Film thicknesses within a set decrease from top to bottom.
This is representative of a scan performed immediately after a number of scans at higher scan rate. The mass changes in (b)–(e) nearly track the electrochemical charges. However, as the number of rapid scans increases, so does the mass gain per cycle for the reduction process, even though the reduction charge remains the same. This suggests that the compositional changes during these more rapid redox processes continue to have contributions from the intercalation of Li+ during reduction, but that additional mass is incorporated during reduction as a function of increasing cycle number. Following the imposition of several rapid scans, as shown in (f), one observes that considerably larger mass changes accompany the redox process at lower scan rates. This behavior, larger mass changes during Li+ intercalation and deintercalation at low scan rates after several reduction/oxidation cycles at higher scan rates, was observed for all of the samples examined as part of this study. Of the two possibilities, solvent insertion and
Fig. 3. Plots of ( ) cathodic charge, as calculated from Dfobs and () experimental cathodic charge vs. DF (see text for details).
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anion co-injection (i.e. with Li+), we favor attributing the excess mass to solvent transport. This is because the negative charge produced on the V2O5 lattice during reduction should comprise a potent electrostatic barrier that would effectively reject anions from entering the film. The origin of the excess mass gain observed after high scan rate cycling will be discussed further below. The magnitude of the excess mass gain following several rapid scan cycles is a roughly constant fraction (ca. 8 – 10%) of the total film mass. Assuming this excess mass is due to transport of AN molecules, this would correspond to transport of 0.22 moles of AN per mole of Li+ or a little less than one AN molecule per 4 Li+ ions. Thus, even though the excess mass gain is quite large (Dfobs increases by a factor of 2.3 while the charge stays constant), the ratio of moles of solvent co-transported per mole of Li+ is not that great, and certainly not close to the value of 4 expected for intercalation and deintercalation of Li+ fully solvated by AN [39]. However, it does represent a significant fraction (ca. 60%) of the total mass gained after a sequence of rapid scans. Fig. 1 also shows a few other interesting features. For example, the distortion in the voltammetric waves that is evident at higher scans rates is a clear indication that kinetic limitations prevent the facile reduction of the vanadium sites in the film. This distortion in the charge passage is mimicked in the mass change, showing that both electron injection and Li+ intercalation during reduction (and vice versa during oxidation) are hindered. It is also notable that the cyclic voltammogram after high scan rate cycling is virtually identical with the first low scan rate experiment, even though the mass change is markedly different. Fig. 2 shows the dependence of the charge harvested from the four films referred to in Table 1as a function of scan rate. Data are shown for both the cathodic charge and the total of the anodic and cathodic charges. These data show that these charge values decrease by 30 – 45% on going from a scan rate of 5 mV/s to a scan rate of 200 mV/s. The magnitude of the decrease depends somewhat on the total film thickness. Fig. 3 shows plots of charge versus DF for the four films referred to in Table 1, all at a scan rate of 5 mV/s and prior to any high scan rate cycling. The open circles show the experimentally measured cathodic charge. The solid circles show the cathodic charge (Qcalc,c) calculated from the measured mass gain during reduction (Dfobs), assuming that this mass gain arises solely from Li+ transport and that one Li+ is intercalated per electron injected into the film. Both plots are shown with best-fit linear regressions through the data points. That these linear regression fits pass through the origin shows that both the charges and mass gains during reduction are directly proportional to film thick-
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Fig. 4. Cyclic voltammograms of spin coated V2O5 xerogel in (a) AN and (b) PC containing 0.5 M LiClO4 at 5mV/s.
ness (i.e. to DF). Further, the good agreement between the experimental charge and Dfobs shows that the mass gains during reduction can be essentially completely accounted for by assuming that one Li+ intercalates for each electron injected into the film during reduction. However, as will be discussed further below, this does not imply that unsolvated Li+ ions are inserting into the film. Rather, it merely implies that the net mass change can be accounted for with the mass of inserted Li+ ions. A final experiment was to make a preliminary examination of the influence of the identity of the solvent on the rudimentary features of the electrochemical responses of these V2O5 sol–gel films. Fig. 4 show a comparison of the cyclic voltammetry in AN and propylene carbonate (PC). As can be seen from the increased peak separation in PC, the electrochemical kinetics seem less facile in PC compared to AN. Given that the results reported here for AN and those reported previously for PC [23] indicate essentially no net solvent transport accompanying the electromigration of Li+ into and out of the V2O5 interlayer spacing, these results might suggest that Li+ desolvation prior to intercalation does occur in both solvent systems, and that this is a more kinetically hindered process in PC.
