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Coordination Abilities of TDI Anion Toward Sodium Cation – Structural and Spectroscopic Studies of Solid and Liquid Glyme-Solvated Electrolyte Systems Maciej Dranka, Grazyna Zofia Zukowska, Piotr Jankowski, Anna Plewa-Marczewska, Tomasz Trzeciak, and Janusz Zachara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09705 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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The Journal of Physical Chemistry

Coordination Abilities of TDI Anion Toward Sodium Cation – Structural and Spectroscopic Studies of Solid and Liquid Glyme-Solvated Electrolyte Systems Maciej Dranka,* Grażyna Z. Żukowska, Piotr Jankowski, Anna Plewa-Marczewska, Tomasz Trzeciak and Janusz Zachara Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland. * M. D.: e-mail, [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Comprehensive structural analysis of sodium 4,5-dicyano-2-(trifluoromethyl)imidazolate (NaTDI) solvates with glymes (1–4), tetrahydrofuran and crown ethers has been performed. Several structural motifs obtained from single-crystal X-ray analysis of complementary series of crystalline adducts with varying O:Na ratios were correlated with spectroscopic and thermal data to provide new information about coordination ability of heterocyclic anions toward sodium cations. Presented results provide basis for developing models of poly(ethylene oxide) electrolytes and liquid systems for sodium ion batteries electrolytes. We have found a wide variety of anion-cation coordination types which allow us to compare them with analogous lithium solvates in terms of Brown’s valence-matching principle and Lewis-acid strength (Sa) parameters. Noticed aggregation modes of sodium salts confirm the occurrence of solvate disproportionation conductivity mechanism at high salt concentrations which can be used for developing new heterocyclic salt systems for sodium batteries.

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


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1. Introduction Sodium conducting electrolytes have been gaining increasing attention in the past few years, as a possible alternative for lithium.1 Low price, resulting from wide availability, compensates for the slightly lower electrochemical potential than that of lithium (−2.73 V vs. −3.04 V) and makes sodium an excellent alternative for efficient energy storage systems. Only recently have Grimaud and co-workers demonstrated that the chemical structure of the salt anion plays a key role in the SEI formation process at the sodium anode and, therefore, has important implications for the cycling performance of batteries employing metallic sodium anodes.2 However, most of the currently known sodium-based systems utilize liquid sodium electrode, which requires the use of electrolyte able to work at high temperature. The application of the 4,5-dicyano-2(trifluoromethyl)imidazole sodium salts (NaTDI) offers the possibility to obtain ambienttemperature working sodium batteries.3 The idea of application of salts based on five-membered heterocyclic ring compounds in lithium or sodium batteries was first proposed by Armand in 2000.4,5 Since then various anions such as 4,5-dicyanoimidazole or 4,5-dicyanotriazoles derivatives have been obtained.6–8 TDI anion seems to be the most promising among them.7,9 The structures of NaTDI–propylene carbonate solvates and properties of the carbonate based electrolytes doped with NaTDI were published by Marczewska and co-workers.3 NaTDI based electrolytes in PC show good conductivity with the values about 4 mS·cm−1 at 20 °C for the salt concentration of 0.5 M. However, the possible ionic association mechanism in the highly concentrated ether based systems doped with NaTDI and its relation to the analogous lithium system remains unknown. Only recently have glyme-ethers been proposed as favorable solvents for several battery systems like Na-ion or Na-O2 batteries.10,11 The explanation of LiTDI aggregation phenomena in aprotic solvents performed in our group was crucial for the


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understanding of conduction mechanism in this system.8,12,13 The most important observation was that glyme solvates of LiTDI salt can provide ionic system with solvent-coordinated free cations accompanied by aggregated polyanionic species formed as a result of disproportionation. Therefore, systems with optimal conducting parameters can also be expected at high salt concentrations. Importantly, the revealed mechanism explains high lithium transference number and high conductivity both in the solid state and in concentrated solutions. This process, solvate disproportionation

