Highly conductive electrolytes derived from nitrile solvents

Article abstract of DOI:10.1149/2.0701507jes

A series of new low molecular weight nitrile solvents has been designed and synthesized for secondary lithium battery electrolytes. Lithium bis(trifluoromethylsulonyl) imide (LiTFSI) salt concentration and temperature dependent ionic conductivity, electrochemical stability and thermal properties have been studied. The best ionic conductivity, 12.9 × 10^-3 S/cm at 25°C, was observed with 1.60M LiTFSI in 2,2-bis(hydroxymethyl)-1,3-propanedi-2-ethylcyano solvent. Both electrolytes and neat solvents displayed freezing point below -80°C.

Full text of DOI:10.1149/2.0701507jes

A1276
Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
0
013-4651/2015/162(7)/A1276/6/$33.00 © The Electrochemical Society
Highly Conductive Electrolytes Derived from Nitrile Solvents
z
Qiang Ma and Braja K. Mandal
Department of Biological and Chemical Science, Illinois Institute of Technology, Chicago, Illinois 60616, USA
A series of new low molecular weight nitrile solvents has been designed and synthesized for secondary lithium battery electrolytes.
Lithium bis(trifluoromethylsulonyl) imide (LiTFSI) salt concentration and temperature dependent ionic conductivity, electrochemical
−
3
◦
stability and thermal properties have been studied. The best ionic conductivity, 12.9 × 10 S/cm at 25 C, was observed with 1.60 M
LiTFSI in 2,2-bis(hydroxymethyl)-1,3-propanedi-2-ethylcyano solvent. Both electrolytes and neat solvents displayed freezing point
◦
below -80 C.
©
2015 The Electrochemical Society. [DOI: 10.1149/2.0701507jes] All rights reserved.
Manuscript submitted February 11, 2015; revised manuscript received April 3, 2015. Published April 16, 2015.
Organic carbonate-based electrolytes are now widely used in com-
(17.095 mmol) of 2-[2-(dimethylamino)ethoxy]ethanol and 1.4626 g
(27.565 mmol) of acrylonitrile were placed in a 50 ml round bottom
flask (RBF) in an ice water bath and stirred. 0.3 ml of tetramethylam-
1–7
mercial Li-ion batteries. A mixture containing 1 molar equivalent
of lithium bis(trifluoromethylsulonyl) imide (LiTFSI) in 1:1 ethylene
carbonate (EC) and dimethyl carbonate (DMC) is widely used as the
standard, because the resulting electrolyte displays ionic conductivity
2
momium hydroxide (TMAH) (25 wt% in H O solution) was added
drop wise to the mixture. After one day stirring at room tempera-
ture (RT), 5.5 ml of 0.1 M HCl solution was added to the mixture.
Dichloromethane (DCM) was used to extract (3X) the product. Af-
−
3
8–10
in excess of 8 × 10 S/cm.
However, for large-scale applications
(
e.g., electric vehicles), a serious disadvantage of using EC/DMC sys-
tem is the limited operating temperature range. The batteries derived
from EC/DMC electrolytes display poor performance at subzero tem-
peratures, because the ionic conductivity drops markedly below 2 ×
2 4
ter drying the extract over Na SO , the solution was eluted through
silica gel using ether and DCM (4:1) as the eluent. Solvent was
◦
rotovaped and the product was dried under high vacuum at 60 C.
−
3
11
10
S/cm, which makes the battery nonfunctional. New solvents
Yield: ∼57%. FTIR: nitrile group (–CN) stretching absorption at
◦
−1
that possess melting point lower than –50 C, boiling point in excess
2250.0 cm , no alcohol group (–OH) stretching absorption at
◦
−1
of 150 C, good solubility of lithium salt (>1 M), low viscosity and
∼3500 cm . NMR (300 MHz, CDCl
3
): δ 3.6 (m), 2.5 (t), 2.4 (t),
chemically inert to electrode materials would be a good alternative to
EC/DMC-based electrolytes.
2.2 (s) ppm.
