Silylated quaternary ammonium salts-ionic liquids with hydrophobic cations

Article abstract of DOI:10.1039/c4ta02675f

Another class of ionic liquids was obtained by a combination of bis(perfluoroalkylsulfonyl)amide anions and O-silylated choline like cations. The protection of the hydrophilic hydroxyl group of hydroxyalkyl substituted quaternary ammonium salts with a trialkylsilyl group changes cations' properties from hydrophilic to hydrophobic; both constituents (cations and anions) of resulting ionic liquids have a hydrophobic character. Their essential physical and chemical properties were investigated. Moderate viscosity and conductivity of ionic liquids follow the Vogel-Tammann-Fulcher model of their dependency on the temperature. All ionic liquids show similar electrochemical stability as anticipated from CV measurements. The cathodic stability for most compounds is below 0 V vs. Li/Li⁺, and anodic decomposition starts typically over more than 5 V vs. Li/Li⁺. These properties suggest that these ionic liquids have potential for the preparation of electrolytes for high voltage lithium batteries. This journal is

Full text of DOI:10.1039/c4ta02675f

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Materials Chemistry A
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PAPER
Silylated quaternary ammonium salts – ionic liquids
with hydrophobic cations†
Cite this: J. Mater. Chem. A, 2014, 2,
15964
*
*
Matjaz Kozelj,‡ Abdelbast Guerfi and Karim Zaghib
ˇ
ˇ
Another class of ionic liquids was obtained by a combination of bis(perfluoroalkylsulfonyl)amide anions and
O-silylated choline like cations. The protection of the hydrophilic hydroxyl group of hydroxyalkyl
substituted quaternary ammonium salts with a trialkylsilyl group changes cations' properties from
hydrophilic to hydrophobic; both constituents (cations and anions) of resulting ionic liquids have a
hydrophobic character. Their essential physical and chemical properties were investigated. Moderate
viscosity and conductivity of ionic liquids follow the Vogel-Tammann-Fulcher model of their
dependency on the temperature. All ionic liquids show similar electrochemical stability as anticipated
from CV measurements. The cathodic stability for most compounds is below 0 V vs. Li/Li+, and anodic
decomposition starts typically over more than 5 V vs. Li/Li+. These properties suggest that these ionic
liquids have potential for the preparation of electrolytes for high voltage lithium batteries.
Received 28th May 2014
Accepted 30th July 2014
DOI: 10.1039/c4ta02675f
www.rsc.org/MaterialsA
exhibit high thermal,16 electrochemical (typically between 0 and
Introduction
5 V vs. Li/Li+) and chemical stabilities and should wet electrodes
properly. The rst problem is stability at low potentials
(cathodic stability), which is attributed mainly to the cations.
Electrochemical intercalation of lithium into graphite anodes in
1-ethyl-3-methylimidazolium (EMI)-based ionic liquids has
stimulated interest because these ionic liquids have low
viscosities and high conductivities. However, these ionic liquids
have narrow electrochemical windows (ꢀ4.2 V). Imidazolium
cations are prone to being reduced/decomposed at the elec-
trode/electrolyte interface when the (carbon) electrode is
polarised to 0.7 V vs. Li/Li+. The strong decomposition reaction
of the cations prevents the normal operation of the cell. In
contrast, tetraalkylammonium salts, including cyclic analogues
(i.e. piperidinium, morpholinium, pyrrolidinium and azepa-
nium), have wider electrochemical windows;1,10 some are stable
well below 0 V vs. Li/Li+, so research should focus on ILs con-
taining these fragments. The introduction of oxygen into alkyl
chains should improve some properties of such ether-contain-
ing ILs; in particular, it should lower their melting point, but it
should have deteriorating impact on their electrochemical
stability.17,18
However, electrochemical stability at high potentials is
connected with the oxidation of anions,10 so anions with high
resistance to oxidation should be used. Most stable are highly
uorinated anions, such as tetrauoroborates,19 hexa-
uorophosphates and peruorinated sulfonylamides.20 ILs
formed with peuorinated sulfonylamides, such as bis(tri-
uoromethanesulfonyl)amide (TFSI),21 bis(uorosulfonyl)-
amide (FSI)22 and (uorosulfonyl)(triuoromethanesulfonyl)-
amide (FTFSI),23 possess the best properties (high stability, wide
liquid range, low viscosity, and acceptable conductivity).
Ionic liquids (IL) are compounds with unique properties and
have been the subject of intense scientic and technological
research in recent decades.1 Their most important property is
their liquid state, which makes them appropriate for the
substitution of simple organic solvents in various processes,
such as organic synthesis,2,3 inorganic synthesis,4 bio-synthesis,5
separations6 and energy production/storage.7,8 Additional
features, such as very low vapour pressure, non-ammability,
good thermal stability, and a wide electrochemical window,
suggest their use as electrolytes in electrochemical systems,8–10
such as Li-ion batteries,11 dye-sensitised solar cells,12 electrical
capacitors,13 electrochromic devices,14 etc.
Most ILs are compounds consisting of organic cations (qua-
ternised nitrogen, phosphorus, tertiarised sulphur, etc.) and
relatively large inorganic anions (tetrauoroborate, per-
uoroalkyl-triuoroborate, triate, bis(triuoromethanesulfonyl)-
amide, bis(uorosulfonyl)amide, etc.). A signicant majority of
ILs contain quaternised nitrogen, also in a heterocyclic form
(imidazolium).