conductivity mechanism,

recently discovered

in

our

team12

and

comprehensively analyzed by means of computational methods14 has a major impact on electrochemical properties of electrolytes based on dicyanoimidazolato salts.15 In this work, we focus on the mechanism of ionic association in the ether based systems doped with NaTDI using single crystal diffraction method combined with Raman spectroscopy to provide the basis for developing the model of poly(ethylene oxide) electrolytes or liquid systems and, furthermore, provide some insight into the possibility of designing disproportionation-based sodium battery electrolytes. 2. Experimental Section 2.1 Synthesis and Crystallization Sodium 4,5-dicyano-2-(trifluoromethyl)imidazolide (NaTDI) was synthesized according to the literature procedure.3 Anhydrous glymes: G1- mono(ethylene oxide), G2- di(ethylene oxide), G3- tri(ethylene oxide), G4- tetra(ethylene oxide) dimethyl ether as well as crown ethers 12crown-4, 15-crown-5 and 18-crown-6 were purchased form Sigma-Aldrich and used as received. All operations were carried out inside an argon-filled glovebox. Single crystals of 1-8 were grown inside the argon-filled glovebox as follows. The mixture of ~100 mg NaTDI and ~0.5 mL of the corresponding solvent was placed in a hermetic glass vial, heated to ~70°C and then


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allowed cool down slowly to room temperature (RT). After a few hours sample was moved to a refrigerator (~4°C), where colorless single crystals suitable for XRD studies grew after a few days. 2.2. Raman Spectroscopy The Raman spectra were collected at room temperature on a 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 the use of Peltier cooled Linkam stage. Spectral analysis was performed with the Omnic software. 2.3. DSC Studies The DSC curves were recorded using a TA Instruments Q200 DSC apparatus in nitrogen flow. The heating rate was equal to 5°C/min. 2.4. X-Ray Crystallography Single crystals of 1-8 suitable for X-ray diffraction studies were selected under a polarizing microscope, mounted in inert oil and transferred to the cold gas stream of the 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.16 The structures were solved with ShelXT17 structure solution program and refined with the ShelXL−201418 refinement package using least-squares minimization which were invoked from Olex2.19 The crystal data and experimental parameters are summarized in Table S1 in the Supporting Information. CCDC 1576879-1576886 contain supplementary crystallographic data for this paper. These data can be obtained

free

of

charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif.


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Na(12C4)2+ TDI− (1). Crystal Data for C22H32F3N4NaO8: monoclinic, space group C2/c, a = 18.4001(6) Å, b = 17.2234(4) Å, c = 18.0327(6) Å, β = 113.199(4)°, V = 5252.7(3) Å3, T = 120.0 K, 24626 reflections measured, 5159 unique (Rint = 0.0412, Rsigma = 0.0276). The final R1 was 0.0335 (I > 2σ(I)) and wR2 was 0.0799 (all data). Raman (selected bands, cm–1): 2222, 1486, 1448, 1304, 1265, 1233, 1113, 1047, 974, 902, 852, 792, 670. Tm = 146°C Na(15C5)·TDI (2). Crystal Data for C16H20F3N4NaO5: orthorhombic, space group P212121, a = 9.52179(9) Å, b = 9.79170(10) Å, c = 21.7103(2) Å, V = 2024.15(3) Å3, Z = 4, T = 100.0(3) K, 117129 reflections measured, 5304 unique (Rint = 0.0361, Rsigma = 0.010). The final R1 was 0.0228 (I > 2σ(I)) and wR2 was 0.0603 (all data). Raman (selected bands, cm–1): 2228, 1494, 1452, 1315, 1265, 1250, 1179, 986, 864, 845, 714, 677. Tm = 92°C. Na(18C6)·TDI (3). Crystal Data for C18H24F3N4NaO6: monoclinic, space group P21/c, a = 9.2468(4) Å, b = 17.7775(8) Å, c = 14.7005(6) Å, β = 102.102(4)°, V = 2362.84(18) Å3, T = 200.0 K, , 20425 reflections measured, 4185 unique (Rint = 0.0341, Rsigma = 0.0242). The final R1 was 0.0480 (I > 2σ(I)) and wR2 was 0.1216 (all data). Raman (selected bands, cm–1): 2238, 1492, 1476, 1444, 1308, 1277, 1250, 1147, 979, 877, 832, 707, 678. Tm = 70°C. Na(G4)·TDI (4). Crystal Data for C32H44F6N8Na2O10: triclinic, space group ܲ1ത, a = 15.02929(14) Å, b = 22.3663(2) Å, c = 25.7490(2) Å, α = 90.0006(8)°, β = 90.0093(7)°, γ = 102.8619(8)°, V = 8438.33(14) Å3, T = 100.0(4) K, 258163 reflections measured, 29951 unique (Rint = 0.050, Rsigma = 0.0213). The final R1 was 0.0558 (I > 2σ(I)) and wR2 was 0.1541 (all data). Raman (selected bands, cm–1): 2239, 1491, 1477, 1445, 1308, 1245, 1178, 1129, 979, 868, 835, 707, 679. Tm = 103°C. [Na(G3)·TDI]2 (5). Crystal Data for C28H36N8O8F6Na2: triclinic, space group ܲ1ത, a = 9.3885(2) Å, b = 10.2921(3) Å, c = 10.7573(3) Å, α = 82.712(2)°, β = 67.370(2)°, γ =