4. Conclusions These results reveal several interesting features of the solvent transport behavior of V2O5 during its reduction and subsequent oxidation in acetonitrile solutions. First, the low scan rate, electrochemically induced intercalation of Li+ into the V2O5 interlayer spacing occurs with essentially no net change in AN content of the xerogel film, as was previously reported for PC [23]. Second, in AN after the films are cycled at higher scan
rates, solvent does appear to accompany Li+ as it moves into and out of the film. The change in solvent transport behavior seems likely to be due to structural changes of the V2O5 matrix that are driven in some way by the rapid electromigration of Li+ into and out of the film. This electrochemically induced change is irreversible in that the films are not observed to return to their original ‘low scan rate’ behavior. Since a consequence of this change is that solvent can reversibly enter and exit the film during Li+ intercalation and deintercalation, the film must be capable of reversibly changing its dimensions in order to create the volume necessary to accommodate solvent incorporation during Li+ intercalation, and also to consume the free volume created during solvent expulsion during Li+ deintercalation. We are uncertain of the origin of the structural changes that drive this change in solvent transport behavior, but one can speculate that they probably arise from the forced intercalation of Li+ at high rates. Whatever their origin, these structural changes will almost certainly have impact on the long term dimensional stability and electroactivity of these inorganic oxides. Third, as expected, the charge that can be harvested from these films decreases as scan rate is increased. This almost certainly is a consequence of slow Li+ electromigration within the interlayer spacing of the V2O5 sol – gel material. It is interesting to compare the solvent transport behavior in PC and AN with previous discussions of the extent of solvent incorporation into V2O5 films as measured by X-ray diffraction. As shown by Baffier and coworkers, the extent of solvent incorporation into V2O5·1.6H2O after simple immersion into the solvent (i.e. in absence of any redox chemistry) depends very much on the identity of the solvent [40]. Specifically, they showed that the interlayer spacing on immersion of V2O5·1.6H2O into AN is 11.5 A, (i.e. no change from that of the parent material) while that on immersion into PC is 21.5 A, . The very large spacing in PC was attributed to PC incorporation into V2O5·1.6H2O. In a related study, the exchange of alkali metal cations for H+ in V2O5·1.6H2O in several organic solvents was also demonstrated [41]. These ion exchange reactions are facile and occur relatively rapidly on immersion of V2O5·1.6H2O into an organic solvent containing the given cation. Unfortunately, PC and AN were not studied. However, for the four organic solvents that were investigated (N, N-dimethylformamide, N-methylformamide, formamide, and dimethylsulfoxide), in every case Li+ exchange into the interlayer spacing occurred with a large increase in the interlayer spacing (to between 16 and 19 A, ). This was interpreted as resulting from simultaneous incorporation of a significant amount of solvent with the Li+ cations due to the large, negative hydration energy of Li+ in high dielectric constant solvents. These results strongly suggest
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that similar behavior should prevail in both PC and AN, namely that the initial immersion of V2O5·1.6H2O into 0.5 M LiClO4 in either solvent should result in Li+ exchange for H+ from the V2O5·1.6H2O lattice with simultaneous incorporation of a significant amount of solvent. Given this situation, there should be more than enough free volume within the interlayer spacing in either AN or PC for solvated Li+ to intercalate during the electrochemically induced intercalation of Li+. However, since the present results and those previously described by Ward and coworkers [23] in propylene carbonate solution unequivocally show that there is no net solvent transport during Li+ intercalation (prior to rapid scan perturbations of the material), one can speculate that while solvated Li+ may actually be the species that intercalates, the free volume restrictions imposed by the relatively rigid V2O5·1.6H2O matrix lead to the simultaneous expulsion of solvent that had been within the interlayer spacing, such that the net solvent transport is extremely small. In other words, assuming that the lattice is rigid and cannot increase its volume to accommodate Li+ intercalation and also that Li+ is solvated both prior to and after its entry into the V2O5 interlayer region, the mass change expected for AN expulsion due to Li+ intercalation is related only to the molar volume of Li+ itself (plus any small, second order effects from electrostriction of AN in the Li+ solvation sphere). Using the ionic radius for Li+ of 0.68 A, gives a molar volume for Li+ of 0.79 cm3/mole, while that for AN is 52.24 cm3/mole. Thus, since the molar volume of Li+ is only 1.5% of that for AN, one would not expect significant mass changes from AN displacement by Li+ if the lattice were to be unable to accommodate volumetric changes due to Li+ intercalation. As seen in Fig. 4, only very subtle differences are observed for the redox behavior of V2O5 in PC and AN. The fact that the electrochemical behavior in PC is less kinetically facile in comparison to that in AN may be the result of slower ion transport in PC, due to greater solvent friction and a somewhat larger solvation sphere. This is likely due to the higher dielectric constant of PC compared to that for AN (64 versus 36, respectively [42]). It is especially significant that the apparent reduction potentials of V2O5 in PC and AN are very nearly identical. This implies that the solvation energy for Li+ likely is not included in the apparent reduction potential in these two solvents. This is consistent with the arguments presented above that Li+ cations do not desolvate prior to entry into the interlayer region in either solvent. Several results in this study have relevance to the potential use of V2O5 and related transition metal oxides as cathode materials in Li secondary batteries. The electrochemically induced changes described above that occur when Li+ electromigration into the interlayer
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spacing is driven at high rates is likely to have a highly detrimental effect on the long term dimensional stability of the material. It seems likely that such structural changes will ultimately lead to loss of conductive pathways within the material which will degrade both the charge storage capacity and the rate of charge extraction. The solvent incorporation and ion exchange processes that occur when this material is immersed into an alkali metal-containing supporting electrolyte imply that the effective molar mass of the material that is used in specific energy and specific capacity calculations should probably include the masses of these species. Finally, taken in total, the results of this study and previous studies strongly suggest that understanding the transport of charge compensating cations within the interlayer spacing of such layered transition metal oxides will necessarily involve understanding the degree of solvation of the cations and how their transport properties are influenced by solvation.