2
a by cyanoethylation of 4-(2-hydroxyethyl) morpholine.—2.2327
In recent years, intensive efforts have been made to introduce new
solvents, leading to improvements in the performance of electrolyte
g (17.0214 mmol) of 4-(2-hydroxyethyl) morpholine and 2.2740
g (42.8571 mmol) acrylonitrile were placed in a 50 ml RBF in
an ice water bath and stirred. 0.2 ml of TMAH was added drop
wise to the mixture. After one day stirring at RT, 4.5 ml of 0.1
M HCl solution was added to the mixture. DCM was used to ex-
1
2–16
systems. The most notable categories include: glymes,
organo
17,18
19
20
sulfone ethers,
fluorinated ethers, organo phosphates and gly-
col borate esters. Several nitrile compounds have also been tested
21
22–26
for applications in Li-ion battery electrolytes.
Among these mate-
2 4
tract (3X) the product. After drying the extract over Na SO , the
rials, glutaronitrile (GLN) and adiponitrile (ADN)-based electrolytes
have been studied most, because of their high thermal (high boiling
point and flash point) and better physical (high dielectric constant
solution was eluted through silica gel using ether and DCM (1:1)
as elute solvent. Solvent was rotovaped and the product was dried
◦
under high vacuum at 60 C. Yield: ∼63%. FTIR: –CN stretching
24
and low viscosity). However, their low ionic conductivity values
−1
absorption at 2250.4 cm , no alcohol group stretching absorption
◦
(
3.6 mS/cm and 1.8 mS/cm at 25 C for GLN and ADN, respectively)
−1
at ∼3500 cm . NMR (300 MHz, CDCl
3
): δ 3.6 (m), 2.6 (m),
24,26
prevent their widespread use.
2
.4 (s) ppm.
In this study, we have designed several new solvents that have
potential to exhibit most of the aforementioned desired characteris-
tics. In this paper, we present synthesis and properties of a series
of new electrolytic solvents that contain aminoether and/or nitrile
groups.
3
a by cyanoethylation of N-methyldiethanolamine.—2.2177 g (18.611
mmol) of N-methyldiethanolamine and 2.7331 g (51.510 mmol) of
acrylonitrile were placed in a 50 ml RBF in an ice water bath and
stirred. 0.7 ml of TMAH was added drop wise to the mixture. After
one day stirring at RT, 6 ml of 0.1 M HCl solution was added to
the mixture. The product was isolated similarly as 2a. Yield: ∼74%.
−
1
1
Experimental
FTIR: –CN stretching absorption at 2242.2 cm , no alcohol group
−
(
–OH) stretching absorption at ∼3500 cm . NMR (300 MHz,
CDCl ): δ 3.7 (t), 3.5 (t), 2.6 (t), 2.3 (s) ppm.
a by cyanoethylation of 2-tert-Butylamino ethanol.—3.1484 g (26.87
Materials.— N-methyldiethanolamine,
2-[2-(dimethylamino)
3
ethoxy]ethanol, 4-(2-hydroxyethyl) morpholine, N,N,N,N-tetrakis (2-
hydroxypropyl) ethylenediamine, dipentaerythritol, pentaerythritol,
4
mmol) of 2-tert-butylamino ethanol, 5.7843 (109.01 mmol) of acry-
lonitrile, 0.8 ml TMAH and 2 ml water were placed in a 50 ml
2
-tert-butylamino ethanol, triethanol amine, tetramethylammomium
hydroxide (25 wt% in H O solution), acrylonitrile were purchased
2
◦
RBF, and stirred from 0 C to RT for 2 days. 6.5 ml of 0.1 M HCl
solution was added to the mixture after the reaction. The prod-
from Sigma-Aldrich Company and used as received without further
purification. Lithium bis(trifluoromethylsulonyl) imide (LiTFSI)
was obtained from 3 M. All the solvents were obtained from Fisher
Scientific Company. Commercially available polypropylene separator
membranes (Celgard 2400) were used exclusively in this study. Key
uct was isolated similarly as 2a. FTIR: −CN stretching absorption
−
1
at 2250.3 cm , no alcohol group (–OH) stretching absorption at
−
1
∼
3500 cm . NMR (300 MHz, CDCl
3
): δ 3.6 (t), 3.4 (t), 2.7 (t), 2.6 (t),
1
.0 (s) ppm.
