TFSI and TDI Anions: Probes for Solvate Ionic ... - ACS Publications

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TFSI and TDI Anions: Probes for Solvate Ionic Liquid and Disproportionation-Based Lithium Battery Electrolytes Piotr Jankowski,*,†,‡,§ Maciej Dranka,† Władysław Wieczorek,†,§ and Patrik Johansson*,‡,§ †

Faculty of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden § ALISTORE-ERI European Research Institute, 33 rue Saint Leu, 80039 Amiens, France ‡

S Supporting Information *

ABSTRACT: Highly concentrated electrolytes based on Li-salts and chelating solvents, such as glymes, are promising as electrolytes for lithium batteries. This is due to their unique properties, such as higher electrochemical stabilities, compliance with high-voltage electrodes, low volatility and flammability, and inertness toward aluminum current collector corrosion. The nature of these properties originates from the molecular-level structure created in either solvate ionic liquids (SILs) or the less common ionic aggregates by disproportionation reactions. The nature of the anion plays a crucial role, and here, we present a computational study using TFSI and TDI anions as probes, revealing increasing differences upon increased salt concentration. TFSI-based electrolytes preferably form SILs, while TDI-based electrolytes form ionic aggregates. The latter lead to an unexpected creation of “free” cationic species even at (very) high salt concentrations and thus promise of ample lithium ion transport.

G

not WCAs) impede the formation of SILs; hence anions, such as BF4−, NO3−, and CF3CO2−, form ordinary concentrated electrolytes rather than SILs.26 In general, the tendency of ions to aggregate reduces their diffusivity (by size) and impedes their migration in an electric field. Thus, extensive aggregation has been seen for a long time as less suited for lithium battery electrolytes.27−32 However, we showed in our previous study that the aggregates can be adapted, as an alternative approach, to achieve superior highly concentrated electrolytes.15 Surprisingly, aggregates are observed at high LiTDI concentrations15,16,33 as TDI is a better WCA than TFSI.34 This is due to the disproportionation reaction forming two types of species (i.e., highly aggregated and solvated). In the case of the highly aggregated species, the ions are more or less immobilized compared to the solvated species, where the lithium cations are coordinated by the glymes only and weakly interact with the anions. The exact balance of glyme vs LiTDI is crucial to enable electrolytes with “free” mobile Li2G422+ species.15 As the origin has not yet been explained and any rationale is useful to optimize this kind of electrolytes, we here employ a computational study to understand the two different types of behavior. The key point is the distinctly different surroundings of the lithium cations in the electrolytes and especially their diversity; in SILs, all anions and cations are equal and thus act similarly, while for the disproportionation-based electrolytes they are unequal. Thus,

rowing demands for high-energy-density storage devices have stimulated research and development of nextgeneration lithium-based battery technologies like Li−metal, Li−sulfur, and Li−air batteries.1,2 Novel electrolytes, including solvents, salts, and additives, are keys to the future success of such technologies. Among the various lithium salts put forward and commonly tested3−9 are lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium 4,5dicyano-2-(trifluoromethyl)imidazole (LiTDI). They represent salts with structurally very diverse weakly coordinating anions (WCAs) where TFSI is elongated and internally very flexible10 and TDI is rigid and semiplanar.11 While these two salts create quite similar lithium ion transport in dilute electrolytes,12 differences appear upon increased salt concentration. Especially when the number of ions and solvent molecules becomes comparable, any small differences in ionic interactions result in significantly altered electrolyte properties.13−16 Henderson and Watanabe have both demonstrated that equimolar (1:1) mixtures of LiTFSI and glyme solvents (i.e., CH3O(CH2CH2O)nCH3, n = 1−4) can act as ionic liquids.17−19 Due to multidentate coordination by the chelating glyme, the “hard” Li+ is turned into a “soft” [Li−glyme]+ complex, drastically weakening any cation−anion interactions. The hereby created so-called solvate ionic liquids (SILs) have high thermal and electrochemical stability, low volatility, low lithium polysulfide solubility, and suppressed aluminum corrosion.20−23 Matching of the length of the glyme with the alkali cation is crucial24,25 (e.g., triglyme (G3) and tetraglyme (G4) have been proven optimal for Li+) in addition to the structure and nature of the anion.23−25 Watanabe showed that anions with strong cation−anion interactions (i.e., © 2017 American Chemical Society