Existing ionic liquids, however, do not solve all the problems
associated with the manufacturing of electrochemical devices,15
especially high power lithium or lithium ion batteries. In
lithium batteries, several electrode materials are used; there-
fore, solvents or electrolytes for use with these materials should
´
´
Direction stockage et conversion d' energie, Institut de Recherche d'Hydro-Quebec,
Varennes, Quebec, Canada. E-mail: zaghib.karim@ireq.ca
† Electronic supplementary information (ESI) available: Detailed experimental
section and NMR spectra of compounds. See DOI: 10.1039/c4ta02675f
‡ Present address: Laboratory for Chemistry of Materials, National Institute of
Chemistry, Ljubljana, Slovenia. E-mail: matjaz.kozelj@ki.si
15964 | J. Mater. Chem. A, 2014, 2, 15964–15971
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Recently there have been some reports on ionic liquid elec- polymers (PTFEs). The prepared antistatic agents were mostly
trolytes based on bis(uorosulfonyl)imide (FSI) for rechargeable solid at room temperature,36 which suggests that it would be
Li batteries. In particular, FSI-based electrolytes containing Li interesting to study the inuence of hydrophobicity on the
ions exhibited practical ionic conductivity, and a natural electrochemical behaviour of ILs.
graphite/Li cell with FSI-based electrolytes containing Li bis-
Therefore, we report the systematic synthesis of a series of a
(triuoromethanesulfonyl)imide (LiTFSI) showed cycle perfor- new type of ILs, which are composed of tetraalkylammonium
mance without any solvent, using 1-ethyl-3-methylimidazolium cations with increased hydrophobicity (one alkyl chain is the
EMIm FSI and EMIm TFSI,24 and using ILs based on bis- (trialkyl-siloxy)ethyl group) and bis(peruorosulfonyl)amide
(uorosulfonyl)imide (FSI)25 as the anion and 1-ethyl-3-methyl- anions (TFSI, FSI, and FTFSI). Most of them are liquids at room
imidazolium and N-methyl-N-propylpyrrolidinium as cations. It temperature; they possess moderate viscosity, moderate
has also been observed that ILs with a TFSI anion cannot be conductivity and good to excellent electrochemical stability.
used alone with graphite electrodes, because only very low
capacities could be reached. The performance of such IL based
electrolytes was improved by the use of various SEI-layer form-
ing additives.26
Results and discussion
The synthesis of choline-like cations could be realised in three
However ILs were probed also as solvents in emerging
battery technologies revealing great potential for further devel-
opment.8 Ether functionalised ILs were used to prepare elec-
trolytes showing promising properties (stabilisation of
superoxide anions with IL cations) for the manufacture of Li–air
batteries.27 Recently, Nohira published electrolytes based on the
Na–FSA/Im12–FSA mixture, showing excellent stability and
conductivity combined with a reversible plating/dissolving on
GC and Al working electrodes. All those desired properties make
those mixtures excellent electrolytes for sodium based
batteries.28 But this also implicates that the development of
sodium batteries is mainly dependent on the manufacture of
stable high capacity Na-cathodes. On the other hand magne-
sium and other polyvalent cation battery technologies are at
their beginnings. The manufacture of stable magnesium elec-
trolytes was demonstrated to be difficult already in the molec-
ular solvents,29 the major obstacle being the passivation of Mg
and irreversibility of plating/dissolution on/from the working
electrode which was not solved yet by the use of ILs,8 but there
are still many possibilities to discover new approaches for the
preparation of suitable electrolytes which will facilitate the
development of Mg batteries.
ways, as proposed in Scheme 1. Methods A and B are the most
suitable for lab preparation, so these two were used in this
study. Method
A includes quaternisation of u-(N,N-dia-
lkylamino)alcohols with an alkylating agent, typically 1-bro-
moalkane; this yields a bromide salt as a product. It is
advantageous to carry out the synthesis at a low temperature
ꢁ
(20–40 C) in a polar solvent, such as acetonitrile or alcohols;
the product is isolated by precipitation in ethyl acetate and
subsequent ltration. The obtained product is in the form of a
colourless hygroscopic solid, which could be recrystallized from
an acetone–alcohol mixture. Applying higher temperatures and
other solvents during alkylation could result in signicant
discoloration of the product; this discoloration is very difficult
to remove without chromatographic methods. Method B is a
reaction between the tertiary amine and u-hydroxy-alkyl halo-
genide. In most cases only chloro derivatives are available, so
higher reaction temperatures are needed to obtain a good yield
of the product. In general, amines are highly volatile, so the use
of a pressure reactor is advisable. We heated both compounds
The hydrophobicity of most ionic liquids is ensured by the
use of highly uorinated anions; there have also been a few
reports on the preparation of hydrophobic cations containing
peruorinated alkyl chains.30,31
Choline-like compounds, possessing a 2-hydroxyethyl group,
are able to form deep eutectic mixtures,32 but are not suitable
for use in electrochemical devices with high operating voltages
because of the presence of labile acidic hydroxyl groups. The
methylation of hydroxyl groups in choline-like compounds may,
however, improve their stability.4,18,19 Another clue for a new
class of ILs was the improvement of the stability of various
oligoethylene glycols (for use as solvents in batteries), achieved
by the protection of the terminal hydroxyl group by various silyl
groups, such as the trimethylsilyl group.33 The preparation of
silylated choline compounds has been reported.34 The intro-
duction of siloxy groups onto various surfaces can make those
surfaces hydrophobic,35 so the same should also be true for
molecules. That concept was proved by Yamazaki, who prepared
ionic compounds containing trialkylsilyl moieties and sug-
Scheme 1 Synthetic possibilities for the preparation of choline-like
gested their use as antistatic agents for low surface energy quaternary ammonium salts.