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74.624(2)°, V = 924.72(4) Å3, T = 100.0 K, 49540 reflections measured, 4638 unique (Rint = 0.0296, Rsigma = 0.0116). The final R1 was 0.0293 (I > 2σ(I)) and wR2 was 0.0771 (all data). Raman (selected bands, cm–1): 2238, 2230, 1491, 1448, 1313, 1279, 1250, 1186, 984, 868, 833, 708, 676. Tm = 84°C. Na(G2)·TDI (6). Crystal Data for C12H14F3N4NaO3: monoclinic, space group P21/c, a = 20.23288(18) Å, b = 9.67201(10) Å, c = 16.11912(14) Å, β = 96.6862(8)°, V = 3132.94(5) Å3, T = 100.0 K, 172118 reflections measured, 8616 unique (Rint = 0.0440, Rsigma = 0.0138. The final R1 was 0.0306 (I > 2σ(I)) and wR2 was 0.0856 (all data). Raman (selected bands, cm–1): 2243, 2231, 1493, 1477, 1450, 1313, 1277, 1174, 1134, 986, 865, 840, 819, 709, 678. Tm = 82°C. Na(G1)0.67·TDI (7) . Crystal Data for C26H20F9N12Na3O4: orthorhombic, space group P21212, a = 37.1989(3) Å, b = 12.80076(13) Å, c = 7.46034(8) Å, V = 3552.42(6) Å3, T = 100.0 K, 159611 reflections measured, 6889 unique (Rint = 0.0532, Rsigma = 0.0125). The final R1 was 0.0322 (I > 2σ(I)) and wR2 was 0.0838 (all data). Raman (selected bands, cm–1): 2257, 2252, 2245, 1503, 1491, 1461, 1449, 1322, 1317, 1311, 1161, 994, 983, 862, 708, 683, 666. Tm = 142°C. Na(THF)·TDI (8). Crystal Data for C20H16F6N8Na2O: monoclinic, space group P21/n, a = 16.0906(2) Å, b = 8.89160(10) Å, c = 16.8789(2) Å, β = 94.9700(10)°, V = 2405.80(5) Å3, T = 100.0 K, 69456 reflections measured, 4296 unique (Rint = 0.0448, Rsigma = 0.0140). The final R1 was 0.0313 (I > 2σ(I)) and wR2 was 0.0875 (all data). Raman (selected bands, cm–1): 2249, 1498, 1457, 1314, 1174, 992, 920, 712, 684, 671. Tm ~135°C.

3. Results and Discussion The series of NaTDI crystalline complexes (Scheme 1) with dimethyl ethers of ethylene oxide oligomers (glymes): mono(ethylene oxide) G1, di(ethylene oxide) G2, tri(ethylene oxide)


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G3, tetra(ethylene oxide) G4, crown ethers 12-crown-4 (12C4), 15-crown-5 (15C5), 18-crown-6 (18C6) and tetrahydrofuran (THF) were obtained and their crystal structures determined by 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 is comprehensively shown in Table 1. NaTDI–glyme system seems to follow the solvate disproportionation conductivity mechanism revealed previously for lithium solvates.12 Some of the structural motifs are similar in part to those distinguished in LiTDI–glymes. However, observed differences between Li and Na salts allow us to validate possible modifications of heterocyclic anions coordination properties which can be useful for future development of electrolytes for sodium batteries.


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Scheme 1. NaTDI salt and ethers used in this study.

Table 1. Cation and Anion Surroundings in Crystalline NaTDI–glyme Solvates.


