Snapshots of the Hydrolysis of Lithium 4,5-DicyanoimidazolateâGlyme

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Snapshots of the Hydrolysis of Lithium 4,5-Dicyanoimidazolate– Glyme Solvates. Impact of Water Molecules on Aggregation Processes in Lithium-Ion Battery Electrolytes Maciej Dranka, Piotr Jankowski, and Grazyna Zofia Zukowska J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11145 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Snapshots of the Hydrolysis of Lithium 4,5-Dicyanoimidazolate–Glyme Solvates. Impact of Water Molecules on Aggregation Processes in Lithium-Ion Battery Electrolytes. Maciej Dranka,* Piotr Jankowski and Grażyna Z. Żukowska Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland. * M. D.: E-mail, [email protected]

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ABSTRACT:

Despite 4,5-dicyano-2-(trifluoromethyl)imidazole lithium salt (LiTDI) exhibits several interesting features in aprotic solvents such as glymes or carbonate esters, little is known about its structural rearrangement after exposure to water. Since the LiTDI salt has been verified as an effective moisture scavenger able to suppress degradation of the LiPF6-based electrolyte, comprehensive knowledge of coordination modes in LiTDI–H2O system, as well as information about the structure of formed hydrates, is desirable. In the present study, we report the impact of water on the LiTDI glyme-based electrolytes investigated by means of the single-crystal X-ray diffraction technique and Raman spectroscopy. We have found that the exposure of lithium 4,5dicyanoimidazolate–glyme solvates to humid air gives rise to the hydrolysis products arising from stepwise addition of water molecules to the lithium coordination sphere. Several structural motifs have been distinguished as preferred coordination modes in LiTDI–H2O system. At high number of available ether oxygen donor centers water molecules cause dissociation of ionic contact pairs and aggregation of cationic species stabilized by crown ethers. Low O:Li molar ratio leads to the formation of LiTDI-glyme-water solvates and LiTDI hydrates. The air-stable LiTDI trihydrate comprises ionic pairs formed by a lithium cation coordinated to an imidazole nitrogen of TDI. Lithium cation coordinated via nitrile groups and bearing water molecules is a basic motif constituting dimeric species of formula [Li(H2O)2TDI]2 which are present in aggregated [Li(H2O)TDI]n chains making up the structure of monohydrate. The discovered motifs have been proved to occur both in the solid and melted hydrated systems of LiTDI. They will be helpful for conducting molecular dynamic calculations and for obtaining information how to manipulate the structure of a Li+-solvation sheath in both hydrated and liquid aqueous electrolytes

based

on

heterocyclic

anions.


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1. Introduction Highly concentrated electrolytes have attracted increased attention in the lithium-ion battery (LIB) research community because of their various new functionalities, which are appearing as the consequences of unique coordination structures formed at a low solvent content. Based on those unique features, concentrated electrolytes are being extensively studied to develop both non-aqueous and aqueous Li-ion batteries with higher capacities and voltages without compromising on safety.1 A new class of Solvent-in-Salt Electrolyte (SiS) developed for the next-generation high-energy rechargeable batteries allows to lower significantly the quantity of highly flammable and often environmentally challenging solvents.2 On the other hand, aqueous electrolyte, obtained as the result of “water-in-salt” electrolyte (WiS) concept raised the possibility of a moisture-tolerant and nonflammable lithium-ion battery. A 2.3 V full Li-ion cell using the LiMn2O4/Mo6S8 electrodes was demonstrated in such electrolyte, providing energy density of about 100 W·h·kg−1.3 Recent revisions of the WiS electrolytes termed “water-inbisalt” (WiBS) using mixed lithium salts achieved unprecedentedly high concentration and widened the electrochemical stability.4,5,6 The efficiency of a solid electrolyte interphase (SEI) formation in aqueous electrolytes depends on the salt concentration leading to effective formation of a protective layer and a reduction in the electrochemical activity of water.4 It is worth noting, that research on the extension of the thermodynamic potential window of water equal to 1.23 V are under rapid progress in these systems.7,

8, 9

The water content and local

structure of aggregated electrolyte seem to be crucial in concentrated aqueous electrolytes. For example, sodium bis(fluorosulfonyl)imide based aqueous electrolytes exhibit a wide electrochemical stability window of up to 2.6 V when the water-to-salt molar ratio falls below 2:1.10 Therefore, aqueous lithium-ion battery has been demonstrated to be one of the most


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promising choices because of its improved safety and lower price when compared to organic electrolyte-based systems.11,12 “Water-in-salt” electrolytes make the use of cost-effective aluminum current collectors for aqueous high-voltage batteries possible.13 It can be expected that the aqueous lithium-ion battery will show the longest cycling life among all aqueous electrolytebased secondary batteries.14 Moreover, combining concentrated electrolytes with solid state electrolytes may also lead to a new class of high energy and safe batteries for a wide range of applications.15 Detailed knowledge about local structure of electrolyte at the microscopic level is still very limited. Understanding the solvation of Li+ cations and aggregations processes is of great importance because of their crucial role in ionic conductivity and formation of a SEI. Our extensive experience in structural research of salts based on five-membered heterocyclic ring compounds such as 4,5-dicyano-2-(trifluoromethyl)imidazole lithium salt (LiTDI) allows us to determine preferred coordination modes in LiTDI–H2O system comprehensively. The application of LiTDI seems to be the most promising choice as an electrolyte for lithium batteries.16,17 While it exhibits several interesting features in aprotic solvents such as high solubility, electrochemical stability and high conductivity, little is known about its properties in water. It is worth noting that highly concentrated or solid glyme solvates of LiTDI tend to disproportionate providing ionic phases with free cations encapsulated by solvent and aggregated polyanionic species.18 Similarly, solvate disproportionation conductivity mechanism was found to be present in a LiTDI–poly(ethylene oxide) (PEO) membranes.19 Recently, the LiTDI salt has been proven to be an effective moisture scavenger with the aim of suppressing the degradation of the LiPF6-based electrolyte.20 However, detailed mechanism of water uptake process is not clear. Moreover, proposed coordination modes between LiTDI and H2O, based on spectroscopic observations and DFT calculations require further examination and evaluation.20 Herein, we have


