Stable Cycling of Lithium Batteries Using Novel Boronium-Cation

Publication Date (Web): December 16, 2009 ... Fax: +61 3 9562 8919., *[email protected] Tel: +1 251 460 7427 Fax: +1 251 460 7359. ... Boron...
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1038 Chem. Mater. 2010, 22, 1038–1045 DOI:10.1021/cm9019815

Stable Cycling of Lithium Batteries Using Novel Boronium-Cation-Based Ionic Liquid Electrolytes† Thomas R€ uther,*,‡ Thuy D. Huynh,‡ Junhua Huang,‡ Anthony F. Hollenkamp,‡ E. Alan Salter,§ Andrzej Wierzbicki,§ Kayla Mattson,§ Adam Lewis,§ and James H. Davis Jr.*,§ ‡

CSIRO Energy Technology, PO Box 312, Clayton South, VIC 3169, Australia, and § Department of Chemistry, University of South Alabama, Mobile, Alabama 36688 Received July 3, 2009. Revised Manuscript Received October 8, 2009

Boronium-cation-based room-temperature ionic liquids (RTILs) were applied for the first time as novel supporting electrolytes in rechargeable Li|LiFePO4 batteries. The physicochemical properties of three different materials (L1L2)BH2-NTf2, 3a (L1, L2 =1-methyl imidazole (mim)), 3b (L1, L2 = 1-butylimidazole (bim), and 3c (L1=trimethylamine (N111), L2=dimethylethylamine (N112)), which are readily synthesized from inexpensive and commercially available starting materials, were established by DSC, TGA, conductivity, and cyclic voltammetry. These RTILs are stable up to between 238 and 335 °C and display sufficient conductivities and electrochemical windows (4.35.8 V) to be compatible with the Li anode of a battery. Stable battery cycling with good capacity retention was possible for >300 cycles with (N111)(N112)BH2-NTf2 þ LiNTf2 solutions at charge discharge rates C/10 and C/5 between 50 and 30 °C. By contrast, a C4mpyr-NTf2 þ LiNTf2 electrolyte system performed less well under the same conditions despite the higher conductivity of C4mpyr-NTf2 compared to the boronium RTIL 3c. Li battery cycling was also possible with the imidazole units containing material (bim)2BH2-NTf2 for 140 cycles at 80 °C. These new materials could emerge as important electrolytes for various electrochemical applications. Introduction Lithium batteries are considered to be the main electrical storage device technology in many applications because of their high energy density. To maximize the performance of rechargeable batteries, e.g., cycle life, cycle rates, safety, etc., many research and development efforts around the world focus on improving electrode materials, electrolytes, and the compatibility with each other.1-6 Room-temperature ionic liquids (RTILs),7-9 because of their unique properties, are now an option to replace conventional electrolytes based on molecular † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. E-mail: [email protected]. Tel: þ61 3 9545 8597. Fax: þ61 3 9562 8919. *[email protected] Tel: þ1 251 460 7427 Fax: þ1 251 460 7359.

(1) Armand, M.; Tarascon, J.-M. Nature 2008, 451, 652. (2) Armand, M.; Tarascon, J.-M. Nature 2001, 414, 359. (3) Armand, M.; Tarascon, J.-M.; Recham, N.; Conference Proceedings of the 3rd Congress on Ionic Liquids; Cairns, Australia, May 31June 4, Royal Society of Chemistry: London, 2009. (4) Borgel, V.; Markevich, E.; Aurbach, D.; Semrau, G.; Schmidt, M. J. Power Soures 2009, 189, 331. (5) Owen, J. R. Chem. Soc. Rev. 1997, 26, 259. (6) Tollefson, J. Nature 2008, 456, 436. (7) Welton, T. Chem. Rev. 1999, 99, 2071. (8) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (9) Buzzeo, M.; Evans, R. G.; Compton, R. G. Chem. Phys. Chem. 2004, 5, 1106. (10) Sakaebi, H.; Matsumoto, H.; Tatsumi, K. Electrochim. Acta. Chem. 2007, 53, 1048. (11) Webber., E.; Blomgren, G. E. In Advances in Lithium Batteries, Ionic Liquids for Li Ion and Related Batteries; van Schalkwijk, W. A., Scrosati, B., Eds.; Kluwer Academic/Plenum Publ.: New York, 2002; p 185.