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comp O:Na ound ratio molecular formula nu mber

aggregation mode

anion environment

used TDI donor centers

1 2 3 4 5 6

8:1 5:1 6:1 5:1 4:1 3:1

Na(12C4)2+ TDI– Na(15C5)·TDI Na(18C6)·TDI Na(G4)·TDI [Na(G3)·TDI]2 Na(G2)·TDI

isolated ions monomer 1D-chain 1D-chain dimer 2D-layer

7

1.33:1

Na(G1)0.67·TDI

3D aggregate

8 a

b

none 1NIm+F 2NCN 2NCN 1NIm+1NCN +F 2NIm+1NCN +F 2NIm+2NCN +F 2NIm+2NCN +F 2NIm+2NCN

1:1 Na(THF)·TDI 3D aggregate 2NIm+2NCN +F for the representation of structural motif see Figure 7

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cation environment

No. of No. of No. coordinat Na+ coordinat of ed coordinat ed TDI Sb/v.u.b ether + Na /struc ion anions oxyge tural number ns motifa 0/ I 8 8 0 1/ II 1 6+1 5 1 2/IV 0.5 8 6 2 2/ IV 0.5 7 5 2 2/ III 0.5 6+1 4 2 3/ V 0.33 6+1 3 3 4/VI 0.25 6+2 4 2 4/VI 0.25 5+1 2 3 4/VI 0.25 5 +1 0 5 6 0 6 4/VI 0.25 5 +1 1 4

Sb/v.u (Lewis-base strength ) defined as anion charge/number of coordinated sodium cations

TDI anion, as shown in Scheme 1, offers several sites available for cation coordination– nitrile groups, ring nitrogens alone or ring nitrogens supported by fluorine atom from trifluoromethyl group. All of the donor centers are able to readily match the coordination sphere of Li+ or Na+ cation in the presence of varying amount of aprotic donor solvent. The 12-crown-4 solvate of NaTDI (1) with the highest O:Na molar ratio comprises sodium cation coordinated by two crown ether rings and isolated from anion as shown in Figure 1a. Thus, it may serve as a convenient model of “free anions” in liquid systems. Similarly, monomeric complex 2 with 15-crown-5 ether (Figure 1b) may serve as the simplest model for ionic contact pair in which anion is coordinated through one imidazole nitrogen supported by fluorine donor center.


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Figure 1. Molecular structures of crystalline NaTDI solvates (a) Na(12C4)2+TDI− (1), (b) Na(15C5)·TDI (2), (c) [Na(G3)·TDI]2 (5). Displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Similar coordination mode is observed for dimeric [Na(G3)·TDI]2 (5) solvate. Na+ cation is coordinated by imidazole nitrogen atom of TDI additionally supported by accompanying F⋯Na dative bond and by four oxygen donor centers from G3 molecule. Remaining coordination site of sodium cation is fulfilled with nitrile nitrogen donor center from adjacent unit. It is worth noting that dimeric structural motif for sodium salt is accomplished with longer glyme than analogous dimeric motif in LiTDI salt. This motif is sustained with G3 molecules for NaTDI salt and G2 molecules for lithium salt of TDI because of the higher coordination number of sodium cation in comparison to the lithium cation. The average observed coordination number (AOCN) of cations determined on the basis of a large number of literature data shows that AOCN of lithium and sodium cations surrounded by O2- anions are equal to 4.9 and 6.4, respectively.20 It is in agreement with the larger effective ionic radius of sodium cation compared to lithium (Na 102 pm, Li 76pm).21 The valence-matching principle given by Brown22,23 states that the most stable structures form when Lewis-acid strength (Sa) of a cation readily matches the Lewis-base strength (Sb) of an anion. Thus, Lewis-acid strength expressed in valence units (v.u.) is a convenient parameter to compare salts with different cations. A quantitative measure of the Sa of a cation is given by the strength of the bonds it forms, namely its formal charge divided by the coordination number.24 Moreover, Lewis-acid strength calculated using the average observed coordination number (AOCN) of cation (Sa = formal cation charge/AOCN) characterizes an idealized acid-strength that would be expected to appear in crystalline phase if ligand packing around the cation were the only factor determining its coordination number.24 These ideal Sa


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values for lithium and sodium cations are equal 0.21 and 0.16 v.u., respectively.20 TDI anion can act as a donor of no more than four centers. Lewis-base strength of the four-coordinated anion can be formally estimated as 0.25 v.u. which is a little higher than Sa of lithium or sodium cations. Hence, an electroneutral donor centers from solvent molecules have to be involved for a proper leveling of cation-anion valences. Thus, it can be concluded that sodium salts of dicyanoimidazoles require at least one additional donor center to maintain the same aggregation level as its lithium analogues. Moreover, according to coordination spheres of Na+ summarized in Table 1, the observed coordination number of cation in mixed anionic TDI donors and electroneutral glyme donors is slightly larger than AOCN of the Na+ and decreases with the lowering O:Na ratio. Lower Lewis-acid strength along with the higher coordination number preferred by Na+ cations results in new structural motifs which were not observed for lithium salts. Tetraglyme, as well as 18-crown-6 ether are able to fully coordinate Na+ in equatorial position leaving two possible sites capable of coordinating anions in axial positions. These sites can be fulfilled with nitrile donor centers from two different TDI anions to form infinite chains as shown in Figure 2.


