Structural Studies of Lithium 4,5-DicyanoimidazolateâGlyme Solvates

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Structural Studies of Lithium 4,5-Dicyanoimidazolate – Glyme Solvates. I. From Isolated Free Ions to Conductive Aggregated Systems. Piotr Jankowski, Maciej Dranka, Grazyna Zofia Zukowska, and Janusz Zachara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01352 • Publication Date (Web): 03 Apr 2015 Downloaded from http://pubs.acs.org on April 11, 2015

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The Journal of Physical Chemistry

Structural Studies of Lithium 4,5Dicyanoimidazolate – Glyme Solvates. I. From Isolated Free Ions to Conductive Aggregated Systems. Piotr Jankowski, Maciej Dranka,* Grażyna Z. Żukowska,* and Janusz Zachara. Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warszawa, Poland.

ABSTRACT: In this paper, we present complementary series of crystal structures of lithium salts containing 4,5-dicyanoimidazolato anions substituted with perfluoroalkyl groups. Singlecrystal X-ray analysis of ten adducts with aprotic solvents: glymes – dimethyl ethers of poly(ethylene glycols) –and crown ethers have been performed to correlate their molecular structures and properties with spectroscopic and thermal data. Comprehensive structure analysis of crystalline materials reveals valuable information about coordination ability of substituted 4,5dicyanoimidazolato anions and provides the basis to develop the model of poly(ethylene oxide) electrolytes and liquid systems. Presented results reveal new aggregation modes at high concentrations of lithium salts involving releasing cations by self-assembly of anionic subnetwork and shed some light on electrochemical performance of TDI anions.

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KEYWORDS: electrolytes, crystal structures, dicyanoimidazoles, aggregation, lithium salts.

1. INTRODUCTION Highly dissociated systems used as charge carriers in battery electrolytes are usually composed of salts characterized by weakly interacting anions with delocalized charge, such as PF6−, AsF6− and TFSI−.1,2 However, the first two undergo decomposition with formation of toxic products in the presence of moisture traces. Due to the above mentioned disadvantage, there is still need for development of new salts. Although salts based on cyano substituted imidazoles or triazoles were known from late 1980’s, the idea of their application in lithium or sodium batteries was first proposed by Armand in early 2000’s.3,4,5 Since then, various salts based on heterocyclic compounds with five-membered rings, such as 4,5-dicyanoimidazole or 4,5-dicyanotriazoles derivatives were obtained.6,7,8 Electrochemical properties of the systems doped with 4,5-dicyano2-trifluoromethyl imidazole lithium salt (LITDI) and 4,5-dicyanotriazole lithium salt (LiDCTA) were intensively studied by Wieczorek and Johansson groups.7 However, the first structural characterization of LITDI salts in the solid state has been performed for acetonitrile solvates9 and ionic liquid mixtures.10 Crystallographic studies allowed for a better understanding of electrochemical properties of TDI salts. X-ray structures of LiTDI and LiDCTA- glyme solvates and sodium salts with propylene carbonate have been reported only recently by Henderson11 and Marczewska,12 respectively. Polymer electrolytes based on poly(ethylene oxide) are known since late 1970’s.13 Conducting properties of PEO solvates depend on the type and amount of the salt used as a dopant, affecting the level of salt dissociation and crystallinity of the systems.14 Although many other polymers and co-polymers have been developed since then, high-molecular-weight poly(ethylene oxide) remains the most popular matrix for solid polymer electrolyte. Unfortunately, the knowledge of


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the exact structure based on diffraction data is unavailable in the case of PEO complexes, due to difficulties with obtaining crystals of required quality.15 Therefore, crystalline complexes of the appropriate salt with short-chain PEO analogues – glymes – can be used as models to overcome this problem.16,17 Complexes of crown ethers may serve as a convenient model of free ions and ionic pairs. Moreover, a class of solid ionic conductors based on glymes and lithium salts called crystalline small-molecule electrolytes was described.18 Adducts consisting of lithium salts dissolved in low molecular weight glymes have promising ionic conductivities and lithium cation transference numbers which depend both on the glyme molecule as well as their crystal structures.19,20 On the other hands, glyme-based liquid systems such as concentrated solutions or solvate ionic liquids have recently been the center of attraction for researchers.21,22 More detailed examination of the interactions between [Li(glyme)]+ complex cations and weakly Lewis basic anions are still necessary to gain further insight into the ion coordination structure and transport mechanism in electrolytes.23, 24, 25 In order to understand the salt association process we attempted thorough systematic analysis of coordination modes in glyme-LiTDI systems with subsequent spectroscopic and thermal characterization. It is an important step to explain the unique properties of the TDI anion, which allows for high conductivity of the LiTDI-based electrolytes even at high salt concentration, in spite of the increase in electrolyte viscosity and salt aggregation.6,7 Herein, we present comprehensive set of crystal structures and Raman spectra of various lithium complexes containing 4,5-dicyanoimidazolato anions with dimethyl ethers of poly(ethylene glycols)glymes and crown ethers.