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investigated the impact of the water on the LiTDI–glyme electrolyte by means of single-crystal X-ray diffraction (XRD) and Raman spectroscopic measurements to reveal preferred coordination motifs in LiTDI–H2O system. These motifs have been proved to occur in the solid and melted hydrated systems of LiTDI and may be helpful as convenient models for further molecular calculations. 2. Experimental Section 2.1 Synthesis and Crystallization All operations were carried out inside an argon-filled glovebox. Anhydrous glymes: G1mono(ethylene oxide), G2- di(ethylene oxide) dimethyl ether as well as crown ethers 15-crown-5 and 18-crown-6 were purchased from Sigma-Aldrich and used as received. Crystalline glyme solvates of lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI) were synthesized according to the literature procedure.18 Single crystals of LiTDI-glyme hydrates 3‒6 suitable for the X-ray measurements were prepared as follows. A portion of the corresponding LiTDI glyme solvate (~20 mg ) was transferred to a microscope slide and covered with an inert oil (Immersion Oil type A, Cargill). Afterward the sample was exposed to humid air for several days resulting in colorless single crystals suitable for XRD studies and temperature dependent Raman experiments. LiTDI hydrates 1a and 1b were crystallized in vials from aqueous solutions at temperatures equal to 20°C and 4°C, respectively. Single crystals of LiTDI dihydrate 2·G1 was obtained from sample 5 heated to 80°C and then cooled slowly to room temperature. 2.2 Spectroscopic Studies 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


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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. Fourier-transform infrared (FT-IR) spectra were recorded on the Avatar System 370 FT-IR spectrometer with a wavenumber resolution of 1 cm−1 (transmission mode) or 2 cm−1 (reflection mode). Temperature dependent FT-IR studies were performed in the −130 to +180°C temperature range using a vacuum isolated temperature-controlled cell. Samples in the form of a thin film (solution or nujol mull) were sandwiched between two BaF2 plates. The system was pumped for 0.5 h prior to use, and the accuracy of the temperature measurements was estimated to be ±2°C. Ambient temperature spectra of solid samples were recorded with use of Golden Gate ATR accessory equipped with diamond crystal. Nuclear magnetic resonance (NMR) spectra were recorded on Varian VNMRS 500 MHz spectrometer. All experiments were performed at room temperature. LiCl dissolved in D2O (7Li NMR) and CF3COOH dissolved in CD3CN (19F NMR and

15

N NMR) were used as external

references for chemical shift utilizing the coaxial insert. 2.3 Thermal characterization The Differential Scanning Calorimetry (DSC) curves were recorded using a TA Instruments Q200 DSC apparatus in nitrogen flow. The heating rate was equal to 5°C/min. Thermogravimetric (TG) IR experiments were completed using a NICOLET 6700 TG-FT-IR at a heating rate of 10 °C min−1 under inert gas. 2.4 X-Ray Crystallography Single crystals of 1‒6 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


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reduction and analysis were performed with the CrysAlisPRO software.21 The structures were solved with ShelXT22 structure solution program and refined with the ShelXL−201723 refinement package using least-squares minimization which were invoked from Olex2.24 The water hydrogen atoms were located from the difference Fourier map. The H-O and H...H distances were restrained to 0.85 and 1.38 Å, respectively. Structures 1a, 1b and 2·G1 were refined as a 2component twins, with crystal domains rotated around [001] direction. Obtained twin ratio for 1a and 1b were 0.465(2):0.535(2) and 0.480(2):0.520(2), respectively. Considering poor data quality for all selected crystals of 2·G1, the results of many refinement trials was still not satisfactory. However, obtained model of structure is reasonable. The crystal data and experimental parameters are summarized in Table S1, Supporting Information. CCDC 1582498‒ 1582504 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. 2.4.1 Li(H2O)3TDI (1a). Crystal data for C6H6F3LiN4O3 (M =246.09 g/mol): monoclinic, space group P21/c, a = 6.9332(4) Å, b = 9.3110(5) Å, c = 15.6864(8) Å, β = 91.590(5)°, V = 1012.25(10) Å3, Z = 4, T = 100.0(1) K, µ(CuKα) = 1.441 mm−1, 3070 reflections measured, 3070 unique (Rsigma = 0.0150) which were used in all calculations. The final R1 was 0.0535 (I > 2σ(I)) and wR2 was 0.1480 (all data). Raman (selected bands, cm−1): 2250, 2245, 1503, 1465, 1322, 1192, 1129, 1006, 712, 683. 2.4.2 Li(H2O)3TDI· H2O (1b). Crystal data for C6H8LiN4O4F3 (M =264.10 g/mol): monoclinic, space group P21/n, a = 9.0805(12) Å, b = 6.7141(6) Å, c = 18.9826(18) Å, β = 91.321(9)°, V = 1157.0(2) Å3, Z = 4, T = 100.0(1) K, µ(MoKα) = 0.152 mm−1, 3488 reflections measured, 3488 unique (Rsigma = 0.0389) which were used in all calculations. The final R1 was 0.0438 (I > 2σ(I))


