Structural Studies of Lithium 4,5 ... - ACS Publications

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Structural Studies of Lithium 4,5-Dicyanoimidazolate−Glyme Solvates. 2. Ionic Aggregation Modes in Solution and PEO Matrix Piotr Jankowski, Maciej Dranka,* and Grazẏ na Z. Ż ukowska* Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warszawa, Poland S Supporting Information *

ABSTRACT: In this paper, we present complementary spectroscopic analyses of lithium 2-trifluoromethyl-4,5-dicyanoimidazole adducts with aprotic solvents like dimethyl ethers of poly(ethylene glycols) (i.e., glymes) and crown ethers. Comparing the XRD structures with Raman spectra we have found fingerprints of various structural motifs such as ionic pairs, dimers, “free ions”, and higher aggregates. Comprehensive analysis of crystalline materials has been performed to correlate molecular structures with spectroscopic data, which give valuable information about the coordination preferences of substituted 4,5dicyanoimidazolato anions and provide the basis for further developing a model for poly(ethylene oxide) electrolytes. Complementary and systematic X-ray studies of glyme adducts enable precise interpretation of the anion−cation and cation−solvent interactions from experimental Raman spectra. This information provides a convenient tool for the characterization of the ionic association interactions within electrolytes.

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INTRODUCTION Progress in developing new materials for lithium batteries, such as new cathodes (e.g., high-voltage metal oxides, sulfur) and anodes requires research on new salts, which should surpass the commercially used LiPF6.1,2 The high conductivity of aprotic electrolytes based on LiPF63−5 and the ability to form a stable solid−electrolyte interface (SEI) with graphite anodes6−9 is accompanied by susceptibility for hydrolysis with the formation of HF.9,10Among a variety of new salts proposed in the past decade to overcome this problem, those representing the imide salts, e.g., lithium bis(trifluoromethanesulfonyl) imide (LiTFSI),1 as well as the family of salts with so-called Hückel anions appear to be particularly interesting.1,11,12 Several heteroaromatic salts representing the last group have already been obtained. Two of them, lithium 4,5-dicyano-1,2,3-triazolate (LiDCTA) and lithium and sodium 2-trifluoromethyl-4,5dicyanoimidazolates (TDI, Scheme 1) attracted major attention. In particular, the properties of LiTDI-doped electrolytes have been intensively studied in recent years.13−16 Knowledge of anion−cation interactions, cation solvation, and the structure of the electrolyte is necessary for effective development of materials for energy conversion and storage.

This can be achieved with the use of theoretical modeling, crystal structure analysis, or spectroscopy data. In the last 10 years significant progress in the understanding of electrolyte systems by combining of the above-mentioned methods has been made. Works of Henderson and co-workers,17 concerning the structure of solvates of lithium salts with aprotic solvents, and of Watanabe’s group,18,19 focusing on the properties of highly concentrated liquid systems, can be considered as different ways to study the same phenomena. Similarly, we have recently applied the structural approach to study dicyanoimidazolate lithium salt systems.14,20−22 The correlation of X-ray and spectroscopy data serves as a convenient tool to clarify the anion−cation and cation−solvent interactions in liquid systems or in polymer membranes. However, knowledge of the exact structure of complexes with high molecular weight poly(ethylene oxide) (PEO) based on diffraction data is often unavailable due to difficulties with the synthesis of the crystals of required quality.23 Crystalline complexes of the appropriate salt with glymes, short-chain PEO analogues, can act as model system to overcome this problem.24 Complexes of crown ethers can serve as convenient models for “free ions” and ionic pairs.25−27 From a spectroscopic point of view, the information about the electrolyte structure can be achieved in two ways: (i) through studies of spectral characteristics of the anion, giving information on the neighborhood of the anion, and (ii) the analysis of the polymer chain conformation, which is important for understanding cation−polymer interactions. Our comprehensive database of the structural fragments identified and characterized by spectroscopic methods has served to model