Acknowledgements This work was supported in full by the Office of Naval Research. Valuable discussions with Kevin White and Dr John Pope are also acknowledged.
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[15] J. Legendre, J. Livage, J. Colloid Interface Sci. 94 (1983) 75. [16] P. Aldebert, N. Baffier, N. Gharbi, J. Livage, J. Mater. Res. Bull. 16 (1981) 669. [17] D.W. Murphy, P.A. Christian, F.J. Disalvo, J.V. Waszczak, Inorg. Chem. 18 (1979) 2800. [18] S. Hub, A. Tranchant, R. Messina, Electrochim. Acta 33 (1988) 997. [19] N. Kumagai, K. Tanno, T. Nakajima, N. Watanabe, Electrochim. Acta 28 (1983) 17. [20] J. Livage, A. Pasturel, C. Sanchez, J. Velel, J. Solid State Ionics 1 (1980) 491. [21] J.M. McGraw, J.D. Perkins, J.-G. Zhang, P. Liu, P.A. Parilla, J. Turner, D.L. Schulz, C.J. Curtis, D.S. Ginley, Solid State Ionics 113-113 (1998) 407. [22] J. Guo, S. Passerini, B.B. Owens, W.H. Smyrl, Proc. Electrochem. Soc. 96-22 (1996) 53. [23] H.-K. Park, W.H. Smyrl, M.D. Ward, J. Electrochem. Soc. 142 (1995) 1068. [24] D.B. Le, S. Passerini, A.L. Tipton, B.B. Owens, W.H. Smyrl, J. Electrochem. Soc. 142 (1995) L102. [25] V.J. Trifonova, Sol. Gel Sci. Technol. 2 (1994) 447. [26] R.T. Atanasoski, H.K. Park, D.B. Le, B.B. Owens, W.H. Smyrl, Proc. Electrochem. Soc. 94-4 (1994) 150. [27] K. West, B. Zachau-Christiansen, T. Jacobsen, S. Skaarup, Electrochim. Acta 38 (1993) 1215.
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[28] D.B. Le, S. Passerini, J. Guo, J. Ressler, B.B. Owens, W.H. Smyrl, J. Electrochem. Soc. 143 (1995) 2009. [29] R.D. Gordon, M.G. Kulkarni, E.L. Cussler, J. Membr. Sci. 127 (1997) 153. [30] K. West, B. Zachau-Christiansen, T. Jacobsen, S. Skaarup, Mater. Res. Soc. Symp. Proc. 293 (1993) 39. [31] C. Julien, A. Gorenstein, J. Power Sources 15 (1995) 373. [32] E. Shouji, D.A. Buttry, Langmuir 15 (1999) 669. [33] T.W. Schneider, D.A. Buttry, J. Am. Chem. Soc. 115 (1993) 12391. [34] D.A. Buttry, M.D. Ward, Chem. Rev. 92 (1992) 1355. [35] R. Borjas, D.A. Buttry, Chem. Mater. 3 (1991) 872. [36] P.T. Varineau, D.A. Buttry, J. Phys. Chem. 85 (1987) 389. [37] N. Gharbi, C. Sanchez, J. Livage, J. Lemerle, L. Ne´jem, J. Jefebvre, Inorg. Chem. 21 (1982) 2758. [38] J. Lemerle, L. Nejem, J. Lefebvre, J. Inorg. Nucl. Chem. 42 (1980) 17. [39] Y. Marcus, Ion Solvation, Wiley, New York, 1985. [40] D. Lemordant, A. Bouhaouss, P. Aldebert, N. Baffier, J. Chim. Phys. 83 (1986) 105. [41] D. Lemordant, A. Bouhaouss, P. Aldebert, N. Baffier, Mater. Res. Bull. 21 (1986) 273. [42] D. Linden, Handbook of Batteries, McGraw-Hill, New York, 1995, p. 36.14.