2. EXPERIMENTAL SECTION


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2.1 Synthesis and crystallization. Lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI) synthesis route was described previously.6 Anhydrous dimethyl ethers of glymes: mono(ethylene glycol) G1, di(ethylene glycol) G2, tri(ethylene glycol) G3, tetra(ethylene glycol) G4 as well as crown ethers 12-crown-4 and 15-crown-5 were purchased form Sigma-Aldrich and used as received. Poly(ethylene oxide) PEO (Mw =5×106 g/mol, Aldrich) was dried under vacuum. Single crystals of 1-10 were grown inside argon-filled glovebox, as describe below. Solutions for DSC measurements with 1:3 molar ratio LiTDI:glyme(mono/di/tri/tetra) were prepared in glass vials and transferred to pans after stirring. Solutions with a ratio of 1:2 and 1:1were prepared in situ in pans due to their oversaturation. All operations were carried out inside the argon-filled glovebox. LiTDI-PEO membranes were obtained as follows. LiTDI was dried under vacuum and added to poly(ethylene oxide) in acetonitrile solution. The obtained solution was poured onto a teflon dish and thin foil was formed after vacuum drying. 2.2. Single Crystals Preparation. Crystals of Li(12C4)2+ TDI− (1) were obtained from solution containing ~20 mg LiTDI and ~40 µL 12C4 (molar ratio: 1:2.5). The mixture prepared in hermetic glass vial was stirred and heated up to ~80°C and then allowed to cool slowly to the room temperature, resulting in a colorless single crystals. Raman (selected bands, cm–1): 2227, 1489, 1444, 1339, 1308, 1267, 1236, 1116, 1096, 1053, 1031, 978, 913, 862. Tm = 91°C. Crystals of Li(15C5)·TDI (2) were obtained analogous to 1 using ~25 mg LiTDI and ~50 µL 15C5 (molar ratio: 1:2). Raman (selected bands, cm–1): 2240, 2228, 1490, 1474, 1450, 1302, 1280, 1261, 1242, 1145, 979, 871. Tm = 78°C.


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Crystals of Li(G1)2·TDI (3) and Li(G1)1·TDI (7) were obtained from solution containing ~30 mg LiTDI and ~50 µL G1 (molar ratio: 1:3). The mixture prepared in hermetic glass vial was stirred and heated up to ~40°C and then allowed to cool slowly to the room temperature. After a few hours sample was moved to fridge (~4°C), where crystals of 7 have grown. Raman (selected bands, cm–1) for 7: 2261, 2240, 1504, 1467, 1324, 1239, 1193, 1009, 872. Tm = 15°C. Putting sample of mixture in the refrigerator (~−20°C) for few days yielded in crystals of 3. Raman (selected bands, cm–1) for 3: 2230, 1494, 1455, 1316, 1277, 1243, 1124, 1024, 991, 876. Tm = 5°C. Crystals of Li(G1)0.5·TDI (10) were obtained from solution containing ~40 mg LiTDI and ~40 µL G1 (molar ratio: 1:2). The mixture prepared in hermetic glass vial was stirred and heated up to ~50°C and then allowed to cool slowly to the room. Raman (selected bands, cm–1): 2259, 2235, 1501, 1498, 1458, 1321, 1317, 1283, 1247, 1188, 1122, 1024, 1005, 996, 876. Tm ~135°C. Crystals of [Li(G2)·TDI]2 (5) were obtained starting from ~40 mg LiTDI and ~60 µL G2 (molar ratio: 1:2). The mixture prepared in hermetic glass vial was stirred and heated up to ~60°C, and then cooled slowly to the RT for a few hours. Next, sample was moved to fridge (~4°C), where crystals of 5 were grown. Raman (selected bands, cm–1): 2251, 2234, 1497, 1479, 1456, 1352, 1313, 1277, 1247, 1179, 1131, 1063, 1025, 990, 873. Tm = 100°C. Crystals of [Li(G2)·PDI]n (6) were obtained in an analogous way to 5 using ~35 mg LiPDI and ~60 µL G2 (molar ratio: 1:2). Raman (selected bands, cm–1): 2254, 2228, 1485, 1312, 1301, 1245, 1151, 1041, 948, 873. Crystals of Li(G3)·TDI (4) and Li(G3)0.5·TDI (8) were obtained from solution containing ~50 mg LiTDI and ~50 µL G3 (molar ratio: 1:1). Mixture was stirred and heated up to ~120°C. Slow cooling to the RT produces single crystals of 4 in the form of blocks which can be easily


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separated from small amounts of remaining powder of 8. Raman (selected bands, cm–1) for 4: 2259, 2233, 1500, 1464, 1454, 1322, 1316, 1277, 1244, 1178, 1124, 1005, 989, 976, 864. Tm = 41°C. Heating up the source mixture of 4 and 8 to ~45°C allows to melt crystals of 4 and filter off pure phase of 8. Raman (selected bands, cm–1) for 8: 2233, 1492, 1454, 1320, 1280, 1234, 1169, 1124, 991, 867. Tm = 110°C. Crystals of [Li2(G4)22+][Li4TDI62−] (9) were obtained from solution containing ~50 mg LiTDI and ~30 µL G4 (molar ratio: 2:1), by heating up to ~130°C. Next, mixture was cooled down to the room temperature. Colorless single crystals suitable for XRD studies have slowly grown after few days. Raman (selected bands, cm–1): 2255, 2229, 1496, 1466, 1454, 1320, 1310, 1271, 1236, 1189, 1129, 1006, 987, 877. Tm = 123°C. 2.3. X-ray crystallography. Selected single crystal was mounted in inert oil and transferred to the cold gas stream of the diffractometer. Diffraction data were measured with mirror monochromated CuKα or graphite monochromated MoKα radiation on an Oxford Diffraction κ-CCD Gemini A Ultra diffractometer. Cell refinement and data collection as well as data reduction and analysis were performed with the CRYSALISPRO software.26 Structures were solved by direct methods using the SHELXS-97 structure solution program and refined by full-matrix least-squares against F2 with SHELXL-201427 and OLEX228programs. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms. The crystal data and experimental parameters are summarized in Table S1, Supporting information. CCDC10448171044826 contain the supplementary crystallographic data for this paper. These data can be


