Lithium cation conducting TDI anion-based ionic liquids

Article abstract of DOI:10.1039/c3cp55354j

In this paper we present the synthesis route and electrochemical properties of new class of ionic liquids (ILs) obtained from lithium derivate TDI (4,5-dicyano-2-(trifluoromethyl)imidazolium) anion. ILs synthesized by us were EMImTDI, PMImTDI and BMImTDI,

Full text of DOI:10.1039/c3cp55354j

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Lithium cation conducting TDI anion-based
ionic liquids†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 11417
Leszek Niedzicki,*a Ewelina Karpierz,a Maciej Zawadzki,b Maciej Dranka,a
Marta Kasprzyk,a Aldona Zalewska,a Marek Marcinek,a Janusz Zachara,a
Urszula Domanskab and Władysław Wieczoreka
´
In this paper we present the synthesis route and electrochemical properties of new class of ionic liquids
(ILs) obtained from lithium derivate TDI (4,5-dicyano-2-(trifluoromethyl)imidazolium) anion. ILs synthesized
by us were EMImTDI, PMImTDI and BMImTDI, i.e. TDI anion with 1-alkyl-3-methylimidazolium cations,
where alkyl meant ethyl, propyl and butyl groups. TDI anion contains fewer fluorine atoms than LiPF6 and
thanks to C–F instead of P–F bond, they are less prone to emit fluorine or hydrogen fluoride due to the
rise in temperature. Use of IL results in non-flammability, which is making such electrolyte even safer for
both application and environment. The thermal stability of synthesized compounds was tested by DSC and
TGA and no signal of decomposition was observed up to 250 1C. The LiTDI salt was added to ILs to form
complete electrolytes. The structures of tailored ILs with lithium salt were confirmed by X-ray diffraction
patterns. The electrolytes showed excellent properties regarding their ionic conductivity (over 3 mS cmÀ1
at room temperature after lithium salt addition), lithium cation transference number (over 0.1), low
viscosity and broad electrochemical stability window. The ionic conductivity and viscosity measurements
of pure ILs are reported for reference.
Received 20th December 2013,
Accepted 14th March 2014
DOI: 10.1039/c3cp55354j
www.rsc.org/pccp
Therefore, new ‘‘tailored’’ anions were studied by our group,
which have been designed and investigated directly for applica-
1. Introduction
No doubt Li (ion) battery is one of the most established devices tion as lithium electrolytes in lithium-ion cells. Several patents
for energy storage.1–3 While there are significant numbers of and papers5–9 have been published before, describing details
publications on the new lithium-ion cell electrode materials about synthesis, production, structure and electrochemical
(or additions to those), very little attention has been devoted to properties of perfluorinated anion based lithium salts. These
new salts used in lithium electrolytes.
new compounds showed thermal stability (over 250 1C, no
That was the reason for the concept of creating new anions for aluminum corrosion and electrochemical stability up to 4.6 V)
lithium ion batteries. The whole idea came from the transposi- and transference number properties superior to the presently
tion of the Hu¨ckel rule predicting the stability of aromatic salt used lithium salts in electrolytes. Also the safety aspects of pure
systems. One of the most common examples of this type of LiTDI (lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide) were
anions (sometimes called ‘‘Hu¨ckel anions’’) is 4,5-dicyanotriazole published recently.10
(DCTA). This particular structure is completely covalently bonded
So far these salts have been studied as a component of solid
and shows very stable 6p (or 10p electron if CN bonds are or liquid electrolytes.5–7 The present paper extends their use to
involved in calculations) configuration. LiDCTA was successfully ionic liquid (IL) based electrolytes.
tested in PEO matrice systems as a promising, improved electro-
Classical solvents in batteries (e.g. EC, DMC, PC) are usually
lyte for rechargeable lithium batteries.4 Unfortunately DCTA toxic and flammable (volatile). The low vapor pressure of ILs
failed as a component of the EC/DMC (1 : 1) battery electrolyte. has very important advantages as compared to other so called
volatile organic chemicals (VOCs). Therefore ILs are neither
a Warsaw University of Technology, Faculty of Chemistry, Department of Inorganic
Chemistry and Solid State Technology, Noakowskiego 3, 00664 Warsaw, Poland.