EQCM measurements of solvent transport during Li+ intercalation in V2O5 xerogel films Eiichi Shouji 1, Daniel A. Buttry * Department of Chemistry, Uni6ersity of Wyoming, Laramie, WY82071 -3838, USA Received 3 August 1999; received in revised form 16 November 1999
Abstract The electrochemical quartz crystal microbalance technique has been used to monitor the mass changes that occur in acetonitrile electrolytes during electrochemically driven Li+ intercalation into V2O5 xerogel films prepared by aqueous sol–gel methods. The mass measurements made with the EQCM reveal that during reduction of the vanadium sites in the sol–gel oxide at low voltammetric scan rates ( 5 5 mV/s) the net compositional change corresponds to insertion of one Li+ per electron injected, with no accompanying solvent (i.e. no net mass change that can be attributed to solvent transfer into or out of the film). However, at higher scan rates ( \50 mV/s), increased mass changes during reduction are attributed to simultaneous acetonitrile transport into the V2O5 film. Based on previous descriptions of solvent swelling of V2O5 thin films in acetonitrile solutions, these results suggest the following: (1) during reduction at low scan rates the intercalation of solvated Li+ takes place simultaneously with some solvent expulsion, such that the net mass change corresponds essentially exclusively to intercalation of one Li+ per electron, (2) at higher scan rates, the rapid electromigration of Li+ into the film drives simultaneous solvent insertion, such that the net compositional change corresponds to a net gain of considerable amounts of solvent during the Li+ intercalation process. This solvent swelling leads to structural changes that allow larger amounts of solvent to transfer into the films during Li+ intercalation for some time after the high scan rate perturbation. The implications of these findings for Li+ secondary battery cathode materials are discussed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Vanadium pentoxide; Cathode; Li battery; EQCM; Quartz crystal microbalance
1. Introduction The high specific capacity of vanadium pentoxide (V2O5), coupled with its relatively attractive redox potential and ability to reversibly intercalate Li+ during reduction of the vanadium sites has made it the subject * Corresponding author. Tel.: +1-307-7666677; fax: +1307-7662807. E-mail address: [email protected] (D. A. Buttry). 1 Present address: Molecular Materials Research Center, Beckman Institute, Mail Code 139-74, California Institute of Technology, Pasadena, CA 91125, USA.
of much study for application as a cathode material in both primary and secondary Li batteries [1 – 17]. A primary goal of much of the recent work in the area has been to achieve the reversible reduction of all of the vanadium sites by two electrons each, so as to achieve the maximum possible charge storage capacity. In the crystalline material, only one of the two vanadium sites in the formula unit can be reversibly reduced at attractive voltages versus Li/Li+. We suspect this is because of irreversible structural changes that occur if more than one electron is injected per formula unit [18 – 21]. To circumvent this issue, several research groups have described the synthesis and characterization of amor-
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 4 7 1 - 0
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phous phases of V2O5 [2,6,22–27]. These amorphous materials can be produced in several ways, including sol–gel methods that give both xerogel and aerogel materials [3,13,15,16,24,28,29]. These sol–gel routes are quite simple, relying on acidification of solutions containing the VO− 3 species, and can be done under mild (e.g. room temperature) conditions. In some cases, it has been possible to achieve reversible charging and discharging corresponding to 3.3 electrons per formula unit [6]. Theoretical calculations suggest that if reversible charging of 4 electrons per formula unit could be achieved at reasonable voltages, the specific energy of a V2O5/Li battery could be as high as 1366 W h/kg [23]. This is much higher than that of many other transition metal-based oxide cathode materials [30,31], and the expectation that such specific energies might be achievable has driven much of the recent effort in the area. The structural changes that occur in crystalline V2O5 during deep charge/discharge cycles presumably arise from stresses that build up in the material. We suspect that these stresses originate from changes in coordination geometry and/or coordination number of the vanadium sites during the redox transitions. While these effects are likely to be responsible for some of the redox-induced structural changes, an additional effect that has been less well studied is the transfer of solvent during the intercalation and deintercalation processes. In fact, we are not aware of any systematic studies of this process for V2O5. The purpose of the present contribution is to report on such measurements for V2O5 xerogel films [32]. The critical measurement comprises monitoring mass changes during the redox process using the electrochemical quartz crystal microbalance (EQCM) [33–36]. A previous study has also reported on the mass changes during reduction of V2O5 xerogel films in propylene carbonate [23]. These were attributed to reversible Li+ intercalation with no simultaneous transfer of solvent. As will be seen below, similar conclusions are reached in the present study in acetonitrile for lower scan rates, while at higher scan rates, the data are consistent with net transfer of solvent into the films during reduction and out during oxidation. Taken together, the known data on this system suggest that a full understanding of the V2O5 compositional changes during charging and discharging will require consideration of solvent transport as well as Li+ intercalation and deintercalation.
2. Experimental
2.1. Materials Acetonitrile (AN) was purchased from Aldrich Chemical Inc., carefully purified by double distillation
in the presence of P2O5 and stored over 3 A, molecular sieves under Ar. Propylene carbonate (PC) was purchased as anhydrous grade from Aldrich Chemical Inc. and purified by vacuum distillation. 18 MV water from a Millipore purification train was used. Sodium metavanadate, lithium perchlorate (99.999% grade) and ion exchange resin (Dowex 50W-X2, 50 – 100 mesh) were purchased from Aldrich Chemical Inc. and used as received.
2.2. Electrochemical measurements Cyclic voltammograms (CVs) were measured using a Wenking style potentiostat that was locally designed and built. Data are reported versus a Ag/AgCl/NaCl(sat) reference electrode, which was used in all experiments. The area of the electrochemically active region of the working electrode was 0.34 cm2, while the piezoelectrically active region was 0.28 cm2 [34]. A platinum wire (diameter: 0.2 mm, length: 60 cm coil) was used as the counter electrode. Other conditions were: 25.0 9 0.1°C, scan rate: 5 – 200 mV/s, in the potential window from −1.0 V to 1.2 V using a quiescent acetonitrile solution containing 0.5 M LiClO4, which was extensively deaerated using Ar gas prior to the experiments.
