ARTICLE pubs.acs.org/JPCC
Inhibition of Self-Aggregation in Ionic Liquid Electrolytes for High-Energy Electrochemical Devices Miriam Kunze,†,‡ Elie Paillard,†,‡ Sangsik Jeong,†,‡ Giovanni B. Appetecchi,§ Monika Sch€onhoff,‡ Martin Winter,†,‡ and Stefano Passerini*,†,‡ †
University of Muenster, Battery Research Laboratory MEET, Corrensstrasse 46, 48149 Muenster, Germany University of Muenster, Department of Physical Chemistry, Corrensstrasse 28/30, 48149 Muenster, Germany § Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Via Anguillarese 301, 00123 Rome, Italy ‡
bS Supporting Information ABSTRACT: Some ionic liquids (ILs) based on pyrrolidinium cations, which are of interest for high-energy electrochemical storage devices, such as lithium batteries and supercapacitors, have a structure similar to that of surfactants and tend to form local aggregates. The investigated ILs consist of the bis(trifluoromethanesulfonyl)imide (TFSI) anion and pyrrolidinium-based cations having a methyl side chain and an ether (ethylmethoxy- or ethylethoxy-) side chain. For such, it is very important to understand if these IL cations tend to aggregate like surfactants because this would affect the ion mobility and thus the ionic conductivity. The aggregation behavior of these ILs was extensively studied with NMR and Raman methods also in the presence of Li+ cations and compared with that of corresponding ILs having no ether group on the cation side chain. 2H NMR spinlattice and spinspin relaxation rates were analyzed by applying the “two step” model of surfactant dynamics. Here we show that whereas the ILs based on pyrrolidinium cations without ether functions tend to form aggregates in which the cations are surrounded by the anions, especially in the presence of lithium cations, those with an ether side chain are not aggregated.
’ INTRODUCTION The wide range of applications of ionic liquids (ILs) depends, besides a melting transition at ambient temperatures, on their favorable properties such as negligible volatility, high chemical, and thermal stability, and in some cases hydrophobicity, high ionic conductivity, and electrochemical stability. Hence, ILs are not only used as “green” solvents for catalysis, chemical synthesis, and separations but also used for electrolyte applications such as the electrodeposition of electropositive metals, light-emitting electrochemical cells, photoelectrochemical cells, electrochemical capacitors, fuel cells, and batteries as well. Rechargeable lithium batteries are currently under development for use in portable electronic, telecommunication, and, especially, electric vehicle propulsion applications. The substitution of flammable and volatile organic solvents with ILs in traditional solvent-salt electrolyte mixtures, which is considered to be a promising approach for the realization of safer, larger batteries, is under investigation worldwide.1 The use of ILs in such a widespread field of applications may be strongly increased or reduced by the IL’s alkyl chain aggregation or nanoscale organization. The tail aggregation well-known for surfactants in solvents can also occur for ILs dissolved in solvents like water, organic solvents, or different r 2011 American Chemical Society
ILs. 27 Changes in the local environment due to aggregation of the IL without any solvent, even for alkyl chains as short as butyl, were already shown by MD simulations. 811 Wideangle X-ray scattering (WAXS) measurements 12,13 and neutron scattering 14 already showed that in the imidazolium- and piperidinium-based ILs local aggregation is present. NMR relaxation measurements provide evidence of the aggregation of alkyl chains in ILs with pyrrolidinium cation. 15 Furthermore, the influence of a dissolved lithium salt on the overall ion mobility and, especially, that of the lithium cation (Li+) is very important for lithium batteries. In this article, the effect caused by the introduction of an oxygen atom into the side chain of a pyrrolidinium cation on the local pyrrolidinium cation aggregation is investigated without and in the presence of a dissolved lithium salt. The effect of the ether side-chain introduction on the Li+ mobility, resulting from the interaction of the oxygen ion pairs with the small Li + cations, is also investigated. Received: June 15, 2011 Revised: August 23, 2011 Published: August 24, 2011 19431
dx.doi.org/10.1021/jp2055969 | J. Phys. Chem. C 2011, 115, 19431–19436
The Journal of Physical Chemistry C Scheme 1. Structure of the Four Investigated the Pyrrolidinium Cations (Pyr14+, Pyr15+, Pyr1[2O1]+, and Pyr1[2O2]+), the Deuterated Pyrrolidinium Cation (Pyr14(D3)+) for the NMR Measurements, and the TFSI Anion
’ EXPERIMENTAL SECTION Synthesis. The investigated ILs consist of the bis(trifluoromethanesulfonyl)imide (TFSI ) anion and either a N-alkyl-N-methyl-pyrrolidinium (Pyr1X) cation or a N-etherN-methyl-pyrrolidinium (Pyr1[mOm]) cation. The alkyl side chains are a butyl or a pentyl chain (Pyr14TFSI or Pyr15TFSI), and the ether side chains are a methoxyethyl or an ethoxyethyl group (Pyr1[2O1]TFSI and Pyr1[2O2]TFSI) (cf. Scheme 1). All ILs are synthesized via a method described in detail elsewhere.16 The chemicals N-methylpyrrolidine (97 wt %), 1-bromobutane (99 wt %), 1-bromopentane (99 wt %), methyl-2-chloro-ethyl ether (98 wt %), ethyl-2-chloro-ethyl ether (99 wt %), 1-bromo4-D3-butane (99 wt %), and ethylacetate (ACS grade, >99.5 wt %) were purchased from Aldrich and purified prior to use (with the exception of ethyl acetate) using activated carbon (Aldrich, Darco-G60) and alumina (acidic, Aldrich Brockmann I). Lithium bis(trifluoromethanesulfonyl)imide, LiTFSI (99.9 wt %, battery grade), was purchased from 3M and used as received. Deionized H2O was obtained using a Millipore ion-exchange resin deionizer. The N-alkyl-N-methylpyrrolidinium bromide precursors (Pyr1xBr) and N-ether-N-methyl-pyrrolidinium chloride precursors (Pyr1[2Om]Cl) were synthesized by reacting N-methylpyrrolidine with the appropriate amount of bromoalkyl/chloroether in the presence of ethyl acetate. The precursors were repeatedly rinsed with ethyl acetate to remove the reagent excess and the soluble impurities. The four ILs were obtained by reacting aqueous solutions of the precursors (Pyr1xBr or Pyr1[2Om]Cl) with the appropriate amount of the imide salt, LiTFSI. The lithium content in the ILs was tested to be