parameters include: thickness 25 μm, porosity 41%, ionic resistivity
2
5a by cyanoethylation of triethanol amine.—15.5415 g (0.104 mol)
of triethanol amine, 28.7204 g (0.541 mol) of acrylonitrile, and
2
.55 ꢀ/cm and tensile strength 140 kgf/cm .
5
ml water were placed in a 100 ml RBF. 3.5 ml TMAH was
Synthesis of low molecular weight nitrile solvents.—1a by
cyanoethylation of 2-[2-(dimethylamino)ethoxy]ethanol.— 2.2769 g
◦
added drop wise and the mixture was stirred from 0 C to RT for
2
days. 0.1 M HCl solution was added to the mixture. The prod-
uct was isolated similarly as 2a. FTIR: −CN stretching absorption
zE-mail: mandal@iit.edu
−1
at 2250.3 cm , no alcohol group (−OH) stretching absorption at

Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
A1277
Table I. Formulations of different concentrations of LiTFSI in varied electrolyte solvents.
Solvent
1a
2a
3a
4a
5a
6a
1b
2b
Concentrations of LiTFSI in different solvents (mol/L)
1.62
1.37
1.18
1.02
0.80
0.61
1.50
1.27
1.11
-
1.78
1.53
1.20
-
1.76
1.38
1.00
-
2.10
1.88
1.61
1.39
1.16
2.04
1.60
1.19
-
1.89
1.61
1.30
-
1
1
.33
.04
-
-
-
-
-
-
-
∼
3500 cm−1. NMR (300 MHz, CDCl
.0 (s) ppm.
a by cyanoethylation of N,N,N,N-tetrakis (2-hydroxypropyl)
3
): δ 3.6 (t), 3.4 (t), 2.7 (t), 2.6 (t),
placed in a 50 ml RBF. 0.8 ml TMAH was added drop wise to the
solution, and the mixture was stirred from 0 C to RT for 4 days.
◦
1
0
.1 M HCl solution was added to the mixture. The isolated procedure
6
−
1
was similar to 2a. FTIR: –CN stretching absorption at 2251.4 cm , no
–
δ 3.7 (t), 3.4 (s), 2.6 (m) ppm.
ethylenediamine.—6.5466 g (22.39 mmol) of N,N,N,N-tetrakis (2-
Hydroxypropyl) ethylenediamine and 9.7678 g (184.09 mmol) of
acrylonitrile were placed in a 50 ml RBF in an ice water bath and
stirred. 1.8 ml of TMAH was added drop wise to the mixture. After
four days stirring at RT, 10.0 ml of 0.1 M HCl solution was added to
the mixture. DCM and water were used to extract (3X) respectively.
−
1
OH stretching absorption at ∼3500 cm . NMR (300 MHz, CDCl
3
):
Preparation of electrolytes.— As depicted in Table I, several elec-
trolytes containing different concentrations of LiTFSI were prepared.
A separator was dipped into each electrolyte solution. The soaked
separator was ready to be used as a liquid electrolyte sample. Ionic
conductivities, electrochemical stabilities and thermal properties of
each electrolyte solutions were tested.
2 4
After drying the extract over Na SO , the solution was eluted through
silica gel using ether and DCM (6:4) as elute solvent. Solvent was
rotovaped and the product was dried under high vacuum at 60 C.
