Article pubs.acs.org/JPCC
Anion Coordination Interactions in Solvates with the Lithium Salts LiDCTA and LiTDI Dennis W. McOwen,† Samuel A. Delp,† Elie Paillard,† Cristelle Herriot,† Sang-Don Han,† Paul D. Boyle,‡ Roger D. Sommer,‡ and Wesley A. Henderson*,†,§ †
Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering and ‡X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States § Electrochemical Materials & Systems Group, Energy & Environment Directorate, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Lithium 4,5-dicyano-1,2,3-triazolate (LiDCTA) and lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI) are two salts proposed for lithium battery electrolyte applications, but little is known about the manner in which the DCTA− and TDI− anions coordinate Li+ cations. To explore this in depth, crystal structures are reported here for two solvates with LiDCTA(G2)1:LiDCTA and (G1)1:LiDCTAwith diglyme and monoglyme, respectively; and seven solvates with LiTDI(G1)2:LiTDI, (G2)2:LiTDI, (G3)1:LiTDI, (THF)1:LiTDI, (EC)1:LiTDI, (PC)1:LiTDI, and (DMC)1/2:LiTDIwith monoglyme, diglyme, triglyme, tetrahydrofuran, ethylene carbonate, propylene carbonate, and dimethyl carbonate, respectively. These latter solvate structures are compared with the previously reported acetonitrile (AN)2:LiTDI structure. The solvates indicate that the LiTDI salt is much less associated than the LiDCTA salt and that the ions in LiTDI, when aggregated in solvates, have a very similar TDI−···Li+ cation mode of coordination through both the anion ring and cyano nitrogen atoms. Such coordination facilitates the formation of polymeric ion aggregates, instead of dimers. Insight into such ion speciation is instrumental for understanding the electrolyte properties of aprotic solvent mixtures with these salts.
■
INTRODUCTION Electrolyte interactions largely govern lithium battery performance (power, cycle life, and safety characteristics), but such interactions remain poorly understood. As lithium cell chemistries for advanced batteries have rapidly progressed with new cathodes (high-voltage metal oxides, sulfur, air, etc.) and anodes (Li metal, Li−Si alloy, etc.), it has become evident that the state-of-the-art salt LiPF6which has been predominantly used in commercial Li-ion battery electrolytes for more than a quarter of a centurymay no longer be optimal. LiPF6 has long been the salt of choice, as electrolytes with this salt have some of the highest ionic conductivity values measured for aprotic solvent-based electrolytes.1−4 Such electrolytes also react to form both a stable interface with Al metal at high potentials5−7 (the current collector used for cathodes) and a favorable solid−electrolyte interface (SEI) with graphite anodes when carbonate solvents are used.8−11 However, the PF6− anion also contains labile P−F bonds which result in the salt readily undergoing hydrolysis12,13 and having a relatively low thermal stability.14,15 The HF, common to LiPF6-based electrolytes, from such instability is known to have a deleterious impact on cell performance, especially at elevated temperature.16−18 New electrolyte salts have therefore been avidly sought, but identifying promising anions which meet the myriad property requirements for lithium battery electrolytes © 2014 American Chemical Society
has been quite challenging. In recent years, lithium salts having two structurally similar anions have been proposed for battery electrolytes: 4,5-dicyano-1,2,3-triazolate (DCTA − )also known as 1,2,3-triazole-4,5-dicarbonitrile (TADC−)and 2trifluoro-4,5-dicyanoimidazolide (TDI−):
Despite their structural similarities, lithium salts with these anions are found to result in remarkably different electrolyte properties. LiDCTA was first prepared by Michot in 199519 and reported in two publications in 2003.20,21 Although LiDCTA was found to be highly soluble in polyethylene oxide (PEO), the ionic conductivity of the resulting polymer electrolytes was found to be relatively low.21 The electrolyte consisting of 1 M LiDCTA in propylene carbonate (PC) is also much less Received: December 24, 2013 Revised: March 17, 2014 Published: March 21, 2014 7781
dx.doi.org/10.1021/jp412601x | J. Phys. Chem. C 2014, 118, 7781−7787
The Journal of Physical Chemistry C
Article
conductive than the corresponding electrolyte with LiPF6.22 When paired with organic cations, the DCTA− anion readily forms low melting salts (i.e., ionic liquids) with a low viscosity,23,24 perhaps suggesting that this anion may be weakly coordinating due to the extensive negative charge delocalization across the anion’s structure. The relatively low conductivity of electrolytes with LiDCTA, however, contradicts this notion. In addition, the 1 M PC-LiDCTA electrolyte is highly reactive with an Al electrode at 99.0%), dichloromethane (CH2Cl2, Sigma Aldrich, > 99.9%), npentane (Sigma Aldrich, > 99%), and ethanol (EtOH, Fisher Scientific, denatured, reagent grade) were used as-received. The solvents used for solvate crystallization (Chart 1) acetonitrile (AN, Sigma Aldrich, 99.9%), dimethyl carbonate (DMC, Sigma Aldrich, > 99%), propylene carbonate (PC, Novolyte, > 99%), 1,2-dimethoxyethane (monoglyme or G1, Sigma Aldrich, anhydrous 99.5%), diethylene glycol dimethyl ether (diglyme or G2, Sigma Aldrich, anhydrous 99.5%), and triethylene glycol dimethyl ether (triglyme or G3, Sigma Aldrich, 99%)were stored over 3 Å molecular sieves prior to use. Tetrahydrofuran (THF, Fisher Scientific, > 99.9%) was distilled over Na metal with benzophenone indicator. Ethylene carbonate (EC, Novolyte, electrolyte-grade 99.9%) was used as received. These solvents were confirmed to be anhydrous (