E-mail: lniedzicki@ch.pw.edu.pl; Fax: +48 22 628 2741; Tel: +48 22 234 7421
b Warsaw University of Technology, Faculty of Chemistry,
flammable nor explosive and the risk of prolonged pollution
into the air is substantially reduced.11,12 In consequence, new
solvents, such as ILs, are considered as alternative technologies
which offer minimal evaporation into the atmosphere allowing
simple recycling and reuse. Using simple syntheses as proposed
in our manuscript is a good way of limiting any unwanted
byproducts and wastes. These are the clear environmental
Department of Physical Chemistry, Noakowskiego 3, 00664 Warsaw, Poland
† Electronic supplementary information (ESI) available. CCDC 940806–940808.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cp55354j
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aspects of such type of electrolytes. It should be definitely
2.1.2. 1-Ethyl-3-methylimidazolium 4,5-dicyano-2-(trifluoro-
mentioned that ILs offer a huge environmental advantage when methyl)imidazolide (EMImTDI). 2.53 g 1-ethyl-3-methylimid-
considering them as so-called ‘‘green chemistry’’ compounds in azolium chloride (as received, IoLiTec 98%) (0.0173 mol) in
battery industry. One of the principles of green chemistry put 10 ml of dichloromethane was added to a solution of 3.31 g
special attention to the new approach toward industrial proce- lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (synthesized)
dures where any kinds of wastes are not treated anymore simply (0.0172 mol) in 10 ml of water. The mixture was stirred at rt for
as byproducts of the chemical processes so the simple synthesis 4 h and was left for separation of the phases for 6 h. After that,
route proposed here fulfils these requirements.
the phases were separated by draining. The water phase was
The richness of structural variations and chemistry behavior13,14 extracted with 10 Â 8 ml of dichloromethane (until the water
makes ILs attractive candidates for many applications.15 The phase was colorless). Then the organic phase was extracted with
interest in ILs as electrolytes for lithium (ion) batteries is increasing 2 Â 4 ml of water (removal of the residual lithium chloride).
with the popularity of lithium batteries as energy storage for Solvents were removed in vacuo and the product was further
everyday use and with social pressure on green technologies dried under vacuum at 80 1C for 12 h. Thus, we obtained 1-ethyl-
development.16 The reason is obvious: ILs offer high conductivity, 3-methylimidazolium 4,5-dicyano-2-(trifluoromethyl)imidazolide
non-flammability, wide electrochemical and thermal stability (5.00 g, 97.9%).
windows.17 However, up until now, there have been many dis-
M = 296.25 g molÀ1; Tm = 334.16 K; DHm = 52.84 kJ molÀ1
;
advantages of these new materials. One of them has been large Tg = 174.84 K; DCp = 280 J molÀ1 KÀ1
.
decrease of conductivity with the addition of lithium salt to IL,
which limits possible charge and discharge rates of the cell
containing such electrolyte. Other disadvantages have included
Elemental analysis: found: C, 49.0; N, 28.3; H, 4.0. Calc. for
12H11N6F3: C, 48.7; N, 28.3; H, 3.7%.
C
1H NMR: dH(400 MHz; d6-acetone; Me4Si): 1.561 (3H, t,
melting point higher than room temperature, which constrains 3JHH = 7.2 Hz, N-CH2CH3), 4.061 (3H, s, N-CH3), 4.403 (2H, q,
applications of such materials to a limited number. A very low 3JHH = 7.2 Hz, N-CH2CH3), 7.734 (2H, d, JHH = 28.8 Hz,
3
lithium cation transference number (less than 0.1) that impacts N-CHCH-N), 9.066 (1H, s, N-CH-N).
charge–discharge cycle efficiency has also been an issue.
13C NMR: dC(100 MHz; d6-acetone; Me4Si): 15.475, 36.580,
In this paper we introduce for the very first time a new class 45.710, 116.056, 120.052, 123.056, 124.739, 137.002, 148.844,
of ILs potentially able (due to its atomic and electronic struc- 206.468.
ture) to overcome the majority of existing electrochemical
2.1.3. 1-Methyl-3-propylimidazolium 4,5-dicyano-2-(trifluoro-
disadvantages of this kind of materials. Also a low content of methyl)imidazolide (PMImTDI). 18.20 g 1-methyl-3-propyllimid-
fluorine in the proposed electrolyte composition might create a azolium chloride (as received, IoLiTec 98%) (0.1133 mol) in
new added value of ‘‘green’’ aspect of ILs.
100 ml of water was added to a solution of 24.03 g lithium 4,5-
dicyano-2-(trifluoromethyl)imidazolide (synthesized) (0.1270 mol)
in 100 ml of water. The mixture was stirred at rt for 4 h, after
which 100 ml of dichloromethane was added and the phases were
separated. The water phase was extracted with 50 ml of dichloro-
methane. Then the organic phase was extracted with 10 Â 25 ml
of water (removal of residual lithium chloride). The solvents
were removed in vacuo, and the product was further dried
under vacuum at 80 1C for 24 h. Thus, we obtained 1-methyl-
3-propylimidazolium 4,5-dicyano-2-(trifluoromethyl)imidazolide
(30.16 g, 85.8%).
2. Experimental
2.1. Synthesis
2.1.1. LiTDI. Lithium 4,5-dicyano-2-(trifluoromethyl)imid-
azolide synthesis route was described previously.18 Fig. 1
shows the structural formula of LiTDI and all subsequently
obtained ILs.