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obtained

free

of

charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif. 2.4. Raman spectra were collected on Nicolet Almega Raman dispersive spectrometer. 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 with use of Peltier cooled Linkam stage. 2.5. FTIR spectra were recorded on Perkin-Elmer 2000 FT-IR system with a wavenumber resolution of 1 cm−1. Electrolytes were sandwiched between two NaCl plates and placed in the FT-IR temperature-controlled cell; the accuracy of the temperature was estimated to be 1°C. The analysis of Raman and IR spectra was performed with the Omnic software. 2.6. DSC studies were performed using TA Instruments Q200DSC apparatus in nitrogen flow. The heating rate was equal to 5°C/min. 2.7. Electrochemical measurements were performed using computer-interfaced multichannel potentiostat with frequency response analyzer option Bio-Logic Science Instruments VMP3. Electrochemical impedance spectroscopy was carried out within 500-0.5 kHz frequency range with 10 points per decade, 5 measurements per frequency and 5 mV AC amplitude. Lithium transference number was calculated using polarization method introduced by Bruce and Vincent29 with ∆V=15 mV. Presented value of transference number is an average of six measurements. Samples for all electrochemical measurements were prepared inside argon-filled glovebox. 3. RESULTS AND DISCUSSION The crystalline complexes of LiTDI with dimethyl ethers of glymes: mono(ethylene glycol) G1, di(ethylene glycol) G2, tri(ethylene glycol) G3, tetra(ethylene glycol) G4, crown ethers 12crown-4 (12C4) and 15-crown-5 (15C5) were obtained and their crystal structures determined by


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XRD were compared with Raman spectra. We have found a wide variety of structural motifs in the crystal structures of the obtained solvates including ionic pairs, dimers, free ions and higher aggregates as shown in Table 1. The results have further been used as a model for the interpretation of aggregation modes in the liquid and solid PEO- LiTDI electrolytes.

Table 1. Numeration of compounds and lithium coordination sphere description. Average Li:O ratio

Compound Molecular formula number

Chemical formula

Li coordination Molecular sphere assembly

C16H32LiO8·C6F3N4

8O

isolated ions

1:8

1

Li(12C4)2+ TDI−

1:5

2

Li(15C5)·TDI C16H20F3LiN4O5

5O +NCN

monomer

1:4

3

Li(G1)2·TDI

C14H20F3LiN4O4

4O +NIm +F

monomer

1:4

4

Li(G3)·TDI

C14H18F3LiN4O4

4O +NIm +F

monomer

1:3

5

[Li(G2)·TDI]2 C24H28F6Li2N8O6

3O +NCN +NIm +F

dimer

1:3

6

[Li(G2)·PDI]n C13H14F5LiN4O3

3O +NCN +NIm

1D - chain

1:2

7

Li(G1)·TDI

2NCN +2NIm +F C20H20F6Li2N8O4

1D - ribbon 4O +NIm +F 2NCN +2NIm +F

1:2

8

Li(G3)0.5·TDI

C40H36F12Li4N16O8

1D - ribbon 4O +NIm +F

1:1.67

9

[Li2(G4)22+]

(C36F18Li4N24)

2NCN +2NIm +F

[Li4TDI62−]

(C20H44Li2O10)

5O

Li(G1)0.5·TDI

C16H10F6Li2N8O2

2NCN +2NIm +F 1:1

10

2O +2NIm +F

2D - anionic layers 3D aggregate

We have previously proposed solvation and dissociation mechanism for LiTDI salt family in acetonitrile, based on conductivity measurements.9 Changes observed in ionic conductivity


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during dilution of saturated LiTDI solution in MeCN were explained with stepwise dissociation of the [LiTDI·MeCN]2 dimers starting with the weakest Li−Nimidazole bonds. This results in the formation of ionic pairs in which TDI anion is coordinated to Li with cyano group and which may subsequently dissociate to form fully solvated lithium cations. As we show below, the mechanism of stepwise dissociation is also consistent with aggregation modes of LiTDI salts in glyme solutions. Structural motifs observed in salts Li(12C4)2+TDI− (1), Li(15C5)·TDI (2) and [Li(G2)·TDI]2 (5) are depicted in Figure 1.

Figure 1. Molecular structures of crystalline LiTDI solvates a) Li(12C4)2+ TDI− (1), b) Li(15C5)·TDI (2), c) [Li(G2)·TDI]2 (5). Increasing the number of solvent donor centers per lithium cation, which can be expressed by an average Li:O molar ratio (see Table 1) leads to the separation of lithium cations and TDI anions. The 12-crown-4 solvate of LiTDI (1) possesses the highest Li:O molar ratio. Molecular structure comprises lithium cation trapped between two ether rings and isolated from anion. Monomeric complex 2 with 15-crown-5 ether may serve as the simplest model for ionic pair in which anion is coordinated through nitrile group to the metal center. What is important, there are other types of ionic pairs as shown in Figure 2a-b.


