Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes

Article pubs.acs.org/JPCB

Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes and Na Salts Toshihiko Mandai,† Risa Nozawa,† Seiji Tsuzuki,‡ Kazuki Yoshida,† Kazuhide Ueno,† Kaoru Dokko,†,§ and Masayoshi Watanabe*,† †

Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan § Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: We prepared a series of binary mixtures composed of selected Na salts and glymes (tetraglyme, G4, and pentaglyme, G5) with different salt concentrations and anionic species ([X]−: [N(SO2CF3)2]− = [TFSA]−, [N(SO2F)2]− = [FSA]−, ClO4−, PF6−) and studied the effects of concentration, anionic structure, and glyme chain length on their phase diagrams and solvate structures. The phase diagrams clearly illustrate that all the mixtures form 1:1 complexes, [Na(G4 or G5)1][X]. The thermal stability of the equimolar mixtures was drastically improved in comparison with those of diluted systems, indicating that all the glyme molecules coordinate to Na+ cations to form equimolar complexes. Single-crystal X-ray crystallography revealed that [Na(G5)1][X] forms characteristic solvate structures in the crystalline state irrespective of the paired anion species. A comparison of the solvate structures of the glyme−Na complexes with those of the glyme−Li complexes suggests that the ionic radii of the coordinated alkali-metal cations have substantial effects on the resulting solvate structures. The Raman bands of the complex cations were assigned by quantum chemical calculations. Concentration dependencies of cationic and anionic Raman spectra show good agreement with the corresponding phase diagrams. In addition, the Raman spectra of the 1:1 complexes strongly suggest that the glymes coordinate to Na+ cation in the same way in both liquid and crystalline states. However, the aggregated structure in the crystalline state is broken by melting, which is accompanied by a change in the anion coordination. temperatures and behave like ionic liquids.17−19 Glymes and Li+ ions form robust complex cations as component cations, and hence, they are classified as “solvate ILs”.20,21 The concept of complex cations, e.g., [Li(glyme)1]+, similarly arises from common anions of typical aprotic ILs, such as BF4−, PF6−, and AlCl4−. The complex ions are adducts of a Lewis acid (BF3, PF5, and AlCl3 as common anions and Li+ as [Li(glyme)]) and a Lewis base (F− and Cl− as common anions and glyme as [Li(glyme)]+ cation). Crown ether-based alkali-metal salt complexes are a structurally similar system, but many of them have relatively high melting points (>100 °C).22−25 In contrast, appropriate combinations of glymes and Li salts result in

1. INTRODUCTION Ionic liquids (ILs) are promising candidates as electrolyte materials for batteries owing to their unique properties, such as negligible vapor pressure, non-flammability, high ionic conductivity, wide electrochemical window, and high thermal, chemical, and electrochemical stability.1−4 The first choice for electrolytes in lithium secondary batteries has been IL-lithium salt binary mixtures.4−14 Lithium-ILs consisting of Li cations as the cationic species and bulky anions have also been studied for this purpose.15,16 However, these two ILs have significant drawbacks such as high viscosity and low Li+ transport property, and consequently, they generally have poor performance in batteries. We have reported that equimolar mixtures of certain Li salts and glymes, i.e., oligo(ethyleneglycol) dimethyl ethers (CH3O(CH2CH2O)nCH3), are in the liquid state at ambient © XXXX American Chemical Society

Received: July 30, 2013 Revised: November 15, 2013

A

dx.doi.org/10.1021/jp407582m | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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Figure 1. Chemical structures of G4, G5, and Na salts.

tigated, focusing on the correlation between their composition and complex formation (phase diagrams). The solvate structures of 1:1 complexes, [Na(glyme)1][X], in the crystalline state were revealed by single-crystal X-ray analyses. From the Raman spectra, quantum mechanical calculations, and the results of structural X-ray analyses, the “liquid state” solvate structure and the coordination manner of the mixtures and 1:1 complexes were elucidated.

molten liquid complexes at ambient temperatures because of the conformational diversity of flexible glyme ligands. Therefore, they behave similar to traditional aprotic ILs.1 The binary mixtures of glymes and Li salts have also drawn the attention of many researchers, and the physicochemical and electrochemical properties of these mixtures, as well as their use as electrolytes for Li-secondary batteries have been reported. Comprehensive studies on the phase diagrams and solvate structures of series of glyme−Li salt mixtures have been performed by Henderson and Brouillette groups.26−30 Bruce et al. have investigated the transport properties of glyme−LiAsF6 complexes, and they found a clear correlation between the solvate structure and the ionic conductivity of the complexes in solid state.31,32 We have also studied systematically the glyme− Li salt mixtures, and found some characteristic aspects of their equimolar mixtures: considerably enhanced oxidative and thermal stability upon complexation,18,19 anomalous solubility,33,34 and strong impact of Lewis basicity of the paired anion species on the nature of the complexes.21 Owing to the remarkable properties mentioned above, the equimolar complexes are fascinating electrolytes for Li secondary batteries using a variety of cathodic and anodic materials, such as LiCoO2, LiMn2O4, and elemental sulfur as the cathode19,33−37 and graphite as the anode.37 As mentioned above, glyme−Li complexes show high potential as electrolytes for Li batteries. On the basis of the similar chemical nature of congeners, a combination of glymes and Na salts should potentially form solvate ILs, and hence, they can be promising electrolytes for Na batteries. Unfortunately, there are few reports on the studies of glyme−Na salt mixtures.38−43 Therefore, fundamental knowledge on these systems is insufficient for applying them as electrolytes. In particular, while phase diagrams usually provide fundamental and important information on the mixtures, there are no phase diagrams for the binary mixtures of glymes and Na salts. In this paper, we report the first systematic study for a series of glyme−Na salt mixtures, hereafter abbreviated as [Na(glyme)n][X], on phase diagrams, solvate structures, and thermal properties. Typical anions of ILs, such as bis( t r ifluoromethanes ulfonyl)amide ([TFSA] − ), bis(fluorosulfonyl)amide ([FSA]−), perchlorate (ClO4−), and hexafluorophosphate (PF6−), were selected as counteranions X (Figure 1). On the basis of the preferential coordination number of 5−7 and stable complexation with certain crown ethers for Na+ cation,38−48 tetraglyme (G4) and pentaglyme (G5) with five and six oxygen atoms within a single molecule, respectively, were selected as ligands. A series of binary mixtures of these glymes and Na salts with different concentrations, [Na(glyme)n][X], were prepared and inves-