Scheme 1. Structure of 2-Trifluoromethyl-4,5dicyanoimidazolate (TDI) Anion

Received: February 24, 2015 Revised: April 8, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/acs.jpcc.5b01826 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Table 1. Cation and Anion Environments in Li−Glyme Solvates anion environment

a

cation environment total Li+ coordination number

compounda

no. of coordinated Li+/ structural motif

1 2

Li(12C4)2+TDI− Li(15C5)·TDI

none/I 1/II

none 1 NCN

8 6

3 4 5 6 7

Li(G1)2·TDI Li(G3)·TDI [Li(G2)·TDI]2 [Li(G2)·PDI]n Li(G1)·TDI

8

Li(G3)0.5·TDI

9

[Li2(G4)22+][Li4TDI62−]

1/III 1/III 2/IV 2/V 3/VI 2/V 3/VI 2/V 3/VII

1 1 1 1 2 1 2 1 1

5 5 5 5 5 4 5 4 5

10

Li(G1)0.5·TDI

2/VIII 3/VI

2 NIm+ F 2 NIm + 1 NCN + F

used TDI donors

NIm+ F NIm+ F NIm + 1 NIm + 1 NIm + 1 NIm + 1 NIm + 1 NIm + 1 NIm + 2

NCN NCN NCN NCN NCN NCN NCN

The structures of the solvates 1−10 have been published previously

+F +F +F +F +F +F +F

number of ether oxygens 8 5

+ + + + + + + + +

1 1 1 1 1 1+1 1 1+1 1

4 4 3 3 4 none 4 none 5

4+1+1 4+1+1

none none

4+1+1

2

chain conformation: torsion sequence

aggregation mode

tgg−tgg−tgg−tgg tg′t−tgt−tg′t−tgt−ggt tgg−tgt−ggg−tgt−tg′t ggt−tg′t−tg′t−tg′t−tgt tg′t−tgt−tg′t−tgg−tgt

isolated ions monomer

tgt−tg′g′−ggt tgt−tg′t tgt−tg′t tgt

monomer monomer dimer 1D chain 1D ribbon

tgt−tg′t−g′g′t

1D ribbon

tgt−tg′g′−tgt−tg′t

2D anionic layers 3D aggregate

tgt

22

with the use of a Peltier-cooled Linkam stage. If not stated otherwise, Raman spectra were recorded at room temperature. Spectral analysis was performed with Omnic software. DSC studies were performed using a differential scanning calorimetry (DSC) technique. The experimental data were registered on a TA Q200 differential scanning calorimeter. The samples were heated at a rate of 5 °C min−1. An empty aluminum pan was used as the reference.

processes existing in the liquid phase and in the polymer system. In complexes of PEO with lithium salts, cations can be decoupled from anions, as in the case of PEO−LiBPh4 (Li:O = 1:5) or PEO−LiXF6 (Li:O = 1:6) (X = P, As, Sb),28,29 or coordinated to anions, like in the majority of PEO−LiX (Li:O = 1:3) systems.30−33 Structures of 1:3 complexes represent aggregate-type coordination and 1:6 complexes usually solventseparated ionic pairs. One of the discovered crystalline phases of PEO−LiTDI represents the aggregate-type, while the other does not seem to belong to any of these types. Herein, we present Raman spectra analyze of various lithium complexes containing 4,5-dicyanoimidazolato anions with dimethyl ethers of poly(ethylene glycols) (i.e., glymes) and crown ethers in order to gain further insight into aggregation stages in solution and in the PEO matrix.

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RESULTS AND DISCUSSION The ability of the salt to dissociate is one of the most important parameters affecting the conducting properties of the electrolyte. One of the tools to achieve knowledge of the types of the ionic species existing in real systems is vibrational spectroscopy. The information on the anion’s neighborhood can be deduced by comparing the Raman or infrared spectra of the analyzed systems with that of model compounds with known structures. A series of LiTDI solvates (1−10) with dimethyl ethers of ethylene oxide oligomers and crown ethers was synthesized, and their structures were resolved on the basis of XRD studies.22 A list of LiTDI solvates obtained is summarized in Table 1. Comparing the structures with Raman spectra, we have found fingerprints of various structural motifs, such as ionic pairs, dimers, “free ions”, and higher aggregates. Various types of anion coordinations existing in the obtained solvates are listed in Table 1 and depicted in Figure 1. Distinguished motifs (I−VIII) depend on the number of coordinated lithium cations and the type of employed anion donor centers. The isolated structural types have been further used as a model for the interpretation of anion coordination modes in the concentrated liquid LiTDI−glymes and solid PEO−LiTDI electrolytes. As shown in Table 1, similar structural motifs repeat in structures of different solvates. For example, among coordination modes depicted in Figure 1, III is found in compounds 3 and 4 and VI is common for 7, 8, and 10. Table 2 presents a summary of the analyzed bands in Raman spectra of