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organic solvents.1,3,4,10,11 The latter have the disadvantage of volatility, combustibility, and thermal and redox instability as well as sometimes limited compatibility with electrode materials, e.g., Li metal anodes.4,12,13 However, to achieve high performance electrochemical devices, it is essential to understand the physicochemical properties of RTILs; in particular, their molecular combinations (cations, anions) are of great interest.14-20 In this regard, recent detailed physicochemical studies focused on different cation structures to gain a more comprehensive understanding of how structural features effect the properties of their salts. Viscosity and hence conductivity, which are macroscopically governed by ion size, molecular weight, density, and ion association, are subtly controlled (12) Xu, K. Chem. Rev. 2004, 104, 4303. (13) Gnanaraj, J. S.; Zinigrad, E.; Asraf, L.; Gottlieb, H. E.; Sprecher, M.; Schmidt, M.; Geissler, W.; Aurbach, G. J. Electrochem. Soc. 2007, 53, 1048. (14) Weing€artner, H. Angew. Chem., Int. Ed. 2008, 47, 654. (15) Seki, S.; Hayamizu, K.; Tsuzuki, S.; Fujii, K.; Umebayashi, Y.; Mitsugi, T.; Kobayashi, T.; Ohno, Y.; Kobayashi, Y.; Mita, Y.; Miyashiro, H.; Ishiguro, S. Phys. Chem. Chem. Phys. 2009, 11, 3509. (16) Zhou, Z.-B.; Matsumoto, H.; Tasumi, K. Chem.;Eur. J. 2006, 12, 2196. (17) Li, H.; Ibrahim, M.; Agberemi, I.; Kobrak, M. N. J. Phys. Chem. B 2008, 129, 124507. (18) Annat, G.; MacFarlane, D. R.; Forsyth, M. J. Phys. Chem. B 2007, 129, 124507. (19) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833. (20) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103.

Published on Web 12/16/2009

r 2009 American Chemical Society

Article

by the geometry of the molecular frame (conformational degree of freedom, symmetry, flexibility), charge delocalization, nature of substituents, and coordination ability. Moreover, these microscopic factors also affect the electrochemical stability of the RTILs to a large degree. This is particularly important for the use in rechargeable Li metal batteries, an area we21 and others, notably Seki,22 Matsumoto,23 and Passerini24 have been promoting in recent years. Li metal batteries offer a ∼25% higher energy density compared to systems using intercalation materials such as LixC as the anode.5 However, today there are only a limited number of electrochemically stable cation-anion combinations available that support reversible cycling of lithium in this application and many more investigations are needed to control the redeposition of metallic Li on the anode surface during cycling.1,10,21-29 In this regard, it was demonstrated in the past for other metals that just changing the anion or the cation of an RTIL can lead to very different deposit morphologies or reduction pathways, which may be the result of solvation layers that adhere relatively strongly to the electrode surface or to multiple interactions (21) (a) Howlett, P. C.; MacFarlane, D. R.; Hollenkamp, A. F. Electrochem. Solid-State Lett. 2004, 7, A97. (b) MacFarlane, D. R.; Howlett, P. C.; Hollenkamp, A. F.; Forsyth, S. M. Australian Patent Appl., WO 2004/082059 A1, 2003. (c) Howlett, P. C.; Brack, N.; Hollenkamp, A. F.; Forsyth, M.; MacFarlane, D. R. J. Electrochem. Soc. 2006, 153, A595. (22) (a) Seki, S.; Mita, Y.; Tokuda, H.; Ohno, Y.; Kobayashi, Y.; Usami, A.; Watanabe, M.; Terada, N.; Miyashiro, H. Electrochem. Solid-State Lett. 2007, 10, A237. (b) Seki, S.; Ohno, Y.; Kobayashi, Y.; Miyashiro, H.; Usami, A.; Mita, Y.; Tokuda, H.; Watanabe, M.; Hayamizu, K.; Tsuzuki, S.; Hattori, M.; Terada, N. J. Electrochem. Soc. 2007, 154, A173. (c) Kobayashi, Y.; Mita, Y.; Seki, S.; Ohno, Y.; Miyashiro, H.; Terada, N. J. Electrochem. Soc. 2007, 154, A677. (d) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Watanabe, M.; Terada, N. Chem. Commun. 2006, 544. (e) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Mita, Y.; Usami, A.; Terada, N.; Watanabe, M. J. Phys. Chem. B 2006, 110, 10228. (f) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita, Y.; Kihira, N.; Watanabe, M.; Terada, N. Electrochem. Solid-State Lett. 2005, 8, A577. (g) Seki, S. Conference Proceedings of the 3rd Congress on Ionic Liquids; Cairns, Australia, May 31-June 4, 2009. (23) (a) Matsumoto, H.; Sakaebe, H.; Tatsumi, K.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 160, 1308. (b) Sakaebe, H.; Matsumoto, H.; Tatsumi, K. J. Power Sources 2005, 146, 693. (c) Matsumoto, H.; H.; Tatsumi, K. J. Power Sources 2005, 146, 45. (d) Sakaebe, H.; Matsumoto, H. Electrochem. Commun. 2003, 5, 594. (24) (a) Kim, G.-T.; Appetecchi, G. B.; Alessandrini, F.; Passerini, S. J. Power Sources 2007, 171, 861. (b) Shin, J.-H.; Henderson, W. A.; Appetecchi, G. B.; Alessandrini, F.; Passerini, S. Electrochim. Acta 2005, 50, 3859. (c) Shin, J.-H.; Henderson, W. A.; Passerini, S. Electrochem. Solid-State Lett. 2005, 8, A125. (d) Shin, J.-H.; Henderson, W. A.; Passerini, S. J. Electrochem. Soc. 2005, 152, A978. (e) Shin, J.-H.; Henderson, W. A.; Passerini, S. Electrochem. Commun. 2003, 5, 1016. (25) Hayamizu, K; Tsuzuki, S.; Seki, S.; Ohno, Y.; Miyashiro, H.; Kobayashi, Y. J. Phys, Chem. C. 2008, 112, 1189. (26) (a) Zhou, Z.-B.; Matsumoto, H.; Tasumi, K. Chem.;Eur. J. 2005, 11, 752. (b) Matsumoto, H.; Kageyama, H.; Miyazaki, Y. Chem. Commun. 2002, 1726. (27) R€ uther, T.; Huang, J.; Hollenkamp, A. F. Chem. Commun. 2007, 5226. (28) Tsunashima, K.; Sugiya, M. Electrochem. Commun. 2007, 9, 2353. (29) Kim, K.-S.; Choi, S.; Demberelnyamba, D.; Lee, H.; Oh, J.; Lee, B.-B.; Mun, S.-J. Chem. Commun. 2004, 828. (30) Moustafa, E. M.; Zein el Abedin, S.; Shkurankov, A.; Zschippang, E.; Saad, A. Y.; Bund, A.; Endres, F. J. Phys. Chem. B. 2007, 111, 4693. (31) Zein el Abedin, S.; P€ ollet, M.; Meiss, S. A.; Janek, J.; Endres, F. Green Chem. 2007, 9, 549. (32) Zein el Abedin, S.; Moustafa, E. M.; Hempelmann, R.; Natter, H.; Endres, F. J. Chem. Phys. Chem. 2006, 7, 1535. (33) Abbot, A. P.; McKenzie, K. J. Phys. Chem. Chem. Phys. 2006, 8, 4265.