2.3. Electrochemical quartz crystal microbalance The EQCM technique has been adequately described elsewhere [33 – 36], and experimental details will not be reviewed here. The proportionality constant relating the calculated mass changes to the experimentally observed frequency changes is 56.6 Hz cm2 mg − 1 for the 1 inch diameter 5 MHz, overtone polished crystals used in this study (Valpey – Fisher, Hopkinton, Mass.). The EQCM thin film gold electrodes were deposited by thermal evaporation using an oil diffusion pumped high-vacuum system (Edwards 306A) as follows: First, quartz crystals were chemically cleaned with a 50/50 (vol%) solution of 30% H2O2/concentrated H2SO4 for 30 min., rinsed copiously with deionized water and dehydrated alcohol, and stored under vacuum. Immediately prior to evaporation, they were plasma-etched with a N2/O2 plasma. Then, a thin Cr adhesion layer (ca. 50 A, ) was deposited, followed immediately by gold (ca. 2000 A, ). A specially tailored H-cell equipped with a number 9 vacuum o-ring joint for EQCM crystal mounting was used for the electrochemical measurements. Silicone o-rings were used to define the area of contact on the front face of the crystal and to prevent solvent contact on the back face. The gold-coated EQCM crystals were placed in AN solution containing 0.5 M LiClO4 as a supporting electrolyte. There was no evidence of electrochemical or gravimetric response from chromium oxide, which can occur if Cr diffuses through the Au layer. This is a result of the very thin Cr adhesion layer used.
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2.4. Synthesis of V2O5 xerogel and spin coating onto EQCM electrodes The V2O5 xerogel solution was prepared as previously described [37,38] using an ion exchange column to treat a solution of sodium metavanadate to produce the protic form, HVO3. This species readily polymerizes to produce a viscous sol–gel solution from which V2O5 can be cast by solvent evaporation. After preparation of the HVO3 solution, hydrolysis and aging were allowed to continue for at least two weeks. The resulting concentration of HVO3 was 10% (w/v). This solution was used to spin-coat xerogel films onto EQCM electrodes at a rotation rate of ca. 5000 rpm using a locally built spin-coater. Then, these electrodes were dried at room temperature for 1 h, followed by drying in an oven at 120°C for 20 h. This treatment gave uniform, adherent films. These films are xerogels, as opposed to aerogels, due to the method by which they are formed (for example, air drying as opposed to supercritical drying, respectively). By using a volumetric pipette to control the droplet volume of the V2O5 xerogel solution used for spin-coating, it was possible to make electrodes that had different thicknesses of the V2O5 layer. The thicknesses used in this study ranged from 50–200 nm, as determined using scanning electron micrographic
Fig. 1. Cyclic voltammograms (left) and EQCM frequency measurements (right) for spin coated V2O5 xerogel on EQCM gold electrode in AN solution containing 0.5 M LiClO4: scan rates are as shown. Curve (f) was done immediately following the sequence of scans (a)–(e).
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examination. These xerogel films are essentially insoluble in common organic solvents, such as acetone, acetonitrile, propylene carbonate, and chloroform. Low angle X-ray diffraction (Scintag XDS2000: CuKa, 40 kV, 30 mA, continuous scan, 1° 2 U/min, 0.01° 2 U /increment (0.60 seconds/point)) gives an interlayer spacing of ca.11.6 A, for all electrodes, consistent with previous reports for this material [37,38].
3. Results and discussion Fig. 1 shows the CVs and EQCM frequency data for a V2O5 film at a variety of scan rates. The left column shows electrochemical data and the right column shows EQCM frequency data. Curve (a) (left) shows the characteristic voltammetry of V2O5 that has been reported by several research groups [2,3,28,32]. This voltammetry is quite stable, and can be repeatedly measured with little change in the shape or magnitude of the current response. The corresponding EQCM frequency data (right) show that mass is gained during the reduction. Quantitative analysis of the total mass gain during reduction and the total reductive charge is consistent with intercalation of one Li+ per electron injected during the reduction process, as has been previously reported at such low scan rates [23]. As can be seen, the subsequent oxidation leads to mass loss, consistent with deintercalation of one Li+ per electron removed from the film during the oxidation process. Repeated scans at 5 mV/s showed essentially identical mass and current responses. Closer examination of the potential dependence of the frequency changes and reductive charges reveals that the majority of the mass gain occurs during the later parts of the reductive scans, predominantly under the second of the voltammetric features in the CV (i.e. under the wave centered at − 0.5 V). This suggests that, even though one would expect that Li+ ions must be inserting during the first reduction wave, there may be loss of mass that compensates for the Li+ insertion. This mass loss could easily be accounted for by expulsion of a small amount of solvent. In principle, the combined transport of solvent and ions can be dissected using isotopically labeled solvents [34]. However, several attempts at such measurements failed because the necessary precision could not be obtained from the measurements. Thus, these results offer only a suggestion that solvent transport may be contributing to the observed mass changes. However, more discussion of this point is presented below. Before proceeding with a discussion of the scan rate dependence, it is worthwhile to first examine the dependence of film thickness on the observed mass changes. Table 1 shows EQCM frequency data from four separate V2O5 films with different film thicknesses. The
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Table 1 Observed and calculated frequency changes for Li+ intercalation into V2O5 Sample number 1 2 3 4
DF (Hz) 1072 1263 1415 1562
Dfobs (Hz) 65 80 87 95
