Snapshots of the Hydrolysis of Lithium 4,5-Dicyanoimidazolate–Glyme

Article Cite This: J. Phys. Chem. C 2018, 122, 3201−3210

pubs.acs.org/JPCC

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 Grazẏ na Z. Ż ukowska Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland S Supporting Information *

ABSTRACT: Despite that 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 the 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 4,5-dicyanoimidazolate−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 the LiTDI−H2O system. A high number of available ether oxygen donor center 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. A 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 a monohydrate. The discovered motifs have been proved to occur in both 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.

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 nonaqueous and aqueous Li-ion batteries with higher capacities and voltages without compromising safety.1 A new class of solvent-in-salt electrolyte (SiS) developed for the next-generation high-energy rechargeable batteries allows us 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 the “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-in-bisalt” (WiBS) using mixed lithium salts achieved an unprecedentedly © 2018 American Chemical Society

high concentration and widened the electrochemical stability.4−6 The efficiency of a solid electrolyte interphase (SEI) formation in aqueous electrolytes depends on the salt concentration, leading to the 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 is under rapid progress in these systems.7−9 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, the aqueous lithium-ion battery has been demonstrated to be one of the most promising choices because of its improved safety and lower price when compared to organic electrolyte-based systems.11,12 “Water-in-salt” electrolytes make use of costReceived: November 10, 2017 Revised: December 21, 2017 Published: January 24, 2018 3201

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

dispersive spectrometer. A diode laser with excitation line 532 nm was used. The spectral resolution for all experiments was about 2 cm−1. Temperature-dependent spectra were obtained with the use of a 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 vacuumisolated 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 a 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 15N 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 TGFT-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 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 two-component twins, with crystal domains rotated around the [001] direction. Obtained twin ratios 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 were still not satisfactory. However, the obtained model of the 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(Cu Kα) = 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.

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 electrolyte-based 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 the local structure of electrolyte at the microscopic level is still very limited. Understanding the solvation of Li+ cations and aggregation processes is of great importance because of their crucial role in ionic conductivity and formation of an SEI. Our extensive experience in structural research of salts based on five-membered heterocyclic ring compounds such as 4,5dicyano-2-(trifluoromethyl)imidazole lithium salt (LiTDI) allows us to determine preferred coordination modes in the 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, the solvate disproportionation conductivity mechanism was found to be present in 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, a detailed mechanism of the 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 investigated the impact of 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 the 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 G1-mono(ethylene oxide) and 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 and 4 °C, respectively. Single crystals of LiTDI dihydrate 2·G1 were 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 3202

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C Table 1. Band Assignments for LiTDI Crystalline Hydrates in Raman Spectrum compound number

molecular formula

crystal structure presented in figure

1a 1b 2·2H2O 2·G1 3 4 5 6a·6b

Li(H2O)3TDI Li(H2O)3TDI·H2O [Li(H2O)2TDI]2·2H2O [Li(H2O)2TDI]2·G1 {Li(15C5(H2O)}+TDI− {Li2(18C6)2(H2O)3}2+2TDI− {Li2(H2O)TDI2}n·nLi(G1)2TDI [Li(H2O)TDI]n·n[Li(G2)TDI]2

1a 1b 2 S3 5 6 8 9

νCN (cm−1) 2250, 2266, 2260 2259 2238 2225, 2267, 2267,

2245 2241

2222 2259 2252, 2236

νCN_Im (cm−1)

δNCN (cm−1)

1322 1303 1318 1318, 1313 1313 1304 1310 1319, 1305

1006 1004 1003 1003, 997 986 980 1001, 990 1001, 997

νOH (cm−1) 3571, 3596, 3670, 3461, 3515 3525 3428 3575,

3514, 3395 3558 3236 3372, 3294

3395, 3298

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 P1,̅ 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(Mo Kα) = 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.

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(Mo Kα) = 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)), 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 P1,̅ 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(Cu Kα) = 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(H 2 O) 2 TDI] 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(H 2 O)} + 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(Mo Kα) = 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(Mo Kα) = 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 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(Cu Kα) = 1.140 mm−1, 34276 reflections measured, 6523 unique (Rint = 0.0525, Rsigma = 0.0302) which were used in all

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) and 18-crown-6 (18C6) were exposed to humid air, and resulting products were analyzed by means of a single-crystal Xray diffraction technique and Raman spectroscopy. 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 Supporting Information (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. 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 the spectral fingerprint of 4 resembles that of a “free” anion. The analysis of the system after hydration may be complicated because of the fact that LiTDI is able to form structurally different hydrates of the same stoichiometry such as 2·2H2O and 1a. Additional information provides a spectral range corresponding to OH stretching vibrations. For example, the presence of a distinct band at 3670 cm−1 allows us to distinguish compound 2·2 H2O from 2·G1, both comprising the same dimer unit. The origin of this band may be attributed to the presence of an isolated water molecule in the structure of the former. Thus, for the convenient and unambiguous identification of hydrated species 3203

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

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. A 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. The crystal structure of 2·2H2O, shown in Figure 2, comprises dimers of dihydrate [Li(H2O)2TDI]2 (2) and two additional cocrystallized water molecules.