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Figure 1. Schematic representation of a boronium cation.

between solvent and solute in the bulk of the RTIL.30-33 In particular, Endres et al. were able to show that the grain size as well as the morphology of aluminum particles electrodeposited from an RTIL are very dependent on the type of cation (1,3-dialkylimidazolium, N-alkylpyrrolidinium, alkylphosphonium) associated with the NTf2 anion.32 On the other hand, the groups of Matsumoto, Seki and others demonstrated in charge-discharge experiments with Li metal batteries how performance is dependent on the nature of the cation component of the RTIL electrolyte.4,10,22b,c,e,23d The most suitable cation structures so far for application in high-energy secondary Li batteries are aliphatic quarternary ammonium and phosphonium entities. However, it is widely acknowledged that when RTILs from these groupings are mixed with a range of suitable lithium salts, the resultant combination of lithium transport and liquid properties is seldom sufficient to meet the demands of an assembled lithium battery. With the important parameters of cation redox-stability, electrode compatibility, and ion structure-dependent grain size/morphology of electrodeposits in mind, we have now turned to novel RTILs based on the boronium cation (Figure 1).34-38 This is a heretofore unused class of ions in energy storage devices, which are easily prepared and provide access to potentially unique structural and electronic properties. The first examples of these hydrophobic RTILs containing the NTf2 anion were reported by the group of Davis, who demonstrated that the boronium cation embodies a versatile platform for structural diversity and formulation of new materials.39,40 This is evident from their general composition L1L2BH2-NTf2, where variation of the ligands L1 and L2 simply require the capacity of L to form a BH3 donor complex. Pertinent to our studies, the group demonstrated by single X-ray structure determination, 1H NMR, and computational studies, respectively, several important points by comparison to the conventional imidazolium counterparts: weaker cation-anion interactions, a greater degree of charge delocalization in the boronium ion, a 0.5 V more negative redox potential, and weaker solution state cation-anion H-bonding. Under these premises, we report for the first time preliminary results of stable reversible cycling of batteries constructed from the (34) Pierce, W.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016. (35) Frankel, R.; Kniczek, J.; Ponikwar, W.; Noth, H.; Polborn, K.; Fehlhammer, W. P. Inorg. Chim. Acta 2001, 312, 23. (36) Denniston, L. M.; Chiusano, M.; Brown, J.; Martin, D. R. J. Inorg. Nucl. Chem. 1976, 38, 379. (37) Douglass, J. E.; Fellman, J. D.; Carpenter, R.; Shih, H.-M.; Chiang, Y.-F. J. Org. Chem. 1969, 34, 3666. (38) Miller, N. E.; Muetterties, E. L. J. Am. Chem. Soc. 1964, 86, 1033. (39) Fox, P. A.; Griffin, S. T.; Reichert, W. M.; Salter, E. A.; Smith, A. B.; Tickell, M. D.; Wicker, B. F.; Cioffi, E. A.; Davis, J. H., Jr.; Rogers, R. D.; Wierzbicki, A. Chem. Commun. 2005, 3679. (40) Davis, J. H., Jr. International Patent WO/2006/125175.

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Li|L1L2BH2-NTf2 þ Li-salt|LiFePO4 system, along with some synthetic and physicochemical aspects of the employed RTILs. A comparison is also made with an electrolyte system based on the (benchmark) RTIL N-butyl-N-methyl pyrrrolidinium bis(trifluoromethansulfonyl)imide C4mpyr-NTf2.41

R€ uther et al.