◦
−
1
Yield: ∼82%. FTIR: –CN stretching absorption at 2250.6 cm , no
−1
–
OH stretching absorption at ∼3500 cm . NMR (300 MHz, CDCl
δ 3.7–3.5 (m), 2.6 (t), 2.4 (d), 1.1 (d) ppm.
b by cyanoethylation of pentaerythritol.—1.0208 g (7.50 mmol)
3
):
Ionic conductivity measurements.— Ionic conductivities for liq-
◦
uid electrolytes were determined from room temperature to 70 C
1
by means of complex impedance measurements using a computer-
controlled Solartron impedance analyzer over the frequency range
of 100 kHz to 0.1 Hz. The samples were sealed and sandwiched
in a Hohsen Cell blocking electrodes and placed in a temperature-
controlled oven.
of pentaerythritol, and 4.1244 g (77.73 mmol) of acrylonitrile were
placed in a 50 ml RBF. 0.5 ml TMAH was added drop wise to the mix-
ture and the solution was stirred from 0 C to RT for 4 days. 0.1 M HCl
◦
solution was added to the mixture. The isolated procedure was similar
−
1
to 2a. FTIR: –CN stretching absorption at 2251.4 cm , no alcohol
−1
group (–OH) stretching absorption at ∼3500 cm . NMR (300 MHz,
CDCl ): δ 3.7 (t), 3.4 (s), 2.6 (m) ppm.
b by cyanoethylation of dipentaerythritol.—1.9947 g (7.84 mmol) of
Differential scanning calorimetry (DSC) measurements.— DSC
data of all samples were obtained using a Mettler differential scanning
calorimeter with liquid nitrogen as cooling element. The samples were
3
2
dipentaerythritol, and 5.9967 g (113.02 mmol) of acrylonitrile were
Scheme 1. Synthesis of ‘a’ group solvents.

A1278
Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
Scheme 2. Synthesis of ‘b’ group solvents.
◦
◦
scanned from −100 C to 150 C. Two segments of DSC scanning
lithium ions between ligands rather than favor chelation with a single
ligand.
◦
included: 1) −100 C isothermal for 10.0 min with Argon purge at 70.0
◦
◦
ml/min, 2) −100∼150 C heating at 10.0 C/min with Argon purge at
7
0.0 ml/min. An empty aluminum pan was used as a reference.
Ionic conductivity studies.— In general, ionic conductivity in-
creases with the increase in salt concentrations due to increase in
number of charge carriers. After the maximum conductivity is reached,
any increase in salt concentrations promotes higher ion aggregation
and higher viscosity of the solution, which simultaneously reduces
both the free-ion number and the ionic mobility, leading to a drop in
Results and Discussion
Electrolytes containing nitrile groups have several advantages. For
31
conductivity. 2–V34iscosity is related to conductivity (σ) via the follow-
example, the nitrile group containing compounds possess high di-
electric constant, due to high polarity of the group. Electrolytes hav-
ing high dielectric constant favor high ionic dissociation. Moreover,
nitrile-based solvents are reported to dissolve high degree (>1 M)
of lithium salts. Both of these attributes are known to enhance ionic
3
ing equation.
q
i
2c
6πμR0
i
σ =
[1]
2
7,28
conductivity of electrolytes.
In this study, all nitrile solvents have been grouped into two
categories: group “a” belongs to aminoether-based compounds
Where q
i
is the charge of the solute, c
i
is the ion concentration, μ
is the viscosity and R is the spherical radius of the solvated ion.
0
(
Scheme 1), while group “b” contains two ether-based compounds
The location of peak point depends on the dielectric constant of the
solvent. Generally speaking, a higher dielectric constant would shift
the occurrence of ion pairing to a higher salt concentration.8
(Scheme 2). Among these two groups, the former has unique advan-
tage of promoting ionic conductivity by structural transport via the
hemi-labile effect (Figure 1). In general, chelates form more stable
complexes than to mono-dentate ligands. This is due to the fact that
when one coordination site is detached, the entire ligand does not “fall
off”, and is, instead, still attached by the other coordination site.29,30
When both coordination sites are chemically equivalent, it is equally
likely for each of the complexes to break off, which, in effect, stabilizes
the chelate. In hemi-labile ligands, such as amino-ethers, the coordi-
nation sites are chemically nonequivalent—one site is weaker than the
other, i.e., oxygen binds more strongly than nitrogen. These will form
more stable complexes than two monodentate ligands, but will allow
much faster ligand exchange as compared to chemically equivalent
chelates. The increase in the rate of ligand exchange results from the
substitution being directed by a ‘trans effect’-like mechanism, which
effectively makes it favorable for the oxygen of another ligand to be
substituted in the position trans to the nitrogen (Figure 1). By directing
ligand substitution in this way, the ionic conductivity will be improved
as a result of allowing the lithium ions to be structurally transferred
between two ligands to the same degree that they are vehicularly
transferred by dragging with the ligands attached during conduction.