Received: May 10, 2017 Accepted: July 25, 2017 Published: July 25, 2017 3678

DOI: 10.1021/acs.jpclett.7b01160 J. Phys. Chem. Lett. 2017, 8, 3678−3682

Letter

The Journal of Physical Chemistry Letters

thus, it can be easier to form and adjust under dynamic conditions. The introduction of glyme in the first coordination shell does not affect this qualitatively (Figure S3). Taking into account these arguments explains the increased role of NIm for the less dynamic higher salt concentrations as well as increased temperatures (Table S4). The appearance of the N Im coordination in the LiTDI−G4 system agrees well with Raman spectroscopy results where this is observed at a concentration of ∼1:2.2.16 Moving to the lower end of the concentration range (i.e., 1:9), the MD simulations predict the LiTDI electrolytes to be comprised almost exclusively of separated ions, while for the LiTFSI electrolytes, ion pairs are predominantly predicted. These differences can also be observed via the total lithium ion coordination numbers (CNLi+). It is almost constant (5.92 ± 0.01) for the LiTFSIbased electrolytes, while it changes from 5.23 to 5.90 (an increase of >10%) for the LiTDI-based electrolytes. The rdf’s for the lithium cation centers only provide average properties. Nevertheless, we can categorize and differentiate between types of cation−anion/solvent complexes by analyzing the distributions via the number of OEther in the first coordination shell. If each Li+ is surrounded by (6−7), (3− 5), or (2 or less) OEther, it would be labeled free, ion pair, or aggregated, respectively (Figure 2 and Table S5). The analysis

no averaging statistical approach is suitable for the latter, and we are forced to analyze the distribution of different complexes formed. Although we are mainly interested in the lithium-based species in the LiTDI and the glyme-based electrolytes, our starting point for the comparative study is the structure of the anions and how they in detail interact with the lithium cations. The radial distribution functions (rdf’s), obtained from MD simulations at 303 K for various electrolytes provide us with coordination numbers and local structure. The overall results for LiTDI and LiTFSI in G4 are shown in Figure 1, and values

Figure 2. Distribution of OEther in the first coordination shell of Li+ for G4-based electrolytes at 303 K. Figure 1. Coordination numbers of Li+ in the different G4-based electrolytes at 303 K.

confirms the totally different character of the electrolytes formed from the two anions. All Li+ ions behave similarly in LiTFSI−glyme, which weakly coordinates one single glyme molecule and a TFSI anion, ensuring a very low dissociation energy and good ion transport properties. In contrast, in the LiTDI−glyme systems, a wide distribution is observed and the relative contributions are highly dependent on the salt concentration (Table 1). The dilute LiTDI-based electrolytes mainly comprise free lithium cations. Upon increased salt concentration, the free ions rapidly decrease (from ca. 68% for 1:9 to ca. 15% for 1:2) together with an increase in ion pairs

for the first coordination shell of lithium (up to 2.5 Å) are presented in Tables S3 and S4. The TDI anion primarily connects to the lithium cations by the nitrogen atoms of the nitrile group (NCN) and the imidazolium ring (NIm), while the TFSI anion predominantly uses oxygen atoms (OTFSI) and its flexibility to maximize the interaction with lithium cations. Hence, neither the nitrogen atom of the TFSI anion (NTFSI) nor the fluorine atoms of the TDI and TFSI anions (FTDI and FTFSI) coordinate the lithium ions. Upon an increased salt concentration, almost no changes occur with respect to the composition of the first coordination shell of the lithium ion in the LiTFSI-based electrolytes. On the other hand, the contribution from the NIm increases significantly in the LiTDI electrolytes, from barely observable for a concentration of 1:3 to 0.62 for the highest concentration of 1:1. The force field of TDI was designed to reproduce the DFT-obtained small preference to coordinate the lithium ion by NIm rather than NCN (Figure S2). However, under dynamic conditions, the preference changes as NCN is sterically less hindered. The DFT calculations show the Li+−NIm interaction to be stronger, but minimal changes in their relative position lead to a rapid increase in energy and, hence, a weaker interaction. In contrast, the Li+−NCN interaction is weaker yet more nondirectional;