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without a solvent in a Parr pressure reactor (316 SS) equipped and the mixture was homogenised in 30 min. The mixture was
with a stirring magnet on a stirring hot plate. The purity of the stirred and heated for few hours, aer which the volatiles were
ꢁ
products obtained by this method was much lower than with removed by drying in a high vacuum at 70 C, reaching a nal
Method A. The products were isolated by dissolution in meth- water content of less than 50 ppm in only eight hours. The ease of
anol, purication by charcoal, ltration and precipitation in dewatering of ILs is connected with a hydrophobic character of
ethyl acetate. In cases of 4-chlorobutanol and 2-chloroethyl-2- ILs, i.e. more hydrophobic ILs are dried more easily than
hydroxyethyl ether, the product was dissolved in water basied hydrophilic ones.19
with NaOH to remove amine hydrochloride from the product.
The newly prepared ILs are listed in Table 1 and consist of
Method C is only suitable for the preparation of hydroxyethyl four groups of compounds, each of them roughly illustrates the
compounds. Those compounds are obtained from the reaction inuence of a particular structural feature change. Group 1 (3a–
between a tertiary amine (or its salt) and ethylene oxide in an e, 3n, 3o and 3p) was obtained by variation of substitution on
appropriate reactor. This method is a major industrial prepa- nitrogen (including heterocycles pyrrolidine and imidazole),
ration of choline chloride, which is used as a food additive for Group 2 (3b and 3f–i) by variation of spacers between nitrogen
animals.37
and oxygen (including the chain containing two oxygen atoms),
The next two steps in the preparation of the silylated ILs are Group 3 by variation of substitution on silicon (3b, 3j and 3k)
presented in Scheme 2. Anion exchange (metathesis) was per- and Group 4 by variation of anions (3b, 3l and 3m).
formed in a water solution,38 because the products are not very
Compounds were characterised by 1H, 13C and 19F NMR. All
water soluble. The starting quaternary ammonium salt 1 was data are consistent with structures, and data on intermediate
dissolved in MQ water, and a solution of appropriate salt (Li or compounds could be found in the ESI.†
K) was added in one portion; in less than a minute, separation
into a two-phase system occurred. The mixture was further
Viscosity
stirred and then the organic phase was diluted with CH2Cl2,
The viscosity of ILs is determined principally by the electrostatic
separated and washed six times with fresh MQ water. Due to the
and van der Waals interaction forces between ions and sterical
factors, including the size, shape and exibility of the ions
hydrophilic hydroxyethyl group, some compounds were soluble
in water to some extent, when the ammonium salt contained
constituting the IL. Sample 3n was solid at RT, so no viscosity
longer alkyl substituents on nitrogen, lower solubility of the
measurement was done. Fig. 1 shows a graph showing the
dependence of viscosity on the temperature for various ILs
prepared in the frame of this work, compared with N1122O1-
nal product in water was observed and higher yields of the
desired product 2 were isolated. Pure ILs 2 were obtained aer
evaporation of the solvent and subsequent drying in a high
TFSI, which has the lowest viscosity on the chart; 3k has the
vacuum (<30 mTorr) at 60 ꢁC; moreover, it was observed that
lowest viscosity of prepared ILs while 3h has the highest
viscosity. It is interesting that the viscosity of 3b and 3h is not
vigorous stirring and purging with dry Ar (vacuum/rell/vacuum)
considerably accelerated the drying. These compounds did not
much higher compared with N1122O1-TFSI despite the intro-
dissolve in CDCl3 at high concentration, indicating the high
duction of the much bulkier trimethylsilyl and triethylsilyl
polarity of the compounds (for NMR DMSO-d6 was used).
groups in place of the methyl group, respectively.
Furthermore, during the extraction process of ILs from water
ꢁ
The viscosities vary from 107 mPa s to 430 mPa s at 25 C;
with CH2Cl2, it was observed that three layers were formed: a
the values of absolute viscosities could be calculated using
water phase, CH2Cl2 saturated with ILs, and an IL layer saturated
the Vogel-Tammann-Fulcher (VTF) equation (eqn (1) and
Table 2).39,40
with CH2Cl2. In order to further increase the hydrophobicity of
the obtained IL 2 and to ensure the electrochemical stability,
silylation using a silazane reagent (hexamethyldisilazane, dime-
thylamino-triethylsilane and diethylamino-ethyldimethylsilane)
was performed. In this manner, the only by-products are
volatile gasses/liquids, which can be easily removed by intense
evacuation combined with inert gas purging. Both reactants were
ꢂB
eTꢂT
0
1
pffiffiffiffi
¼ A
(1)
h
T
In eqn (1) h is the viscosity in Pa s, T is the temperature in K,
mixed without the solvent forming two immiscible layers; and A, B, and T0 are empirical coefficients. A is the pre-expo-
vigorous stirring was applied and the ne emulsion was slowly nential factor proportional to T1/2; B has a dimension of energy
heated under purging with N2. When the temperature reached and is related to the critical free volume for ion transport,
50–60 ꢁC, intense evolution of ammonia (amines) was observed, although it is not related to any simple activation process. T0 is
the temperature at which the transport function ceases to exist
or the liquid structural relaxation becomes zero and may be
considered to be the glass transition temperature.