amount of V2O5 on each crystal was determined by measuring the total frequency change for the quartz crystal (in air and after drying at 120°C for 20 h) that occurred as a result of the spin-coating deposition of the xerogel film. This frequency change is indicated by DF, and is given in the second column of Table 1. As can be seen, this frequency change shows that the thickest film is roughly 1.5 times more massive than the thinnest film. Absolute film thickness measurements were complicated by lack of information on the refractive index and density of this xerogel material, and are not reported here. However, crude calculations suggest that these films have thicknesses in the 50–100 nm range. Table 1 also shows three additional frequency-related quantities, Dfobs, DfDF and DfQ. Respectively, these correspond to the frequency change observed during the reduction of the film at a scan rate of 5 mV/s (presumably due to Li+ intercalation, as discussed above), the frequency change expected to occur during this reduction based on the DF value (i.e. the total mass of V2O5 deposited during the spin-coating process) assuming one Li+ intercalated per electron injected and an extent of reduction of two electrons per V2O5 unit, and the frequency change expected to occur based on the electrochemical charge for the reduction and again assuming intercalation of one Li+ per electron. The calculation of DfDF additionally requires the molar mass of the vanadium pentoxide unit. It is known [13] that the actual formula for this material, when prepared as described in the experimental section, is V2O5·1.6H2O, so this value was used in the calculation. Quantitative analysis of EQCM frequency changes requires rigid film behavior on the part of the xerogel [34]. Previous studies [23] of spin-coated V2O5 xerogels in propylene carbonate solutions employed acoustic impedance measurements that verified this behavior. Further, the constant ratio of DF to Dfobs over a factor of 1.5 in film mass (Table 1) is also indicative of rigid film behavior, since one expects the mass of intercalated Li+ to scale linearly with the total film mass [34]. Thus, in the present study, rigid film behavior is assumed and the Sauerbrey equation is used to calculate mass changes from frequency changes, with the proportionality constant given in the Experimental Section 2.
DfDF (Hz) 72 85 95 105
DfQ (Hz) 63 78 89 97
DF/Dfobs 16.5 15.8 16.3 16.4
The data in the table show excellent agreement between the observed frequency change (mass uptake during reduction) and those predicted based on the assumptions given above. Specifically, the excellent agreement between Dfobs and DfQ clearly shows that during the reduction process at low scan rates one Li+ is intercalated per electron injected into the film with no net accompanying solvent transport, as previously reported for this material in propylene carbonate supporting electrolytes [23]. The good agreement between DfDF and the other two values suggests either that the extent of reduction represented by the voltammograms in Fig. 1 corresponds to one electron per vanadium site (the assumption used in calculating DfDF from DF, vide supra) or that the extent of reduction is greater than 1 e−/V site, but that the film is not 100% electroactive. These data do not allow these two possibilities to be distinguished. The present results indicate that net solvent transfer does not occur during the Li+ intercalation process at low scan rates in acetonitrile solutions. Note that this does not imply that bare, unsolvated Li+ intercalates during reduction, only that the total amount of solvent within the film before and after reduction remains the same. One way for this to occur would be for intercalation of solvated Li+ to cause expulsion of solvent molecules from within the V2O5 interlayer regions such that the net composition (and probably also the total volume) of the system remains constant. Another would be for unsolvated Li+ ions to intercalate after desolvation at the point of entry into the V2O5 interlayer region. In both cases, the behavior would seem to be driven by a tendency for the V2O5 material to resist increasing its free volume. This tendency of redox-active thin film systems to resist creation of additional free volume during redox-induced compositional changes has been previously discussed for organic polymers [35]. This behavior is not unexpected for the present case of a structurally rigid transition metal oxide lattice. Returning now to the scan rate dependence of the redox-induced compositional changes, one can see the voltammetric and EQCM frequency responses for higher scan rates in Fig. 1(b) – (e). Curve (f) shows the data for a slow (5 mV/s) scan rate experiment performed immediately after the sequence of scans (b) – (e).
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Fig. 2. Plots of cathodic (open symbols) and total charges (filled symbols) vs. scan rate for the four films listed in Table 1, all at 5 mV/s (see text for details). Film thicknesses within a set decrease from top to bottom.
This is representative of a scan performed immediately after a number of scans at higher scan rate. The mass changes in (b)–(e) nearly track the electrochemical charges. However, as the number of rapid scans increases, so does the mass gain per cycle for the reduction process, even though the reduction charge remains the same. This suggests that the compositional changes during these more rapid redox processes continue to have contributions from the intercalation of Li+ during reduction, but that additional mass is incorporated during reduction as a function of increasing cycle number. Following the imposition of several rapid scans, as shown in (f), one observes that considerably larger mass changes accompany the redox process at lower scan rates. This behavior, larger mass changes during Li+ intercalation and deintercalation at low scan rates after several reduction/oxidation cycles at higher scan rates, was observed for all of the samples examined as part of this study. Of the two possibilities, solvent insertion and
Fig. 3. Plots of ( ) cathodic charge, as calculated from Dfobs and () experimental cathodic charge vs. DF (see text for details).