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 an ionic pair of composition Li(H2O)3TDI. The crystal structure of the obtained compound is shown in Figure 1a.

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 are presented in Supporting 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 1 h. 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 the 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 the 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). 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 Supporting Information Figure S2). The second observed mass loss completed at 140 °C equal to 6.2%

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).

The 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 tetrahydrate 1b of formula Li(H2O)3TDI·H2O (Figure 1b). This compound melts at 20 °C with subsequent crystallization of hydrate 1a. In the crystal structure of 1b a 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 the fourth H2O molecule acts as lattice water. An average Li−O distance of 1.955 and 1.935 Å for 1a and 1b, respectively, is very similar for 3204

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

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

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

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.

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 is immediately followed by the salt decomposition. Therefore, anhydrous LiTDI salt can be obtained only by drying under vacuum. 3205

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

“free” TDI anions and lithium cations solvated by crown ether and water molecule as depicted in Figure 5b. The water molecule fulfills the remaining coordination site of lithium after desolvation of the TDI anion. This allows the Li cation to remain dissociated even in the solid phase. Therefore, H2O molecules act in terms of Lewis basicity as a stronger donor to the lithium cation than TDI anions. The Li1−O1 bond length between the lithium and oxygen atom from a 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 the crown ether is uncoordinated in the presence of water. Thus, the coordination number of the Li cation is lowered in comparison to the lithium coordination number in the starting Li(15C5)TDI solvate. Similarly, LiTDI solvate with 18-crown-6 ether forms isolated ions in the presence of water which is shown in detail in Figure 6.

The most significant change observed in temperaturedependent spectroscopy experiments is the shift of the νCN maximum toward higher values at about 100 °C resulting from the rearrangement of the structure and switching of the cation coordination from the imidazolium ring to the 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 the hydrated lithium cation via the 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 subambient temperatures for 0.5 M 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, a high number of available ether oxygen donor center water molecules cause dissociation of ionic contact pairs as shown in Figure 5a. The solvate of composition Li(15C5)TDI which is a convenient model of an ionic pair recrystallizes after absorption of moisture resulting in an ionic mixed salt 3 consisting of uncoordinated

Figure 6. Molecular structure of the [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−O1 and Li2−O3 bond lengths between the lithium cation and terminal water molecules equal to 1.897(5) Å and 1.890(5) Å, respectively, are slightly shorter than the 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 an extended hydrogen bonding network links cations and TDI− anions in the crystal lattice of 3, whereas TDI− anions do not act as the acceptors of hydrogen bonds in the crystal structure of 4 (see Supporting Information, Figures S5−S6). Dications in 4 act both as donors and as acceptors of hydrogen bonds which results in further aggregation of solvated lithium cations by noncovalent interactions. Thus, it is the solvated lithium cations and not the TDI− anion itself which

Figure 5. (a) Dissociation process of ionic pair in the presence of water and (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), and Li1−O5 2.102(3). 3206

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

Figure 7. Changes in Raman spectra observed during stepwise hydrolysis of the LiTDI−G1 system after absorption of water.

should be considered as a proper moisture scavenger in glymebased 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 a 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 the 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 the crystalline phase of composition {Li2(H2O)TDI2}n·nLi(G1)2TDI (5). The crystal structure of the obtained mixed glyme−water solvate with O:Li overall ratio of 1.67:1 is presented in Figure 8. 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 a nitrile group [Li1−N1 2.018(4) Å]. A second nitrile group from the terminal TDI anion is bound to water molecules in the layer via a 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 cations exhibit different coordination. A coordination sphere of the first Li+ is completed by a water molecule and the nitrogen atom of the imidazole ring from an adjacent unit, while the second lithium cation is additionally coordinated by an imidazole ring from an adjacent unit and nitrile group of the terminal TDI anion. The 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 cocrystallized monoglyme (G1) as a lattice solvent molecule (Supporting Information, Figure S7).

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

Another complicated chemical transformation taking place after the exposure of dimeric diglyme solvate [Li(G2)TDI]218 to air is presented in detail 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 based 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 3207

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

Figure 9. Stepwise hydrolysis of the LiTDI−G2 system and the molecular structure of [Li(G2)TDI]2 and [Li(H2O)TDI]n cocrystals (6a·6b).