The net effect is a system that kinetically favors the exchange of
The electrolytes, shown in Table I, were tested to find out the op-
timized LiTFSI concentration of each solvent system with reference
to providing highest ionic conductivity. Figures 2 and 3 show that the
temperature dependence of ionic conductivity of each solvent under
different LiTFSI concentrations. The maximum conductivity of each
product at room temperature is listed in Table II along with the op-
timized concentrations. Among all compounds we prepared, only 2a
displayed the conductivity maximum at under 1.0 M LiTFSI concen-
tration. All other compounds required at least 1.5 M concentration
to reach the conductivity maximum. For example, 2b showed con-
ductivity maximum at 1.89 M. Therefore, we may conclude that 2a
has the least dielectric constant, while 2b has the highest dielectric
constant and others are in between. From the structural perspective,
2a has a linear backbone with a nitrile group in one end and the
other end contains a six-member ring, which is preferred to stay in
“chair conformation”. This conformation does not favor the lithium
ion solvation, which has the lower flexibility and reduces the permit-
tivity, resulting in decrease in solubility of lithium salts. By contrast,
2b, because of the presence of maximum number of nitrile groups
(six), possesses a highest dielectric constant, thus increasing both the
N
O
O
N
O
O
N
O
O
Facile Structural
Transport of
Li ion
Kinetically
Thermodynamically
Favored Path
Li
Li
Li
Favored Path
N
N
N
Figure 1. Fast lithium ion structural transport via a ligand exchange mechanism involving a hemi-labile amino-ether ligand.

Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
A1279
Figure 2. Ionic conductivity versus temperature of different LiTFSI concentrations in a group solvents.
respectively).24 An attempt has also been made to improve the con-
ductivity of GLN and ADN solvent systems by mixing with alkyl
carbonates (e.g. EC, DMC, etc.); however, the ionic conductivity val-
ues of these mixtures were only marginally improved (e.g., 3.8 mS/cm
for 1:1 v/v EC:ADN electrolyte containing 1 M LiTFSI and 4.9 mS/cm
solubility lithium salt and shifting the optimized concentration to a
higher point.
Figure 4 demonstrates that the ionic conductivities of best formu-
lations for each solvent system. Apparently, from all of the above
measurements and analysis, we conclude that the best ionic conduc-
tivity was displayed by 1b at 1.60 M LiTFSI solution, which was
26,29
when GLN was combined with EC at the same ratio).
It is im-
1
2.9 mS/cm at room temperature. As a single electrolyte solvent, the
portant to stress that these conductivity numbers are much less than
what we achieved with our single solvent system, especially with 1b.
Other good nitrile-based electrolytes include: 1.62 M of 1a and 1.78 M
conductivity of 1b is 3 to 6 times higher compared to reported con-
ductivities of GLN and ADN at 25 C (3.6 mS/cm and 1.8 mS/cm,
◦

A1280
Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
Figure 3. Ionic conductivity versus temperature of different concentrations of LiTFSI in b group solvents.
Table II. Best ionic conductivities given by certain LiTFSI concentration of each product.