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and wR2 was 0.1392 (all data). Raman (selected bands, cm−1): 2266, 2241, 1503, 1320, 1303, 1004, 768, 686. 2.4.3 [Li(H2O)2TDI]2·2H2O (2·2H2O). Crystal data for C12H12F6Li2N8O6 (M =492.18 g/mol): triclinic, space group ܲ1ത, a = 7.3838(5) Å, b = 8.5505(5) Å, c = 9.9562(6) Å, α = 112.610(6)°, β = 109.205(6)°, γ = 93.780(5)°, V = 534.47(6) Å3, Z = 1, T = 100.0(1) K, µ(CuKα) = 1.365 mm−1, 4994 reflections measured, 1896 unique (Rint = 0.0225, Rsigma = 0.0176) which were used in all calculations. The final R1 was 0.0385 (I > 2σ(I)) and wR2 was 0.1084 (all data). Raman (selected bands, cm−1): 2260, 1503, 1464, 1318, 1189, 1003, 710, 688. 2.4.4 [Li(H2O)2TDI]2·G1 (2·G1). Crystal data for C16H18Li2N8O6F6 (M =546.26 g/mol): monoclinic, space group P21/n, a = 8.653(14) Å, b = 18.007(5) Å, c = 16.68(4) Å, β = 91.5(3)°, V = 2599(7) Å3, Z = 4, T = 120.0(1) K. Raman (selected bands, cm−1): 2259, 1503, 1461, 1313, 1183, 1003, 997, 709, 688. 2.4.5 {Li(15C5(H2O)}+ TDI− (3). Crystal data for C16H22F3LiN4O6 (M =430.31 g/mol): monoclinic, space group P21/n, a = 12.4709(3) Å, b = 8.2509(2) Å, c = 20.2761(7) Å, β = 91.312(2)°, V = 2085.79(10) Å3, Z = 4, T = 120.0(1) K, µ(MoKα) = 0.120 mm−1, 24964 reflections measured, 4256 unique (Rint = 0.0377, Rsigma = 0.0249) which were used in all calculations. The final R1 was 0.0445 (I > 2σ(I)) and wR2 was 0.1109 (all data). Raman (selected bands, cm−1): 2238, 1493, 1451, 1313, 1276, 1183, 1145, 986, 871, 831, 705, 673. 2.4.6 {Li2(18C6)2(H2O)3}2+2TDI− (4). Crystal data for C36H54F6Li2N8O15 (M =966.75 g/mol): orthorhombic, space group P212121, a = 14.2835(3) Å, b = 17.9913(4) Å, c = 18.1462(4) Å, V = 4663.19(18) Å3, Z = 4, T = 120.0(1) K, µ(MoKα) = 0.121 mm−1, 76851 reflections measured, 9466 unique (Rint = 0.0591, Rsigma = 0.0326) which were used in all calculations. The final R1 was


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0.0372 (I > 2σ(I)) and wR2 was 0.0815 (all data). Raman (selected bands, cm−1): 2225, 2222, 1489, 1443, 1304, 1188, 1143, 980, 873, 831, 676. 2.4.7 {Li2(H2O)TDI2}n·nLi(G1)2TDI (5). Crystal data for C26H22F9Li3N12O5 (M =774.37 g/mol): orthorhombic, space group Pbca, a = 19.7040(2) Å, b = 13.95120(10) Å, c = 26.6779(3) Å, V = 7333.61(12) Å3, Z = 8, T = 120.0(1) K, µ(CuKα) = 1.140 mm−1, 34276 reflections measured, 6523 unique (Rint = 0.0525, Rsigma = 0.0302) which were used in all calculations. The final R1 was 0.0459 (I > 2σ(I)) and wR2 was 0.1284 (all data). Raman (selected bands, cm−1): 2267, 2259, 1497, 1455, 1310, 1177, 1001, 990, 709, 688. 2.4.8 [Li(H2O)TDI]n·n[Li(G2)TDI]2 (6a·6b). Crystal data for C18H16F6Li2N8O4 (M =536.27 g/mol): triclinic, space group ܲ1ത, a = 8.5247(4) Å, b = 10.0929(4) Å, c = 15.4277(8) Å, α = 102.766(4)°, β = 99.270(4)°, γ = 98.810(4)°, V = 1252.86(10) Å3, Z = 2, T = 120.0(1) K, µ(MoKα) = 0.130 mm−1, 33633 reflections measured, 5128 unique (Rint = 0.0340, Rsigma = 0.0194) which were used in all calculations. The final R1 was 0.0320 (I > 2σ(I)) and wR2 was 0.0799 (all data). Raman (selected bands, cm−1): 2267, 2236, 1501, 1461, 1319, 1305, 1186, 997, 876, 770, 683. 3. Results and Discussion 3.1 Spectroscopic Characterization of LiTDI Hydrates and Hydrated Glyme‒LiTDI Salts 4,5-Dicyano-2-(trifluoromethyl)imidazole lithium salt (LiTDI) and the series of LiTDI– glyme crystalline solvates18 with dimethyl ethers of ethylene oxide oligomers (glymes): mono(ethylene oxide) G1, di(ethylene oxide) G2 and crown ethers 15-crown-5 (15C5), 18crown-6 (18C6) were exposed to humid air and resulting products were analyzed by means of a single-crystal X-ray diffraction technique and Raman spectroscopy.