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EXPERIMENTAL SECTION Sample Preparation. All operations were carried out inside an argon-filled glovebox. Anhydrous dimethyl ethers of mono(ethylene glycol) (G1), di(ethylene glycol) (G2), tri(ethylene glycol) (G3), and tetra(ethylene glycol) (G4) as well as crown ethers 12-crown-4 and 15-crown-5 were purchased from Sigma-Aldrich and used as received. Poly(ethylene oxide) (PEO) (Mw = 5 × 106 g/mol, Aldrich) was dried under vacuum. Crystalline samples were prepared as previously described.22 Solutions of 0.1−4 M of LiTDI in glyme (mono/di/tri/tetra) were prepared in glass vials, transferred to pans after stirring, and put in quartz tubes before measurements. LiTDI−PEO membranes were obtained as follows. LiTDI was dried under vacuum and added to poly(ethylene oxide) in acetonitrile solution. The solution obtained was poured onto a Teflon dish and a thin foil was formed after vacuum drying. Raman spectra were collected on a Nicolet Almega Raman dispersive spectrometer. A diode laser with excitation line 532 nm was used. The spectral resolution for all experiments was about 2 cm−1. Temperature-dependent spectra were obtained B


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The Journal of Physical Chemistry C

spectral pattern corresponding to the coordination type I was also found for the G2 solvate 11, identified with structure Li(G2)2+TDI− published previously.17 The crystal structure of Li(15C5)·TDI (2) may serve as a model of ionic pairs II, where cations are linked with anions through one nitrile group. Li(G1)2·TDI (3) and Li(G3)·TDI (4) both represent the same type of coordination (contact ion pair III in Figure 1), with lithium cation chelated by ring nitrogen and fluorine atoms. As shown in Table 2, the same anion coordination type results in almost the same positions of the analyzed peaks. Further aggregation leads to dimers IV represented by solvate [Li(G2)· TDI]2 (5), comprising two anions, two cations, and two molecules of diglyme. As a result, bands of the νCN and δNCN vibrations are split, due to the presence of pairs of free and associated nitrile groups as well as free and associated ring nitrogens in each anion. Several other complexes, Li(G1)·TDI (7), Li(G3)0.5·TDI (8), [Li2(G4)22+][Li4(TDI)62−] (9), and Li(G1)0.5·TDI (10) can be described as ionic aggregates. The most common structural motif, VI (see Figure 1), with one nitrile group and both ring nitrogens and fluorine coordinated to lithium, was found in 7, 8, and 10. Some spectral features, namely, bands at ∼2230 and 2260, 1320, and 1005 cm−1, appear to characterize this type of anion. This common pattern in spectra of 7 and 8 is overlapped by additional bands peaking at ∼1316 and 990 cm−1, corresponding to type V in Figure 1 and represented by the LiPDI solvate 6. Structure 9 also consists of anions of two types, but different than in other solvates, shown in Figure 1 as VII and VIII. Hence, the spectral pattern is unique, although it resembles in some degree spectra of other aggregate-type complexes. An interesting spectral feature is possessed by LiTDI-18C6 solvate 12, the X-ray structure of which was not determined. Positions of the bands indicate the coordination type III. One of the possible explanations can be the complexation of two cations by two ether rings and a formation of the anion−cation linkage with the use of imidazolium nitrogens. As was shown, anions in the lithium coordination sphere of LiTDI can be easily substituted with a variety of oxygen ligands from oligoether. Glyme complexes with higher O:Li ratio are stable at subambient to ambient temperatures; a decrease of the O:Li below 4 results in more aggregated structures, e.g., 5, 8, or 9. Heating and subsequent cooling of 5 leads to the formation