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Figure 2. Structural motifs observed in (a) Na(18C6)·TDI (3) and (b) Na(G4)·TDI (4). Lowering of the O:Na ratio leads to the formation of more aggregated species. The crystal structure of G2 solvate shows layers comprised of dimeric units linked by spanned TDI anions (Figure 3). Glyme molecules are located above and below the layer constituting isolated, electroneutral sheets. One of the four imidazole donor centers remains uncoordinated and dicyanoimidazolate anions in crystal lattice of 6 act as tridentate ligands. TDI anions and G2 oxygen donor centers occupy three coordination sites of the Na+ cation each and all sodium cations possess the same surrounding.


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Figure 3. Dimeric units forming layers in the crystal structure of Na(G2)TDI (6). Further decrease in the O:Na molar ratio leads to a three-dimensional (3D) aggregated system with concurrent acid-base disproportionation process taking place. This process provides cations which differ in the number of anionic ligands existing in their coordination spheres. The crystal structure of the G1 solvate 7 with the formula Na(G1)0.67·TDI (Na:O molar ratio equal to 1:1.33) reveals the presence of dicyanoimidazolate anions acting as tetradentate ligands connected with four different Na+ cations as shown in Figure 4. Lewis-base strength of the four-coordinated anion can be formally estimated as 0.25 v.u.


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Figure 4. Anion coordination mode in Na(G1)0.67·TDI (7). Closer inspection of structural motifs observed in 7 shows that each Na+ cation possesses a different, mixed nitrogen-oxygen or purely nitrogen surrounding as depicted in detail in Figure 5.

Figure 5. Four different sodium cations present in the crystal structure of G1 solvate (7). Sodium cations coordinated by five or six TDI anions can be considered as structural fragments exhibiting properties of aggregated polyanions. Other cations are bound with a lower number of TDI anions and the remaining sites in the coordination sphere are fulfilled with one or two glyme molecules. They can be considered as terminal cations which are able to dissociate relatively


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easily during electrochemical processes. The tendency to diversify metal centers by mutual exchange between oxygen and nitrogen donors in coordination sphere of cation for highly aggregated structures origins from the discrepancy between Sb of anions and Sa of cations that cannot be easily offset by solvent donor centers at low O:Na ratio. In such cases, disproportionation may arise enabling the occurrence of solvate disproportionation conductivity mechanism (SDCM) in aggregated electrolytes. This process, recently discovered in our team for lithium TDI salts, has a major impact on electrochemical properties of electrolytes based on dicyanoimidazolato salts.12 Nevertheless, disproportionation window in sodium salts is relatively narrow, because at O:Na molar ratio equal 1:1 NaTDI solvates form electroneutral aggregates as shown in Figure 6. Every sodium cation is coordinated by four tetradentate TDI anions and one THF molecule in the manner which has been observed previously for NaTDI-propylene carbonate solvate.3 Resulting three-dimensional framework composed of Na+ cations and TDI anions constituting square grid framework with THF-molecules occupying cavities.

Figure 6. ORTEP plot of 3D square grid supramolecular network of 8 projected onto the (001) plane. Color code: gray C; red O; green: F, blue: N, cyan: Na.


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In this case monodentate solvent does not induce the disproportionation process. We presume that in order to expand the applicable disproportionation window the Sb of anion should be a little lower to better match the Sa of sodium cation. It can be achieved by introducing additional nitrile nitrogen donor centers into the structure of anion. Our preliminary electrochemical measurements show that employing percyano-substituted heterocyclic anions may be beneficial for developing new classes of electrolytes for sodium batteries.25 3.1. Spectroscopic Characterization of Sodium Solvates 3.1.1. Raman Fingerprints of Structural Motifs Detailed knowledge of the exact structure based on diffraction data is often unavailable in the case of PEO complexes because of difficulties in obtaining crystals of required quality. The same problem exists for liquid and amorphous phases. In order to overcome this problem it is convenient to retrieve spectroscopic fingerprints of crystalline solvates with short-chain PEO analogues, glymes, of known structure, and use them as models to deduce the structure of noncrystalline complexes. The assumption that coordination modes as well as conformations of the polyether chains are reflected in the Raman spectrum works reasonably well. Basing on XRD studies we have distinguished 6 different structural motifs present in NaTDI solvates with dimethyl ethers of ethylene oxide oligomers and crown ethers (Figure 7).