One would expect that increasing the ion size should
increase the viscosity of the ionic liquid, but the results suggest
that in silylated ionic liquids this is not necessarily true. In our
class of ILs, we have three different sites on the cation, where
the size could be increased: groups on nitrogen, the length of
Scheme 2 Preparation of silylated ionic liquids.
15966 | J. Mater. Chem. A, 2014, 2, 15964–15971
the linker between the nitrogen-silyl group and substitution on
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Table 1 List of prepared silylated ionic liquids
IL no.
Abbreviation
R1
R2
R3
R4
R5
R6
R
ANI
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3na
3ob
3pc
N1112OTMS
N1122OTMS
N1222OTMS
N2222OTMS
N1132OTMS
N1123OTMS
N1124OTMS
N1126OTMS
N1122O2OTMS
N1122OEDMS
N1122OTES
N1122OTMS
N1122OTMS
PY12OTMS
Me
Me
Me
Et
Me
Me
Et
Me
Et
Et
Et
Pr
Et
Et
Et
Et
Et
Et
Et
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
CH2
CH2
CH2
CH2
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
TFSI
FSI
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Pyr
Im
MIm
CH2
(CH2)2
(CH2)3
(CH2)5
(CH2)2OCH2
CH2
CH2
CH2
CH2
CH2
Et
Me
Me
Me
Me
Me
FTFSI
TFSI
TFSI
TFSI
Im12OTMS
MIm12OTMS
3-CH2
3-CH2
a
b
c
N-(2-Trimethylsiloxyethyl)-N-methylpyrrolidinium TFSI. 1-(2-Trimethylsiloxyethyl)-3-methylimidazolium TFSI. 1-(2-Trimethylsiloxyethyl)-2,3-
dimethylimidazolium TFSI.
silicon. Different structural features have diverse inuences on
the viscosity of the corresponding ionic liquids.
The changing size of the ammonium cation does not inu-
ence the viscosity as drastically as the elongation of the spacer
between silicon and nitrogen. Surprisingly, the IL with the
biggest triethylsilyl group has the lowest viscosity; the same rule
can be observed in the case of different anions: the FSI IL is the
most viscous in this system, while the TFSI IL is the least
viscous. This feature may be a consequence of the decreased
symmetry of molecules, increased steric hindrance of cations
and high dispersion of charge density in anions, which prevent
strong inter-ionic bonding.
Conductivity
The ionic conductivity of the ILs is not clearly dependent on
their molecular mass but is well in accordance with viscosities.
Fig. 1 Viscosity of selected ionic liquids between ꢂ20 and 80 ꢁC (3b
was measured only above 10 ꢁC, due to solidification around 0 ꢁC).
Table 2 Table of coefficients in the Vogel-Tammann-Fulcher equa-
tion for dependence of absolute viscosity of prepared ILs versus
temperature
IL
A [K1/2 Paꢂ1 sꢂ1
]
B [K]
T0 [K]
h @ 25 ꢁC [Pa s]
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3p
2265 ꢃ 103
385.6 ꢃ 103
208.9 ꢃ 103
198.8 ꢃ 103
416.7 ꢃ 103
2017 ꢃ 103
1046 ꢃ 103
1264 ꢃ 103
896.4 ꢃ 103
1054 ꢃ 103
4260 ꢃ 103
570.2 ꢃ 103
1251 ꢃ 103
745.3 ꢃ 103
2084
1261
1014
984.4
1230
1768
1669
1758
1586
1773
1983
1550
1719
1453
82.94
136.8
164.7
171.2
147.5
128.5
130.1
128.4
121.4
98.00
103.4
112.7
108
0.122
0.111
0.165
0.202
0.146
0.287
0.340
0.430
0.152
0.115
0.107
0.129
0.116
0.193
Fig. 2 Dependence of conductivity of ionic liquids on the temperature
between RT and 90 ꢁC.
137.2
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On Fig. 2, conductivities of 3a–e versus temperature are plotted.
The conductivities for 3a–e vary from 1.44 mS cmꢂ1 to 1.10 mS
cmꢂ1 at 25 ꢁC, which is signicantly lower if compared with
data in ref. 19 on ILs containing quaternary ammonium cations
and TFSI, BF4 and RfBF3 anions. For example, 3b, which differs
from N1122O1-TFSI by the size of the OH blocking group (TMS
or Me, respectively), has more than two times lower conduc-
tivity. The values of conductivities could be calculated using the
Vogel-Tammann-Fulcher (VTF) equation (eqn (2) and Table 3).39
ꢂB
eTꢂT
0
pffiffiffiffi
k ¼ A
(2)
T
In eqn (2), k is conductivity in S cmꢂ1, T is temperature in K,
and A, B, and T0 are empirical coefficients, which resemble the
coefficient from eqn (1): A is a pre-exponential factor propor- Fig. 3 Linear sweep voltammograms of various ionic liquids.
tional to T1/2; B has a dimension of energy and is related to the
critical free volume for ion transport, although it is not related
to any simple activation process. T0 is the temperature at which
the transport function ceases to exist or the liquid structural
relaxation becomes zero, and may be considered to be the glass
transition temperature. However, A, B, and T0 obtained by
tting conductivity data are different from eqn (1); one would
expect that T0 values should be very similar, but actually they
differ signicantly, which is not yet understood but it has been
observed before for the solution of lithium electrolytes.39
windows (DE) are summarised in Table 4. In general, the anodic
stability of the IL is limited mostly by the decomposition of
anions; Eano shows some dependence on the structure of the
counter cation. Prepared ILs contain very stable peruorinated
sulphonamides, which exhibit excellent oxidation stability;20 so
the obtained results are in accordance with expectations: all ILs
are stable over 5 V vs. Li metal, and the majority are around 5.5
V. The cathodic stability of ILs is governed by the decomposi-
tion of cations, which are very complex in the present cases.