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Raman fingerprints obtained for pure phases of crystalline LiTDI hydrates covering assignment of the characteristic bands are presented in Table 1. Additionally, collected full-range spectra are shown in Supplementary Materials (vide spectra). These fingerprints have been correlated with structural motifs retrieved from X-ray structure analysis and further used for gaining information on` the molecular assembly in hydrated systems. Table 1. Band Assignments for LiTDI Crystalline Hydrates in Raman Spectrum

Molecular formula

Crystal structure presented in figure

νCN (cm−1)

νCN_Im (cm−1)

1a

Li(H2O)3TDI

1a

2250, 2245

1322

1006

1b

Li(H2O)3TDI· H2O

1b

2266, 2241

1303

1004

2·2H2O

[Li(H2O)2TDI]2·2H2O

2

2260

1318

1003

2·G1

[Li(H2O)2TDI]2·G1

S3

2259

1318, 1313

1003, 997

3

{Li(15C5(H2O)}+ TDI−

5

1313

986

4

{Li2(18C6)2(H2O)3}2+2TDI−

6

1304

980

3525

5

{Li2(H2O)TDI2}n·nLi(G1)2TDI

8

2238 2225, 2222 2267, 2259 2267, 2252, 2236

3571, 3514, 3395 3596, 3558 3670, 3236 3461, 3372, 3294 3515

1310

1001, 990

3428

1319, 1305

1001, 997

3575, 3395, 3298

Compound number

6a·6b

[Li(H2O)TDI]n·n[Li(G2)TDI]2

9

νOH δNCN −1 (cm ) (cm−1)

It is worth noting that Raman band positions of the nitrile group coordinated to the lithium cation and involved in hydrogen bonds are often very close to each other. Compounds 3 and 4, both possessing anions decoupled from cations, may serve as an example. However, in the structure of 3 water molecules form hydrogen bonds with anion’s nitrogens, while in the structure of 4 they do not. Therefore, only spectral fingerprint of 4 resembles that of a “free” anion. The


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analysis of the system after hydration may be complicated because of fact that LiTDI is able to form structurally different hydrates of the same stoichiometry such as 2·2H2O and 1a. An additional information provides spectral range corresponding to OH stretching vibrations. For example, the presence of distinct band at 3670 cm−1 allows to distinguish compound 2·2 H2O from 2·G1, both comprising the same dimer unit. Origin of this band may be attributed to the presence of isolated water molecule in the structure of the former. Thus, for the convenient and unambiguous identification of hydrated species in various systems, spectroscopic studies should be supported by structural investigations. This knowledge enables us to get insight into the role of LiTDI as a promising electrolyte additive in lithium-ion batteries.20 3.2 Crystal Structure Analysis of LiTDI Hydrates LiTDI salt crystallizes from aqueous solution at room temperature as a LiTDI trihydrate (1a) in the form of ionic pair of composition Li(H2O)3TDI. The crystal structure of obtained compound is shown in Figure 1a.


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Figure 1. X-ray structure of (a) trihydrate Li(H2O)3TDI (1a) and (b) tetrahydrate Li(H2O)3TDI· H2O (1b). Selected bond lengths (Å) for 1a: Li1‒N2 2.101(4), Li1‒O1 1.947(4), Li1‒O2 2.015(4), Li1‒O3 1.903(4); 1b: Li1‒O1 1.968(5), Li1‒O2 1.882(5), Li1‒O3 1.955(5), Li1‒N4 2.032(5). Lithium cation solvated with three water molecules is coordinated to the nitrogen atom of an imidazole ring, with the Li1‒N2 bond length equal to 2.101(4) Å. It is also worth noting that LiTDI trihydrate 1a is the final product of many experiments on the hydrolysis of LiTDI solvates. Trihydrate 1a is stable in air at ambient temperature and can be considered as the thermodynamically preferred phase in the solid state. On the other hand, crystallization of the LiTDI salt from water solution at 4°C results in the formation of a tetrahydrate 1b of formula Li(H2O)3TDI· H2O (Figure 1b). This compound melts at 20°C with subsequent crystallization of


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hydrate 1a. In the crystal structure of 1b Li+ cation bound with three water molecules is coordinated to a nitrile nitrogen atom, with the Li1‒N4 bond length equal to 2.032(5) Å , and fourth H2O molecule acts as lattice water. Average Li‒O distance of 1.955 and 1.935 Å for 1a and 1b, respectively, is very similar for both hydrates. Hydrolysis experiments conducted at elevated temperatures revealed yet another preferred type of coordination motif based on dimeric subunits formed by two lithium cations linked with two spanning TDI anions via nitrile groups. New dimeric structural fragment of LiTDI hydrate was found in the crystal structure of 2·2H2O obtained by recrystallization of the LiTDI salt from aqueous solution at 80°C. Crystal structure of 2·2H2O, shown in Figure 2, comprises dimers of dihydrate [Li(H2O)2TDI]2 (2) and two additional cocrystalized water molecules.