Table 1. Distribution of Complexes and Calculated Concentrations of Cation Charge Carriers for LiTDI−G4 Electrolytes at 303 K distribution of complex types (%) solvated

3679

salt concentration

free ions

ion pairs

1:1 1:2 1:3 1:9

19 15 21 68

40 72 78 32

aggregated cationic charge carriers (m) 41 13 1 0

0.44 0.23 0.25 0.31


Letter


Figure 3. Mechanism of the polyanion creation under dynamic conditions determined via MD simulation of a LiTDI−G4 1:2 electrolyte at 303 K.

for practical battery application is the dynamics and the concentration of Li+ carriers enabling transfer of cations between the electrodes. Interestingly, the calculated concentration of cationic charge carriers is not a simple function of salt concentration for the LiTDI−G4 system (Table 1) with 0.44 m for 1:1 being higher than the 0.31 m for 1:9. Additionally, the presence of polyanionic aggregates can hinder anion transfer and thereby enhance the lithium transference number. Comparative analysis of the LiTFSI- and LiTDI-based electrolytes shows the significant differences in the organization at high concentrations, reflecting their completely disparate structural character. The flexible TFSI anion is a multidentate, chelating ligand, while the rigid structure of TDI restricts the coordination severely to mainly act as a monodentate ligand, which, in turn, eases the initiation of polyanion formation. Moreover, the presence of two different types of coordination centers in TDI (i.e., low-energy but less kinetically stable NIm and high-energy, kinetically preferred NCN) plays a crucial role in stabilization of polyanions as the energetically preferred coordination with NIm occurs only at higher concentrations when the lithium ion already is immobilized by N CN coordination from other anions. The “side effect” of the growth of the polyanionic structure is the formation of free lithium cations that readily ensures lithium ion transport properties. Creation by disproportionation is central for these new types of concentrated electrolytes, and the concentrated LiTDI−G4 electrolytes seem to be an attractive choice for further experimental studies targeting the unique properties of disproportionation-based electrolytes, including their possible usage in next-generation lithium batteries.

and aggregates. However, a further increase in salt concentration from the 1:2 concentration yields re-creation of free ions (ca. 18% for 1:1); hence, this decrease is not monotonic. This is connected with the creation of negatively charged aggregates− polyanions (Figure 3). Indeed, when the TDI anion concentration becomes high enough, the ion pairs start to interact with TDI anions, resulting in weakened interactions between cations and glymes. Eventually, the Li+ has two or less employed OEther atoms, and subsequently, the glyme is easily released. As the Li+ ion is rather immobilized by NCN coordination by a few TDI anions, some of the anions may change coordination from NCN to NIm to stabilize the polyanionic aggregate. The described growth is connected with the release of redundant Li+ cations, which are coordinated by glymes and form new charge carriers. The disproportionation behavior of the LiTDI−glyme systems agrees very well with the experimental data for crystalline samples15 and with Raman spectroscopy data for LiTDI−G4 electrolytes.16 In the LiTFSI-based electrolytes, the chelating nature of the TFSI anion allows it to be efficiently coordinated by Li+ and therefore difficult to entirely release and initiate any polyanion aggregate creation. The polyanionic structures have already been observed for LiTFSI/ionic liquid systems, where there is an excess of TFSI anions vs Li+ cations; hence, the disproportionation process is not necessary.35,36 While all of the above details refer to G4 as the solvent, similar behavior is observed for the G3 systems (Figure S6, Tables S5 and S6). The shorter glyme chain does not hinder the dissociation process, but the free ions are fewer (ca. 14% and 19% for the G3 and G4 1:1 electrolytes, respectively), which results in a reduced diversity in the first lithium ion coordination shell. For example, most ion pairs use 4 OEther atoms, unlike 4−5 OEther atoms for G4. The smaller variance of the first lithium ion coordination shell composition explains the reduced liquid range of LiTDI−G3 electrolyte. The 1:1 electrolyte forms crystals that melt at 41 °C, while the LiTDI−G4 electrolyte does not crystallize until extremely high salt concentrations (e.g., 3:1), yet very sluggishly (days).15 From a practical perspective, this fundamental feature may also affect the desolvation dynamics of the cation at the anode surface in a working cell.37,38 Although all of the above has focused on the different types of lithium complexes, the most important electrolyte property