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Figure 2. Structural motifs observed in a) Li(G1)2·TDI (3), b) Li(G3)·TDI (4) and c) [Li(G2)·PDI]n (6) solvates. Molecular structures of LiTDI salt with two molecules of monoglyme (3) and one of triglyme (4) are analogous and contain ionic pairs of TDI anions coordinated to lithium cations through one of the imidazole nitrogens. The crystal structures of these two complexes were determined independently by Henderson,11 but without further spectroscopic characterization and DSC verification. We have found, that additional interaction between fluorine and lithium cation is present with Li⋯F distances of 2.642(2) and 2.630(5) Å for 3 and 4, respectively. It is worth to notice that coordination modes of TDI anion involving imidazole nitrogen are always supported by accompanying alkali metal ion⋯F dative bond. It is also revealed in the crystal structures of diglyme solvate 5 (Figure 1c) in which TDI anion acts as a bridging ligand. Decrease in the number of oxygen donor centers in 5 brings about the formation of the dimeric ionic pair which can be considered as the first step of aggregation process. TDI is coordinated in an unsymmetrical fashion through the nitrogen atom of the imidazole ring [Li1−N1 2.071(3) Å] and one of the cyano groups [Li1’−N4 2.052(3) Å] forming ten-membered central Li(NCCN)2Li ring with additional Li1⋯F1 contacts of 2.765(2) Å.


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An alternative to dimeric structural motif is a chain utilizing two diagonally placed nitrogen donor centers. Such structural motif has not been isolated from LiTDI systems. Replacing of TDI anion with a similar 4,5-dicyano-2-pentafluoroethyl imidazole (PDI) anion enabled us to isolate adduct, which proves the possibility of the formation of the chain coordination polymers (Figure 2c). Chain and dimeric forms are probably exchangeable and may coexist in glyme solution. Such behavior was not found in the case of acetonitrile solvates9 and all of the isolated crystalline adducts were dimeric. Further decrease in the number of solvent oxygen donor centers per Li cation generally leads to further aggregation into 3D structures through bridging anions. Accordingly the presence of aggregates causes high viscosity in solute systems and, therefore, a decrease in conductivity. Structural fragments found in this kind of 3-D aggregates observed for monoglyme solvates of LiTDI (Li:O molar ratio = 1:1) are depicted in Figure 3.

Figure 3. a-b) Two different coordination spheres of Li+ cations in Li(G1)0.5·TDI (10) crystal structure; c) structural fragment showing coordination modes of TDI anions in 10. The crystal structure of 10 comprises two lithium cations having different coordination. The first Li+ is surrounded by four TDI anions, while the second one by two TDI anions and one


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monoglyme molecule. For LiTDI salts systems the increase in conductivity was observed up to 0.7-1 mol/L, which is quite puzzling.6,7 High ability of this salt to dissociate with the formation of uncoordinated free anions and simple ionic pairs cannot explain the high conductivity of the LiTDI containing systems. Such adducts are present in rather dilute solutions and are stable in the solid state at low temperatures. Surprisingly, we have found, that there is another way of agglomeration, leading to a new type of species, which have not been considered before. These structures are formed at room temperature by disproportionation of lithium centers at high concentrations of complexes with Li:O ratio equal to 1:2. We were able to isolate two solvates Li(G1)·TDI (7) and Li(G3)0.5·TDI (8) of this type. In both cases, single crystal X-ray analysis shows a 1D ladder-like assembly of the [Li(TDI)2]nn− polyanions lacking oxygen atoms in Li coordination sphere as depicted in Figure 4.


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Figure 4. Two isostructural coordination polymers formed in solvates a) Li(G1)·TDI (7) b) Li(G3)0.5·TDI (8). Characteristic dimeric motifs with ten-membered Li(NCCN)2Li ring are linked with one another through the spanning TDI anions which are coordinated in an unsymmetrical fashion through imidazole nitrogen atom and one of the cyano groups, forming a 1D coordination polymer. These chains are decorated with auxiliary glyme solvated lithium cations which are weakly coordinated through second imidazole nitrogen atom [Li1−N1 2.124(11) Å]. The coordination sphere of this lithium cation is a distorted octahedron analogous to coordination spheres observed in 3 and 4. It can be assumed that terminal lithium cations should dissociate easily at the first step, recovering the polyanionic chain. Interestingly, almost identical anionic species have been observed in crystal structures of salts formed by addition of LiTDI to imidazolium based ionic liquids.10 Moreover, one could expect that proper amounts of glymes can promote dissociation of the terminal lithium cations. Thus, exact combination of fitting oligo(ethylene oxide) with LiTDI salt should provide ionic system with aggregated polyanion and free cations encapsulated by solvent. We have confirmed this concept in case of tetraglyme solvates of LiTDI. Formation of polyanionic layers in the form of nets with large meshes is observed at the molar ratio of tetraglyme to lithium salt equal to 1:3 (Li:O = 1:1.67). Remaining Li(I) cations are linked via two bridging tetraglyme helically twisted solvent molecules to give a centrosymmetric dinuclear cation Li2(G4)22+(Fig. 5).


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Figure 5. X-ray structure of ionic LiTDI tetraglyme solvate 9: dinuclear Li2(G4)22+ cations placed between polyanionic mesh layers of composition (Li4TDI6)n2n-. Such anionic agglomerates are less mobile than individual anions and cause the lithium cation to have higher transference number. As a result, lower local viscosity and higher ionic conductivity should be achieved even though over a half of the Li cations is bound in anionic layers.

Electrochemical Testing Electrochemical measurements for crystalline [Li2(G4)22+] [Li4TDI62−] (9) were performed to test the above hypothesis. We have obtained extraordinarily high conductivity at 30°C for solid sample equal to 1.8·10−5 S·cm−1. Conductivity curve shown in Figure 6 can be divided into two regions: above and below melting point of the crystals (~123°C).


