Ion Association and Ion Solvation Effects at the Crystalline

to the minor amorphous phase, for the n g 6 range. For all-amorphous ... ion pairs for n g 8, but a significant amount (≈24%) for n ) 6. This increa...
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7254

J. Phys. Chem. B 2000, 104, 7254-7258

ARTICLES Ion Association and Ion Solvation Effects at the Crystalline-Amorphous Phase Transition in PEO-LiTFSI Ludvig Edman† Department of Experimental Physics, Umeå UniVersity, SE-901 87 Umeå, Sweden ReceiVed: January 6, 2000; In Final Form: March 20, 2000

From band shape analyses of the strong 740 cm-1 Raman mode of the TFSI anion, we deduce that the crystalline state of the P(EO)nLiTFSI system contains a very small amount of ion pairs (≈7%), most probably belonging to the minor amorphous phase, for the n g 6 range. For all-amorphous samples, we found a small amount of ion pairs for n g 8, but a significant amount (≈24%) for n ) 6. This increase in ion pair formation coincides with a decrease in the relative solvation of lithium cations by polymeric ether oxygens, as detected from a careful study of the 863 cm-1 polymer-cation “breathing mode”. We therefore propose that the local ionic structure is preserved during the melting of n g 8 compositions, but that there is a changeover from a predominately ether oxygen lithium coordination to a combined ether oxygen and anionic coordination of the lithium cations for more concentrated samples upon melting.

Introduction Solid polymer electrolytes (SPEs), i.e., salts dissolved in high molar mass polymers, have attracted considerable attention for use as electrolytes in all-solid secondary batteries, with applications in, for instance, portable electronic devices and electric vehicles.1,2 The PEO-LiTFSI systemsLi(CF3SO2)2N (LiTFSI) complexed with poly(ethylene oxide) (PEO)sis an especially promising candidate considering its relatively high ionic conductivity3,4 and positive cationic transference number.5 Initially, it was also reported to be of an all-amorphous constitution at ambient temperature for ether oxygen-to-lithium ratios (from hereon referred to as n) ranging from 8 to 10;6 a noteworthy observation since significant ionic transport takes place only in the amorphous phase.7 Later reports have, however, shown that a eutectic system consisting of crystalline PEO and a crystalline P(EO)6LiTFSI complex dispersed in a minority amorphous phase is the preferred thermodynamic state, at least for highmolecular-weight polymers,8 and that the reported “crystallinity gap” is a consequence of extremely slow recrystallization kinetics for n values ranging from 5 to 12.9 The phase diagrams of SPEs have been thoroughly studied,6,8-10 and it appears as though those which crystallize into a n ) 6 structure, e.g., PEO-LiTFSI, exhibit a higher ionic conductivity in the amorphous phase than those that only crystallize into structures with lower n values.6,10 MacGlashan et al. made use of X-ray diffraction data to show that the crystalline structure of P(EO)6LiAsF6 significantly differs from systems of lower n values in that the lithium ions are contained in a cylinder formed by two interlocked PEO chains, and that each cation is coordinated to 5 out of 6 possible ether oxygens with no coordination to the anions.11 In contrast, crystalline complexes of n ) 3 stoichiometry are built up of lithium ions located inside a helix formed by one PEO chain, with each †

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cation being coordinated by 3 out of 3 possible ether oxygens and by 2 oxygens from neighboring anions, e.g., P(EO)3LiTFSI12 and P(EO)3LiCF3SO3.13 MacGlashan et al. argue that if the absence of ionic coordination in the crystalline complex of P(EO)6LiAsF6 is also retained in the amorphous phase, it would allow ions to migrate independently, thus explaining the high ionic conductivity in the amorphous phase of compounds which crystallize into n ) 6 structures.11 The ionic interactions in SPEs are readily studied with vibrational spectroscopy (IR and Raman) since internal modes of several anions, such as CF3SO3 and ClO4, shift with increasing cationic coordination, subsequently allowing for a qualitative assessment of the ionic environment. (However, we wish to point out that some interpretations of electrochemical results indicate a higher ionic association than what has been concluded from spectroscopy.14) It is further possible to evaluate the ion solvation process through careful analysis of cationdependent polymer modes in vibrational spectra.15,16 Recently, Frech et al. also performed an IR study on a P(EO)3LiCF3SO3 electrolyte at the crystalline-amorphous phase transition, from which they concluded that the crystalline n ) 3 complex only exhibited a loss in long-range order from melting.17 To our knowledge, no material has yet been published on the morphological effects of the melting of an n ) 6 crystalline compound, and with this in mind, and with the aim of testing the intriguing proposal of MacGlashan et al.,11 we have chosen to perform an extensive Raman spectroscopy study on the PEO-LiTFSI system as a function of composition and temperature. This system is also appealing for further studies due to its aforementioned potential in actual applications.2 Experimental Section P(EO)nLiTFSI electrolytes were prepared by dissolving preweighed quantities of PEO (Polysciences, MW ) 5 × 106 g/mol) and LiTFSI (3M Company) in anhydrous acetonitrile.