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COMPUTATIONAL METHODS All DFT calculations were performed using the B3LYP functional39,40 and the 6-311+G(d) basis set, as implemented in the Gaussian0941 package. The MD simulations were performed using the GROMACS42 package using the OPLSbased force field. Parameters for G3, G4, and Li+ were obtained from the studies of Jorgensen et al.,43,44 and the TFSI parameters were from Köddermann et al.,45 all shown to work well for simulating LiTFSI in PEO.46 The parameters for the TDI anion were determined based on DFT calculations,47 adjusted to be consistent with the OPLS force field (Tables S1 3680


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(2) Younesi, R.; Veith, G. M.; Johansson, P.; Edström, K.; Vegge, T. Lithium Salts for Advanced Lithium Batteries: Li−metal, Li−O2, and Li−S. Energy Environ. Sci. 2015, 8, 1905−1922. (3) Zhang, S. S. New Insight into Liquid Electrolyte of Rechargeable Lithium/Sulfur Battery. Electrochim. Acta 2013, 97, 226−230. (4) Kim, S.; Jung, Y.; Park, S.-J. Effect of Imidazolium Cation on Cycle Life Characteristics of Secondary Lithium−sulfur Cells Using Liquid Electrolytes. Electrochim. Acta 2007, 52, 2116−2122. (5) Shin, E. S.; Kim, K.; Oh, S. H.; Cho, W. I. Polysulfide Dissolution Control: The Common Ion Effect. Chem. Commun. 2013, 49, 2004− 2006. (6) Chen, J.; Han, K. S.; Henderson, W. A.; Lau, K. C.; Vijayakumar, M.; Dzwiniel, T.; Pan, H.; Curtiss, L. A.; Xiao, J.; Mueller, K. T.; et al. Restricting the Solubility of Polysulfides in Li-S Batteries Via Electrolyte Salt Selection. Adv. Energy Mater. 2016, 6, 1600160. (7) Dominko, R.; Patel, M. U. M.; Lapornik, V.; Vizintin, A.; Koželj, M.; N. Tušar, N.; Arčon, I.; Stievano, L.; Aquilanti, G. Analytical Detection of Polysulfides in the Presence of Adsorption Additives by Operando X-Ray Absorption Spectroscopy. J. Phys. Chem. C 2015, 119, 19001−19010. (8) Li, F.; Zhang, T.; Yamada, Y.; Yamada, A.; Zhou, H. Enhanced Cycling Performance of Li-O2 Batteries by the Optimized Electrolyte Concentration of LiTFSA in Glymes. Adv. Energy Mater. 2013, 3, 532−538. (9) Liu, Y.; Suo, L.; Lin, H.; Yang, W.; Fang, Y.; Liu, X.; Wang, D.; Hu, Y.-S.; Han, W.; Chen, L. Novel Approach for a High-EnergyDensity Li−air Battery: Tri-Dimensional Growth of Li2O2 Crystals Tailored by Electrolyte Li+ Ion Concentrations. J. Mater. Chem. A 2014, 2, 9020−9024. (10) Canongia Lopes, J. N.; Shimizu, K.; Pádua, A. A. H.; Umebayashi, Y.; Fukuda, S.; Fujii, K.; Ishiguro, S. Potential Energy Landscape of Bis(fluorosulfonyl)amide. J. Phys. Chem. B 2008, 112, 9449−9455. (11) 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. (12) Berhaut, C. L.; Porion, P.; Timperman, L.; Schmidt, G.; Lemordant, D.; Anouti, M. LiTDI as Electrolyte Salt for Li-Ion Batteries: Transport Properties in EC/DMC. Electrochim. Acta 2015, 180, 778−787. (13) Brouillette, D.; Irish, D. E.; Taylor, N. J.; Perron, G.; Odziemkowski, M.; Desnoyers, J. E. Stable Solvates in Solution of Lithium Bis(trifluoromethylsulfone)imide in Glymes and Other Aprotic Solvents: Phase Diagrams, Crystallography and Raman Spectroscopy. Phys. Chem. Chem. Phys. 2002, 4, 6063−6071. (14) Aguilera, L.; Xiong, S.; Scheers, J.; Matic, A. A Structural Study of LiTFSI−Tetraglyme Mixtures: From Diluted Solutions to Solvated Ionic Liquids. J. Mol. Liq. 2015, 210, 238−242. (15) 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. (16) Jankowski, P.; Dranka, M.; Ż ukowska, G. Z. Structural Studies of Lithium 4,5-Dicyanoimidazolate−Glyme Solvates. 2. Ionic Aggregation Modes in Solution and PEO Matrix. J. Phys. Chem. C 2015, 119, 10247−10254. (17) Henderson, W. A.; McKenna, F.; Khan, M. A.; Brooks, N. R.; Young, V. G.; Frech, R. Glyme−Lithium Bis(trifluoromethanesulfonyl)imide and Glyme−Lithium Bis(perfluoroethanesulfonyl)imide Phase Behavior and Solvate Structures. Chem. Mater. 2005, 17, 2284−2289. (18) Henderson, W. A. Glyme−Lithium Salt Phase Behavior. J. Phys. Chem. B 2006, 110, 13177−13183. (19) Mandai, T.; Yoshida, K.; Ueno, K.; Dokko, K.; Watanabe, M. Criteria for Solvate Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 8761−8772. (20) Yoshida, K.; Nakamura, M.; Kazue, Y.; Tachikawa, N.; Tsuzuki, S.; Seki, S.; Dokko, K.; Watanabe, M. Oxidative-Stability Enhancement