2. EXPERIMENTAL SECTION Materials. G4, NaClO4, and NaPF6 in battery-grade were purchased from Kishida Chemical, and Na[FSA] was purchased from Mitsubishi Materials. H[TFSA] and Na2CO3 were purchased from TCI and used as received. G5 was supplied by Nippon Nyukazai. The glymes used in this study were distilled under high vacuum over sodium metal before use. The residual water contents in these liquids, measured by Karl Fischer titration, were less than 50 ppm. Na[TFSA] was synthesized by the neutralization of H[TFSA] with Na2CO3 in methanol according to a previously published procedure.49 All Na salts were dried under high vacuum at 80 °C for 24 h, and stored in an Ar-filled glovebox. A series of glyme−Na salt binary mixtures, with molar ratios (Na salt/glyme) in the range from 0.167 to 0.6 (n = 0.8−5), was prepared by mixing stoichiometric Na salts and glymes in the Ar-filled glovebox. To achieve complete mixing, the mixtures were heated up to 80 °C and stirred for 3 days. Because decomposition of NaPF6 would occur easily in the mixing, only equimolar glyme−NaPF6 binary mixtures were prepared upon dropwise addition of a chloroform solution of each glyme to the NaPF6/chloroform suspension, followed by removing the solvent under vacuum. Measurements. Melting points (Tm), glass transition temperatures (Tg), and other thermal transition temperatures (Tx) were determined using a differential scanning calorimeter (DSC6220, Seiko). The samples were hermetically sealed in aluminum pans in a glovebox. Typical measurements were conducted at a scanning rate of 5 °C min−1 in a temperature range of −150 to 100 °C under a nitrogen atmosphere. To ensure experimental reliability of each DSC result, the samples were thermally crystallized/annealed several times. The onsets of endothermic peaks were taken as Tm and Tx, and the onset in heat capacity change was taken as Tg. A thermogravimetric (TG) measurement was performed on a TG/TDA 6200 (Seiko) to estimate the thermal stability of the studied mixtures. For assessing temperature dependent stability, the samples were heated from room temperature to 550 °C at a heating rate of 10 °C min−1. Isothermal stability of the samples was evaluated by monitoring the weight loss of the samples keeping at 100 or 150 °C for an hour. B



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Figure 2. Phase diagrams of [Na(G4)n][X] paired with [TFSA]− (left), [FSA]− (center), and ClO4− (right) anions.

and solid immiscible binary systems. The Tm values of the systems decrease with the addition of NaX to pure G4, and Tm minima can be observed at a NaX mole fraction of 0.1−0.25. Further addition of NaX leads the elevation of the Tm values. The Tm maxima can be observed at 1:1 molar ratio. These diagrams suggest the formation of stable complexes ([Na(G4)1][X]) in the equimolar binary mixtures. DSC curves (Figure S1, Supporting Information) suggest the presence of metastable complexes with a certain molar ratio other than 1:1. For example, for [Na(G4)n][TFSA], some sharp endothermic peaks are observed at 40 °C before the melting peaks appear. This peak is largest in the [Na(G4)1.4][TFSA] curve, suggesting the presence of a 1.4:1 (or 1.5:1) phase with a Tm value of 40 °C. This sample was thermally crystallized/annealed repeatedly to ensure the complete crystallization of the neat 1.4:1 phase; however, the single phase could not be isolated. The peaks due to the melting of solvent at ca. −25 °C can be observed in the wide concentration range. These results strongly suggest that, although the mixtures of Na[TFSA] and G4 may have potential to form metastable phases other than an equimolar phase, formation of these intermediate phases is less favorable compared to that of the equimolar one. On the other hand, the DSC curves for [Na(G4)n]ClO4 also suggest the presence of 1.4:1 (or 1.5:1) and 1.2:1 phases with Tm of −5 and 60 °C, respectively. Peculiar peaks were found in [Na(G4)1.4]ClO4 and [Na(G4)1.2 ]ClO 4 systems (Figure S1, Supporting Information), and similar results were reproducibly observed. Although these phases have not been isolated yet, this observation suggests that [Na(G4)n]ClO4 may form metastable phases with a molar ratio of 1.4:1 and/or 1.2:1. The other endothermic peaks which cannot be assigned are also reproducible even though the sample pans were repeatedly cooled/annealed. These complicated peaks for the mixtures are plotted in Figure 2 as Tx. The Tm values of [Na(G4)1][X] are listed in Table 1. Although the values of Tm for [Na(G4)1][X] are higher than

Suitable crystals of [Na(G4)1][TFSA], [Na(G4)1]ClO4, [Na(G5)1][TFSA], [Na(G5)1]ClO4, and [Na(G5)1]PF6 for single-crystal X-ray diffraction were obtained by slow condensation of chloroform or acetone solutions. Because of the hygroscopicity of the crystals, each single crystal was mounted on a glass pin coated with a minimal amount of manicure to avoid adsorbing moisture. Crystal evaluations and data collections were performed with a Rigaku Mercury70 diffractometer using graphite monochromated Mo Kα radiation. Diffraction measurements on [Na(G4)1][TFSA], [Na(G4)1]ClO4, [Na(G5)1]ClO4, and [Na(G5)1]PF6 were carried out at −50 °C, except for that on [Na(G5)1][TFSA] at −100 °C, under a stream of nitrogen gas. The structures were solved by direct methods SIR9250 and refined by fullmatrix least-squares SHELXL-97.51,52 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were introduced at ideal positions and refined using the riding model. All calculations were performed using a Crystal Structure crystallographic software package.53 Raman spectra measured with a portable Raman system (RMP-330, Jasco) were excited by a 532 nm line of LD-excited solid laser. The resolution was ca. 2 cm−1. Raman spectra of all samples, except for [Na(G4)1][TFSA] and [Na(G5)1][TFSA] in the liquid state, were measured in the range from 370 to 1570 cm−1 at ambient temperature (22 ± 2 °C). To confirm the coordination environment of the equimolar complexes in the liquid state, the liquid state spectra were collected at 80 ± 0.1 °C using a hot stage with temperature controller (mK1000, Instec). All Raman spectral bands were analyzed for different concentrations or different states, and suitable spectral ranges were adopted to analyze the variations of Raman shifts and wave shapes attributed to representative vibrational modes of each component. The ranges were as follows: glyme, from 780 to 900 cm−1; [TFSA], from 720 to 760 cm−1; [FSA], from 700 to 770 cm−1; ClO4, from 920 to 960 cm−1. Calculations. Ab initio molecular orbital calculations were carried out using the Gaussian 03 program package.54 Normal frequency analyses for [Na(G4)1][TFSA] and [Na(G5)1][TFSA] were performed for the gas phase at the HF/6-311G** level.