10.1021/jp000082d CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000

Crystalline-Amorphous Phase Transition in PEO-LiTFSI The solutions were stirred on a magnetic plate for at least 36 h while the volatile solvent was allowed to evaporate. Further drying took place under dynamical vacuum for another 18 h, after which the samples were transferred to quartz cuvettes. All sample handling was performed under a dry argon atmosphere in a glovebox and at ambient temperature. The latter precaution was taken in view of the sluggish transformation to a crystalline state which has been observed for a relatively wide composition range.9 FT-Raman spectra were recorded using a Bruker IFS 66 with a Raman module FRA 106 and a continuous Nd:YAG laser (1064 nm) in a 180° backscattering geometry. The spectra are averages from several recordings with a total acquisition time varying from 3 to 30 h. To optimize signal-to-noise ratios with respect to peak separation and acquisition time, a wavenumber resolution of 3.5 cm-1 was used in conjunction with the HappGenzel apodization function. An evacuated temperature controller allowed measurements to be performed at different temperatures with a stability of (0.3 °C. To perform qualitative and quantitative comparisons, each Raman spectrum was set to an equal intensity scale by first subtracting a linear background18 and then setting the integrated intensity of the CH2 scissoring vibrations located between 1425 and 1510 cm-1 to a constant value, in close agreement with a procedure carried out by Rey and co-workers.19 This “calibration” was double-checked against a number of TFSI modes, and an observed 1/n scaling of the integrated intensity corroborated the validity of this approach. Spectral analyses were carried out by fitting the band envelope of interest to one or more four-component Voigt functions (i.e., convolutions of Lorentzian and Gaussian line shapes), a feature included in the software package PeakFit.20 Results and Discussion The vibrational spectra of the PEO-LiTFSI system have previously been studied with IR19,21,22 and Raman spectroscopy,19,22 all measurements being performed on samples heated during preparation, thus making the electrolytes amorphous for stoichiometries located within the “crystallinity gap”, i.e., for 5 e n e 12.9 Wen et al. used band shifts in the 1100-1400 cm-1 region, initially appearing for an n value of 10, as indicators of ionic association;21 whereas Rey et al. tentatively attributed an asymmetrical broadening of the δSCF3-mode, first observed in an n ) 6 electrolyte, to ion pair formation.22 The latter approach was also used by Bakker et al. in a study of PEO-TFSI electrolytes (M ) Mg, Ca, Sr, Ba), where they used a cation-dependent shift of the 740 cm-1 band, appearing for n < 9, as a probe of ion pair formation.23 This latter vibration located at ≈740 cm-1 in TFSI systems, and at ≈753 cm-1 in closely related CF3SO3 systems, has routinely been attributed to a symmetric deformation of the CF3-group (δSCF3) of a “free” anion, but Rey et al. (TFSI)19 and Huang et al. (CF3SO3)24 proved this to be an oversimplification through potential energy distribution calculations. Instead, it appears as the 740 cm-1 mode involves a complex mixing of a number of internal coordinates which makes the entire TFSI molecule expand and contract.19 Such a vibration involves a large change in the polarizability, and it is expected that it will be affected by cationic coordination and type,23 in parallel with, for instance, cationic-induced shifts in the Ag(2) “breathing mode” of C60.25,26 This assumption is further strengthened by a theoretical study on vibrational frequencies stemming from ion pair formation, where one of the conclusions was that the relatively large shift of this mode observed in most M-TFSI systems (M ) Ca, Sr,

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Figure 1. Raman spectra at 22 °C for crystalline P(EO)nLiTFSI electrolytes (from top to bottom: n ) 6, 8, 11, 20) with no preceding heat treatment. The vertical dotted line indicates the free ion position located at 740 cm-1.