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Figure 2. Crystal structure of [Li(H2O)2TDI]2·2H2O (2·2H2O). Selected bond lengths (Å): O1‒ Li1 1.926(3), O2‒Li1 1.902(3), N3i‒Li1 2.051(3), N4‒Li1 2.054(3). Symmetry codes: i 1−x, 1−y, 1−z.

Thus, hydrate 2·2H2O can be regarded as a LiTDI dihydrate water solvate of formula [Li(H2O)2TDI]2·2H2O. After heating to 140°C 2·2H2O loses one water molecule per lithium cation giving stable dihydrate [Li(H2O)2TDI]2 (2). Raman spectra of pure phase 2 is presented in Supplementary Information (SI) in Figure S1. It should be also pointed out, that the hydrate 2·2H2O is easy to prepare just by heating of trihydrate 1a at 80°C for one hour. Because both compounds have the same stoichiometry of LiTDI·3H2O, this reaction proceeds without evolution of water. Thus, drying hydrate 1a in such conditions is not effective. Moreover, both hydrates, 1a and 2·2H2O heated to 140°C lose only one water molecule per Li cation giving the same hydrate 2. Our findings show that the unraveled mechanism of the LiTDI salt hydration is more complicated than that given previously.20 For the determination of Li(H2O)3TDI (1a) crystalline hydrate dehydration process we have recorded TGA and DTGA curves supplemented by the series of FT-IR spectra of evolved gaseous products. Results are presented in Figure 3. We have found that initial mass loss observed below 50°C (around 4 %, Figure 3) is due to the evaporation of the absorbed water and according to Raman fingerprints the remaining product is trihydrate Li(H2O)3TDI (1a).


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Figure 3. (a) TGA and DTGA curves for dehydration of LiTDI·3H2O (1a) at heating rate of 10 °C min−1, (b) 3D FT-IR spectra of evolved gaseous products, (c) FT-IR spectra of evolved water recorded at 140 and 330°C.

The slow decomposition of this hydrate takes place above 75°C with the subsequent release of water and the formation of dihydrate [Li(H2O)2TDI]2 (2). This process accelerates above the melting point of trihydrate at 110°C, as shown in DSC trace (see Supplementary Figure S2). The second observed mass loss completed at 140°C equal to 6.2 % corresponds to the release of one water molecule per lithium cation. The resulting LiTDI dihydrate 2 which melts at 180°C is stable up to 250°C. The next mass loss (12.2%, ~2H2O) takes place between 260 and 360°C and


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is immediately followed by the salt decomposition. Therefore, anhydrous LiTDI salt can be obtained only by drying under vacuum. The detailed knowledge about correlation between hydrates of known structures, their spectroscopic characteristic and thermal behavior gives us also the opportunity to interpret structural rearrangement reflected in the temperature-dependent FT-IR spectra of LiTDI saturated water solution shown in Figure 4.

Figure 4. Influence of the temperature on the FT-IR spectra of LiTDI saturated solution in water.

The most significant change observed in temperature-dependent spectroscopy experiments is the shift of νCN maximum towards higher values at about 100°C resulting from the rearrangement of the structure and switching of the cation coordination from the imidazolium ring to nitrile group. This change corresponds to the dehydration process of LiTDI trihydrate 1a and the formation of dihydrate [Li(H2O)2TDI]2 (2), which is stable also after subsequent cooling of the solution down to 20°C. At the temperature range from −20°C to +100°C trihydrate Li(H2O)3TDI (1a) is present in freshly prepared solution. According to our best knowledge, coordination of hydrated lithium


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cation via imidazole nitrogen atom is preferred in aqueous LiTDI solution in a wide range of concentrations (Supporting Information, Figure S3). However, both types of lithium coordination with imidazole or nitrile nitrogen occur at the temperature range from −10°C to +10°C. On the basis of our spectroscopic measurements shown in Figure 4 and detailed temperature-dependent FT-IR spectra collected in sub-ambient temperatures for 0.5M aq LiTDI solution shown in Figure S4 (SI) we may postulate that there is probably another type of coordination below −10°C. It involves probably TDI anions coordinated to lithium cations with both nitrile groups forming infinite chains. 3.3 Crystal Structure Analysis of Hydrated Glyme‒LiTDI Salts We have found that stepwise addition of water molecules in glyme‒LiTDI systems strongly depends on O:Li molar ratio and starts from the insertion of H2O into the coordination sphere of lithium. As a result, at high number of available ether oxygen donor centers water molecules cause dissociation of ionic contact pairs as shown in Figure 5a. Solvate of composition Li(15C5)TDI which is a convenient model of ionic pair recrystallizes after absorption of moisture resulting in an ionic mixed salt 3 consisting of uncoordinated “free” TDI anions and lithium cations solvated by crown ether and water molecule as depicted in Figure 5b.


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Figure 5. (a) Dissociation process of ionic pair in presence of water, (b) crystal structure of mixed {Li(15C5)(H2O)}+TDI− solvate (3) with thermal ellipsoids drawn at the 50% probability level. Selected bond lengths (Å): Li1‒O1 1.940(3), Li1‒O2 2.205(3), Li1‒O3 2.120(3), Li1‒O4 2.142(3), Li1‒O5 2.102(3).