Materials. Lithium iron-(II)-phosphate (LiFePO4) was obtained from Phostech Lithium and contains 1.71 wt % residual carbon. N-Butyl-N-methyl pyrrrolidinium bis(trifluoromethansulfonyl)imide, C4mpyr-NTf2, was obtained from Merck (quality: ultra pure, electronic grade). Lithium bis(trifluoromethansulfonyl)imide, LiNTf2, was obtained from 3 M and was dried at ∼140 °C under a vacuum for 2 days prior to use. Solutions of LiNTf2 in the RTILs of this work were prepared by thorough mixing of the components overnight in a glovebox. All RTILs and solutions were stored against Li metal (finely cut from Li foil) contained in a drying tube connected to a flask for at least one week in a drybox (see Figure S1 in the Supporting Information). This treatment guaranteed water levels below the detection limit (10-50 ppm depending on sample amount) of the Karl Fischer method used. Water Content and Impurity. The water content in the RTILs was determined by coulometric Karl Fischer titration (Methrom 756 KF Coulometer). The levels of residual lithium and halide ions in the ionic liquid were estimated by inductively coupled plasma mass spectrometry (Thermo Elemental X Series ICPMS) and ion chromatography (Dionex DX 300 ion-chromatograph), respectively.42 Thermal Stability. Thermal gravimetric analysis (TGA) was performed on a thermal analysis system (TA Instruments, TGA 2050). An average sample weight of 10 mg was loaded into a platinum pan and heated at a rate of 10 °C min-1 (except for isothermal measurements at 100 °C) over a range of approximately 25-600 °C under a slow stream of nitrogen gas. The decomposition temperature (Tdec) was taken as the temperature at which a 5% mass loss is observed. Phase Transition. Calorimetric measurements were performed on a differential scanning calorimeter (TA Instruments, DSC 2910). The temperature calibration was performed with the following standard samples: cyclopentane (solid-solid transitions, -151.16 and -135.06 °C), n-heptane (-90.56 °C), n-octane (-56.76 °C), cyclohexane (solid-solid transition: -87.06 °C; melting point: 6.54), n-decane (-29.64 °C), n-dodecane (-9.65 °C), water (0 °C), n-octadecane (28.24 °C) p-nitrotoluene (melting point: 51.64 °C), hexatriacontan (solid-solid transitions, 72.14 and 73.94 °C; melting point: 75.94 °C) and indium (melting point: 156.6 °C) used as reference. An average weight of 1-5 mg of each sample was hermetically sealed in an aluminum pan (glovebox) and cooled to -140 °C then heated again at a rate of 10 °C min-1, under a flow of helium gas. Specific Conductivity. The specific conductivity (σ) was measured with a conductivity meter (Agilent precision LCR meter 4284A) coupled with a PC controlled temperature controller (Eurotherm) in a custom-made glass cell in combination with a custom-made Pt electrode. The glass cell/electrode combination was housed in the center of a custom-made brass block next to a thermocouple. Data were collected at each step of individual temperature programs using a temperature equilibration

time of 5 min and a temperature deviation of ΔT = 0.5 °C. The cell/electrode combination was calibrated prior to each measurement with 0.01 M KCl solutions (cell constant ∼2 cm-1). The assembly and method was validated by C4mpyr-NTf2 (Merck, high purity grade) and comparing results with reported values. The data were recorded over a frequency range of 20 to 1 000 000 Hz between 20 and 85 °C. Electrochemical Measurements. Cyclic voltammetry was performed (drybox) on potentiostats Autolab PGSTAT 302 interfaced and monitored with a PC. The standard three-electrode configuration consisted of a Pt electrode (surface area: 1.49  10-3 cm2, determined from the Randles-Sevcik equation, using D = 2.3  10-5 cm-2 s-1 and 5 mM ferrocene/0.1 M tetrabutylammonium hexafluorophosphate) served as the working electrode, a Pt wire was used as the counter electrode, and the reference electrode was a Ag wire immersed in 10 mM solution of AgOTf in C4mpyr-NTf2 and contained in a Teflon tube with a glass sinter tip.43 All potentials are given versus the Ag|Agþ redox couple. The data for each salt were collected in the liquid state; prior to each scan the working electrode was washed with (i) methanol, (ii) deionisised water in presence of 3 μm alumina, (iv) polished with 0.3-1 μm alumina, (v) rinsed with water followed by HPLC grade acetone and finally dried. The cathodic and anodic limits were arbitrarily defined as the potentials at which the current density reached 1  10-3 A cm-2. Electrode Materials and Battery Cycling. LiFePO4 (75%), Shawinigan carbon black (15%), and polyvinylidene difluoride (PVDF) binder (10%) were combined in a glass jar and then thoroughly mixed by adding alumina spheres and slowly rotating the jar for several hours. Subsequently, N-methyl pyrrolidone (NMP) solvent was added until the resultant slurry had a free-flowing texture that was coated on to an aluminum foil (30 μm thickness) current collector using a K-bar roller. The coated electrodes were then left to dry in an air stream (fume hood) overnight before being vacuum-dried at 120 °C for 72 h. This led to an average active material loading density of 2-3 mg cm-2. The CR2032 coin cell contained a metallic lithium anode (1.4 cm diameter), a Solupor 7PO7C separator (1.6 cm diameter), an electrolyte solution of 0.4-0.5 mol kg-1 LiNTf2 salt in the respective RTIL and a 1.3 cm diameter disk cathode. All cells were assembled in an argon filled glovebox at ambient temperature. Cell performance was evaluated between 3.0 (end of discharge) and 3.8 V (top of charge) on a Maccor 4000 multi channel battery test station. Galvanostatic cycling was conducted at various chargedischarge rates (C/time) and temperatures. For temperature control the cells were kept in thermostatic ovens. The synthesis of all three new boronium RTILs is quite simple, and utilizes an approach pioneered by Ryskewitsch, et al.44 for the synthesis of boronium iodide salts and later refined by us for the preparation of boronium-based ionic liquids.39 The synthesis of iodide salts, which are intermediates in the preparation of the target RTILs, is carried out in a one-pot fashion in air. In this approach, a commercial borane complex of trimethylamine is dissolved in toluene and oxidized with I2 to form N111-BH2I in situ. In the case of the synthesis of 3a and 3b, slightly more than two equivalents of the requisite 1-alkyl imidazole is then added, which (upon heating) displace both the B- bound iodide and trimethylamine, resulting in the formation of symmetrical bis(1 - alkylimidazole)BH2þIsalts. Each of these is, in turn, treated with potassium