Figure 1. Coordination modes of the TDI anion found in glymes and corona ether solvates based on the XRD studies.22

the complexes obtained and, additionally, of LiTDI complexes with high molecular weight PEO. We selected bands corresponding to CN triple-bond stretching, C−N imidazole ring stretching, NCN ring deformation, and CF3 deformation, which are particularly sensitive probes of ionic association. The last one, CF3 deformation, gives a single band for I−III coordination types, centered at 673−678 cm−1, and at least one additional band between 680 and 686 cm−1 for all other structures. Moreover, the upshift of any of these bands is a clear indicator of ionic association, in the sense of the lithium−donor center linkage. Various structures of LiTDI−glymes solvates can be considered as following steps in the salt dissociation, with a rising number of sites in the Li+ coordination shell substituted by ether oxygens. Structural differences between the adducts obtained are clearly reflected in the Raman spectra. The positions of the anion’s characteristic bands in 1 are close to those observed in diluted LiTDI−glyme solutions.34 The

Table 2. Band Assignments for LiTDI Crystalline Complexes in Raman Spectraa no.

molecular formula

struct motifs

1 2 3 4 5 7 8 9 10 11 12 13 14 15

Li(12C4)2+TDI− Li(15C5)·TDI Li(G1)2·TDI Li(G3)·TDI [Li(G2)·TDI]2 Li(G1)·TDI Li(G3)0.5·TDI [Li2(G4)22+][Li4TDI62−] Li(G1)0.5·TDI Li(G2)2+TDI− b Li(18C6)xTDIc Li(G2)yTDIc,d PEO−LiTDI phase α PEO−LiTDI phase β

I II III III IV V + VI V + VI VII + VIII VI I III α β

νCN/cm−1 2225 2228, 2230 2230 2250, 2259, 2259, 2258, 2261, 2226 2231, 2258, 2231 2260,

2239

2233 2235 2234 2230, 2240 2228 2233 2232

νCN Im/cm−1

δNCN/cm−1

δCF3/cm−1

1307 1302 1316 1320 1313 1321, 1316 1322, 1316 1322, 1312 1324 1308 1312 1313 1318 1319

977 979 991 991 988, 976 1005, 996 1005, 996, 989, 977 1007, 990 1009 978 986 1001, 993 995 1001, 992

674, 664 678, 673 676 675, 667 680, 672, 668 682, 674 683, 674 686, 674, 668 684, 674 672 674 686, 679 674 684, 672

ρCH2 + νCO/cm−1 861, 843 871, 828, 872, 840 865, 844, 871, 842, 875, 841 867, 843 877, 830 876, 841 871, 840, 870, 833 872, 838 878, 839, 875, 848

812 828 831

832

815

a

Stronger bands are indicated by bolded numbers. bIdentification of the compound on the basis of the Raman spectrum. cX-ray crystal structure not determined. dy < 1. C


Article


Figure 2. Raman spectra of LiTDI−G4 solutions in spectral ranges corresponding to (a) νCN, (b) νCN ring, (c) δNCN, and (d) δCF3 vibrations.

of yet another crystalline phase, 13, stable in the 100−120 °C range. Spectral characteristics of the νCN range, with two strong peaks at 2258 and 2233 cm−1, resemble that of 7 or 8, but the positions of the bands involving ring vibrations, in particular a lack of the bands between 1000 and 1010 cm−1, indicate the presence of coordination type V. Ether Chain Vibrations. In complexes with cyclic ligands, the coordination number can be 6−8; the 8-fold coordination is obtained by the formation of a sandwich complex of lithium with two 12-crown-4 molecules.25,26 The ionic radius of the cation alters the conformation of the polyether chain. This effect is known and has been described for a variety of systems.35 Complexes of triglyme and tetraglyme are of particular interest due to the presence of an ether chain long enough to be coordinated by two cations, thus better mimicking PEO behavior in comparison with shorter glymes. Molecules of longer glymes (G3, G4) are usually wrapped around cations in the form of a helix. Both Raman and infrared spectra deliver important information concerning the conformation of the polyether chain in liquid and solid complexes. This can be deduced, for example, on the basis of the position of the band of CH2 rocking and C−O stretching vibrations, which have several maxima in the 870−840 cm−1 region. In the solid state, all glymes adopt a (tgt)n conformation with the two main bands in the Raman spectrum at ∼860 and 840 cm−1 corresponding to the vibrational modes of crystalline PEO.36,37 A collection of the torsional angles found in solvates 1−10 is given in the Supporting Information. In the presence of lithium salt, the formation of Li−O bond leads to a change in the torsional sequences and appearance of the new band between 870 and 890 cm−1, attributed to a “breathing mode”, similar to that observed in crown ether complexes. In Li(12C4)2+TDI− complex (1), two bands in this range were found, the stronger one with a maximum at 861 cm−1 and the other, much weaker, at 814 cm−1. These values are similar to those reported for other Li(12C4)2+ complexes38 and correspond to a (tgg)4 conformation sequence, verified by XRD results. The only conformation adopted by a monoglyme in LiTDI adducts is tgt; in the Raman spectra we observed two bands at ∼875 and 840 for all G1 complexes. The spectral pattern of diglyme complex 5 is similar to that for LiSbF6−G2 and LiCF3SO3−G2 complexes, which is not surprising, as the conformation on the basis of XRD experiments is the same, i.e., tgt-tg′t.39 Chains of longer glymes tend to wrap around two