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Figure 6. The conductivity curve for the crystalline solid state sample of 9. Electrolyte melting is accompanied by a rise in activation energy from 15.4 kJ·mol−1 for the crystalline phase to 18.4 kJ·mol−1 in the liquid phase. It results in a faster decrease in conductivity for amorphous phase when temperature is lowered. After the sample was cooled to 30°C, the measured value of conductivity (8.7·10−6 S·cm−1) is about two times lower than for the initial crystalline sample. The conductivity rises slowly at RT to recover the initial value after about four days. Remarkably, the conductivity of this crystalline phase is of the same order of magnitude as of amorphous phase of PEO-based electrolyte. Thus, this system is better conductive than typical polymer solid electrolyte comprising up to 80% of PEO-crystalline phase. Moreover, exceptionally high lithium transference number (t+=0.82) determined for the crystalline material at 30°C confirms tremendous cation contribution in the general conductivity of this system at relatively high conductivity. On the basis of our results it might be concluded, that insisting on systems with fully isolated simple anions is not necessary. The disproportionation leading to the formation of the polyanion in which part of the lithium is bound, is still advantageous; the remaining dicationic species are free, and the lithium


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transference numbers is high, due to the additional immobilization of the anions. Thanks to the unique properties of TDI anion, the system in presence of electroneutral donor solvent tends to disproportionate with formation of solvent coordinated free cations. Therefore, systems with the optimal conducting parameters can be expected also at a high salt concentration solutions with small, well-adjusted amount of solvent, where the overall number of charge carriers is much larger than in diluted ones. Disproportionation of lithium centers proceeds towards cations with pure nitrogen and pure oxygen coordination spheres and can be ended in releasing of solvated cations as depicted in Figure 7.

Figure 7. Mutual exchange between oxygen and nitrogen donor centers in coordination sphere of lithium. This process is probably driven by a preference of lithium cations acting as a hard acid to fill its coordination spheres by hard oxygen donors rather than nitrogen ones. Moreover, minor changes in observed coordination number of lithium varying from 5 to 6 at entirely different ligand surroundings can support smooth ligand exchange around lithium cation. Thus, TDI anion can be considered as a ligand with a unique ability to readily match to coordination sphere of Li+ cation. Furthermore, it has no tendency to forcing geometry around metal cation and can easily employ


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subsequent donor centers to increase its coordination number. Such a process is observed when deficiency in oxygen donor centers leads toward disproportionated 3-D aggregates. Spectroscopic Study We have found that three of the bands observed in the Raman spectra of the TDI anion are particularly suitable as probes for analysis of ionic association: bands corresponding to CN triple bond stretching (~2225 cm−1), imidazole ring stretching (~1305 cm−1) and NCN ring deformation (~978 cm−1)19. These values are attributed to the free anions and were observed in diluted LiTDI-glyme solutions at concentrations equal to or lower than 0.05M as well as in spectra of Li(12C4)2+ TDI− complex (1) in the solid state. In solutions with higher salt concentrations we observed gradual shift of the band to higher frequencies and finally band splitting. These additional bands clearly come from some associated ionic species. Figure 8 and Table 2 present a comparison of Raman spectra of the obtained complexes and, additionally, LiTDI complexes with high-molecular-weight PEO.

Figure 8. Raman spectra of crystalline LiTDI-glyme complexes in spectral ranges corresponding to a) νCN, b) νCN_Im and c) δNCN vibrations. Table 2. Band assignments for LiTDI crystalline complexes in Raman spectrum.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

νCN/ cm−1 νCN_Im/ cm−1

δNCN / cm−1

ωCH2 / cm−1

2225

1307

977

861, 843

Li(15C5)·TDI

2239, 2228

1302

979

871, 812

Li(G1)2·TDI

2230

1316

991

872, 840

2230

1320

991

865, 828

844,

[Li(G2)·TDI]2

2250, 2233

1313

988, 976

871, 831

842,

Li(G1)·TDI

2259, 2235

1321, 1316

1005, 996

875, 841

Li(G3)0.5·TDI

2259, 2234

1322, 1316

1005, 996, 867, 843 989, 977

[Li2(G4)22+][Li4TDI62 ]

2258, 2230

1322, 1312

1007, 990

877, 830

Li(G1)0.5·TDI

2261, 2240

1324

1009

876, 841

2226

1308

978

871, 832

840,

2231

1318

995

878, 815

839,

Compound Molecular formula number Li (12C4)2+ TDI

1 2

3 4

−

Li(G3)·TDI

5

7

8

9

−

10

12

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Li(G2)2+TDI

Li(EO)xTDI

−*

828,

* Identification of the compound on the basis of Raman spectrum

Li(15C5)·TDI (2) solvate which melts at 79°C is stable in solutions. Thus, it may serve as a model of ionic pairs in which cations are linked with anions through one nitrile group. The asymmetry introduced by the complexation of only one of the nitrile groups results in band splitting of the CN triple bond vibration and in the appearance of two distinct bands, at 2239 and 2228 cm−1 for coordinated and free nitrile group, respectively. The νCN_Im as well as δNCN vibrations give single bands of almost symmetric shape, which reflects lack of the ring-cation