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anion co-injection (i.e. with Li+), we favor attributing the excess mass to solvent transport. This is because the negative charge produced on the V2O5 lattice during reduction should comprise a potent electrostatic barrier that would effectively reject anions from entering the film. The origin of the excess mass gain observed after high scan rate cycling will be discussed further below. The magnitude of the excess mass gain following several rapid scan cycles is a roughly constant fraction (ca. 8 – 10%) of the total film mass. Assuming this excess mass is due to transport of AN molecules, this would correspond to transport of 0.22 moles of AN per mole of Li+ or a little less than one AN molecule per 4 Li+ ions. Thus, even though the excess mass gain is quite large (Dfobs increases by a factor of 2.3 while the charge stays constant), the ratio of moles of solvent co-transported per mole of Li+ is not that great, and certainly not close to the value of 4 expected for intercalation and deintercalation of Li+ fully solvated by AN [39]. However, it does represent a significant fraction (ca. 60%) of the total mass gained after a sequence of rapid scans. Fig. 1 also shows a few other interesting features. For example, the distortion in the voltammetric waves that is evident at higher scans rates is a clear indication that kinetic limitations prevent the facile reduction of the vanadium sites in the film. This distortion in the charge passage is mimicked in the mass change, showing that both electron injection and Li+ intercalation during reduction (and vice versa during oxidation) are hindered. It is also notable that the cyclic voltammogram after high scan rate cycling is virtually identical with the first low scan rate experiment, even though the mass change is markedly different. Fig. 2 shows the dependence of the charge harvested from the four films referred to in Table 1as a function of scan rate. Data are shown for both the cathodic charge and the total of the anodic and cathodic charges. These data show that these charge values decrease by 30 – 45% on going from a scan rate of 5 mV/s to a scan rate of 200 mV/s. The magnitude of the decrease depends somewhat on the total film thickness. Fig. 3 shows plots of charge versus DF for the four films referred to in Table 1, all at a scan rate of 5 mV/s and prior to any high scan rate cycling. The open circles show the experimentally measured cathodic charge. The solid circles show the cathodic charge (Qcalc,c) calculated from the measured mass gain during reduction (Dfobs), assuming that this mass gain arises solely from Li+ transport and that one Li+ is intercalated per electron injected into the film. Both plots are shown with best-fit linear regressions through the data points. That these linear regression fits pass through the origin shows that both the charges and mass gains during reduction are directly proportional to film thick-
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Fig. 4. Cyclic voltammograms of spin coated V2O5 xerogel in (a) AN and (b) PC containing 0.5 M LiClO4 at 5mV/s.
ness (i.e. to DF). Further, the good agreement between the experimental charge and Dfobs shows that the mass gains during reduction can be essentially completely accounted for by assuming that one Li+ intercalates for each electron injected into the film during reduction. However, as will be discussed further below, this does not imply that unsolvated Li+ ions are inserting into the film. Rather, it merely implies that the net mass change can be accounted for with the mass of inserted Li+ ions. A final experiment was to make a preliminary examination of the influence of the identity of the solvent on the rudimentary features of the electrochemical responses of these V2O5 sol–gel films. Fig. 4 show a comparison of the cyclic voltammetry in AN and propylene carbonate (PC). As can be seen from the increased peak separation in PC, the electrochemical kinetics seem less facile in PC compared to AN. Given that the results reported here for AN and those reported previously for PC [23] indicate essentially no net solvent transport accompanying the electromigration of Li+ into and out of the V2O5 interlayer spacing, these results might suggest that Li+ desolvation prior to intercalation does occur in both solvent systems, and that this is a more kinetically hindered process in PC.