Table 1. Melting Points (Tm) of the [Na(glyme)1][X] Series Tm (°C)

3. RESULTS AND DISCUSSION 3-1. Phase Diagrams and Thermal Properties. G4-Na Salt Series. The phase diagrams for the binary mixtures of G4 and three NaX salts (X = [TFSA], [FSA], and ClO4) are shown in Figure 2. The three diagrams are analogous and independent of the paired anionic species. They are typical of liquid miscible C

X

G4

G5

[TFSA] [FSA] ClO4 PF6

71.7 89.6 81.2 130.6

31.7 41.3 82.3 81.5



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Figure 3. (a) Temperature dependent and (b) isothermal TG curves of [Na(G4)n][TFSA] with pure G4 and Na[TFSA]. Isothermal stability was evaluated by monitoring the mass loss keeping temperature at 100 °C.

Figure 4. Phase diagrams of [Na(G5)n][X] paired with [TFSA]− (left), [FSA]− (center), and ClO4− (right) anions.

Supporting Information). These results strongly suggest the only [Na(G4)1]+ complex cations have exceptionally high thermal stability and these cations are still present in the molten state. Furthermore, the results of isothermal stability measurements at 100 °C also prove higher stability of equimolar complexes because of the observation of negligible weight loss under this condition. The remarkable high thermal stability of the 1:1 mixture strongly suggests that all glyme molecules form [Na(G4)1]+ complexes in the mixture. G5-Na Salt Series. The phase diagrams of the binary mixtures composed of G5 and Na salts are illustrated in Figure 4. Similar to [Na(G4)n][X] systems, their diagrams suggest that equimolar mixing of G5 and Na salts also leads to the formation of complexes. The peaks due to the melting of solvent at ca. −20 °C can be observed in the wide concentration range, except for [Na(G5)n][TFSA]. For [Na(G5)n][TFSA], no thermal events in addition to glass transition are observed in the range of 0.33−0.5 (Figure S4, Supporting Information), which is the so-called crystallinity gap. It is known that crystallinity gaps also exist in some glyme−Li systems,26 and several factors, such as solvate structures, steric interaction, and flexibility of components, cause these phenomena. The gap observed for [Na(G5)n][TFSA] probably appears because of the similar factors. In the DSC curves for [Na(G5)n]ClO4 (Figure S4, Supporting Information), endothermic peaks are observed at ca. 50 °C in the range of 0.25−0.6 before melting. The peak intensity and shape vary depending on the salt concentration, and these DSC results are reproducible through thermal cycling measurements. As well as [Na(G4)n]ClO4, observation of these

those for the corresponding Li−glyme equimolar complexes [Li(G4)1][X],26 the [Na(G4)1][X] complexes, except for [Na(G4)1]PF6, melt below 100 °C. Figure 3 shows temperature dependent and isothermal TG curves for the binary mixtures of Na[TFSA] and G4. The TG results for [Na(G4)n][X] (X = FSA and ClO4) are included in the Supporting Information (Figures S2 and S3). As shown in Figure 3a, the temperature dependent thermal stability of the mixture gradually increases with salt concentration, and the equimolar complex ([Na(G4)1][TFSA]) has significantly high stability up to 200 °C. It is noted that the mass loss of these systems below 410 °C is attributed to the evaporation of glymes. The high stability of the equimolar mixture suggests that the complexation of glymes with metal cations inhibits the evaporation. The strong interaction between the cations and the oxygen atoms of the glymes makes the equimolar complex thermally stable.19 In dilute solutions (n > 1), the weight losses occur in three steps. Uncoordinated and/or weakly coordinated glymes exist in the solutions, and first, they evaporate at ca. 150 °C, leading to yield [Na(G4)1][TFSA] at ca. 220 °C. Second, desolvated glyme molecules evaporate from [Na(G4)1][TFSA] in the temperature range from 220 to 410 °C to yield Na[TFSA] at 410 °C. Finally, decomposition of Na[TFSA] occurs. Similar results can be observed for [Na(G4)n][FSA] and [Na(G4)n]ClO4 systems (Figure S2, Supporting Information). DSC results suggested the mixtures of Na[TFSA] and G4 may form a 1.4:1 (or 1.5:1) metastable phase; however, the presence of this phase is indistinct in the temperature dependent TG curves for the diluted mixtures. This phenomenon is apparent in particular for the case of [Na(G4)n]ClO4 (Figure S2, D



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Figure 5. (a) Temperature dependent and (b) isothermal TG curves of [Na(G5)n][TFSA] with pure G5 and Na[TFSA]. Isothermal stability was evaluated by monitoring the mass loss keeping the temperature at 150 °C.

Table 2. Crystallographic Data of [Na(G4)1][TFSA], [Na(G4)1]ClO4, [Na(G5)1][TFSA], [Na(G5)1]ClO4, and [Na(G5)1]PF6 chemical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) μ (mm−1) temp (°C) reflections collected independent reflection, Rint R1 [I > 2σ(I)] wR2 (all data) GooF

[Na(G4)1][TFSA]

[Na(G4)1]ClO4

[Na(G5)1][TFSA]

[Na(G5)1]ClO4

[Na(G5)1]PF6

C12H22F6NNaO9S2 525.41 triclinic P1̅ 8.221(3) 9.012(3) 15.966(5) 80.724(10) 88.677(10) 68.189(9) 1082.9(6) 2 1.611 0.3603 −50 9177 5521, 0.0280 0.0524 0.1503 1.065

C10H22ClNaO9 344.72 monoclinic P21 12.539(4) 9.781(3) 13.473(5) 90 91.773(4) 90 1651.5(9) 4 1.386 0.2938 −50 12241 6299, 0.0295 0.0815 0.2429 1.020