18

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coordination. Li(G1)2·TDI (3) is an example of different type of contact ionic pair, with lithium cation chelated by imidazole nitrogen and fluorine atoms. In the Raman spectrum it is characterized by a single band at 2230 cm−1 resulting from nitrile stretching which confirms lack of interactions of this group with lithium. On the other hand, νCN_Im and δNCN bands (at 1316 and 991 cm−1, respectively) are shifted to higher wavenumbers in comparison to bands observed in 2 (1302 and 979 cm−1 for νCN_Im and δNCN, respectively) indicating coordination of imidazole nitrogen to Li. Analogous coordination type of TDI anion present in crystal structure of Li(G3)·TDI (4) results in almost the same positions of the analyzed peaks, as shown in Figure 9a-c and Table 2. The next stage of association process can be ascribed to dimers of Li(G2)·TDI (5). A motif consisting of two anions, two cations and two molecules of diglyme presents a model of dimeric unit. In such system, both bands corresponding 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 imidazole nitrogen in each anion. The νCN_Im vibration band is asymmetric, with a maximum at 1313 cm−1 and a broad shoulder at 1305 cm−1. The first value, which is shifted towards higher wavenumbers than in 1, corresponds to lithium-coordinated imidazole nitrogen atom whilst the second one to the free imidazole nitrogen. Li(G1)·TDI (7), Li(G3)0.5·TDI (8), [Li2(G4)22+][Li4(TDI)42−] (9) and Li(G1)0.5·TDI (10) complexes should be considered as aggregates, comprising two types of cations: i) coordinated solely with anions and ii) coordinated with various number of ether oxygens and TDI anions as well as several types of anions. One type of anions with one nitrile group and both imidazole nitrogen and fluorine coordinated to lithium is common for the first three solvates. Bands at ~2230 and 2260, 1320 and 1005 cm−1 appear to characterize this type of anion and are found in spectra of 7, 8 and 10. In case of Li(G1)·TDI (7) and Li(G3)0.5·TDI (8) this common pattern is overlapped by additional bands


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peaking at ~1310 and 990 cm−1. Both of these solvates have anions of the same coordination type, which results in almost the same anion’s spectral characteristic. Presence of the associated and non-associated imidazole nitrogens is reflected in the splitting of bands corresponding to ring vibrations, with the downshifted part stemming from non-associated nitrogen donor centers. Structure 9 consists of two types of anions, but different, than in other solvates. Hence the spectral pattern is unique, though resembling the spectra of aggregate complexes 8 and 10 to some degree. Thermal behavior and phases’ identification As mentioned above, some crystalline phases of LiTDI-glymes adducts have been recently described by Henderson at al.11 However, our comprehensive studies have demonstrated that these systems may form a wide variety of crystalline adduct species, with Li:O stoichiometry varying from 1:8 to 1:1. Therefore, we have performed DSC measurements in LiTDI-glymes systems for both known crystalline adducts and liquid solution mixtures. Figure 9 presents the obtained DSC curves.


20

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Figure 9. DSC curves of crystalline and liquid LiTDI-glymes systems with thermal effects corresponded to phase transitions of the various defined solvates. What is important, we carefully verified the occurrence and structure of specific phases by means of temperature dependent Raman spectroscopy. Presence of the same phases was also confirmed by FTIR studies. Positions of the bands characteristic for uncoordinated anions in 1 is close to that observed in diluted LiTDI-glyme solutions. The same pattern was also found for the


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G2 solvate 12 with a melting point of 20°C, which allows us to identify it as Li(G2)2+ TDI− published previously.11 On the basis of the spectroscopy measurements we have also identified an additional phase 11 with the composition of Li(G1)3+TDI−, comprising free anions and fully solvated cations. It is worth to notice, that high solvates like 3-4, 11-12 are supposed to occur rather in dilute solutions and are mostly stable in the solid state at low temperatures. For LiTDI-G1 systems with 1:1, 1:2 and 1:3 molar ratio we observed thermal effect at −35, 5, 15 and ~135°C corresponding to Li(G1)3+TDI− (11), Li(G1)2·TDI (3), Li(G1)1·TDI (7) and Li(G1)0.5·TDI (10) solvates, respectively. Cooling of the LiTDI-G1 1:2 solution at Raman spectrometer down to −25°C resulted in precipitation of crystals which possessed the spectral features resembling that of Li(G1)2·TDI (3) contact pairs. Heating the sample to about −5°C resulted in melting of this phase and bands characteristic for complex 7 appeared at 5°C. Moreover have found that from 0°C much weaker bands at 2233 and 2256 cm−1 appeared in the spectra, representative for νCN of Li(G1)0.5·TDI (10). Crystals of 7 disappeared above 15-20°C and the only crystalline phase found in the sample was that of 10. We observed two melting points at ~20 and 100°C in DSC curves of LiTDI-diglyme solutions and the crystalline adduct . The lower one is characteristic for ionic solvate of composition Li(G2)2+TDI− (12) described previously11 while the higher one corresponds to a system more rich in a salt [Li(G2)·TDI]2 (5) with the dimeric structure shown in Figure 1c. Raman spectra reveal mixture of crystallized phases of 12 and 5 in broad range of temperatures. Crystals of 12 dominate up to 5°C and disappear above 20°C. 5 can be observed below 120°C. LiTDI-G3 1:1 and 1:2 solutions exhibit melting points at about 40°C, attributed to melting of Li(G3)·TDI phase (4). The spectra recorded below 40°C correspond to 4. After melting of this phase at 45°C we found only crystals of 8 in the sample, which disappeared at 110°C. No thermal effects were observed for LiTDI-G4


22

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systems. This is consistent with our observation, that crystallization of 9 (Tm=123°C) takes place slowly during few days. Thus, tetraglyme solutions may be considered as supersaturated. LiTDIPEO membranes with Li:O ratio equal to 1:8 are characterized by melting points of 39°C and 83°C corresponding to the melting of the PEO phase and the solvate complex, respectively. The results of XRD analysis of the obtained adducts and Raman data allows us to make some assumptions concerning TDI anions environment in the LiTDI complexes with high molecular weight PEO. In the polymer based systems uncoordinated anions exist mostly in the amorphous phase. However, our data obtained for the crystalline PEO-LiTDI system indicate the same type of coordination, as in Li(G3)·TDI (4), i.e. existence of NIm- linked ionic pairs. Further work to explore the structural aspects of this family of glyme adducts in amorphous and liquid systems more fully is in progress. 4. CONCLUSION The structural and spectroscopic analysis of the crystalline solvates formed by lithium salts of dicyanoimidazoles with aprotic solvents like glymes -dimethyl ethers of poly(ethylene glycols)or crown ethers allow us to compare mutual competition in the interactions between such solvent molecules and dicyanoimidazole anions. Decreasing the number of solvent donor centers, which can be expressed by decreasing the average Li:O molar ratio, leads to aggregation starting from isolated ions and ionic pairs through dimeric species, chains and ribbons to layers and 3-D nets (Figure 10).