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Figure 7. Coordination modes of the TDI anion found in glymes and corona ether solvates based on the XRD studies.

Several coordination motifs recognized in LiTDI glyme solvates have their analogues among the NaTDI solvates. Some of them, e.g. I, II and III, which served as models for “free anions”, ionic contact pairs and dimers, are common for LiTDI and NaTDI solvates, while others, such as IV or VI, are found exclusively in NaTDI adducts. Knowing the structural motifs of crystalline solvates 1-8 we have collected their Raman spectra (see Supplementary material, Figure S1) and analyzed them to choose bands sensitive to coordination mode of anions. Three bands


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corresponding to νCN, νN_Im and δNCN have been selected as probes for anion environment analysis. Data collected in Table 2, correlating positions of the analyzed bands in Raman spectra with structural motifs, can be further used for elucidating structure of liquid, amorphous or unknown solvates. Table 2. Band Assignments for NaTDI Crystalline Complexes in Raman Spectrum. compound number

molecular formula

1

Na(12C4)2+ TDI−

I

2222

1304

974

2

Na(15C5)·TDI

II

2228

1315

986

IV IV III V VI VI

2238 2239 2238, 2230 2243, 2231 2257, 2252, 2245 2249

1308 1308 1313 1313 1322, 1317, 1311 1314

979 979 984 986 994, 983 992

3 4 5 6 7 8

Na(18C6)·TDI Na(G4)·TDI [Na(G3)·TDI]2 Na(G2)·TDI Na(G1)0.67·TDI Na(THF)·TDI

structural motif

νCN (cm−1)

νCN_Im (cm−1)

δNCN

(cm−1)

The behavior of the selected νCN, νN_Im and δNCN bands follows the same rules which have been discussed for LiTDI solvates.13 Upshift or splitting of any of these bands is a clear indicator of a given ionic association, in the sense of the particular cation- donor center linkage. For example, the spectral fingerprint of motif IV forming chains through nitrile groups has Raman shift of νCN band typical for aggregates and the νCN_Im and δNCN shifts almost the same, as for motif I with uncoordinated imidazole nitrogens. The main difference between the lithium and sodium salt solvates representing the same coordination motif, for example dimers III, is the higher number of ether oxygens in the cation coordination sphere of the NaTDI solvates. The observed shift in the position of the key bands in NaTDI solvates spectra is weaker, than in their LiTDI analogues because of the lower polarizing effect of the Na+ cation. This effect strongly depends on Lewisacid strength and is weaker for sodium cation. The weaker polarizing ability of sodium cation is also reflected in the position of the so-called breathing M+-O mode in Raman spectra of NaTDI solvates.


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3.1.2. Thermal Behavior and Phase Identification In real electrolyte systems, depending on the individual thermal history of the sample, the occurrence of multiple phases is very common. Therefore, identification of specific phases in solid precipitates which contain several crystalline solvates has to be carefully confirmed by means of temperature-dependent Raman spectroscopy. Figure 8 shows DSC traces of the glyme solvates of NaTDI salt. Besides the thermal effects corresponding to the melting points of crystalline solvates 4-7 there are several additional effects. They can be correlated with new phases A-E obtained in Raman temperature experiments and their molecular assembly may be postulated on the basis of structural motif fingerprints. Additionally, we have found two highly aggregated solvates during temperature dependent Raman experiments. Phases labeled F and G occur after several heating-cooling cycles conducted for NaTDI–G2 and NaTDI–G3 solutions, respectively. These phases are still present in samples at temperatures as high as 200°C. Thus, their detailed thermal stability remains unknown. Obtained results are summarized in Table 3.


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Figure 8. DSC curves of crystalline NaTDI–glymes solvates.