C14H26F6NNaO10S2 569.46 orthorhombic Pbca 13.5214(5) 16.1932(8) 22.1855(9) 90 90 90 4857.6 8 1.557 0.3308 −100 38917 6912, 0.0426 0.0604 0.1588 1.164

C12H26ClNaO10 388.77 monoclinic Cc 12.1380(11) 30.924(3) 15.341(2) 90 107.119(4) 90 5503.4(10) 12 1.408 0.2771 −50 233.98 9963, 0.0419 0.0580 0.1569 1.052

C12H26F6NaO6P 434.29 monoclinic P21/n 9.568(3) 17.117(6) 12.039(4) 90 96.437(4) 90 1959.2(11) 4 1.472 0.2414 −50 16330 5272, 0.0453 0.0742 0.2171 1.093

(poor R1 and wR2 values). The structures obtained for these complexes are not the exact crystal structures, but they provide useful information especially on the effects of anions on the coordination environment. Thus, the solvation and packing structures of [Na(G4)1][FSA] and [Na(G5)1][FSA] are included in the Supporting Information (Figures S12 and S13). In this section, we discuss the effects of glyme chain length, anionic structures, and alkali metal species on the solvate structures. G4-Na Salt Series. Figure 6 illustrates the solvate structures of [Na(G4)1][TFSA] and [Na(G4)1]ClO4 in the crystalline state. In the crystal of [Na(G4)1][TFSA], the Na+ cation is coordinated by five oxygen atoms from a single G4 and two oxygen atoms from two different [TFSA]− anions (sevencoordinate), resulting in the formation of column-like aggregated structures along the b-axis (Figure 6a). The solvate structure of [Na(G4)1][TFSA] is classified as AGG-type (aggregate) solvate. On the other hand, the coordination manner of the anions to Na+ cation in the [Na(G4)1]ClO4 crystal differs substantially (Figure 6b), although the coordination number of the Na+ cation is identical. The Na+ cation is coordinated by five oxygen atoms of a single G4 and two

peaks suggests the presence of intermediate phases (probably a 1.5:1 phase) other than the equimolar phase. These peaks are also shown in Figure 4 as Tx. The Tm values of [Na(G5)1][X] are lower than those of [Na(G4) 1][X] counterparts, with an exception of the equimolar mixture with NaClO4 (Table 1). Temperature dependent and isothermal TG curves for [Na(G5)n][TFSA] series are shown in Figure 5. The data for [Na(G5)n][FSA] and [Na(G5)n]ClO4 are shown in the Supporting Information (Figures S5 and S6). Three-step weight loss can also be seen in the temperature dependent TG measurements for the diluted solutions, [Na(G5)n][X] (n > 1). The isothermal stabilities of [Na(G5)n][TFSA] improve with increasing salt concentration, as seen for [Na(G4)n][TFSA]. These results also support the formation of the highly stable [Na(G5)1]+ complex. 3-2. Solvate Structures in the Crystalline State. The crystallographic data of [Na(glyme)1][X] are summarized in Table 2, and the thermal ellipsoid models of the solved structures are shown in Supporting Information (Figures S7− S11). Due to the considerably low crystallinity and high hygroscopicity of equimolar [Na(G4)1][FSA] and [Na(G5)1][FSA] complexes, refinements could not be accomplished E



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[Na(G4)1][TFSA]. The difference probably arises from different ionic radii of the alkali-metal cations. G5-Na Salt Series. The solvate structures in [Na(G5)1][TFSA], [Na(G5)1]ClO4, and [Na(G5)1]PF6 crystals are depicted in Figure 7. They are classified as CIP-I-type (contact ion pair with a monodentate anion) solvates, with the exception of [Na(G5)1]ClO4. Contrary to [Na(G4)1][X] solvates, the effects of anion on the structures of [Na(G5)1][X] solvates are

Figure 6. Packing diagrams for [Na(G4)1][TFSA] along the a-axis (a, upper) and [Na(G4)1]ClO4 along the c-axis (b, lower). H atoms are omitted for clarity. Deep yellow, Na; red, O; gray, C; blue, N; yellow, S; green, F; pale green, Cl. Two independent solvates exist in [Na(G4)1]ClO4.

oxygen atoms of a single ClO4− anion in bidentate form, resulting in CIP-II-type (contact ion pair with a bidentate anion) solvate structure. The packing structure of [Na(G4)1]ClO4 is also different from that of Na(G4)1][TFSA], as illustrated in Figure 6. The CIP-type ion pairs are arranged in a zigzag pattern in the crystal of [Na(G4)1]ClO4, whereas [Na(G4)1][TFSA] solvates are aligned in one-dimensional order. Both solvate and packing structures of [Na(G4)1][FSA] crystal are similar to those of [Na(G4)1][TFSA] crystal (Figure S12, Supporting Information). The different packing probably arises from the different coordination manner of the anion species. Although the solvate structures observed in [Na(G4)1][TFSA] and [Na(G4)1]ClO4 crystals are different, the coordination manner of the glyme to Na+ cations is similar in the two structures. In both solvates, the glyme wraps around the central Na+ cation, as seen in [Na(15-crown-5)]+. The conformations of the glymes in the two crystals are similar: gg′ttg′t-tg′t-tgt and tgt-tg′t-g′g′t-tgt for [Na(G4)1][TFSA] and [Na(G4)1]ClO4, respectively (Table S1, Supporting Information, t and g mean trans and gauche conformations, respectively). The Na−Oglyme distances in [Na(G4)1][TFSA] crystal (2.426−2.496 Å) are close to those in [Na(G4)1]ClO4 crystal (2.380−2.494 Å), as listed in Table S2 (Supporting Information). The [TFSA]− anions in the [Na(G4)1][TFSA] crystal coordinate to Na+ cations in a unique manner (Figure 6a). Indeed, AGG-type crystalline structures have also been reported for the glyme−Li solvates with [TfO]−, ClO4−, and AsF6− anions,28 but they are quite different from that of