cations, forming a helix or cylinder. The torsional sequence found for the G4 solvate 9 is a combination of tgt and tgg, the same as in LiBF4·G4 solvate.40 G3 solvates 4 and 8 are characterized by various chain conformations: tgt−tg′g′−ggt and tgt−tg′t−g′g′t, different from tgt−tg′t−tgt or tgt−tgt−tgt reported for complexes of LiClO4, LiAsF6, and LiCF3SO3.35 The spectral pattern of chain vibrations is similar for 4 and 8, with the strongest band at ∼865 cm−1. Li(15C5)·TDI (2) is unique among other solvates. Although each of the four ether rings comprising a unit has a different geometry, the spectrum is dominated by bands characteristic of the combination of tgt and tg′t sequences. Determination of Ionic Aggregation Modes in Solution and PEO Matrix. Previously, on the basis of single crystal X-ray diffraction measurements, we have proposed a disproportionation mechanism for the LiTDI salt family in glymes. The three structural motifs, I−III serve as models for noncoordinated free anions (I) and contact ion pairs (II and III) for Raman analysis at ambient temperatures. Dimers IV and chains V can be formed at high salt concentrations. On the basis of the above arguments, we can ascribe bands in the Raman spectra of the electrolytes to the structural motifs shown in Figure 1. Figure 2 presents a comparison of the Raman spectra of LITDI solutions in tetraglyme. In the diluted solutions, at concentrations equal to or lower than 0.05 M, we observed the maxima of the νCN, νCN Im, and δNCN at 2222, 1305, and 978 cm−1, respectively, the positions being attributed to the spectroscopically free anions I. In solutions with higher salt concentrations, there is a gradual shift of the bands to higher frequencies and finally splitting, as shown in Figure 2. An exemplary deconvolution of the νCN spectral range is shown in the Supporting Information (Figure S16). These additional bands, at ∼2230, 1315, and 990 cm−1, are clearly related to associated ionic species, in this case ionic pairs III, as can be concluded on the basis of data in Table 2. The same values were also found for triglyme and diglyme solutions, which suggests similar anion environments. A further increase of the salt concentration above 1 M results in the formation of shoulders at ∼2250 and 680 cm−1, corresponding to higher ionic aggregates: dimers or chains. It is interesting to note also that the melting of the crystalline phase of 2, i.e. ionic pair II, leads to a rearrangement of the system and the formation of type III associates. These spectral data indicate that ionic pairs D


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Figure 3. Model of the LiTDI association in glyme solutions.