Studies on alkylated choline-like compounds have revealed that
alkyl ether chains on small quaternary ammonium compounds
are unfavourable for producing stable ILs.19 We imposed the
synthesis of silylated compounds in order to explore the effect
of the silicon protection group on electrochemical properties of
compounds obtained in that manner. Structurally diverse
cations were studied and the inuence of substitution on
reduction stability was examined. A silylated choline cation (3a)
Electrochemical stability
The electrochemical stability of ionic liquids is very important
for their use in electrochemical applications. We used standard
linear sweep voltammetry with platinum wire electrodes versus
metallic lithium working and counter electrodes, thus imitating
conditions in a typical lithium metal battery. On Fig. 3, linear
sweep voltammograms of the representative silylated ionic
liquids in comparison with methylated N1122O1-TFSI are
depicted. In general, silylation improves the cathodic stability of
ILs (3b and 3i); in the case of silylated choline, the stability is
not much improved (3a); the imidazolium IL undergoes
reduction at 1.21 V vs. Li. On platinum wire electrodes in the
rst scan at RT; scan rate: 1 mV sꢂ1, reference and counter
electrode: a sheet of lithium metal. The anodic and cathode
limits for each IL were arbitrarily dened as a current exceeding
the value of 0.03 mA. Cathodic (Ecat) and anodic (Eano) potential
limits versus Li metal and the corresponding electrochemical
Table 4 Cathodic (Ecat) and anodic (Eano) potential limits versus Li
metal and the corresponding electrochemical windows (DE) for
various ionic liquids determined with a Pt wire electrode
ILa
Ecat [V]
Eano [V]
DE [V]
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3o
1.54
<0
<0
5.56
5.6
4.02
>5.6
>5.44
>5.58
5.41
3.47
5.28
4.75
5.45
5.23
4.96
>5.51
>5.56
4.39
5.12
5.44
5.58
5.49
5.45
5.48
5.4
5.51
5.26
5.04
5.51
5.56
5.6
<0
0.08
1.98
0.2
0.65
0.06
0.03
0.08
<0
Table 3 Table of coefficients in the Vogel-Tammann-Fulcher equa-
tion for the dependence of absolute conductivity of selected ILs versus
temperature
IL
A [mS K1/2 cmꢂ1
]
B [K]
T0 [K]
k @ 25 ꢁC [mS cmꢂ1
]
<0
1.21
0.30
3a
3b
3c
3d
3e
4.35
3.71
4.23
3.87
3.66
604.0
568.1
576.8
553.7
556.4
182.7
184.6
189.6
193.4
192.47
1.35
1.44
1.21
1.14
1.10
N1122O1-TFSIa
5.52
a
N-Ethyl-N-2-methoxyethyl-N,N-dimethylammonium bis(triuoro-
methanesulfonyl)amide.
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is not stable due to insufficient steric shading of positive acetone, ethyl acetate (Fischer), 2-bromoethanol, 3-chlor-
nitrogen and starts to decompose at 1.54 V vs. Li. By changing opropanol, 3-chlorobutanol, 6-chlorohexanol, 2-chloroethyl-2-
one methyl group with an ethyl group, a very stable N-ethyl-N-(2- hydroxyethyl ether, diethyl ether, acetonitrile, diethylamine (Alfa
trimethylsiloxyethyl)-N,N-dimethylammonium (N1122OTMS, Aesar), (dimethylamino)triethylsilane, ethyldimethylchlorosilane
3b) cation was obtained (stable below 0 V vs. Li). By increasing (Gelest), LiTFSI (3 M), and triic amide (Rhodia). K-FSI,45
the substitution on nitrogen, other stable cations were obtained K-FTFSI23 and (diethylamino)ethyldimethylsilane46 were syn-
(3c–e); the stability was detrimentally affected by increasing the thesised according to published methods.
length of the ethyleneoxy linker between nitrogen and silicon;
1H, 13C and 19F NMR spectra were recorded using a 300 MHz
the propylene group caused the worst effect; 3f is reduced at Varian Mercury equipped with an SMS-100 sample changer and
1.98 V, while butylene and hexylene show moderate inuence a 500 MHz Varian VNMRS. D2O and DMSO-d6 were used as
increasing with the linkers' length. When a linker resembling a solvents for halides and DMSO-d6 for TFSI, FTFSI and FSI
short di(ethyleneoxide) chain is used (3i), the stability is not hydroxy compounds and CDCl3 was used for silylated ILs.
affected. An N1122O fragment was used to study the inuence Chemical shis are reported relatively to solvents peaks, and for
of various silyl groups on reduction stability; the slight variation 19F NMR uorobenzene was used as the internal reference
of methyl and ethyl groups on silicon does not affect stability, (ꢂ113.10 ppm).
which is in all three compounds nearly the same (3b, j and k).