Figure 7. Packing diagrams for [Na(G5)1][TFSA] (a, upper), [Na(G5)1]ClO4 (b, middle), and [Na(G5)1]PF6 (c, lower) along the b-axis, respectively. H atoms are omitted for clarity. Deep yellow, Na; red, O; gray, C; blue, N; yellow, S; green, F; light green, Cl; cream orange, P. Both 7- and 8-coordinate solvates exist in [Na(G5)1]ClO4. F



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for Na[TFSA],54 resulting in the situation where ClO4− anion interacts with Na+ cation in a 1:1 relationship. Thus, the stronger interaction is probably the cause of the CIP-type solvate structure in the [Na(G4)1]ClO4 crystal. Comparison with Glyme-Li Solvates. Henderson et al. have already reported the solvate structures for a series of glyme−Li complexes.27−29 The coordination manner of glymes and anions in the glyme−Li complexes changes drastically with glyme chain length and the paired anion species. On the other hand, the effects of these factors on the solvate structures of [Na(glyme)1][X] are not significant, and the solvate structures are similar to 15-crown-5-based complexes. The different ionic radii can be a reason for this difference. Although it is well-known that Li, Na, and K salts form stable complexes with 12-crown-4, 15-crown-5, and 18-crown-6 ethers, respectively,22,45,46,56,57 the ionic radius of Li+ cation is slightly larger than the cavity size of 12-crown-4 ether. In fact, binary mixtures of 12-crown-4 ethers and certain Li salts form 2:1 sandwich complexes even when an equimolar mixture was prepared.23 Thus, in the case of glyme−Li complexes, it can be anticipated that solvate structures analogous to 12-crown-4 ether complexes are less favorable than other structures, such as polymeric and dimeric crystalline solvates in the form of [Lin(glyme)n] and [Li2(glyme)2], respectively. On the other hand, because the ionic radius of Na+ cation matches the cavity size of 15-crown-5 ether, Na+ cation has a high affinity with the crown ether and certain glymes. Thus, the coordination of certain glymes to Na+ cations produces highly stable complexes. Moreover, the Na−Oanion distance in [Na(G4)1][TFSA] is considerably longer than Li−Oanion in [Li(G3)1][BETI], suggesting that a larger Na+ cation leads to weaker interionic interaction compared with Li+ cation. This is also related to the fact that Na+ cations are prone to form stable 1:1 complexes with G4 or G5, similar to the complex of Na+ and 15-crown-5 ether. Therefore, it can be concluded that the akin structure observed in [Na(glyme)1][X], independent of both the glyme chain length and the paired anion species, originates from the relatively large Na+ cation. Correlation between Solvate Structures and Thermal Properties. For typical ILs, there are many reports on the correlation between crystal structures and thermal properties.58−60 Interestingly, a correlation between the solvate structure and Tm is observed for the equimolar glyme−Na salt complexes. As listed in Table 1, the Tm value for [Na(G4)1]ClO4 is comparable to that for [Na(G5)1]ClO4, although the values for other [Na(G4)1][X] complexes are higher than those for the corresponding [Na(G5)1][X]. It is likely that the solvate structure of the crystal is one of the important factors determining the Tm. In typical salts, increase in the cation and/or anion size leads to lower Tm because of weaker Coulombic interactions. However, the results observed for the complexes paired with ClO4− anions suggest that the ionic volumes of the complex cations are not related to Tm values. Thus, the packing structures rather than the similar coordination (ion pair) structures affect effectively their phase transition behavior. As illustrated in Figures 7c and S14 (Supporting Information), the packing structures for [Na(G4)1]ClO4 and [Na(G5)1]ClO4 are similar, which gives comparable Tm values for these complexes. On the other hand, the packing structures with other anions change drastically with glyme chain length. [Na(G4)1][TFSA] and [Na(G4)1][FSA] form aggregated structures connected by amide anions (Figures 6a and S12, Supporting Information),

not clearly observed. The seven-coordinate Na+ cation is coordinated by six oxygen atoms of a single G5 and one oxygen or fluorine atom of an anion, that is, an oxygen atom of [TFSA]− or ClO4− anions or a fluorine atom of PF6− anion (see Figures S9−S11, Supporting Information). For [Na(G5)1]ClO4, there are three independent ion pairs in the unit cell. Among them, one ion pair consists of bidentate ClO4− anions, resulting in eight-coordinate Na+ cations (CIP-II). As described above, the coordination manner of the glymes in [Na(G5)1]+ is highly different from that in the glyme−Li solvates. The structure of four ethylene oxide (EO) units containing five oxygen atoms in G5 is close to the planar structure of the [Na(15-crown-5)]+ complex. However, the last oxygen atom in the remaining EO unit coordinates to the Na+ cation from the foreside of the crown plane constructed by four EO units. The conformations of the glyme in these solvate cations in [Na(G5)1][TFSA], [Na(G5)1]ClO4 (the CIP-II solvate), and [Na(G5)1]PF6 are tg′t-tgt-tg′t-tgg-tgt, tgt-tg′t-tgttg′g′-tg′t, and tg′t-tgt-tg′t-tgg-tgt, respectively. The conformation of the fourth EO unit of G5 in these three solvates is tgg. The torsion angles of C8−C9−O5−C10 bonds (the fourth EO unit) are approximately 75°, which leads to the upstanding conformation of the fifth EO unit from the 15-crown-5 plane, and enables the foreside coordination of the oxygen atom in the fifth EO unit to the Na+ cation. The anions coordinate to Na+ cations from the opposite side, leading to the formation of characteristic CIP solvates. The Na−Oglyme distances in the [Na(G5)1][TFSA], [Na(G5)1]ClO4, and [Na(G5)1]PF6 crystals are 2.399−2.476, 2.387−2.623, and 2.444−2.523 Å, respectively. Among the three independent ion pairs in the [Na(G5)1]ClO4 crystal, a longer Na−Oglyme distance, 2.623 Å, was found in the CIP-II solvate. This phenomenon could arise from interionic interaction. Stronger interaction between the bidentate ClO4− anion and coordinated Na+ cation would screen the positive charge of Na+ cation, resulting in the weakening of the iondipole interaction between the Na+ cation and the ligand glyme. In other words, strong ion−ion (Na−X) interaction leads to weak ion−solvent (Na−glyme) interaction. Consequently, the Na−Oglyme distances for that solvate become longer. As shown in Figure 7, although the solvate types in these three complexes are identical, the assembling structures of the solvates are different, depending on the paired anion species. For [Na(G5)1]ClO4 and [Na(G5)1]PF6, the complex cations and anions are arranged along the b-axis, but the CIP ion pairs are arranged in a slightly different manner for the solvates paired with ClO4− and PF6− anions. On the other hand, the CIPs in [Na(G5)1][TFSA] are arranged alternately, resulting in a zigzag pattern. The bulkiness of the [TFSA]− anion probably causes the resulting peculiar packing diagrams. The comparison of [Na(G4)1][TFSA] and [Na(G5)1][TFSA] reveals that the glyme length affects the solvate structures in the crystalline state. The coordination numbers of Na+ cations in both solvate structures are equal (seven). Thus, the difference in the solvate types is attributed to different oxygen atom number of glymes. In contrast, the solvate structures of [Na(G4)1]ClO4 and [Na(G5)1]ClO4 belong to similar solvate types. The dependence of the solvate type on the anion is closely related to the dissociation energy of NaX salts, because the cation−anion interactions of the salts are important for the solvate structures. The intrinsic dissociation energies of the Na salts calculated by density functional theory suggest that ion−ion interaction is stronger for NaClO4 than G