PEO membranes with O:Li equal to 8:1 and 3:1 is presented in Figure 4. Samples with higher PEO content consist mainly of crystalline phase α, while the second sample contains phase β. For phase α, the position of the νCN band, 2231 cm−1, and the almost symmetric shape of this peak, as well as the positions of the νCN Im (1318 cm−1) and δNCN (995 cm−1) bands, indicate the presence of the crystalline phase comprising ionic pairs III, i.e., contact ionic pairs. PEO complexes with this type of coordination are not common, but similar structures were described for triglyme complexes with LiBF4 and LiTFSI.41 For the crystalline phase β, we observed a split of the νCN band, along with other spectral features, which indicates the presence of both free and coordinated nitrile groups and the exchange of the coordination type III by the coordination type V. Raman mapping experiments confirmed the multiphase character of the electrolytes obtained and the presence of domains with different structures. Figure 5 presents the salt distribution obtained from the Raman profile for the PEO− LiTDI membrane with O:Li equal to 8:1. Areas with low salt content are characterized by the same PEO spectral pattern as observed in pure polymer, while the spectral pattern extracted from areas rich in salt originates from the crystalline phase α. At low salt content (O:Li equal to or higher than 16:1) the main bands are accompanied by weaker bands at lower wavenumbers, approximately 2222, 1305, and 977 cm−1.34 The latter values are in agreement with the data obtained for 1 and ascribed to free anions. In polymer-based systems, such anions, decoupled from cations and spectroscopically free, may exist in the amorphous phase. An interesting behavior was found for crystalline phase α. An increase in the temperature results in distinct changes in the νCN and δNCN spectral range, as shown in Figure 6. Melting of the LiTDI−PEO crystalline phase α leads to partial redissociation of the ionic pairs; bands of the complex at 2231 and 995 cm−1 are replaced by peaks at 2226 and 978 cm−1, which are typical for free anions. Shoulders at 2245 and 985 cm−1, visible at elevated temperatures, indicate the presence of dimer- or chain-type complexes in the amorphous phase. The existence of the higher aggregates in a molten system is also supported by the presence of a complex band in the δCF3 range, with maxima at ∼680 and 669 cm−1. The first of these is related to coordination types IV or V. Chain Conformations. The conformation sequence of the polymer chain provides further insight into the structure of the polymer complex. It strongly depends on the size of the cation, the stoichiometry of the complex, and the properties of the anion. The spectral range corresponding to coupled CO stretching and CH2 rocking vibrations (800−900 cm−1) is particularly sensitive to changes in the torsional angles, which is reflected in the number and positions of the bands. If the cation is decoupled from the anion, as in the case of LiBPh4−PEO or LiX−PEO (Li:O = 1:6) (where LiX = LiPF6, LiClO4, or LiBF4), polymer chains form cylindrical tunnels.23,42 Each

in which a cation is connected via imidazole nitrogen is privileged in all of the oligoether-based liquid systems. Figure 3 presents models of salt aggregation in the glyme solutions. When the salt concentration increases, we observe formation of the ionic pairs III, which in highly concentrated solutions undergo recombination, leading to dimers (IV) or chains (V). In these ionic species, anions utilize both imidazole nitrogen and nitrile nitrogen to link cations. In the highly concentrated solutions (above 4 M), aggregation does not end with ionic pairs, which gradually combine into higher aggregates. In these particular systems, we can confirm the presence of coordination chains V, which in Raman spectra are distinguishable from ionic pairs III by the combined spectral fingerprints of the νCN and δNCN. A further decrease in the solvent content results in the disproportionation of the lithium centers and a subsequent crystallization of system. It has to be stressed that the spectral pattern of molten solvates resembles spectra of both liquid and solid systems. Polymer Electrolytes. Membranes based on high molecular weight PEO contain several crystalline phases and also contain some amounts of the amorphous phase. The multiphase character of polymer systems and the nonhomogeneity of the polymer membranes make the analysis of the Raman spectra even more difficult. A polymer chain is able to adopt various torsional sequences, depending on the type and the concentration of the salt. Therefore, to understand the structure of the polymer-based systems, we need both information concerning the anion’s coordination sphere as well as information on the conformations of the polymer chain. Using data collected in Tables 1 and 2 found for short chain glyme complexes, supported by DSC results, we are able to describe the structure of PEO-based systems. Results of the DSC analysis are collected in Table 3. Table 3. DSC Data for LiTDI−PEO Samplesa O:Li

Tt1/°C

Tt2/°C

3:1 8:1 16:1 32:1 160:1

38 39 39 41

89 82 89

Tt3/°C

61 66

a Tt1, Tt2, and Tt3 correspond to melting temperatures of LiTDI−PEO phases α and β and crystalline PEO.