(41) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164. (42) Hao, F.; Haddad, P.; R€ uther, T. Chromatographia 2008, 67, 495.

(43) Snook, G. A.; Best, A. S.; Pandolfo, A. G.; Hollenkamp, A. F. Electrochem. Commun. 2006, 8, 1405. (44) Garrett, J. M.; Ryschkewitsch, G. E. Inorg. Synthesis; Parry, R. W., Ed.; McGraw Hill: New York, 1970, 12, 132.

Experimental Section

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Figure 2. Differential scanning calorimeter thermogram of (mim)2BH2-NTf2 3a (green line), (bim)2BH2-NTf2 3b (red line), (N111)(N112)BH2-NTf2 3c (black line) recorded at a heating rate of 10 °C min-1 after cooling from room temperature to -140 °C.

Scheme 1. Synthesis of Boronium Cation Based RTILs (mim)2BH2-NTf2 3a, (bim)2BH2-NTf2 3b, (N111)(N112)BH2-NTf2 3c

bis(trifluoromethanesulfonyl)amide which results in the separation of the desired RTILs as water-immiscible second phases. The synthesis of 3c differs only in the use of the non-heterocyclic dimethyl ethylamine as the nucleophile and its addition in a 1:1 molar equivalence, which results in the formation of the mixed donor ligand iodide salt 2c. Anion metathesis of this salt is effected in the same manner as with 3a and 3b. To obtain electrolyte materials sufficiently pure for the use in coin cell cycling, we successively washed the hydrophobic products of each synthesis with deionized water (Ag-halide test), dried them under high vacuum (∼5  10-2 mbar at 40-50 °C), and finally stored them against Li metal (see Figure S1 in the Supporting Information). Details of the synthesis and characterization of the materials 3a-3c are provided in the Supporting Information.

Results and Discussion Synthesis and Characterization. The synthesis of the boronium RTILs is accomplished from readily available and relatively inexpensive starting materials in a straightforward three-step procedure as illustrated in Scheme 1.39,44 Its simplicity allows for a great freedom in design so that a large variety of cation structures with different

physicochemical properties can be accessed with ease. The product RTILs separate, after ion-exchange with M-NTf2, as hydrophobic phases. The thermal properties of the three compounds were determined by differential scanning calorimetry (DSC) and by thermal gravimetric analysis (TGA). The solidliquid phase transitions of the boronium RTILs are presented in Figure 2 and their TGA traces are collected in the supplement Figure S2 (temperature ramp) and S3 (isotherm). The data for the glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), and decomposition temperature (Td) are listed in Table 1. Two types of behavior are observed for the materials, the first type is represented by 3a showing only a glass transition at -65 °C. The second type of behavior, glass transition at low temperature to form a super cooled liquid followed by a crystallization (Tc exothermic peak) and final melting was observed for 3b (Tc =2.3 °C, Tm = 28 °C) and 3c (Tc=-40.5 °C, Tm=15.4 °C) . None of the materials displayed single or multiple solid-solid phase transitions as is often seen in other RTILs, independent of the anion. The thermal stability (see Figure S2 in the Supporting Information) of 3a, b is >300 °C and

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Table 1. Thermal and Electrochemical Properties of 3a-3c compd

Tg (°C)

Tc (°C)

Tm (°C)

ΔSm (J/g)

Td (°C)

σa (mS cm-1)

Ecath (V)

Ean (V)

ΔE (V)

(mim)2BH2-NTf23a (bim)2BH2-NTf23b (N111)(N112)BH2-NTf23c C4mpyr-NTf2b

-65 -69.7 -79.2 -87

n.a. 2.3 -40.5 ∼ -50

n.a. 28.1 15.4 -18

n.a. 31.1 34.9

335 315 238 431

1.23 0.62 1.56 2.37c

-2.8 -2.9 -3.8

1.5 1.6 2.0

4.3 4.5 5.8 5.5

a At T = 25 °C; Ecath = cathodic limiting potential; Ean = anodic limiting potential, ΔE = electrochemical window (Ean þ Ecath). bLiterature values from from ref 41. c This was a Merck product measured in our laboratory.