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Figure 8. Concentration dependence of the Raman spectra in the range 780−900 cm−1 for [Na(G4)n][X] with [TFSA] (left), [FSA] (center), and ClO4 (right) anions. The Raman spectrum of pure G4 was also included in each figure. The captions “ss” and “ls” in each figure correspond to “solid state” and “liquid state”, respectively.

Figure 9. Concentration dependence of the Raman spectra in the range 780−900 cm−1 for [Na(G5)n][X] with [TFSA] (left), [FSA] (center), and ClO4 (right) anions. The Raman spectrum of pure G5 was also included in each figure. The captions “ss” and “ls” in each figure correspond to “solid state” and “liquid state”, respectively.

[TFSA] and [Na(G5)1][FSA] can be explained in terms of the conformational diversity of the counteranion. 3-3. Raman Spectra. In the case of binary mixtures of crown ethers and certain alkali-metal salts, a characteristic Raman band appears at 870 cm−1 when the mixtures form complexes.68 This band corresponds to the combination of CH2 rocking and COC stretching modes of the coordinating ether molecules, and is called “ring breathing mode”. The intense band also appears at ca. 870 cm−1 when a glyme coordinates to Li cation to form a crown-like complex.30 In the presence of dimeric solvates in the form of [Li2(glyme)2]2+, that band appears at ca. 890 cm−1, shifted to the high-frequency side.69 Therefore, this band is one of the fingerprint modes for the formation of 1:1 complexes, and we can speculate the complex structures from the band position. Complex Cations. Raman bands corresponding to the intramolecular vibrational modes of the ligand glymes appear in the range 780−900 cm−1.30,69 In this study, we took these bands into account to discuss the coordination states of the glyme molecules. Figures 8 and 9 show the concentration dependence of the Raman spectra for [Na(G4)n][X] and [Na(G5)n][X], respectively. The spectral correction was performed by using the salt concentration and the peak areas of representative anion bands. In Figures 8 and 9, the Raman intensity at ca. 855 cm−1, assignable to pure glymes, decreases with an increasing amount of Na salts, and simultaneously, a sharp peak, corresponding to the breathing mode, emerges at ca. 870 cm−1. This result suggests that the binary mixtures of glymes and Na salts form crown-like complexes, as found by crystallography, and formation of the [Na(glyme)1]+ complex

whereas [Na(G5)1][TFSA], [Na(G5)1]PF6, and [Na(G5)1][FSA] belong to CIP-type solvates, and certain packing structures related to these CIPs are observed (Figures 7 and S13, Supporting Information). Therefore, the higher Tm values observed for [Na(G4)1][X] than for the corresponding [Na(G5)1][X] can be explained by different packing structures. In addition, this means that the solvate structure of [Na(G4)1]PF6 can be anticipated to belong to AGG type, although the exact crystal structure has not been solved yet. The Tm values of [Na(G5)1][X] with different anions follow the order ClO4− ≈ PF6− > [FSA]− > [TFSA]−. Since the coordination manner in these complexes is similar, it is suggested that the differences in Tm values arise predominantly from the anions, that is, from the interaction energy with Na+ cation and conformational diversity. The calculated dissociation energies of Na salts at the G4MP2 level follow the order ClO4− > PF6− > [FSA]− > [TFSA]− with values 502.6, 494.4, 490.1, and 479.9 kJ mol−1, respectively.55 This order agrees well with the order of Tm values for [Na(G5)1][X], indicating that the interionic interactions between Na+ cations and counteranions affect significantly the Tm. In addition to the dissociation energy, it is well-known that the conformational diversity of the components also influences the thermal properties of typical ILs.61−64 Amide-type anions have cisoid and transoid conformers with a very little rotational barrier.65−67 Tm can be written as Tm = ΔHm/ΔSm, where ΔHm and ΔSm are the enthalpy and entropy of fusion, respectively. The multiple conformers lead to larger ΔSm, and hence, many organic salts paired with these anions tend to have lower Tm values. Therefore, the substantially lower Tm values of [Na(G5)1]H



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proceeds by consuming the free glymes upon the addition of Na salts. In the case of 1:1 mixtures, the broad Raman bands attributable to various conformers (vibrational modes) of free glymes vanished, indicating that almost all the glyme molecules coordinate to Na+ cations to form crown-like complexes. Except for [Na(G5)1][TFSA], because no shifts were observed in the vibrational frequencies of the breathing modes, the similar complex cations, [Na(G4 or G5)1]+, exist even at low salt concentrations. These results are consistent with results of DSC curves shown in Figures S1 and S2 (Supporting Information), where the phase transition behavior of free glymes vanished at the equimolar composition and the thermal transitions of the complexes can be seen even at low concentrations. On the other hand, an apparent spectral change can be seen in [Na(G5)n][TFSA], where the breathing mode of the equimolar mixture shifts to the low-frequency side. Because the bands for [Na(G5)n][TFSA] (n > 1) appear at positions nearly identical to those for [Na(G4)n][X], this result suggests that the coordination manner of G5 to Na+ cations in [Na(G5)1][TFSA] (solid state) is slightly different from that in [Na(G5)n][TFSA] (n > 1, liquid state). Furthermore, the prominent peaks at ca. 835 cm−1 emerge with increasing concentration of salts. This indicates that not only the breathing mode at ca. 870 cm−1 but also the Raman band at ca. 835 cm−1 can be identified as a fingerprint mode for the glyme−Na complex cation formation. As listed in Tables 3 and 4, characteristic Raman bands can be confirmed at ca. 815 cm−1 in the Raman spectra of

Figure 10. Measured Raman spectra and calculated bands in the range 780−900 cm−1 for [Na(G5)1][TFSA] (a) and [Na(G4)1][TFSA] (b).