Table 3. Identification of an Additional Phases Found in DSC Traces and Raman Temperature Experiments.

phase A B C D E F G

νCN (cm−1) 2228 2226 2246, 2234 2249, 2238 2252 2250 2249

νCN_Im (cm−1) 1310 (sh), 1305 1307 1314 1320 (sh), 1307 1318 1313 1320

δNCN/ (cm−1) 985, 979 979 988 990, 979 995 993 996

Tm ( °C)

anion coordination mode

15 12 76 79 176 not determined not determined

II I VII VII VI VI VI

supposed molecular assembly contact pairs ionic chain chain 3-D aggregate 3-D aggregate 3-D aggregate

Liquid NaTDI–G1 system during cooling process forms additional phase A with a melting point of 15°C. Spectral characteristic of anion in this phase resembles that observed in the spectrum of 2 and is typical of ionic contact pairs with anion coordinated in the manner specific for the structural motif II. Thus, phase A can be described as a contact pair with TDI anion coordinated with imidazole ring nitrogen to sodium cation solvated by G1 molecules. For the NaTDI–G3 system we observed thermal effects at 84 and 12°C attributed to the solvate 5 and a new phase labeled as B. We have identified this new phase as an ionic solvate with anions of type I. Thus, phase B can be regarded as a solvate comprising uncoordinated free TDI anions and fully solvated cations encapsulated by G3 molecules. It is interesting that after melting of the crystals of 5 and B the spectral pattern of III and I is replaced by new sets of bands, which points to the rearrangement of the structure and a change of the coordination type in the liquid. During the second cycle of cooling-heating program on DSC curve a new phase C with melting point of 76oC appears. Raman spectra indicates that in this phase two donor centers of TDI anion, one nitrile and one imidazole nitrogen atom, are involved with two possible coordination modes, III or VII. On the basis of the spectroscopic measurements we can reject dimeric motif III as not


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matched with the fingerprint of the new phase. Thus, it could be assumed that phase C is an infinite chain based on motif VII utilizing two diagonally placed nitrogen donor centers of TDI anion which is the coordination mode alternative to dimeric structural motif. Despite the fact that we were not able to retrieve structural motif VII for sodium salts, its spectral pattern resembles the pattern of analogous motif found for lithium salt LiPDI.12 The same spectral pattern as in C was found for phase D in NaTDI–G4 system with a melting point of 79°C. Thus, the formation of the same coordination motif can be assumed. Additionally, we have found that NaTDI salt forms aggregates E-G possessing probably similar three-dimensional square grid framework composed of Na+ cations and TDI anions as for Na(PC)·TDI solvate3 and Na(THF)·TDI solvate 8.

3.1.3. Liquid Systems Raman spectra of triglyme, tetraglyme and diglyme solutions exhibit very similar position of three bands selected for the analysis of an anion coordination. These bands centered at ~2225 cm−1, ~1305 cm−1 and ~980 cm−1 are observed in all NaTDI–glyme solutions up to 0.5 M and correspond to spectroscopically “free anions”. It is interesting, that this type of complex is privileged in all studied NaTDI–glyme solutions. At higher salt concentrations of salt shoulders or additional bands shifted approx. by 8 cm−1 toward higher wavenumbers appear whose intensity gradually increase with an increase in salt content (see Supplementary material, Figure S2). Maxima of these additional bands corresponds to values of motif II representing ionic contact pair. The same type of contact pair exists in liquid after melting crystals of solvates 4 and 5.


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Figure 9. Raman spectra of crystalline phases detected in NaTDI–G3 (top) and NaTDI–G4 (bottom) systems. Numbering of crystalline phases and its structural motifs are given in parentheses.

It is worth to note that spectral features of crystalline phases obtained after recrystallization of 4 and 5 from melt suggest that new crystalline solvates are formed. Recrystallization of 5 leads to the formation of an aggregate in which TDI anion is in the same environment as in NaTDI–THF solvate. The split of the bands in the spectrum of recrystallized solvate 4 indicate formation of an aggregated structure comprising anions of type V.