Figure 3. Temperature-dependent specific conductivity of (mim)2BH2NTf2 3a, (bim)2BH2-NTf2 3b, (N111)(N112)BH2-NTf2 3c recorded on a Pt electrode.

therefore comparable to or slightly higher than other imidazolium based RTILs. The Bisamine salt 3c has an ∼100 °C lower stability and displays two distinct steps before decomposition of the final fragment at ∼400 °C. Similar thermal behavior was reported earlier by Miller38 for bisamineboronium salts (e.g., Tdec [(N111)(N112)BH2][Cl]=195 °C). The mechanism is related to the ease of displacement of the amine donor by the anion. The long-term stability was estimated for two salts 3b and 3c at 100 °C, where 3c had shown some signs of early weight loss in the temperature ramp TGA. Over a period of 24 h, both compounds showed good stability with a weight loss not exceeding 0.3 wt % (see Figure S3 in the Supporting Information). Conductivities for the boronium-based RTILs were measured over a temperature range of 20-85 °C and a plot of the data (σ vs T) is shown in Figure 3. The estimated conductivities are only slightly lower than that of the commercial (Merck) C4mpyr-NTf2 measured under the same conditions but which has a considerably smaller cation formula weight (142.26 g mol-1). The lowest conductivity was estimated for 3b, which was expected because this cation has the highest formula weight (261.19 g mol-1) among the three materials. Among the boronium RTILs, 3c exhibits the highest conductivity (1.56 mS cm-1) despite its higher formula weight (187.15 g mol-1) compared to 3a. This relatively high conductivity may be explained by weaker ion interactions owing to structural features like lower symmetry and flexibility, giving rise to a greater degree of freedom.15,16,23c,26a This, and a greater degree of charge delocalization than in conventional counterparts,39 may account for the conductivities when comparing the boronium based RTILs with C4mpyr-NTf2. These data

also demonstrate the opportunities that are embodied in the (L1L2)BH2 cation template for improving physicochemical properties, including viscosity and conductivities, through easy permutation of the donor ligands L. As expected, conductivities are lower after addition of 0.4-0.5 mol kg-1 LiNTf2 to the RTIL, the addition being required when using the RTIL as a battery electrolyte system. A conductivity vs temperature plot is shown for 3a + LiNTf2 in Figure S4 of the Supporting Information. Electrochemistry. Previous reports revealed that alicyclic and cyclic quarternary ammonium cations and phosphonium cations combined with weakly coordinating anions NTf2,21a,b,22f,27,28,41,45,46 RBF3,16,26a FSI23a,47 have sufficient electrochemical stability to allow reversible cycling of >3 V lithium batteries, in particular devices containing a lithium metal anode. By contrast, earlier studies showed that RTILs based on 1,3-dialkylimidazolium cations are not always suitable as electrolytes in lithium batteries because of their limited electrochemical windows (∼ 4 V).10,22a-22c,22e,23c,d,48-51 Hence, there has been particular interest in the relationship between cation and anion structures of RTILs and their electrochemical windows, which stimulated the search for new electrolytes that allow for the stable reversible cycling of safe, highenergy-density lithium metal batteries. Cyclic Voltammetry. To establish the electrochemical stability of the (L1L2)BH2-NTf2 RTIL in the present study, the salts were investigated by cyclic voltammetry on a platinum electrode at ambient temperature. A [Ag/ Agþ] redox couple [10 mM AgOTf in C4mpyr-NTf2] was used as a stable reference electrode.43 A representative voltammogram is shown in Figure 4 and the measurement data for the cathodic limit (Ecath) and the anodic limit (Ean) are listed in Table 1, in which the values of Ecath and Ean are defined as the potential when the current density reaches 1.0 mA cm-2. The highest electrochemical stability was observed for the Bisamine coordinated boronium salt which has an Ecath close to -4 V and an Ean of close to þ2 V. The electrochemical window is thus comparable with that of the most stable cyclic (45) Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. B. 1998, 102, 8858. (46) Egashira, M.; Okada, S.; Yamaki, J.; Dri, D. A.; Bonadies, F.; Scrosati, B. J. Power Sources 2004, 138, 240. (47) Armand, M. International Patent WO/09/400025. (48) Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalayarasundaram, N.; Gr€azel, M. J. Inorg. Chem. 1996, 35, 1168. (49) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881. (50) Garcia, B.; Lavallee, S.; Perron, G.; Michot, N.; Armand, M. Electrochim. Acta 2004, 49, 4583. (51) Egashira, M.; Nakagawa, M.; Watanabe, I.; Okada, S.; Yamaki, J. J. Power Sources 2005, 146, 685.

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Figure 4. Cyclic voltammograms of (mim)2BH2-NTf2 3a (red line) and 3c (N111)(N112)BH2-NTf2 3c (black line) obtained on a platinum working electrode; scan rate: v = 100 and 50 mV s-1, respectively; temperature: ambient; reference electrode Ag|AgOTf (10 mM AgOTf in C4mpyrNTf2).