[TFSA] and [Na(G5)1][TFSA] in the range 780−900 cm−1. The selected Raman frequencies obtained from the calculations are listed in Table 5. The atomic coordinates for initial

Table 3. Selected Vibrational Frequencies of the Coordinating Glymes and Counteranions in the Measured Raman Spectra for [Na(G4)1][X]

Table 5. Selected Vibrational Frequencies of the Coordinating Glymes and [TFSA]− Anions in the Calculated Raman Spectra for [Na(G4)1][TFSA] and [Na(G5)1][TFSA]

Raman shift (cm−1) [Na(G4)1][TFSA] [Na(G4)1][FSA] [Na(G4)1]ClO4 coordinating glyme

counter anions

870

866

869

854 841 832 741

851 835 827 725

852 837

Raman shift (cm−1)

Raman shift (cm−1)

anion (S−N stretching)

706

877 857 849 842 838 818 706

geometries were obtained from the crystal structures. While slight differences can be seen in the observed and calculated Raman shifts, the calculated frequencies agree well with the measured ones. For [Na(G5)1][TFSA], the measured Raman band observed at ca. 815 cm−1 corresponds to the calculated band at 818 cm−1, and the corresponding vibrational mode was assigned to the predicted mode, as mentioned above. A similar result can be seen in the experimental and calculated Raman spectra for [Na(G5)1]PF6 (Figure S13, Supporting Information). Because this band appears to be independent of the paired anion species, we can predict the solvate structure from the band at ca. 815 cm−1 in the Raman spectra. Paired Anion Species. In addition to the Raman bands of complex cations, the bands assignable to each anion also provide useful information on the formation of the complexes. To consider the effect of concentration on the coordination environment of each anion, the Raman spectra involving the representative vibrational modes of each anion for [Na(G4)n]-

[Na(G5)1][TFSA] [Na(G5)1][FSA] [Na(G5)1]ClO4

counter anions

[Na(G5)1][TFSA]

879 858 845 840 826

935

Table 4. Selected Vibrational Frequencies of the Coordinating Glymes and Counteranions in the Measured Raman Spectra for [Na(G5)1][X]

coordinating glyme

[Na(G4)1][TFSA] coordinating glyme

863

867

864

846 835 828.0 814 743

851 834

850 833

815 723

816 935

[Na(G5)1][X]. Since these bands appear only in the spectra of [Na(G5)1][X] independently of the paired anion species, these bands arise from the vibrational modes of the EO units coordinating to Na+ cations from the foreside of the 15-crown5 plane. Vibrational analysis was carried out using ab initio calculations to assign the Raman spectra. Figure 10 shows the experimental and calculated Raman spectra for [Na(G4)1]I



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Figure 11. Concentration dependence of the Raman spectra in the range of representative vibrational modes of each anion for [Na(G4)n][TFSA] (left), [Na(G4)n][FSA] (center), and [Na(G4)n]ClO4 (right). The captions “ss” and “ls” in each figure correspond to “solid state” and “liquid state”, respectively.

Figure 12. Concentration dependence of the Raman spectra in the range of representative vibrational modes of each anion for [Na(G5)n][TFSA] (left), [Na(G5)n][FSA] (center), and [Na(G5)n]ClO4 (right). The captions “ss” and “ls” in each figure correspond to “solid state” and “liquid state”, respectively.

the others. Certainly, this observation partially corresponds to the fact that the binary mixtures of G4 and Na[TFSA] could form a 1.4:1 (1.5:1) phase other than equimolar composition; however, a neat 1.4:1 phase has not been isolated. In addition, the peak shift is within the resolution of the spectrometer. Hence, it is difficult to conclude the presence of a 1.4:1 (or 1.5:1) phase in G4−Na[TFSA] mixtures from these spectral variations. On the other hand, for [Na(G5)n][TFSA], the position of the anionic band slightly shifts to the high-frequency side with increasing salt concentration, implying that the coordination manner of [TFSA]− anions is concentrationdependent. It has been reported for the glyme−Li[TFSA] mixtures that the bands corresponding to the S−N stretching mode of [TFSA]− anion also shift to the high-frequency side with increasing concentration,29 because [TFSA]− anions enter from uncoordinating (solvent-separated ion pair, SSIP) to coordinating (CIP) state. The spectral shifts in [Na(G5)n][TFSA] with increasing concentration are probably attributed to a similar coordination variation of [TFSA]− anion. In the case of [Na(glyme)n][FSA], the bands corresponding to the S−N stretching mode of [FSA]− anions shift to the highfrequency side with increasing concentration, as well as [Na(G5)n][TFSA]. This spectral variation can be attributed to the change in the coordination manner of [FSA]− anions. According to the resemblance of the structures of [TFSA]− and [FSA]− anions, it is strongly suggested that Na[FSA] salts are dissociated, and [FSA]− anions are not involved in the coordination to Na+ cations in diluted systems. With increasing concentration, the glyme−Na[FSA] mixtures will form CIP and/or AGG solvates.