The water content was determined by using a Mettler Toledo
An N1122OTMS cation was also used to study the inuence of V10 Compact Volumetric Karl-Fischer titrator. The water
different anions, but those showed practically no inuence on content in all investigated ILs was below 50 ppm.
cathodic reactions, so the reduction stability is the same for all
Viscosity (absolute) was measured using an Anton Paar
three compounds (3b, l and m). When the unsaturated hetero- Physica MCR 301 instrument equipped with a parallel-plate
cyclic imidazolium cation is used for the construction of the system (PP50-SN5204) with Peltier temperature control, which
silylated cation, an intense reduction reaction is observed at enabled measuring viscosity vs. temperature at constant shear
1.21 V vs. Li. By comparing the stabilities of silylated cations, we rate of dg/dt ¼ 500/s with a linear temperature increase.
can conclude that silylation of the OH group has a benecial Determination of coefficients in the Vogel-Tammann-Fulcher
effect on electrochemical stability when compared with methyl- equation has been done using curve tting (provided by Origin
ation.19 Increased stability may be s consequence of many 8.1 programme) on experimental data between ꢂ20 and 80 C.
ꢁ
factors but the most important is denitely the sterically
The conductivity of the neat ILs was measured between room
shielding of cations combined with the higher dispersion of temperature (ꢀ23 ꢁC) and 90 ꢁC using a MMulty Conductimeter
charge density. These results suggest that the majority of these made by Materials Mates Italia S.r.L in a sealed cell, consisting
ILs could be used as electrolytes in lithium batteries with a high of two parallel platinised Pt electrodes. The temperature
43
voltage cathodes (LiNiPO4,41 LiMnPO4,42 and Li(Mn, Ni, Co)Ox
)
program was very slow; measurement in the complete range was
as well as with metallic lithium or graphite anodes. For nished in 12 hours. Determination of coefficients in the Vogel-
compounds that are not stable below 1.4 V vs. Li, lithium titanate Tammann-Fulcher equation has been done using curve tting
could be used as the anode.44
(provided by Origin 8.1 programme) on experimental data.
Electrochemical stability was determined in a 5 ml beaker
The other point is the stability of the passivation lm, which
is composed of IL molecules and salt, which should occur with type three electrode cell having a Pt wire as the working elec-
these materials. It should be noted that, despite the range of trode, lithium metal (as a sheet) as the counter electrode and
electrochemical stability of ILs, there must be a chemical another sheet of lithium metal as the quasi reference electrode
affinity with the surface of the active material in order to obtain (QRE). Measurements were performed in a glovebox with
a long cycle life of the battery. This subject will be addressed in atmosphere containing less than 1 ppm oxygen and water.
further investigations.
Linear sweep voltammetry was performed using a VMP3
Moreover, the prepared ILs are also candidates for the Modular 16 Channels Potentiostat/Galvanostat/EIS, manufac-
preparation of sodium based electrolytes, but the properties like tured by BioLogic Science instruments. The CV curve was
solubility of appropriate sodium salt in such a system must be measured between 0 and 6 V vs. Li at a scan rate of 1 mV sꢂ1
checked, because sodium ions need a solvent with stronger from an open circuit potential to 6 V and back to 0 V.
solvation power in comparison with lithium. The low polarity of
,
the prepared IL which reects also in the hydrophobic character
may however prevent solubility of sodium salts. Also those
Experimental details
features need to be explored in the future.
For demonstration of the preparation of 3b: N-ethyl-N-(2-trimethyl-
siloxyethyl)-N,N-dimethylammonium bis(triuoromethanesul-
fonyl)amide (N1122-OTMS-TFSI, 3b) is given; this compound is
prepared in three steps:
Experimental section
General
N-Ethyl-N-2-hydroxyethyl-N,N-dimethylammonium bromide
The following reagents were used as received: choline chloride, (1b). In a 500 ml round-bottom ask equipped with a magnetic
2-(dimethylamino)ethanol, 2-(diethylamino)ethanol, N-methyl- stirrer were placed 45.3 g (0.508 mol) of 2-dimethylaminoetha-
pyrrolidine, hexamethyldisilazane, CH2Cl2, bromo-ethane, nol dissolved in 150 ml of MeCN. To this solution, a mixture of
1-bromopropane, triethylamine (from Aldrich), methanol, 61.5 g (0.550 mol) ethyl bromide and 60 ml of MeCN was added
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Journal of Materials Chemistry A
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dropwise over a period of 1.5 h using a water bath (at 20 ꢁC) for group, which suggests that silylation of the OH group makes
cooling during the addition. Aer half of the EtBr was added, a compounds more cathodically stable. The most stable
snow white crystalline product started to precipitate from the compounds are stable below 0 V vs. Li. As anions, highly elec-
solution. The mixture was stirred over a weekend (57 h), and trochemically stable FSI, TFSI and FTFSI were used, which
then vacuum ltered, washed with a small amount of acetone, yielded ILs with anodic stability over 5 V vs. Li metal. Those
and dried in a vacuum oven at 60 ꢁC. The ltrate was evaporated properties are highly desirable for any liquids that are used for
to dryness and an additional product was isolated. Altogether, the preparation of electrolytes for electrochemical devices,
98.13 g (72%) of N-ethyl-N-2-hydroxyethyl-N,N-dimethylammo- especially for high energy density Li or Li-ion devices with a
nium bromide were obtained.
high operating voltage, where electrolytes are simultaneously
N-Ethyl-N-2-hydroxyethyl-N,N-dimethylammonium bis(tri- and constantly exposed to low and to high voltages. The
uoromethanesulfonyl)amide (2b). In a 250 ml round-bottom majority of the prepared ILs satisfy these demands so we hope
ask, solutions of 52 g (0.263 mol) of N-ethyl-N-2-hydroxyethyl- that these compounds will contribute to the fabrication of
N,N-dimethylammonium bromide in 70 ml MQ water and 78 g better lithium (and sodium) batteries.