Samples with O:Li ratios ranging from 3:1 to 16:1 are characterized by two melting temperatures, at 39 °C (phase α) and 80−89 °C (phase β). These are ascribed to the melting of the two different PEO−LiTDI crystalline phases. It is worth noting that there are no visible thermal effects in the range 62− 65 °C, temperatures whch correspond to the melting of the pure PEO. This effect occurs for samples with O:Li ratios equal to 32:1 and higher. A comparison of Raman spectra of LiTDI− E


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Figure 4. Raman spectra of LiTDI−PEO membranes with O:Li = 8:1 (phase α) and 3:1 (phase β). Spectral ranges corresponding to (a) νCN, (b) νCN ring, (c) δNCN, and (d) δCF3 vibrations.

Figure 6. Raman spectra of the LiTDI−PEO (Li:O = 1:8) recorded in the range 20−70 °C.

of these bands are close to those observed in the helical double cation [Li2(G4)22+] complex 9. Thus, it is likely that a similar structural motif to those observed for LiX−PEO (Li:O = 1:6) (X = PF6, AsF6, SbF6)32,42 also exists in the crystalline phase α. However, differences in the spectral pattern suggest that one of the two polymer chains is exchanged. Ether oxygens are replaced by imidazole nitrogen atoms from coordinated anions (Figure 7). In conclusion, the local structure of lithium cation can be ascribed as coordination type III, but the overall polyether chain adopts a helical conformation. The spectral characteristics of phase II indicates a similar chain conformation. However, Figure 5. (a) Raman profile showing the salt distribution on the PEO−LiTDI membrane (O:Li = 8:1), obtained from the ratio of the areas of bands of the νCN (salt) and νCH (polymer). (b) Raman spectra recorded in salt-rich (red curve, area A) and polymer-rich (blue curve; area B) parts of the membrane.

cation is coordinated by oxygens from the same23 or two different polyether molecules.42 On the other hand, LiX−PEO (Li:O = 1:3) complexes can be considered as aggregates.33,43−45 In this case, each anion coordinates two cations, which results in the zigzag chain conformation, tgt−tg′t−tgt, where all of the C−C bonds are g or g′.44 The spectral pattern of PEO−LiTDI membranes with O:Li = 32:1 is typical for crystalline PEO, with two peaks due to the rocking CH2 vibrations at 861 and 845 cm−1 and three peaks in the DLAM spectral range at 232, 277, and 364 cm−1. An increase in the salt content results in the formation of the crystalline phase α characterized by bands at 878, 839, and 815 cm−1. The positions and relative intensities

Figure 7. Proposed structure of LiTDI−PEO crystalline phases (a) α and (b) β. F


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the part of the chain coordinating the cation folds in a crown ether-like flat structure. The remaining coordination sites on the lithium cation are linked with TDI anions in an unsymmetrical fashion by diagonal imidazole and nitrile nitrogens.

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CONCLUSION The comprehensive analysis of Raman data performed for a series of crystalline solvates provided the basis for the correlation of spectral patterns with various types of structural motifs. Comparing the XRD structures with Raman spectra, we have found fingerprints of various structural motifs such as ionic pairs, dimers, “free ions”, and higher aggregates. On the basis of the results obtained, we postulate a dissociation mechanism in the LiTDI−oligoethers systems. The spectral data obtained for crystalline compounds allowed us to propose coordination types for various ionic species existing in LiTDI solutions, giving valuable information about the coordination preferences of substituted 4,5-dicyanoimidazolato. These Raman studies help to establish the coordination motifs present in solid PEO-based complexes and provided valuable information to develop a model for poly(ethylene oxide) electrolytes doped with heteroaromatic salts.

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ASSOCIATED CONTENT

S Supporting Information *

List of torsion angles in the glymes’ chains and detailed spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.D. e-mail: [email protected]. *G.Z.Ż e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Warsaw University of Technology. The authors would like to thank Dr. A. Zalewska for differential scanning calorimetry (DSC) measurements and Prof. J. R. Stevens for proof-reading and corrections.

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ABBREVIATIONS USED MeCN, acetonitrile; DSC, differential scanning calorimetry; LiTDI, lithium 2-trifluoromethyl-4,5-dicyanoimidazole; LiPDI, lithium 2-pentafluoroethyl-4,5-dicyanoimidazole; PEO, poly(ethylene glycol) dimethyl ether; G1, mono(ethylene glycol); G2, di(ethylene glycol); G3, tri(ethylene glycol); G4, tetra(ethylene glycol); 12C4, crown ether 12-crown-4; 15C5, crown ether 15-crown-5

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REFERENCES

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