[X] and [Na(G5)n][X] are extracted and shown in Figures 11 and 12, respectively. The band correction method is identical to that for the complex cations. The spectra clearly indicate that the salt concentration effects on the anionic Raman spectra depend on the paired anion species. As shown in Figures 6a and 7a, while the solvate structure for [Na(G4)1][TFSA] is somewhat similar to that for [Na(G5)1][TFSA], the coordination manner of [TFSA]− anion to Na+ cations is different. In [Na(G4)1][TFSA], two sulfonyl oxygen atoms within a single [TFSA]− anion coordinate to two different Na+ cations. On the other hand, one oxygen atom within a single [TFSA]− anion coordinates to Na+ cation in [Na(G5)1][TFSA]. This difference would lead to changes in the corresponding Raman shift. In the case of glyme−Li salt mixtures with [TFSA]− anions, it is well-known that the Raman active S−N stretching modes are particularly sensitive to changes in the environment of the anions.30 In the case of [Na(glyme)1][TFSA], the S−N stretching mode of [TFSA]− anions is different for [Na(G4)1][TFSA] and [Na(G5)1][TFSA] in the crystalline state, indicating that a different coordination manner of the [TFSA]− anion in [Na(glyme)1][TFSA] also affects the S−N stretching modes of [TFSA]− anions. The difference in anionic bands observed in the spectra for [Na(G4)1][FSA] and [Na(G5)1][FSA] can be explained in a similar manner. The band position of [TFSA]− anion for [Na(G4)1.2][TFSA] and [Na(G4)1.4][TFSA] seems to be slightly shifted to the low-frequency side compared to those for other compositions, suggesting that the coordination manner of [TFSA]− anion in these two mixtures is different from those in J



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Figure 13. Cationic (a) and anionic (b) Raman spectra of [Na(G4)1][TFSA] and [Na(G5)1][TFSA] in the crystalline (lower) and liquid (upper) states. The dashed vertical lines in the spectra are guides for the eye.

variation was observed in both the anionic and cationic spectra for [Na(G5)n]ClO4, while the presence of intermediate phases is also suggested by their DSC curves. This result would mean that the binary mixture of G5 and NaClO4 forms CIP-type solvates like the structure of the equimolar complex in a wide concentration range. Solvate Structures in the Liquid State. Comparing the Raman spectra of crystalline and liquid states of 1:1 complexes can provide important information on structural differences at these states. In particular, information on the coordination manner of [TFSA]− anion in the liquid state for [Na(G4)1][TFSA], which forms an aggregated structure connected by [TFSA]− in the crystalline state, is most intriguing. The cationic and anionic Raman spectra in the crystalline and liquid states for [Na(glyme)1][TFSA] are shown in Figure 13. The physical state does not affect the breathing modes for [Na(G4)1][TFSA]. On the other hand, the band assignable to the breathing mode for [Na(G5)1][TFSA] in the crystalline state shifts significantly by melting. The spectral shape for [Na(G5)1][TFSA] in the liquid state is nearly identical to that for [Na(G4)1][TFSA]. This result strongly suggests that the solvate cation structure, [Na(G5)1]+, is different in liquid and crystalline states, even though the glymes still coordinate to Na+ cations. In contrast, in the case of anionic spectra, although no significant change in the anionic band can be confirmed for [Na(G5)1][TFSA], the band for [Na(G4)1][TFSA] in the crystalline state shifts to the high-frequency side upon melting, and the resulting band position is consistent with that for [Na(G5)1][TFSA]. As shown in Figures 6a and 7a, [TFSA]− anions in [Na(G4)1][TFSA] coordinate to Na+ cations in a manner different from [Na(G5)1][TFSA], leading to different solvate typesAGG for the former and CIP for the latter. Therefore, it can be concluded that the AGG structure of [Na(G4)1][TFSA] in the crystalline state is broken by melting, and consequently, the solvate cations and anion are predominantly as CIP in the liquid state.

For [Na(glyme)n]ClO4, the intense bands observed at around 935 cm−1 are known to correspond to the symmetrical stretching mode of ClO4− anions. The structural analyses on [Na(glyme)1]ClO4 revealed that only bidentate ClO4− anions exist in the crystalline [Na(G4)1]ClO4 (CIP-II), whereas the crystalline [Na(G5)1]ClO4 is composed of a mix of bidentate and monodentate ClO4− anions (CIP-I and CIP-II). Focusing on the [Na(G5)1]ClO4 spectra, a small shoulder can be seen at ca. 925 cm−1, which probably arises from CIP-I ClO4− anion, because it is absent in the [Na(G4)1]ClO4 spectra. Also, Henderson et al. reported for glyme−LiClO4 mixtures that the band corresponding to CIP-I ClO4− anions appeared at around 927 cm−1.70 Therefore, the peaks observed at 935 cm−1 would be assigned to the symmetrical stretching modes of CIP-II ClO4− anions. Characteristic concentration dependence can be seen in the [Na(G4)n]ClO4 spectra. Although an intense band appears at 935 cm−1 for the equimolar composition, it shifts to the lowfrequency side in diluted compositions. Cvjetićanin reported that the bands corresponding to uncoordinating ClO4− anions, namely, SSIP, can be observed at lower frequency than those corresponding to CIP for 1 M solution of NaClO4 in PC/H2O mixed solvents.71 Assuming that the spectra of [Na(G4)n]ClO4 also exhibit similar behavior, the obtained spectra would imply that SSIP-type solvates predominantly exist in the diluted composition. As mentioned above, for the [Na(G4)n]ClO4 system, the presence of 1.4:1 (or 1.5:1) and 1.2:1 phases is suggested from DSC curves (Figure S1, Supporting Information). Their anionic Raman spectra show good agreement with the DSC results, implying the formation of 1.4:1 and 1.2:1 phases. Moreover, the band position of breathing modes, representative cationic vibrational modes, is irrespective of the salt concentration. This observation suggests that these intermediate complexes are composed of predominantly SSIP-type solvates involving the solvated cations analogous to the structure of [Na(G4)1]+ cation. In contrast, no significant K



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Interestingly, the liquid state [Na(G4)1][TFSA] and [Na(G5)1][TFSA] possess appreciable ionic conductivity (approximately 0.6 mS cm−1 at 30 °C for [Na(G5)1][TFSA]) and high ionicity (data not shown) comparable to that of typical imidazolium-based ILs and [Li(G3)1][TFSA] without any other additional solvent, although they form CIP in the liquid state as detected by Raman spectroscopy. [Li(G3)1][TFSA] also forms non-charged CIP-type solvate in the liquid state.18,30 If such CIP structure is persistent in the liquid state, it should have low ionic conductivity and low ionicity. This discrepancy is understandable if the 1:1 mixtures undergo ion-exchange reactions, i.e., dynamic coordination exchange between [Na(glyme)1]+ cation and [TFSA]− anion, in the liquid state. A CIP structure in the liquid state detected by Raman spectroscopy is a snapshot with a time scale

Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes

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