(0.274 mol) of LiTFSI in 80 ml MQ water were mixed together
under vigorous stirring. Phase separation occurred at once, but
stirring was continued for another 4 hours at room tempera-
ture. Then, 100 ml of CH2Cl2 were added and the phases
Acknowledgements
separated. The water phase was extracted with 50 ml of CH2Cl2 The authors thank V. Gariepy and J. Trottier for the technical
´
and the combined organic phases were washed 6 times with help. M.K. thanks Institut de Recherche d'Hydro-Quebec/McGill
100 ml of MQ water. A clear colourless solution was obtained University for generous postdoctoral grant.
and poured into a round-bottom ask. The solvent was removed
using a rotary evaporator and then under high vacuum at 60 ꢁC.
In this manner, 70 g (67%) of pure N-ethyl-N-2-hydroxyethyl-
N,N-dimethylammonium bis(triuoromethanesulfonyl)amide
Notes and references
as a colourless liquid were obtained.
1 P. Wasserscheid and T. Welton, Ionic liquids in synthesis,
Wiley-VCH, Weinheim, 2nd edn, 2008.
2 T. Welton, Chem. Rev., 1999, 99, 2071–2084.
3 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576.
4 T. Alammar, H. Noei, Y. Wang and A.-V. Mudring, Nanoscale,
2013, 5, 8045–8055.
5 F. J. Hernandez-Fernandez, A. Rios, L. J. Lozano-Blanco and
C. Godinez, J. Chem. Technol. Biotechnol., 2010, 85, 1423–
1435.
6 L. B. Escudero, A. C. Grijalba, E. M. Martinis and
R. G. Wuilloud, Anal. Bioanal. Chem., 2013, 405, 7597–7613.
7 S. Zhang, J. Sun, X. Zhang, J. Xin, Q. Miao and J. Wang, Chem.
Soc. Rev., 2014, DOI: 10.1039/C3CS60409H.
N-Ethyl-N-(2-trimethylsiloxyethyl)-N,N-dimethylammonium
bis(triuoromethanesulfonyl)amide (3b). To a 250 ml round-
bottom ask containing 80 g (0.20 mol) of neat N-ethyl-N-2-
hydroxyethyl-N,N-dimethylammonium TFSI, 31 g (0.20 mol) of
hexamethyldisilazane (HMDS) were added at room temperature
as a gentle stream of nitrogen was passed through the apparatus
to facilitate the removal of forming ammonia. The mixture was
slowly heated to 60–70 ꢁC and stirred so that a ne emulsion of
HMDS in ILs formed. A vigorous evolution of gaseous ammonia
started as the temperature reached 60 ꢁC and ended aer a few
minutes. The mixture was heated and stirred for an additional
four hours aer the end of the vigorous reaction. Then, the
remaining HMDS, which was in a separate layer on top of the
desired product, was evaporated under high vacuum. The
round-bottom ask was then relled six times with argon and
8 D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle,
P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe,
P. Simon and C. A. Angell, Energy Environ. Sci., 2014, 7,
232–250.
ꢁ
again evacuated. The product was heated to 70 C during this
procedure. Finally, the apparatus was cooled down under
vacuum and relled with argon. In this manner, 94 g (quant.) of
9 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and
B. Scrosati, Nat. Mater., 2009, 8, 621–629.
the title compound in the form of a colourless liquid were 10 H. Ohno, Electrochemical aspects of ionic liquids, Wiley,
obtained.
Hoboken, N.J, 2nd edn, 2011.
11 B. Garcia, S. Lavallee, G. Perron, C. Michot and M. Armand,
Electrochim. Acta, 2004, 49, 4583–4588.
Conclusion
¨
12 P. Wang, S. M. Zakeeruddin, J.-E. Moser and M. Gratzel, J.
A new family of low melting ionic liquids with hydrophobic
Phys. Chem. B, 2003, 107, 13280–13285.
´
cations and anions were prepared by silylation of choline-like 13 R. Lin, P.-L. Taberna, S. Fantini, V. Presser, C. R. Perez,
(u-hydroxyalkyl-trialkylammonium) quaternary ammonium
salts possessing hydroxyl groups. The inuence of substitution
F. Malbosc, N. L. Rupesinghe, K. B. K. Teo, Y. Gogotsi and
P. Simon, J. Phys. Chem. Lett., 2011, 2, 2396–2401.
on nitrogen, the length of the spacer between the nitrogen and 14 A. Brazier, G. B. Appetecchi, S. Passerini, A. S. Vuk, B. Orel,
hydroxyl group and various silyl groups on their physico-
chemical properties were explored. All but one are liquid at
F. Donsanti and F. Decker, Electrochim. Acta, 2007, 52,
4792–4797.
room temperature. These ILs exhibit better cathodic stability 15 A. Brandt and A. Balducci, J. Power Sources, 2014, 250, 343–
than their analogues that have an OH group blocked by a methyl
351.