4. Conclusions These results reveal several interesting features of the solvent transport behavior of V2O5 during its reduction and subsequent oxidation in acetonitrile solutions. First, the low scan rate, electrochemically induced intercalation of Li+ into the V2O5 interlayer spacing occurs with essentially no net change in AN content of the xerogel film, as was previously reported for PC [23]. Second, in AN after the films are cycled at higher scan
rates, solvent does appear to accompany Li+ as it moves into and out of the film. The change in solvent transport behavior seems likely to be due to structural changes of the V2O5 matrix that are driven in some way by the rapid electromigration of Li+ into and out of the film. This electrochemically induced change is irreversible in that the films are not observed to return to their original ‘low scan rate’ behavior. Since a consequence of this change is that solvent can reversibly enter and exit the film during Li+ intercalation and deintercalation, the film must be capable of reversibly changing its dimensions in order to create the volume necessary to accommodate solvent incorporation during Li+ intercalation, and also to consume the free volume created during solvent expulsion during Li+ deintercalation. We are uncertain of the origin of the structural changes that drive this change in solvent transport behavior, but one can speculate that they probably arise from the forced intercalation of Li+ at high rates. Whatever their origin, these structural changes will almost certainly have impact on the long term dimensional stability and electroactivity of these inorganic oxides. Third, as expected, the charge that can be harvested from these films decreases as scan rate is increased. This almost certainly is a consequence of slow Li+ electromigration within the interlayer spacing of the V2O5 sol – gel material. It is interesting to compare the solvent transport behavior in PC and AN with previous discussions of the extent of solvent incorporation into V2O5 films as measured by X-ray diffraction. As shown by Baffier and coworkers, the extent of solvent incorporation into V2O5·1.6H2O after simple immersion into the solvent (i.e. in absence of any redox chemistry) depends very much on the identity of the solvent [40]. Specifically, they showed that the interlayer spacing on immersion of V2O5·1.6H2O into AN is 11.5 A, (i.e. no change from that of the parent material) while that on immersion into PC is 21.5 A, . The very large spacing in PC was attributed to PC incorporation into V2O5·1.6H2O. In a related study, the exchange of alkali metal cations for H+ in V2O5·1.6H2O in several organic solvents was also demonstrated [41]. These ion exchange reactions are facile and occur relatively rapidly on immersion of V2O5·1.6H2O into an organic solvent containing the given cation. Unfortunately, PC and AN were not studied. However, for the four organic solvents that were investigated (N, N-dimethylformamide, N-methylformamide, formamide, and dimethylsulfoxide), in every case Li+ exchange into the interlayer spacing occurred with a large increase in the interlayer spacing (to between 16 and 19 A, ). This was interpreted as resulting from simultaneous incorporation of a significant amount of solvent with the Li+ cations due to the large, negative hydration energy of Li+ in high dielectric constant solvents. These results strongly suggest
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that similar behavior should prevail in both PC and AN, namely that the initial immersion of V2O5·1.6H2O into 0.5 M LiClO4 in either solvent should result in Li+ exchange for H+ from the V2O5·1.6H2O lattice with simultaneous incorporation of a significant amount of solvent. Given this situation, there should be more than enough free volume within the interlayer spacing in either AN or PC for solvated Li+ to intercalate during the electrochemically induced intercalation of Li+. However, since the present results and those previously described by Ward and coworkers [23] in propylene carbonate solution unequivocally show that there is no net solvent transport during Li+ intercalation (prior to rapid scan perturbations of the material), one can speculate that while solvated Li+ may actually be the species that intercalates, the free volume restrictions imposed by the relatively rigid V2O5·1.6H2O matrix lead to the simultaneous expulsion of solvent that had been within the interlayer spacing, such that the net solvent transport is extremely small. In other words, assuming that the lattice is rigid and cannot increase its volume to accommodate Li+ intercalation and also that Li+ is solvated both prior to and after its entry into the V2O5 interlayer region, the mass change expected for AN expulsion due to Li+ intercalation is related only to the molar volume of Li+ itself (plus any small, second order effects from electrostriction of AN in the Li+ solvation sphere). Using the ionic radius for Li+ of 0.68 A, gives a molar volume for Li+ of 0.79 cm3/mole, while that for AN is 52.24 cm3/mole. Thus, since the molar volume of Li+ is only 1.5% of that for AN, one would not expect significant mass changes from AN displacement by Li+ if the lattice were to be unable to accommodate volumetric changes due to Li+ intercalation. As seen in Fig. 4, only very subtle differences are observed for the redox behavior of V2O5 in PC and AN. The fact that the electrochemical behavior in PC is less kinetically facile in comparison to that in AN may be the result of slower ion transport in PC, due to greater solvent friction and a somewhat larger solvation sphere. This is likely due to the higher dielectric constant of PC compared to that for AN (64 versus 36, respectively [42]). It is especially significant that the apparent reduction potentials of V2O5 in PC and AN are very nearly identical. This implies that the solvation energy for Li+ likely is not included in the apparent reduction potential in these two solvents. This is consistent with the arguments presented above that Li+ cations do not desolvate prior to entry into the interlayer region in either solvent. Several results in this study have relevance to the potential use of V2O5 and related transition metal oxides as cathode materials in Li secondary batteries. The electrochemically induced changes described above that occur when Li+ electromigration into the interlayer
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spacing is driven at high rates is likely to have a highly detrimental effect on the long term dimensional stability of the material. It seems likely that such structural changes will ultimately lead to loss of conductive pathways within the material which will degrade both the charge storage capacity and the rate of charge extraction. The solvent incorporation and ion exchange processes that occur when this material is immersed into an alkali metal-containing supporting electrolyte imply that the effective molar mass of the material that is used in specific energy and specific capacity calculations should probably include the masses of these species. Finally, taken in total, the results of this study and previous studies strongly suggest that understanding the transport of charge compensating cations within the interlayer spacing of such layered transition metal oxides will necessarily involve understanding the degree of solvation of the cations and how their transport properties are influenced by solvation.
Acknowledgements This work was supported in full by the Office of Naval Research. Valuable discussions with Kevin White and Dr John Pope are also acknowledged.
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