Article pubs.acs.org/JPCB
Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes Daniel M. Seo,† Paul D. Boyle,‡ Roger D. Sommer,‡ James S. Daubert,† Oleg Borodin,§ and Wesley A. Henderson*,†,∥ †
Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States § Electrochemistry Branch, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States ∥ Electrochemical Materials & Systems Group, Energy & Environment Directorate, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temperature range. Four new crystalline solvate structures (PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, respectivelyhave been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI−···Li+ cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI− anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI−···Li+ cation interactions possible and the overlap of the corresponding spectroscopic data signatures.
1. INTRODUCTION Understanding the molecular-level interactions in electrolyte mixtures is crucial because such interactions determine the electrolyte properties and correspondingly influence the performance, safety, and longevity of batteries. The most widely utilized methods for scrutinizing such interactions are Fourier transform infrared (FTIR) and Raman spectroscopy. Upon coordination with Li+ cations, the vibrational bands of the anions are shifted due to changes in the anion’s electron density and structure. The variation in these shifts is dependent upon the manner in which one or more Li+ cations are coordinated. Detailed information about the molecular-level interactions is therefore obtainable if one can correlate the experimental data to specific forms of coordination. Without such a key, however, the evaluation of experimental spectroscopic data is fraught with pitfalls as a misinterpretation of the data leads to highly erroneous conclusions about ionic association interactions within electrolytes. To address this problem, previous studies have been reported for LiClO4,1,2 LiBF4,3 and LiDFOB (i.e., lithium difluoro(oxalato)borate)4 which examine the anion band variation of crystalline solvates with known anion···Li+ cation coordination to identify solvent-separated ion pair (SSIP), contact ion pair (CIP), and aggregate (AGG) coordination in which the anions are coordinated to zero, one, or more than one Li+ cations, © 2014 American Chemical Society
respectively. Such studies provide electrolyte characterization tools which may be broadly utilized for a given lithium salt. The present study continues this evaluation for the widely studied salt lithium bis(trifluoromethanesulfonyl)imide (i.e., LiTFSI or LiN(SO2CF3)2):
The TFSI− anion has extensive charge delocalization (i.e., multiple resonance structures). The anion is also flexible with two low-energy conformations: a cisoid form (C1) with the CF3 groups on the same side of the S−N−S plane and a transoid form (C2) with the CF3 groups on opposite side of the plane.5−9 In solution, an equilibrium exists between these conformations with the C2 conformation expected to be lower in energy from gas-phase calculations.7 In crystalline phases, however, the TFSI− anion often has only one conformationeither C1 or C2. In addition, the TFSI− anion is able to readily adopt bidentate coordination to a single Li+ cation with two oxygen atoms. Received: May 21, 2014 Revised: October 20, 2014 Published: October 27, 2014 13601
dx.doi.org/10.1021/jp505006x | J. Phys. Chem. B 2014, 118, 13601−13608
The Journal of Physical Chemistry B
Article
Chart 1. Example of TFSI−···Li+ Cation Coordination (from Crystal Structures): (a) SSIP-C1, (b) SSIP-C2, (c) CIP-I-C2, (d) CIP-II-C1, (e) AGG-Ia-C2, (f) AGG-Ib-C2, (g) AGG-IIb-C1, (h) AGG-IIb-C2, and (i) AGG-III-C2a
liquid mixtures is a difficult challenge. This is due to both the diversity present in the manner in which the anions may be coordinated and the overlap of Raman bands for these different forms of coordination. Significant information may still be learned about anion coordination in solution, however, when spectroscopic information is combined with other methods of electrolyte characterization.10−12 As an example of its utility, the information obtained from the present work has been directly used to evaluate the solution structural interactions within acetonitrile (AN)nLiTFSI liquid electrolytes.11
2. EXPERIMENTAL METHODS 2.1. Materials. LiTFSI (battery grade) was purchased from 3M. The salt was dried at 120 °C under high vacuum overnight. The solvents (Chart 2) ethylene glycol dimethyl ether, Chart 2. Structures and Acronyms of the Solvents Studied
1,2-dimethoxyethane or monoglyme (G1, anhydrous, 99.5%), diethylene glycol dimethyl ether, 2-methoxyethyl ether or diglyme (G2, anhydrous, 99.5%), triethylene glycol dimethyl ether or triglyme (G3, 99%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), N,N,N′,N′-tetramethylethylenediamine (TMEDA, purified by re-distillation, ≥99.5%), ethylene carbonate (EC, 99%), 1,4,7,10-tetraoxacyclododecane or 12-crown-4 (12C4, 98%), phenanthroline (PHEN, ≥99%), neocuproine or 2,9-dimethyl[1,10]phenanthroline (2,9DMPHEN, ≥99.0%), 2,6-lutidine or 2,6-dimethylpyridine (2,6-DMPy, ≥99.0%), and acetonitrile (AN, anhydrous, 99.9%) were purchased from either Fisher Scientific or SigmaAldrich. These solvents were dried over 3 Å molecular sieves (for the liquids), and the water content of the solvents was verified prior to their use to be negligible (