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Figure 10. Aggregation process involving disproportionation mechanism observed in Li-TDIglyme systems. This process has a major impact on electrochemical properties of electrolytes based on dicyanoimidazolato salts. System tends to disproportionate with the formation of solvent coordinated free cations and aggregated polyanionic species in the presence of electroneutral donor solvents. Therefore, systems with optimal conducting parameters can also be expected at high salt concentration solutions. Small, well-adjusted amount of solvent provides a system, where the overall number of charge carriers is much larger than in diluted solutions and additional immobilization of anions occurs. What is important, revealed mechanism explains high lithium transference number and high conductivity both in the solid state and in concentrated solutions. This, in turn, leads to the better understanding of the unique coordination properties of TDI anion. Moreover, this mechanism may be more general, applying to other systems. ASSOCIATED CONTENT Supporting Information. Crystal data for the single crystal X-ray structures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


24

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Corresponding Author * E-mail: [email protected] (M. D), [email protected] (G. Z. Ż). ACKNOWLEDGMENT 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 P. Guńka for helpful discussions, proof-reading and corrections. ABBREVIATIONS MeCN, acetonitrile; DSC, differential scanning calorimetry; LiTDI, lithium 4,5-dicyano-2(trifluoromethyl)imidazolate; LiPDI, lithium 4,5-dicyano-2-(pentafluoroethyl)imidazolate; 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. REFERENCES

(1) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303-4418. (2) Seo, D. M.; Allen, J. L.; Gardner L. A.; Han S.; Boyle P. D.; Henderson W. A. Electrolyte Solvation and Ionic Association: Cyclic Carbonate and Ester-LiTFSI and -LiPF6 Mixtures. ECS Trans. 2013, 50, 375-380. (3) Pentacyclic Anion Salts or Tetrazapentalene Derivatives and Their Uses as Ionic Conducting Materials, US 7906235 B2.


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(4) Egashira, M.; Scrosati, B.; Armand, M.; Béranger, S.; Michot, C. Lithium Dicyanotriazolate as a Lithium Salt for Poly(ethylene oxide) Based Polymer Electrolytes. Electrochem. Solid St. 2003, 6, A71-A73. (5) Johansson, P.; Jacobsson, P. New Lithium Salts on the Computer: Fction or Fact? Electrochim. Acta 2001, 46, 1545-1552. (6) Niedzicki, L.; Zukowska, G. Z.; Bukowska, M.; Szczecinski, P.; Grugeon, S.; Laruelle, S.; Armand, M.; Panero, S.; Scrosati, B.; Marcinek, M.; et al. New Type of Imidazole Based Salts Designed Specifically for Lithium Ion Batteries. Electrochim. Acta 2010, 55, 1450-1454. (7) Niedzicki, L.; Kasprzyk, M.; Kuziak, K.; Zukowska, G. Z.; Marcinek M.; Wieczorek W.; Armand M., Liquid Electrolytes Based on New Lithium Conductive Imidazole Salts. J. Power Sources 2011, 196, 1386-1391. (8) Johansson, P.; Béranger, S.; Armand, M.; Nilsson, H.; Jacobsson, P. Spectroscopic and Theoretical Study of the 1,2,3-Triazole-4,5-dicarbonitrile Anion and Its Lithium Ion Pairs. Solid State Ionics 2003, 156, 129-139. (9) Dranka, M.; Niedzicki, L.; Kasprzyk, M.; Marcinek, M.; Wieczorek, W.; Zachara, J. An Insight into Coordination Ability of Dicyanoimidazolato Anions Toward Lithium in Presence of Acetonitrile. Crystal Structures of Novel Lithium Battery Electrolyte Salts. Polyhedron, 2013, 51, 111-116.


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(10) Niedzicki, L.; Karpierz, E.; Zawadzki, M.; Dranka, M.; Kasprzyk, M.; Zalewska, A.; Marcinek, M.; Zachara, J.; Domanska, U.; Wieczorek, W. Lithium Cation Conducting TDI Anion-Based Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 11417-11425. (11) McOwen, D. W.; Delp, S. A.; Paillard, E.; Herriot, C.; Han, S. D.; Boyle, P. D.; Sommer, R. D.; Henderson, W. A. Anion Coordination Interactions in Solvates with the Lithium Salts LiDCTA and LiTDI. J. Phys. Chem. C 2014, 118, 7781-7787. (12) Plewa-Marczewska, A.; Trzeciak, T.; Bitner, A.; Niedzicki, L.; Dranka, M.; Żukowska, G. Z.; Marcinek, M.; Wieczorek W. New Tailored Sodium Salts for Battery Applications. Chem. Mater. 2014, 26, 4908-4914. (13) Wright, P. V. Electrical Conductivity in Ionic Complexes of Poly(Ethylene Oxide). British Polym. J. 1975, 7, 319-327. (14) Payne, D. R.; Wright P. V. Morphology and Ionic Conductivity of Some Lithium Ion Complexes with Poly(Ethylene Oxide). Polymer, 1982, 23, 690-693. (15) Henderson, W. A.; Brooks, N. R.; Young, V. G. Jr. Single-Crystal Structures of Polymer Electrolytes. J. Am. Chem. Soc. 2003, 125, 12098-12099. (16) Henderson, W. A.; Brooks, N. R.; Brennessel, W. W.; Young, V. G. Jr. Triglyme-Li+ Cation Solvate Structures: Models for Amorphous Concentrated Liquid and Polymer Electrolytes (I). Chem. Mater. 2003, 15, 4679-4684.