Ba), and in our case M ) Li, indicates a nitrogen-oxygen anionic coordination of the cation.27 An upshift with increasing ionic association is also observed for the “δSCF3”-mode in PEO-LiCF3SO3 systems, where free ions are located at 752 cm-1, ion pairs at 758 cm-1, and positive ion triplets at 763 cm-1, respectively.17,28-30 In Figure 1, we present Raman spectra in the 728-752 cm-1 region for non preheated P(EO)nLiTFSI (n ) 6, 8, 11, 20) electrolytes at 22 °C. Considering the absence of obvious peak shifts and broadening effects of the 740 cm-1 mode, we conclude that the majority anionic species is spectroscopically free TFSI ions for all concentrations investigated. The lack of direct ionic interactions in non preheated complexes of P(EO)nLiTFSI for n g 6 is consistent with the samples being at thermodynamic equilibrium and subsequently consisting of crystalline PEO and crystalline P(EO)6LiTFSI in different proportions dispersed in a minor dilute amorphous phase.8,9 In neither of the two majority components is ionic association to be expected if the crystalline structure of the n ) 6 complex is identical to that of the PEO-LiAsF6 system, as described by McGlashan et al.11 In Figure 2, we present Raman spectra for the same wavenumber region at 70 °C. Under these conditions, we are dealing with single-phase amorphous electrolytes, since 70 °C is well above the highest melting temperature of the investigated samples (which is ≈57 °C for the crystalline PEO phase in P(EO)20LiTFSI).9 Here, an increasing asymmetric broadening of the 740 cm-1 peak toward higher wavenumbers with increasing salt concentration is observed, in line with a previously performed IR study on the same system at 80 °C.22 Those authors compared the integrated intensity of this band to its height and suggested that significant ion pairing takes place only for n < 8 stoichiometries.22 We instead chose to make use of a sophisticated peak fitting procedure where one peak of Voigt character was set at 740 cm-1 (i.e., the free ion position), and the remaining peak(s), also of Voigt character, was allowed to be positioned by the residual, i.e., the difference between

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Edman TABLE 1: Relative Proportions in % of Different Ionic Species in the P(EO)nLiTFSI System

P(EO)20LiTFSI P(EO)11LiTFSI P(EO)8LiTFSI P(EO)6LiTFSI

Figure 2. Raman spectra at 70 °C for all-amorphous P(EO)nLiTFSI electrolytes (from top to bottom: n ) 6, 8, 11, 20). The vertical dotted line indicates the free ion position located at 740 cm-1.

Figure 3. The symbols represent measured Raman spectra for a P(EO)6LiTFSI electrolyte at 22 °C preheated sample (triangles), 70 °C (circles), 22 °C not preheated sample (squares). The two lower solid lines in each spectrum correspond to the two peaks of Voigt character set by the band analysis procedure, whereas the upper solid line represents the sum of these two fitted functions. The two vertical dotted lines correspond to the free ion position at 740 cm-1 and the contact ion pair position at 746 cm-1, respectively.

the measured spectra and the pre-set 740 cm-1 peak. Following this procedure, one remaining peak, consistently located at 746 cm-1, was found, with the magnitude typically increasing with increasing salt concentration. We chose to attribute this peak to ion pair formation, in accord with arguments presented above. The outcome of this procedure is shown in Figure 3 for a P(EO)6LiTFSI electrolyte, where the two lower solid lines in each of the three spectra represent fitted peaks at 740 and 746