Water molecule fulfills remaining coordination site of lithium after desolvation of TDI anion. This allows Li cation to remain dissociated even in the solid phase. Therefore, H2O molecules act in terms of Lewis basicity as a stronger donor to lithium cation than TDI anions. The Li1‒O1 bond length between lithium and oxygen atom from water molecule equals 1.940(3) Å and is significantly shorter than other lithium‒oxygen ether contacts which vary from 2.102(3) to 2.205(3) Å. The fifth oxygen donor center from crown ether is uncoordinated in the presence of water. Thus, the coordination number of Li cation is lowered in comparison to the lithium coordination number in the starting Li(15C5)TDI solvate. Similarly, LiTDI solvate with 18crown-6 ether forms isolated ions in the presence of water which is shown in details in Figure 6.


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Figure 6. Molecular structure of [Li2(18C6)2(H2O)3]2+2TDI− (4) solvate. Lithium coordination spheres are highlighted with larger ellipsoids and broader bonds. Selected bond lengths (Å): Li1‒ O1 1.890(5), Li2‒O3 1.897(5), Li1‒O2 2.004(5), Li2‒O2 2.116(5), Li1‒O4 2.288(6), Li1‒O5 1.922(5), Li1‒O6 2.450(6), Li2‒O10 1.990(5), Li2‒O11 2.273(5), Li2‒O15 2.155(5).

Lithium cations in mixed water-ether solvate 4 are coordinated only with three oxygen donor centers of 18C6 crown rings and one terminal water molecule. The Li1‒O1and Li2‒O3 bond lengths between lithium cation and terminal water molecules equal to 1.897(5) Å and 1.890(5) Å, respectively, are slightly shorter than Li‒H2O bond length observed in 3. Two similar units are further linked with a third bridging water molecule to give an aggregated dication of formula [(H2O)(18C6)Li‒(H2O)‒Li(18C6)(H2O)]2+. It is worth noting that extended hydrogen bonding network links cations and TDI− anions in the crystal lattice of 3 whereas TDI− anions do not act


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as the acceptors of hydrogen bonds in the crystal structure of 4 (see: Supporting Information, Figure S5–6). Dications in 4 act both as donors and acceptors of hydrogen bonds which results in further aggregation of solvated lithium cations by non-covalent interactions. Thus, it is the solvated lithium cations and not TDI− anion itself which should be considered as a proper moisture scavenger in glyme-based electrolytes. More complicated structural rearrangement after absorption of water was, however, found in highly concentrated LiTDI−glyme systems. In the area of low O:Li ratio we have previously discovered disproportionation mechanism with redistribution of donor centers in the coordination sphere of cations.18 Herein, in the presence of water, the salt concentrations are also recognized as a key to developing new functionalities for battery electrolytes. Figure 7 summarizes changes in Raman spectra observed during stepwise hydrolysis of LiTDI−G1 system after absorption of water.

Figure 7. Changes in Raman spectra observed during stepwise hydrolysis of LiTDI−G1 system after absorption of water. Slow absorption of water by concentrated LiTDI−G1 systems or for example by crystals of 2:1 Li(G1)1TDI solvate18 covered by inert oil allow for the isolation of crystalline phase of composition {Li2(H2O)TDI2}n·nLi(G1)2TDI (5). Crystal structure of the obtained mixed glymewater solvate with O:Li overall ratio of 1.67:1 is presented in Figure 8.


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Figure 8. (a) Fragment of the crystal structure of {Li2(H2O)TDI2}n·nLi(G1)2TDI hydrate (5), (b) structural motifs I and II constituting aggregated layers in 5.

Compound 5 can be described as an aggregated hemihydrate of formula {Li2(H2O)TDI2}n. It consists of infinite electroneutral layers with H2O:Li molar ratio of 0.5:1 which are additionally decorated with Li(G1)2TDI ionic pairs coordinated through nitrile group [Li1−N1 2.018(4) Å]. Second nitrile group from terminal TDI anion is bound to water molecules in the layer via hydrogen bond. In the hemihydrate layer two structural motifs, I and II, can be distinguished. Motif I is based on dimeric subunits formed by two lithium cations linked with two spanning TDI anions via nitrile groups. In the structural motif I, forming hemihydrate layers, lithium


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cations exhibit different coordination. Coordination sphere of the first Li+ is completed by water molecule and the nitrogen atom of the imidazole ring from adjacent unit, while the second lithium cation is additionally coordinated by an imidazole ring from adjacent unit and nitrile group of the terminal TDI anion. Structural motif II in the form of a ten-membered Li(NCCN)2Li ring is formed by TDI anions coordinated in an unsymmetrical fashion through the nitrogen atom of the imidazole ring and nitrile groups. This motif is common for different TDI solvates.18,25 Thus, compound 5 comprising two structural motifs I and II can be regarded as an intermediate between solvated and hydrated structures. Hemihydrate 5 is highly hygroscopic and, when exposed to air further, absorbs more moisture yielding after several minutes a new crystalline phase– LiTDI trihydrate 1a. However, the hemihydrate 5 after exposure to moisture and heating to 80°C recrystallizes giving centrosymmetric dihydrate [Li(H2O)2TDI]2 (2·G1) with cocrystalized monoglyme (G1) as a lattice solvent molecule (Supporting information, Figure S7). Another complicated chemical transformations taking place after the exposure of dimeric diglyme solvate [Li(G2)TDI]218 to air are presented in details in Figure 9. In the first step of the hydrolysis process we were able to separate cocrystals of [Li(G2)TDI]2 and [Li(H2O)TDI]n (6a·6b) consisting of the starting diglyme [Li(G2)TDI]2 solvate (6a) with a LiTDI monohydrate (6b) of composition LiTDI·H2O. The structure of LiTDI·H2O proposed basing on DFT calculations20 differs from that which we have found in the crystal structure of 6a·6b. This shows that both, spectroscopic studies and theoretical calculations, if possible, should be supported by XRD structural investigations. The exact structure of LiTDI monohydrate retrieved from X-ray measurement reveals aggregated infinite polymeric chain of composition [Li(H2O)TDI]n.