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(17) Henderson, W. A.; Brooks, N. R.; Young, V. G. Jr. Tetraglyme-Li+ Cation Solvate Structures: Models for Amorphous Concentrated Liquid and Polymer Electrolytes (II). Chem. Mater. 2003, 15, 4685-4690. (18) Zhang, C.; Andreev, Y. G.; Bruce, P.G. Crystalline Small-Molecule Electrolytes. Angew. Chem., Int. Ed. 2007, 46, 2848-2850. (19) Zhang, C.; Ainsworth, D.; Andreev, Y. G.; Bruce, P. G. Ionic Conductivity in the Solid Glyme Complexes [CH3O(CH2CH2O)nCH3]:LiAsF6 (n = 3,4). J. Am. Chem. Soc. 2007, 129, 8700-8701. (20) Zhang, C.; Lilley, S. J.; Ainsworth, D.; Staunton, E.; Andreev, Y. G.; Slawin, A. M. Z.; Bruce,

P.G.

Structure

and

Conductivity

of

Small-Molecule

Electrolytes

[CH3O(CH2CH2O)nCH3]:LiAsF6 (n = 8−12). Chem. Mater. 2008, 20, 4039-4044. (21) Mandai, T.; Yoshida, K.; Ueno, K.; Dokko, K.; Watanabe, M. Criteria for Solvate Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 8761-8772. (22) Ueno, K.; Yoshida, K.; Tsuchiya, M.; Tachikawa, N.; Dokko, K.; Watanabe, M. GlymeLithium Salt Equimolar Molten Mixtures: Concentrated Solutions or Solvate Ionic Liquids? J. Phys. Chem. B 2012, 116, 11323-11331. (23) Mandai, T.; Yoshida, K.; Tsuzuki, S.; Nozawa, R.; Masu, H.; Ueno, K.; Dokko, K.; Watanabe, M. Effect of Ionic Size on Solvate Stability of Glyme-Based Solvate Ionic Liquids. J. Phys. Chem. B 2015, 119, 1523-1534.


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(24) Zhang, C.; Ueno, K.; Yamazaki, A.; Yoshida, K.; Moon, H.; Mandai, T.; Umebayashi, Y.; Dokko, K.; Watanabe, M. Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties. J. Phys. Chem. B 2014, 118, 5144-5153. (25) Zhang, C.; Yamazaki, A.; Murai, J.; Park, J-W.; Mandai, T.; Ueno, K.; Dokko, K.; Watanabe M. Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids, Part 2: Importance of Solvate-Structure Stability for Electrolytes of Lithium Batteries. J. Phys. Chem. C 2014, 118, 17362-17373. (26) CRYSALISPRO Software system, Agilent Technologies, Oxford, UK, 2014. (27) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112-122. (28) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Cryst. 2009, 42, 339-341. (29) Bruce, P.; Vincent, C. A. Steady State Current Flow in Solid Binary Electrolyte Cells. J. Electroanal. Chem. 1987, 225, 1-17.


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For Table of Contents Only.


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50x50mm (300 x 300 DPI)



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Figure 1. Molecular structures of crystalline LiTDI solvates a) Li(12C4)2+ TDI− (1), b) Li(15C5)∙TDI (2), c) [Li(G2)∙TDI]2 (5). 39x9mm (300 x 300 DPI)


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Figure 2. Structural motifs observed in a) Li(G1)2∙TDI (3), b) Li(G3)∙TDI (4) and c) [Li(G2)∙PDI]n (6) solvates. 60x43mm (300 x 300 DPI)



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Figure 3. a-b) Two different coordination spheres of Li+ cations in Li(G1)0.5∙TDI (10) crystal structure; c) structural fragment showing coordination modes of TDI anions in 10. 77x70mm (300 x 300 DPI)


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Figure 4. Two isostructural coordination polymers formed in solvates a) Li(G1)∙TDI (7) b) Li(G3)0.5∙TDI (8). 106x134mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. X-ray structure of ionic LiTDI tetraglyme solvate 9: dinuclear Li2(G4)22+ cations placed between polyanionic mesh layers of composition (Li4TDI6)n2n-. 50x30mm (300 x 300 DPI)


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Figure 6. The conductivity curve for the crystalline solid state sample of 9. 70x58mm (600 x 600 DPI)



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Figure 7. Mutual exchange between oxygen and nitrogen donor centers in coordination sphere of lithium. 47x12mm (300 x 300 DPI)


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Figure 8. Raman spectra of crystalline LiTDI-glyme complexes in spectral ranges corresponding to a) νCN, b) νCN_Im and c) δNCN vibrations. 61x45mm (600 x 600 DPI)



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Figure 9. DSC curves of crystalline and liquid LiTDI-glymes systems with thermal effects corresponded to phase transitions of the various defined solvates. 156x299mm (600 x 600 DPI)


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Figure 10. Aggregation process involving disproportionation mechanism observed in Li-TDI-glyme systems. 43x10mm (600 x 600 DPI)


Structural Studies of Lithium 4,5-DicyanoimidazolateâGlyme Solvates

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Structural Studies of Lithium 4,5-DicyanoimidazolateâGlyme Solvates