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4. Conclusion Structural motifs retrieved from single-crystal X-ray analysis of crystalline NaTDI adducts with glymes, tetrahydrofuran and crown ethers were correlated with spectroscopic and thermal data to provide new information about coordination ability of heterocyclic anions toward sodium cations. We have found that, similarly to lithium salts, decreasing the number of electroneutral solvent donor centers by lowering average O:Na molar ratio leads to aggregation starting from isolated ions and ionic pairs through dimeric species and chains to layers and 3D structures. The system also tends to disproportionate at high salt concentrations with the formation of solvent coordinated cations and aggregated polyanionic species. However, detailed analysis reveals that in terms of the valence-matching principle sodium cation with the ideal value of Lewis-acid strength equal to 0.16 v.u prefers six anions in its surroundings, which is difficult to obtain by TDI anions. TDI anion acting as tetradentate ligand possesses the maximum Lewis-base strength of 0.25 v.u. which matches better Lewis-acid strength of lithium cations. Sodium cations have higher coordination numbers than lithium and should be preferably stabilized with heterocyclic anions bearing higher number of nitrile donor centers. Preliminary results seem to confirm this assumption and detailed work on percyano substituted heterocyclic anions is in progress. Moreover, the approach based on valence-matching principle and Lewis-acid strength parameters provide new physical insight into electrolyte systems and allows us to understand structural features of heterocyclic anions better.

Associated Content Supporting Information


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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 Corresponding Authors * M. D.: e-mail, [email protected]. Notes The authors declare no competing financial interest. Acknowledgments. This work has been supported by the Warsaw University of Technology. Abbreviations Sa, Lewis-acid strength; Sb, the Lewis-base strength, MeCN, acetonitrile; DSC, differential scanning calorimetry; AOCN, average observed coordination number; TDI, 4,5-dicyano-2(trifluoromethyl)imidazolate anion; 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, 18C6, crown ether 18crown-6. References (1)

Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636–11682.


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Lutz, L.; Alves Dalla Corte, D.; Tang, M.; Salager, E.; Deschamps, M.; Grimaud, A.; Johnson, L.; Bruce, P. G.; Tarascon, J.-M. Role of Electrolyte Anions in the Na–O2 Battery: Implications for NaO2 Solvation and the Stability of the Sodium Solid Electrolyte Interphase in Glyme Ethers. Chem. Mater. 2017, 29, 6066–6075.

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Michot, C.; Armand, M.; Gauthier, M.; Choquette, Y. Pentacyclic Anion Salts or Tetrazapentalene Derivatives and Their Uses as Ionic Conducting Materials. US6835495 B2, December 28, 2004.

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Bitner-Michalska, A.; Krztoń-Maziopa, A.; Żukowska, G.; Trzeciak, T.; Wieczorek, W.; Marcinek, M. Liquid Electrolytes Containing New Tailored Salts for Sodium-Ion Batteries. Electrochimica Acta 2016, 222, 108–115.

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(10) Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. A Rechargeable Room-Temperature Sodium Superoxide (NaO2) Battery. Nat. Mater. 2013, 12, 228–232. (11) Jache, B.; Binder, J. O.; Abe, T.; Adelhelm, P. A Comparative Study on the Impact of Different Glymes and Their Derivatives as Electrolyte Solvents for Graphite CoIntercalation Electrodes in Lithium-Ion and Sodium-Ion Batteries. Phys. Chem. Chem. Phys. 2016, 18, 14299–14316. (12) Jankowski, P.; Dranka, M.; Żukowska, G. Z.; Zachara, J. Structural Studies of Lithium 4,5-Dicyanoimidazolate–Glyme Solvates. 1. From Isolated Free Ions to Conductive Aggregated Systems. J. Phys. Chem. C 2015, 119, 9108–9116. (13) Jankowski, P.; Dranka, M.; Żukowska, G. Z. Structural Studies of Lithium 4,5Dicyanoimidazolate–Glyme Solvates. 2. Ionic Aggregation Modes in Solution and PEO Matrix. J. Phys. Chem. C 2015, 119, 10247–10254. (14) Jankowski, P.; Dranka, M.; Wieczorek, W.; Johansson, P. TFSI and TDI Anions: Probes for Solvate Ionic Liquid and Disproportionation-Based Lithium Battery Electrolytes. J. Phys. Chem. Lett. 2017, 8, 3678–3682.


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Table of Contents Image


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Molecular structures of crystalline NaTDI solvates (a) Na(12C4)2+TDI− (1), (b) Na(15C5)∙TDI (2), (c) [Na(G3)∙TDI]2 (5). Displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity. 80x207mm (300 x 300 DPI)


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Raman spectra of crystalline phases detected in NaTDI–G3 (top) and NaTDI–G4 (bottom) systems. Numbering of crystalline phases and its structural motifs are given in parentheses. 175x124mm (300 x 300 DPI)


(trifluoromethyl)imidazolate Anion toward Sodium ... - ACS Publications

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