quarternary ammonium salts reported so far.45,16,27 The electrochemical windows of the two bis imidazole salts are exceeding 4 V and appear to be similar or slightly larger than found for other RTILs composed of imidazolium based cations.23c,d,49,50,52 In support of the voltammetric measurements, gas-phase values of E0red for the cations of 3a and 3c were computed in the manner of Baik and Friesner53 by carrying out calculations on the parent and reduced species. Significantly, the computed -3.04 V value of E0red for 3a is quite close to the -2.9 V determined experimentally. In contrast, the computed and experimental values for 3c differ by nearly 0.5 V, being -3.34 V and -3.8 V, respectively. Although this discrepancy may be attributable to the character of the computational cation as being in the gas phase, the relative trend is the same: a boronium cation bound to two imidazolium rings is considerably easier to reduce than is a boronium cation in which both nitrogen ligands are tertiary amines. Further underscoring the potential utility of such computations for “prescreening” in the design of cations for battery applications, the E0red of a “mixed” cation bearing one N-methyl imidazole and one trimethylamine moiety was computed to be -3.40 V, logically in line with the foregoing computational and experimental results.39 It is fundamental for the present work to demonstrate the feasibility of electrodeposition of the very electropositive Li metal from the novel boronium cation RTILs. Previously, reversible electrodeposition of Li was mainly achieved with quarternary ammonium and phosphonium cation based RTILs. Because the cyclic voltammogram of (mim)2BH2-NTf2 3a shows a Ecath close to the reduction potential of Li in RTIL media, we have doped a sample of 3a with 0.45 mol kg-1 of LiNTf2 and recorded the voltammogram for this solution (Figure 5). Reversible Li deposition - stripping was observed at Eo -3.52 V, with Li deposition occurring at Ecath -3.92 V, approximately 1.1 V more negative than the cathodic limiting potential of the neat RTIL (52) Sakaebe, H.; Matsumoto, H.; Tatsumi, K. Chem.;Eur. J. 2004, 10, 6581. (53) Baik, M.-H.; Friesner, R. A. J. Phys. Chem. A 2002, 106, 7407.

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Figure 5. Cyclic voltammogram of reversible electrodeposition of Li in (mim)2BH2-NTf2 3a þ 0.45 mol kg-1 LiNTf2 obtained on a Pt electrode; scan rate: v = 20 mV s-1; reference electrode Ag|AgOTf (10 mM AgOTf in C4mpyr-NTf2); temperature: ambient.

(mim)2BH2-NTf2 3a. Negative shifts for the Ecath of 1 V, which have been observed for other cations in the presence of Li salts, are attributed to the formation of a protective layer on the electrode surface resulting from breakdown products of the lithium salt during reduction.4,21a,c,23b,24d,49 However, in most cases, it is not possible to obtain reversible Li depositionstripping for 1,3-alkylimidazolium RTILs containing lithium salts. The exact reason for this behavior is still unclear,4,10,22a,b,e,23a,c,48 although some of the adverse effects during battery cycling can be eliminated by reducing the acidity of the imidazolim cation, either through the replacement of the acidic proton in C2 position with the less acidic CH3 group22b or through increasing the length of the alkyl chain on one of the nitrogen hetero atoms.22a Cycling of Li|LiFePO4. Coin cells of Li|LiFePO4 were assembled with electrolytes based on RTILs of each of the boronium cations, together with 0.4 - 0.5 mol kg-1 LiNTf2 (1.4 cm diameter disk). Galvanostatic cycling was performed at different cycling rates (C/time) and temperatures. Figure 6 shows the charge-discharge capacity as a function of cycle number for the most efficient RTIL containing two amine ligands at the boronium center. For comparison the data for the C4mpyr-NTf2/LiNTf2 electrolyte system collected under the same conditions are also represented in this figure. It is clear from this data collection that the boronium cation based electrolyte shows very good cycling performance, gaining 145 mA h g-1 after a few cycles at 50 °C relative to the theoretical value of 170 mA h g-1 for LiFePO4. A relatively small capacity drop to 130 mA h g-1 but stable cycling was observed when the operating temperature was lowered to 30 °C. The Coulombic efficiency was quite close to 1 (ratio of discharge capacity/to charging capacity) except for the initial few cycles at 50 °C, and this indicates that the entire electrolyte was stable during the charge-discharge cycles. The initial low capacity build up may be partly related to a decrease in polarization resistance because of the formation of a stable SEI layer that allows for better ion conduction.4,21a,22b,23b,24a

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Figure 6. Galvanostatic charge-discharge cycling of Li|(N111)(N112)BH2-NTf2 þ 0.44 mol kg-1 LiNTf2|LiFePO4 and Li|C4mpyr-NTf2 þ 0.5 mol kg-1 LiNTf2|LiFePO4 cells, (a) cycling performance at different charge rates and temperatures, (b) long-term cycling of Li|(N111)(N112)BH2-NTf2 þ 0.44 mol kg-1 LiNTf2|LiFePO4 at C/5 and 30 °C.