T ) 22 °C (non preheated) free ions/ion pairs

T ) 70 °C free ions/ ion pairs

T ) 22 °C (preheated) free ions/ion pairs

93/7 93/7 94/6 92/8

96/4 94/6 93/7 76/24

96/4 94/6 76/24

cm-1, respectively, and the upper solid line represents the sum of these two fitting functions. An excellent agreement between measured data (represented by symbols in Figure 3) and fitted values was found for all concentrations investigated. The summarized results of the peak fitting process are presented in Table 1 for PEO-LiTFSI electrolytes at 22 °C (non preheated), 70 °C, and 22 °C (preheated), respectively. Under the initial condition, a small but constant amount of ion pairs was found for all compositions. Such an invariability and the reported11 absence of ion pair formation in the crystalline n ) 6 phase for the related PEO-LiASF6 system lead us to the conclusion that the small amount of ion pairs originate in the minor amorphous phase. For all-amorphous samples (T ) 70 °C), we find a small but slightly increasing amount of ion pairs with increasing salt concentration for n > 6, but a significant portion (≈24%) in the n ) 6 composition; these results are in good agreement with previous studies on similar systems.22,23 It thus appears as though the proposal regarding a preserved structure during the crystalline-amorphous phase transition brought forward by McGlashan et al.11 might have some merit for n g 8 concentrations in the P(EO)nLiTFSI system, but that the local ionic structure of the n ) 6 complex clearly is affected by the melting transition as detected by a large increase in ion pair formation. The Raman spectra of the pristine LiTFSI salt (not shown) reveals a single strong peak located at 749 cm-1 in this wavenumber region; subsequently, we conclude that our observation of significant ion pair formation during the melting of P(EO)6LiTFSI is not an artifact coupled to salt precipitation. For preheated electrolytes at 22 °C, the persistence of a large amount of ion pairs in the n ) 6 complex once again underlines the extremely slow recrystallization kinetics for the salt-rich concentration range. For the more dilute compositions, only a small amount of ion pairs is present. In Figure 4, Raman spectra for a selected part of the C-O stretching and CH2 rocking region is presented for non preheated PEO-LiTFSI electrolytes at 22 °C. The peaks indicated by dotted lines include the 845 and 860 cm-1 bands of crystalline PEO,31 while the 863 cm-1 band is due to the so-called “breathing mode” of polymer segments involved in lithium ion solvation.15,16,19 The intensities of the crystalline PEO modes decrease with increasing salt concentration and are absent in the P(EO)6LiTFSI electrolyte, as expected from the phase diagram,8,9 whereas the 863 cm-1 band increases in magnitude with increasing salt content. A new band located at 849 cm-1, clearly noticed for the n ) 6 composition, might be due to a small amount of crystalline P(EO)3LiTFSI, necessary in order to compensate for the minor dilute amorphous phase, or to crystal field splitting effects.32 This latter phenomenon also makes quantitative evaluations of crystalline polymer phases dubious, and we have therefore refrained from doing so. For all-amorphous conditions, quantitative analyses of the aforementioned polymeric “breathing mode” have revealed the extent of ether oxygen coordination of cations in solvents such as crown ethers in methanol solution,33 liquid poylethylene glycols,34 and PEO.35 Rey et al. found a linear increase in the relative intensity of this “breathing mode” with 1/n, i.e., the

Crystalline-Amorphous Phase Transition in PEO-LiTFSI

Figure 4. Raman spectra performed at 22 °C on, from top to bottom, pristine PEO, P(EO)20LiTFSI, P(EO)11LiTFSI, and P(EO)6LiTFSI. None of the electrolytes had been exposed to elevated temperatures before the measurement. The dotted lines mark crystalline PEO modes (C-O stretch, CH2 rock), located at 845 and 860 cm-1, and the polymeric “breathing mode”, located at 863 cm-1. The latter mode is used as a measure of the amount of lithium ions solvated by polymeric ether oxygens.

lithium-to-ether oxygen ratio, for n > 8 in the P(EO)nLiTFSI system.35 An observation in relatively good agreement with data obtained by us, as presented in Figure 5, from which a linear increase in the relative intensity of the 863 cm-1 mode with 1/n is observed for n > 6. From our observations regarding ionic coordination and solvation in the amorphous phase, it thus appears that a majority of the lithium cations are coordinated solely by polymeric ether oxygens for n values exceeding 6, and that anions contribute to the solvation in the more concentrated P(EO)6LiTFSI sample. Conclusions In this paper, we demonstrate that the 740 cm-1 mode of the TFSI anion is a useful probe of ionic interactions, and we use this tool to investigate the extent of ion pair formation as a function of salt concentration, temperature, and physical state (crystalline or amorphous) in the important P(EO)nLiTFSI system. It is shown that the crystalline phase contains a very small amount of ion pairs for n g 6, and that those few aggregates which do exist most probably reside in the minor amorphous phase. For all-amorphous samples (T ) 70 °C), the amount of ion pairs is still small for n g 8 but significant in the more salt-rich P(EO)6LiTFSI sample. For preheated samples at 22 °C, the persistence of a large amount of ion pairs for the latter concentration once again demonstrates the slow recrystallization kinetics for salt-rich samples of this system. A quantitative analysis of the cationic-induced 863 cm-1 polymeric band reveals a constant relative solvation of the lithium cations by polymeric ether oxygens for n values of 8 and above in all-amorphous samples. For a higher salt concentration (n ) 6), an observed decrease of the relative ether oxygen solvation coincides with the reported increase in contact ion pair formation, as reported above. We therefore suggest that