and S2), and verified by reproduction of the recently published crystalline structures of LiTDI solvates with G3 and G4 (with cell dimension errors < 1%).15 Using the random algorithm implemented in GROMACS, in total, 16 systems were created with different ratios of salt (LiTFSI, LiTDI) to glyme (G3, G4): 1:1, 1:2, 1:3, and 1:9. Each system consisted of 96 ion pairs, except the four 1:9 systems, reduced to 48 ion- pairs due to the large number of solvent molecules. All systems were subsequently submitted to energy minimization and simulations under NVT and NPT conditions to achieve starting geometries, after which they were equilibrated at three different temperatures, 303, 348, and 393 K, each for 5 ns. The MD production runs were 15 ns using a time step of 2 fs and a leapfrog algorithm under NPT and periodic boundary conditions at 1 bar and a V-rescale thermostat (coupling time 0.5 ps) and a Parrinello−Rahman barostat (coupling time 2 ps). A particle-mesh Ewald summation routine was used for the long-range forces (cutoff 14 Å). The analysis of trajectories was performed using tools implemented in the GROMACS package, except the distribution of ether oxygen atoms in the lithium ion coordination sphere, where data were exported to MATLAB by the Gro2mat program48 and then analyzed using our own code, which for each of the simulation frames checked surroundings of each of the lithium cations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01160. Developed force field parameters for the TDI anion, DFT data, and MD results for G3-based electrolytes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P. Johansson). *E-mail: [email protected] (P. Jankowski). ORCID

Piotr Jankowski: 0000-0003-0178-8955 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Muhammad Abdelhamid for all of the comments to the manuscript. All calculations have been carried out at the Wroclaw Centre for Networking and Supercomputing (http://www.wcss.pl), Grant No. 346. Support by ALISTORE-ERI for doctoral studies and by the Chalmers Areas of Advance: Energy and Materials Science within the joint Materials for Energy Applications profile supported by the Chalmers Battery Initiative for travel scholarships to P. Jankowski is gratefully acknowledged. P. Johansson acknowledges both the Swedish Energy Agency for a basic research grant via the Swedish Research Council and continuous support by many of Chalmers Areas of Advance: Energy, Materials Science, and Transport.

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