However, another contribution may also come from an improved metallic lithium surface arising from repeated dissolution-deposition of Li during cycling. To establish the limits of cycling performance for this electrolyte composition, we collected data at faster cycling rates C/5 and C/2, both at 30 °C. Still reasonable cycling, C=114 mA h g-1, was maintained at a rate of C/5, but increasing the rate to C/2 caused a significantly sharper drop in capacity to ∼62 mA h g-1. By contrast, cells constructed under the same conditions with the C4mpyr-NTf2 electrolyte system did not perform as well. At 50 and 30 °C (both charge rate C/10) the capacities were on average ∼20 and 40 mA h g-1 lower than those observed for (N111)(N112)BH2-NTf2 and a gradual decrease in capacity occurred. This is surprising because the estimated conductivity of the employed C4mpyr-NTf2 is 2.3 mS cm-1 compared to 1.56 mS cm-1 for the boronium-based RTIL, indicating that chemical or physical processes other than mere ion conduction in the bulk electrolyte appear to influence the battery performance; i.e., ion conduction does not necessarily reflect lithium ion transport in the bulk electrolyte.22a,b A similar observation was also made by other groups for pyrrolidinium and piperidinium cation-based RTIL, where the less conductive piperidinium derivative showed better cycling performance in both LiCoO2/Li and high voltage LiNi0.5Mn1.5O4/Li cells.4,23c,d In this context, a comparison with the ternary PEO-LiNTf2-Cxmpyr-NTf2 (x=number of carbons in side chain) polymer electrolyte systems studied

R€ uther et al.

by Passerini et al. is also interesting.24a-d These systems with ionic conductivities of ∼0.1 mScm-1 (20 °C, in the presence of LiNTf2) exhibit remarkable capacities of 125 mA h g-1 (30 °C) and 150 mA h g-1 (40 °C), respectively, at a charge rate of C/10. After estimating the limits of operational parameters, cycle rate, and temperature, we also investigated the longterm cycling of the Li|(N111)(N112)BH2-NTf2 þ LiNTf2| LiFePO4 cell using the same cell immediately following the first 50 cycles described above. Figure 6b shows the charge discharge capacity as a function of cycle number for a C/5 cycle rate at 30 °C and a representative voltagetime profile is given in Figure S5. The battery reached its cycle life defined as 80% original capacity after 200 cycles, throughout which a Coulombic efficiency of close to 1.0 was maintained. After this period the cell could still be cycled with good capacity and Coulombic efficiencies for more than 50 cycles at 50 °C (see Figure S6 in the Supporting Information). Cells constructed with other short alkyl chain alicyclic quarternary ammonium RTIL electrolytes like N1113-NTf2 were only stable for the first few cycles.23d Initial cycling of cells containing the (bim)2BH2-NTf2 electrolyte solution at a C/10 rate and 50 °C was erratic (a plot of capacity vs cycle number is presented in the Figure S7 in the Supporting Information). On the first charge, a capacity higher than the theoretical value was observed followed by a charge discharge capacity in the second cycle of only 20 to 30 mA h g-1. A gradual but steady increase in capacity was observed in subsequent charge discharge cycles reaching approximately 65 mA h g-1. During this cycling period, the Coulombic efficiency also increased initially from very low 0.27 to 0.95 in the eighth cycle before dropping back to approximately 0.8. This behavior may, apart from the formation of an SEI layer, also involve other processes.23b Improved battery cycling was observed upon increasing the temperature to 80 °C when the RTIL is significantly more conductive, exceeding the values observed for 3a and 3c at 50 °C. However, cycling was less stable than that of cells containing the 3a electrolyte and each time a cell is restarted, overcharging is observed, followed by an increase in discharge efficiency similar to that observed in the initial cycles at 50 °C. At a cycling rate of C/10 and 80 °C, 140 charge-discharge cycles were achieved before the cell lost approximately 20% of the initial capacity. During this period, the initially significant gap between charge and discharge capacity narrowed and the discharge efficiency became more stable after approximately 80 cycles, approaching an efficiency of 0.9. These data suggest that irreversible processes are occurring during battery cycling. Although reversible Li deposition-stripping was possible with the (mim)2BH2-NTf2 electrolyte, battery cycling with this material did not show a reproducible profile. The cells had noisy voltage-time curves for charging developed large voltage transients, which is indicative of the formation of dendritic lithium growths and partial shortcircuiting of the cell. In addition, large portions of capacity were lost in the first few cycles. This behavior

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may be attributed to the acidic nature of the proton in C2 position, which is not compatible with the Li anode over longer periods of time.22a,b In summary, the physico and electrochemical properties of boronium cation based ionic liquids were investigated for the first time with a view of applying these materials as novel electrolytes in lithium metal rechargeable batteries. Conductivities that are approaching that of commercial C4mpyr-NTf2 and the electrochemical windows of 4.3 V and 5.8 V for the bisimidazole and the bisamine-substituted cations, respectively, render these molten salts suitable electrolytes in Li|LiFePO4 cells. Consequently, very good capacity (145 mA h g-1) and cycling stability was observed for the bisamine salt, which are surprisingly better than those of C4mpyrNTf2 under the same reaction conditions despite the higher conductivity of the latter. Hence, the novel

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boronium-cation-based molten salts are an interesting addition to the few examples of electrolytes with sufficient stability for stable Li metal battery cycling. Acknowledgment. The authors thank the CSIRO Energy Transformed Flagship program for financial support and Dr Graeme Snook for helpful discussions during revision of the manuscript. We also thank the Alabama Supercomputing Authority for computational resources and Ms. Kayla Mattson and Mr. Adam Lewis for assistance with synthesis and acquisition of NMR data. Supporting Information Available: Details of computational studies, synthetic procedures for 3a-3c, 300 MHz NMR spectra of 3a-3c, and ESI-MS data of 3a-3c. Figures of TGA analysis, battery cycling, drying procedure of electrolyte materials (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.”