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Figure 5. The integrated intensity of the polymer-cation “breathing mode” (with the 1425-1510 CH2 scissoring region used as an internal standard) plotted as a function of 1/n, i.e., the lithium-to-ether oxygen ratio, for all-amorphous electrolytes at 70 °C. The dotted straight line is a guide to the eye and represents a “complete” ether oxygen solvation of the lithium ions.

for all-amorphous P(EO)nLiTFSI electrolytes, lithium cations predominately are solvated by ether oxygens for n g 8, and by a combination of ether oxygens and TFSI anions for n ) 6. In the crystalline phase, our results strongly indicate an ionic structure in agreement with that of the closely related PEOLiAsF6 system. To summarize, it appears as though the proposal brought forward by McGlashan et al. regarding a preserved ionic structure during the crystalline-amorphous phase transition might have some merit for dilute samples, but that the salt-rich P(EO)6LiTFSI exhibits a significant increase in ion pair formation upon melting. Acknowledgment. The author is indebted to the foundation Blanceflor Boncompagni-LudoVisi, ne´ e Bildt for generous financial support and to Professor Per Jacobsson at Chalmers University of Technology for extraordinary hospitality and use of excellent laboratory facilities. Part of this work was also supported by NFR, the Swedish natural science research council. References and Notes (1) For recent reviews on polymer electrolytes, see: (a) Ferry, A. In Recent Research DeVelopments in Macromolecules Research; Pandalai, S. G., Ed.; Research Signpost: Trivandrum, India, 1999; Vol. 4, p 79. (b) Ferry, A.; Doeff, M. M. In Current Trends in Polymer Science; Richard, R., Ed.; Research Trends: Trivandrum, India, 1998; Vol. 3, p 117. (2) Oman, H. MRS Bull. 1999, November, 33. (3) Armand, M.; Gorecki, W.; Andre´ani, R. Second International Symposium on Polymer Electrolytes 1990, 91. (4) Doeff, M. M.; Edman, L.; Sloop, S. E.; Kerr, J.; De Jonghe, L. C. J. Power Sources 2000, 89, 227. (5) Edman, L.; Doeff, M. M.; Ferry, A.; Kerr, J.; De Jonghe, L. C. J. Phys. Chem. B 2000, 104, 3476. (6) Valle´e, A.; Besner, S.; Prud′homme, J. Electrochim. Acta 1992, 37, 1579.

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Edman (22) Rey, I.; Lasse`gues, J. C.; Grondin, J.; Servant, L. Electrochim. Acta 1998, 43, 1505. (23) Bakker, A.; Gejji, S.; Lindgren, J.; Hermansson, K.; Probst, M. M. Polymer 1995, 36, 4371. (24) Huang, W.; Wheeler, R. A.; Frech, R. Spectrochim. Acta 1994, 50A, 985. (25) Science of Fullerenes and Carbon Nanotubes; Dresselhaus, M. S., Dresselhaus, G., Eklund, P. C., Eds.; Academic Press: San Diego, 1995; pp 376-379. (26) Edman, L.; Ferry, A.; Jacobsson, P. Macromolecules 1999, 32, 4130. (27) Gejji, S. P.; Suresh, C. H.; Babu, K.; Gadre, S. R. J. Phys. Chem. B 1999, 103, 7474. (28) Huang, W.; Frech, R. Polymer 1994, 35, 235. (29) Frech, R.; Huang, W. J. Solution Chem. 1994, 23, 469. (30) Ferry, A.; Ora¨dd, G.; Jacobsson, P. J. Chem. Phys. 1998, 108, 7426. (31) Matsuura, H.; Fukuhara, K. J. Polym. Sci. Polym. Phys. 1986, 24, 1383. (32) Koenig, J. L. Spectroscopy of Polymers; Elsevier: Amsterdam, 1999; pp 195-196. (33) Takeuchi, H.; Arai, T.; Harada, I. J. Mol. Struct. 1986, 146, 197. (34) Kasatani, K.; Sato, H. Chem. Lett. 1986, 991. (35) Rey, I.; Bruneel, J.-L.; Grondin, J.; Servant, L.; Lasse`gues, J.-C. J. Electrochem. Soc. 1998, 145, 3034.