Solvent
1a
2a
3a
4a
5a
6a
1b
2b
LiTFSI Concentration (M)
Ionic Conductivity (mS/cm at 25 C)
1.62
9.97
0.80
5.68
1.50
8.15
1.78
9.04
1.76
3.06
1.61
4.07
1.60
12.9
1.89
6.97
◦
and tetraglyme, our polynitrile solvents possess unsymmetrical co-
ordination sites. As discussed earlier (see Figure 1), these types of
solvents are expected to enhance structural transport, leading to higher
ionic conductivity. Therefore, in the design, the coordination sites of
the aforementioned HLAELs-based nitrile compounds are chemically
nonequivalent, one site is weaker than the others, i.e., amine nitrogen
and nitrile nitrogen are much weaker than ether oxygen. This strat-
egy is expected to allow much faster lithium ion transport compared
to chemically equivalent chelates (e.g., those obtain from triglyme
and tetraglyme). The enhancement of lithium ion transport leads to
increase in ionic conductivity. Among the molecules investigated in
this study, 1b, 1a and 4a contain four asymmetric coordination sites
for stable chelates with lithium ions. By contrast, one molecule of 1b
has ability to accommodate two lithium ions (more charge carriers)
through 4 + 4 coordination sites, which explains why this material
exhibits the best ionic conductivity. 2b has the similar structure as 1b
with extra coordination sites (6 + 6), which allows superior chelation,
but at the cost of lower ionic conductivity because of higher viscosity
resulting from higher molecular weight of 2b. However, at elevated
temperature the chain movements become faster, and the extra sites
favor lithium ion transfer. This explains why ionic conductivity of
Figure 4. Ionic conductivities of best formulations of each solvent.
2b increased sharply compared to 1b with increase of temperatures.
Other molecules have not given satisfactory conductivity values be-
cause they possess ether wrong coordination number (5a and 6a) or
inappropriate spaces between coordination sites (2a, 3a and 6a).
of 4a, which showed 9.97 mS/cm and 9.04 mS/cm, respectively, at
room temperature. An electrolyte containing 1.89 M LiTFSI in 2b
also posted a good conductivity number (6.97 mS/cm at 25 C). Sur-
◦
prisingly, the increment of ionic conductivity at elevated temperatures
was abruptly higher compared to other electrolytes.
From the molecular structure point of view, we recognize that
oxygen, nitrogen and nitrile groups can provide electron lone pairs
to coordinate with a lithium ion to form a complex. In this study
Differential scanning calorimetry (DSC) studies.— The DSC re-
sults (freezing/melting point) for the best electrolytes and that of cor-
responding neat solvents are presented in Table III. Only 1a and 1b
performed a slightly lower T
while other materials displayed the same T
It is noteworthy that all materials exhibited very low T
melting points of amine-based materials were lower than ether-based
m
for the electrolyte than the neat solvent,
with or without the salt.
. Additionally,
m
(
Schemes 1 and 2), all nitrile solvents contain at least four coordination
m
sites. Unlike symmetrical poly-coordinated solvents, such as triglyme
Table III. Thermal property studies of each product (Tm: melting point).
Samples
1a
2a
3a
4a
5a
6a
1b
2b
◦
Tm of neat solvent ( C)
−83.0
−83.8
−82.7
−82.7
−82.2
−82.2
−83.3
−83.3
−83.1
−83.1
−83.1
−83.1
−76.4
−78.8
−76.4
−76.4
◦
Tm of electrolytes ( C)

Journal of The Electrochemical Society, 162 (7) A1276-A1281 (2015)
A1281
◦
compounds. All amine-based materials showed T
m
around −83 C,
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◦
while group “b”, ether-based compounds displayed near −77 C. As
◦
mentioned earlier, high melting point of EC/DMC (T
m
= 39–40 C
9
. M. C. Smart, B. V. Ratnakumar, and S. Surampudi, Journal of Electrochemical So-
◦
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In this study, a series of new low molecular weight polynitrile sol-
vents based on hemi-labile amino-ether ligands have been designed
and synthesized. Their performance as liquid electrolyte solvents has
been investigated. The effects of lithium salt concentration and struc-
tural relationship properties were studied and discussed. Due to their
asymmetric molecular design and fast lithium ion transport properties,
several compounds presented displayed impressive ionic conductiv-
ities: 1.60 M in 1b displayed the best ionic conductivity of 12.9
mS/cm, 1.62 M LiTFSI in 1a and 1.78 M in 4a at 25 C performed
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