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Figure 9. Stepwise hydrolysis of LiTDI−G2 system and the molecular structure of [Li(G2)TDI]2 and [Li(H2O)TDI]n cocrystals (6a·6b). The coordination sphere of lithium in its basic dimeric motif as shown in Figure 9 is fulfilled with two nitrile groups and one water molecule. Thus, fourth coordination of Li site has to be coordinated with donor center from another dimeric unit constituting aggregated structure of the monohydrate. Water molecule is coordinated to lithium cation with relatively short Li‒O bond whose length equals 1.877(2) Å. Temperature dependent spectroscopic experiments supported by Raman fingerprints of LiTDI hydrates shown in Figure 10 make tracing further rearrangement of diglyme solvate [Li(G2)TDI]2 (6a) under hydrolysis conditions possible.


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Figure 10. Changes in Raman spectra observed during stepwise hydrolysis of LiTDI−G2 system. Crystals of 6a·6b left in the air lose diglyme and slowly transform to trihydrate 1a. However, when 6a·6b is heated to about 80°C it recrystallizes resulting in the mixture of water free diglyme solvate 6a and dimeric LiTDI dihydrate 2 (Figure 9 and 10). Thus, the model of water consumption to form electroneutral inert dihydrate [Li(H2O)2TDI]2 with simultaneous recovery of ionic pairs is in agreement with the improvement of Li-ion battery electrochemical parameters.20

Figure 11. Structural motifs constituting (a) hemihydrate, (b) monohydrate, (c) dihydrate, (d) trihydrate and (e) tetrahydrate of LiTDI.


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4. Conclusion We have investigated a new class of hydrates based on LiTDI salt. Both structure and chemistry of these materials are complex due to the strong influence of lithium-donor interactions leading to aggregation processes and the presence of additional hydrogen bonds. These interactions should be taken into consideration when interpreting and conducting future theoretical calculations. The exposure of lithium 4,5-dicyanoimidazolate–glyme solvates to humid air gives rise to the hydrolysis products because of stepwise addition of water molecules as summarized in Figure 11. Structural motifs retrieved from single-crystal X-ray analysis of crystalline adducts with glymes and water provide new information about preferred coordination modes in LiTDI−H2O system and coordination ability of heterocyclic anions in hydrated systems. We have found that, the small amount of water introduced into the salt induces significant changes of the Li+ environment. At high O:Li molar ratio dissociation of ionic contact pairs is observed. Additionally, aggregation between cations occurs. Nevertheless, coordination of H2O to positively charged lithium fragment could bring water molecules near to the electrodes. On the other hand, at low O:Li molar ratio and at low water content electroneutral aggregated hydrates are formed. In concentrated systems and at elevated temperatures molecules of the dimeric dihydrate [Li(H2O)2TDI)]2 are produced. Our findings show how LiTDI could play a key role as an additive in achieving higher stability of electrolytes that contain moisture without the sacrifice of ionic conductivity. Understanding of solvation processes of LiTDI in the presence of water, as well as detailed knowledge of the coordination modes present in LiTDI-glyme-water system, is also the first step in developing highly concentrated and solid hydrated electrolytes for battery application.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. Tables S1–S3 comprising crystal data, experimental and refinement parameters for compounds 1–6 and hydrogen bonds for 3–4; Figures S1–S16 including full range FT-IR and Raman spectra, depiction of hydrogen bond networks in 3–4 and DSC trace of 1a (PDF) X-ray crystallographic data for 1–6 (CIF) AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. ORCID Maciej Dranka: 0000-0003-0987-5762 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work has been supported by the Warsaw University of Technology. The authors would like to thank Dr. P. Guńka for helpful discussions, proof-reading and corrections. ABBREVIATIONS DSC, differential scanning calorimetry; TDI, 4,5-dicyano-2-(trifluoromethyl)imidazolate anion; G1, mono(ethylene oxide); G2, di(ethylene oxide); 15C5, 15-crown-5; 18C6, 18-crown-6.


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Suo, L.; Borodin, O.; Sun, W.; Fan, X.; Yang, C.; Wang, F.; Gao, T.; Ma, Z.; Schroeder, M.; von Cresce, A.; et al. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chem. Int. Ed. 2016, 55, 7136–7141.

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Ding, M. S.; von Cresce, A.; Xu, K. Conductivity, Viscosity, and Their Correlation of a Super-Concentrated Aqueous Electrolyte. J. Phys. Chem. C 2017, 121, 2149–2153.

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Yamada, Y.; Usui, K.; Sodeyama, K.; Ko, S.; Tateyama, Y.; Yamada, A. Hydrate-Melt Electrolytes for High-Energy-Density Aqueous Batteries. Nat. Energy 2016, 1, 16129.

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Tomiyasu, H.; Shikata, H.; Takao, K.; Asanuma, N.; Taruta, S.; Park, Y.-Y. An Aqueous Electrolyte of the Widest Potential Window and Its Superior Capability for Capacitors. Sci. Rep. 2017, 7, 45048.