15970 | J. Mater. Chem. A, 2014, 2, 15964–15971
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
16 L. Chancelier, A. O. Diallo, C. C. Santini, G. Marlair, T. Gutel, 31 Y. R. Mirzaei, B. Twamley and J. n. M. Shreeve, J. Org. Chem.,
S. Mailley and C. Len, Phys. Chem. Chem. Phys., 2014, 16,
1967–1976.
17 W. A. Henderson, J. V. G. Young, D. M. Fox, H. C. De Long
and P. C. Trulove, Chem. Commun., 2006, 3708–3710.
18 N. Terasawa, S. Tsuzuki, T. Umecky, Y. Saito and
H. Matsumoto, Chem. Commun., 2010, 46, 1730–1732.
2002, 67, 9340–9345.
32 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and
V. Tambyrajah, Chem. Commun., 2003, 70–71.
33 L. Zhang, L. Lyons, J. Newhouse, Z. Zhang, M. Straughan,
Z. Chen, K. Amine, R. J. Hamers and R. West, J. Mater.
Chem., 2010, 20, 8224–8226.
19 Z.-B. Zhou, H. Matsumoto and K. Tatsumi, Chem.–Eur. J., 34 E. Lukevics, L. Liberts and M. G. Voronkov, Russ. Chem. Rev.,
2005, 11, 752–766. 1970, 39, 953–963.
20 M. Ue, A. Murakami, K. Omata and S. Nakamura, Book of 35 A. Vilcnik, I. Jerman, A. Surca Vuk, M. Koˇzelj, B. Orel,
ˇ
ˇ
ˇ ˇ
ˇ ˇ
ˇ
Abstracts of 41st Battery Symposium in Japan, 2000, pp. 292–
B. Tomsic, B. Simoncic and J. Kovac, Langmuir, 2009, 25,
293.
5869–5880.
21 M. Armand, US 5072040, 1991.
36 S. Yamazaki, M. Torii and M. Usui, JP 2010095473, 2010.
ˇ
´
ˇ
ˇ´
´
22 M. Beran, J. Prıhoda, Z. Zak and M. Cernık, Polyhedron, 2006, 37 H. C. Klein and W. A. Di Salvo, US 2,870,198, 1959.
25, 1292–1298.
38 G. B. Appetecchi, S. Scaccia, C. Tizzani, F. Alessandrini and
S. Passerini, J. Electrochem. Soc., 2006, 153, A1685–A1691.
39 G. Y. Gu, S. Bouvier, C. Wu, R. Laura, M. Rzeznik and
K. M. Abraham, Electrochim. Acta, 2000, 45, 3127–3139.
40 C. Seeton, Tribol. Lett., 2006, 22, 67–78.
ˇ´
23 M. Beran, J. Prıhoda and J. Taraba, Polyhedron, 2010, 29,
991–994.
24 M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko and M. Kono,
J. Power Sources, 2006, 162, 658–662.
25 A. Guer, S. Duchesne, Y. Kobayashi, A. Vijh and K. Zaghib, J. 41 C. M. Julien, A. Mauger, K. Zaghib, R. Veillette and H. Groult,
Power Sources, 2008, 175, 866–873. Ionics, 2012, 18, 625–633.
26 N. Wongittharom, C.-H. Wang, Y.-C. Wang, G. T.-K. Fey, 42 K. Zaghib, M. Trudeau, A. Guer, J. Trottier, A. Mauger,
H.-Y. Li, T.-Y. Wu, T.-C. Lee and J.-K. Chang, J. Power
Sources, 2014, 260, 268–275.
R. Veillette and C. M. Julien, J. Power Sources, 2012, 204,
177–181.
´
27 H. Nakamoto, Y. Suzuki, T. Shiotsuki, F. Mizuno, S. Higashi, 43 D. Liu, J. Hamel-Paquet, J. Trottier, F. Barray, V. Gariepy,
K. Takechi, T. Asaoka, H. Nishikoori and H. Iba, J. Power
Sources, 2013, 243, 19–23.
28 K. Matsumoto, T. Hosokawa, T. Nohira, R. Hagiwara,
P. Hovington, A. Guer, A. Mauger, C. M. Julien,
J. B. Goodenough and K. Zaghib, J. Power Sources, 2012,
217, 400–406.
´
A. Fukunaga, K. Numata, E. Itani, S. Sakai, K. Nitta and 44 A. Guer, M. Dontigny, P. Charest, M. Petitclerc, M. Lagace,
S. Inazawa, J. Power Sources, 2014, 265, 36–39. A. Vijh and K. Zaghib, J. Power Sources, 2010, 195, 845–852.
ˇ´
29 H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman, 45 M. Beran and J. Prıhoda, Z. Anorg. Allg. Chem., 2005, 631, 55–
A. E. Rodnyansky, A. G. Oliver, W. C. Boggess and
J. Muldoon, Nat. Commun., 2011, 2, 427.
59.
´
´
46 K. Szabo, N. L. Ha, P. Schneider, P. Zeltner and E. s. Kovats,
30 T. L. Merrigan, E. D. Bates, S. C. Dorman and J. H. Davis Jr,
Chem. Commun., 2000, 2051–2052.
Helv. Chim. Acta, 1984, 67, 2128–2142.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 15964–15971 | 15971