Figure 10. Changes in Raman spectra observed during stepwise hydrolysis of the LiTDI−G2 system.

free diglyme solvate 6a and dimeric LiTDI dihydrate 2 (Figures 9 and 10). Thus, the model of water consumption to form an 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

investigations. The exact structure of LiTDI monohydrate retrieved from X-ray measurement reveals an aggregated infinite polymeric chain of composition [Li(H2O)TDI]n. 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, the fourth coordination of the Li site has to be coordinated with a donor center from another dimeric unit constituting the aggregated structure of the monohydrate. A water molecule is coordinated to the 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. 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

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 4,5dicyanoimidazolate−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 3208

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C

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

■

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.

retrieved from single-crystal X-ray analysis of crystalline adducts with glymes and water provide new information about preferred coordination modes in the 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 the LiTDI−glyme−water system, is also the first step in developing highly concentrated and solid hydrated electrolytes for battery application.

■

■

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

■

(1) Yamada, Y. Developing New Functionalities of Superconcentrated Electrolytes for Lithium-Ion Batteries. Electrochemistry 2017, 85, 559−565. (2) Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (3) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. Water-in-Salt” Electrolyte Enables High-Voltage Aqueous Lithium-Ion Chemistries. Science 2015, 350, 938−943. (4) Suo, L.; Borodin, O.; Sun, W.; Fan, X.; Yang, C.; Wang, F.; Gao, T.; Ma, Z.; Schroeder, M.; von Cresce, A.; et al. Advanced HighVoltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chem., Int. Ed. 2016, 55, 7136−7141. (5) 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. (6) 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. (7) 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. (8) Daubinger, P.; Kieninger, J.; Unmussig, T.; Urban, G. A. Electrochemical Characteristics of Nanostructured Platinum Electrodes - a Cyclic Voltammetry Study. Phys. Chem. Chem. Phys. 2014, 16, 8392−8399. (9) Yang, C.; Chen, J.; Qing, T.; Fan, X.; Sun, W.; Cresce, A.; Ding, M. S.; Borodin, O.; Vatamanu, J.; Schroeder, M. A.; et al. 4.0 V Aqueous Li-Ion Batteries. Joule 2017, 1, 122−132. (10) Kühnel, R.-S.; Reber, D.; Battaglia, C. A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2017, 2, 2005− 2006. (11) Kim, H.; Hong, J.; Park, K.-Y.; Kim, H.; Kim, S.-W.; Kang, K. Aqueous Rechargeable Li and Na Ion Batteries. Chem. Rev. 2014, 114, 11788−11827. (12) Suo, L.; Han, F.; Fan, X.; Liu, H.; Xu, K.; Wang, C. Water-inSalt” Electrolytes Enable Green and Safe Li-Ion Batteries for Large Scale Electric Energy Storage Applications. J. Mater. Chem. A 2016, 4, 6639−6644.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11145.

■

REFERENCES

X-ray crystallographic data for 1−6 (CIF) 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)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maciej Dranka: 0000-0003-0987-5762 Piotr Jankowski: 0000-0003-0178-8955 Notes

The authors declare no competing financial interest. 3209

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210

Article

The Journal of Physical Chemistry C (13) Kuhnel, R.-S.; Reber, D.; Remhof, A.; Figi, R.; Bleiner, D.; Battaglia, C. Water-in-Salt” Electrolytes Enable the Use of CostEffective Aluminum Current Collectors for Aqueous High-Voltage Batteries. Chem. Commun. 2016, 52, 10435−10438. (14) Wang, Y.; Yi, J.; Xia, Y. Recent Progress in Aqueous Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 830−840. (15) Zheng, J.; Lochala, J. A.; Kwok, A.; Deng, Z. D.; Xiao, J. Research Progress towards Understanding the Unique Interfaces between Concentrated Electrolytes and Electrodes for Energy Storage Applications. Adv. Sci. 2017, 4, 1700032. (16) Niedzicki, L.; Kasprzyk, M.; Kuziak, K.; Ż ukowska, G. Z.; Armand, M.; Bukowska, M.; Marcinek, M.; Szczeciński, P.; Wieczorek, W. Modern Generation of Polymer Electrolytes Based on Lithium Conductive Imidazole Salts. J. Power Sources 2009, 192, 612−617. (17) Karpierz, E.; Niedzicki, L.; Trzeciak, T.; Zawadzki, M.; Dranka, M.; Zachara, J.; Ż ukowska, 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.; Ż ukowska, 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.; Ż ukowska, G. Z.; Dranka, M.; Marczewski, M. J.; Ostrowski, A.; Korczak, J.; Niedzicki, L.; Zalewska, A.; Wieczorek, W. Understanding of Lithium 4,5-Dicyanoimidazolate−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, G. M. SHELXT − Integrated Space-Group and Crystal-Structure Determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (23) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (24) 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. Crystallogr. 2009, 42, 339−341. (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.

3210

DOI: 10.1021/acs.jpcc.7b11145 J. Phys. Chem. C 2018, 122, 3201−3210