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Yang, C.; Chen, J.; Qing, T.; Fan, X.; Sun, W.; Cresce, A. von; Ding, M. S.; Borodin, O.; Vatamanu, J.; Schroeder, M. A.; et al. 4.0 V Aqueous Li-Ion Batteries. Joule 2017, 1, 122–132.

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(17) Karpierz, E.; Niedzicki, L.; Trzeciak, T.; Zawadzki, M.; Dranka, M.; Zachara, J.; Żukowska, G. Z.; Bitner-Michalska, A.; Wieczorek, W. Ternary Mixtures of Ionic Liquids for Better Salt Solubility, Conductivity and Cation Transference Number Improvement. Sci. Rep. 2016, 6, 35587. (18) 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. (19) Jankowski, P.; Żukowska, G. Z.; Dranka, M.; Marczewski, M. J.; Ostrowski, A.; Korczak, J.; Niedzicki, L.; Zalewska, A.; Wieczorek, W. Understanding of Lithium 4,5Dicyanoimidazolate–Poly(Ethylene Oxide) System: Influence of the Architecture of the Solid Phase on the Conductivity. J. Phys. Chem. C 2016, 120, 23358–23367. (20) Xu, C.; Renault, S.; Ebadi, M.; Wang, Z.; Björklund, E.; Guyomard, D.; Brandell, D.; Edström, K.; Gustafsson, T. LiTDI: A Highly Efficient Additive for Electrolyte Stabilization in Lithium-Ion Batteries. Chem. Mater. 2017, 29, 2254–2263. (21) CrysAlisPRO Software System; Rigaku: Oxford, U.K., 2016. (22) Sheldrick,

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(25) 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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Table of Contents Graphic


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Figure 1. X-ray structure of (a) trihydrate Li(H2O)3TDI (1a) and (b) tetrahydrate Li(H2O)3TDI∙ H2O (1b). Selected bond lengths (Å) for 1a: Li1‒N2 2.101(4), Li1‒O1 1.947(4), Li1‒O2 2.015(4), Li1‒O3 1.903(4); 1b: Li1‒O1 1.968(5), Li1‒O2 1.882(5), Li1‒O3 1.955(5), Li1‒N4 2.032(5). 77x131mm (300 x 300 DPI)



Figure 2. Crystal structure of [Li(H2O)2TDI]2∙2H2O (2∙2H2O). Selected bond lengths (Å): O1‒Li1 1.926(3), O2‒Li1 1.902(3), N3i‒Li1 2.051(3), N4‒Li1 2.054(3). Symmetry codes: i 1−x, 1−y, 1−z. 80x76mm (300 x 300 DPI)


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Figure 3. (a) TGA and DTGA curves for dehydration of LiTDI∙3H2O (1a) at heating rate of 10 °C min−1, (b) 3D FT-IR spectra of evolved gaseous products, (c) FT-IR spectra of evolved water recorded at 140 and 330°C. 176x121mm (300 x 300 DPI)



Figure 4. Influence of the temperature on the FT-IR spectra of LiTDI saturated solution in water. 176x86mm (300 x 300 DPI)


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Figure 5. (a) Dissociation process of ionic pair in presence of water, (b) crystal structure of mixed {Li(15C5)(H2O)}+TDI− solvate (3) with thermal ellipsoids drawn at the 50% probability level. Selected bond lengths (Å): Li1‒O1 1.940(3), Li1‒O2 2.205(3), Li1‒O3 2.120(3), Li1‒O4 2.142(3), Li1‒O5 2.102(3). 80x85mm (300 x 300 DPI)



Figure 6. Molecular structure of [Li2(18C6)2(H2O)3]2+2TDI− (4) solvate. Lithium coordination spheres are highlighted with larger ellipsoids and broader bonds. Selected bond lengths (Å): Li1‒O1 1.890(5), Li2‒O3 1.897(5), Li1‒O2 2.004(5), Li2‒O2 2.116(5), Li1‒O4 2.288(6), Li1‒O5 1.922(5), Li1‒O6 2.450(6), Li2‒O10 1.990(5), Li2‒O11 2.273(5), Li2‒O15 2.155(5). 62x110mm (300 x 300 DPI)


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Figure 7. Changes in Raman spectra observed during stepwise hydrolysis of LiTDI−G1 system after absorption of water. 169x47mm (300 x 300 DPI)



Figure 8. (a) Fragment of the crystal structure of {Li2(H2O)TDI2}n∙nLi(G1)2TDI hydrate (5), (b) structural motifs I and II constituting aggregated layers in 5. 80x125mm (300 x 300 DPI)


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Figure 9. Stepwise hydrolysis of LiTDI−G2 system and the molecular structure of [Li(G2)TDI]2 and [Li(H2O)TDI]n cocrystals (6a∙6b). 152x91mm (300 x 300 DPI)



Figure 10. Changes in Raman spectra observed during stepwise hydrolysis of LiTDI−G2 system. 169x66mm (300 x 300 DPI)


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Figure 11. Structural motifs constituting (a) hemihydrate, (b) monohydrate, (c) dihydrate, (d) trihydrate and (e) tetrahydrate of LiTDI. 173x56mm (300 x 300 DPI)


Snapshots of the Hydrolysis of Lithium 4,5-DicyanoimidazolateâGlyme

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