M = 310.28 g molÀ1; Tm = 286.85 K; DHm = 28.09 kJ molÀ1
Tg = 203.81 K; DCp = 246 J molÀ1 KÀ1
;
.
Elemental analysis: found: C, 50.0; N, 26.9; H, 4.2; calc. for
13H13N6F3: C, 50.3; N, 27.1; H, 4.2%.
C
1H NMR: dH(400 MHz; d6-acetone; Me4Si): 0.943 (3H, t, 3JHH
=
3
7.2 Hz, CH2CH3), 1.981 (2H, hex, JHH = 7.2 Hz, CH2CH2CH3),
3
4.067 (3H, s, N-CH3), 4.309 (2H, t, JHH = 7.2 Hz, N-CH2CH2),
7.720 (2H, d, 3JHH = 19.2 Hz, N-CHCH-N), 9.068 (1H, s, N-CH-N).
13C NMR: dC(100 MHz; d6-acetone; Me4Si): 10.598, 23.915,
36.565, 51.853, 115.942, 119.954, 123.260, 124.663, 137.100,
148.749, 206.892.
2.1.4. 1-Butyl-3-methylimidazolium 4,5-dicyano-2-(trifluoro-
methyl)imidazolide (BMImTDI). 6.38 g 1-butyl-3-methylimid-
azolium chloride (as received, IoLiTec 98%) (0.0365 mol) in
100 ml of dichloromethane was added to a solution of 7.70 g
lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (synthesized)
Fig. 1 Structural formula of LiTDI salt and ionic liquids: EMImTDI, PMImTDI
and BMImTDI.
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(0.0401 mol) in 100 ml of water. The mixture was stirred at rt for
2.3.1. Crystal data for EMImTDI (1). C12H11F3N6 (M = 296.27):
%
4 h, after which the phases were separated. The water phase triclinic, P1, a = 8.63283(15) Å, b = 8.86281(12) Å, c = 10.25772(13) Å,
was extracted with 10 Â 10 ml of dichloromethane. Then the a = 87.4723(11)1, b = 78.8034(13)1, g = 66.0794(15)1, V = 703.24(2) Å3,
organic phase was extracted with 2 Â 5 ml water (removal of Z = 2, m(MoKa) = 0.119 mmÀ1, Dcalc. = 1.399 g cmÀ3, 111 869
residual lithium chloride). The solvents were removed in vacuo reflections measured (7.24 r 2Y r 58.98), 3902 unique (Rint
=
and the product was further dried under vacuum at 80 1C 0.0237). The final R1 was 0.0408 (I 4 2s(I)) and wR2 was 0.1090
for 12 h. Thus, we obtained 1-butyl-3-methylimidazolium (all data).
4,5-dicyano-2-(trifluoromethyl)imidazolide (10.14 g, 85%).
M = 324.30 g molÀ1; no Tm; Tg = 203.63 K; DCp = 311 J molÀ1 KÀ1
2.3.2. Crystal data for LiTDI–PMImTDI (2). C19H13F6LiN10
%
.
(M = 502.33): triclinic, P1, a = 8.28368(13) Å, b = 11.45842(18) Å,
Elemental analysis: found: C, 51.8; N, 25.8; H, 4.6; calc. for c = 12.57923(20) Å, a = 74.9663(14)1, b = 77.5238(13)1, g =
C
14H15N6F3: C, 51.8; N, 25.9; H, 4.6%.
78.6965(13)1, V = 1113.43(3) Å3, Z = 2, m(CuKa) = 1.157 mmÀ1
,
1H NMR: dH(400 MHz; d6-acetone; Me4Si): 0.877 (3H, t, Dcalc. = 1.498 g cmÀ3, 60 286 reflections measured (7.38 r 2Y r
3JHH = 7.2 Hz, CH2CH3), 1.302 (2H, hex, 3JHH = 7.6 Hz, CH2CH2CH3), 133.34), 3923 unique (Rint = 0.0330). The final R1 was 0.0275
1.809 (2H, tt, JHH = 7.2 Hz, JHH = 7.6 Hz, CH2CH2CH2), 3.943 (I 4 2s(I)) and wR2 was 0.0711 (all data).
(3H, s, N-CH3), 4.152 (2H, t, JHH = 7.2 Hz, N-CH2CH2-), 7.375
(2H, m, N-CHCH-N), 9.025 (1H, s, N-CH-N).
3
3
3
2.3.3. Crystal data for LiTDI–BMImTDI (3). C20H15F6LiN10
%
(M = 516.36): triclinic, P1, a = 8.42410(11) Å, b = 11.66815(17) Å,
13C NMR: dC(100 MHz; d6-acetone; Me4Si): 13.427, 19.759, c = 12.85431(18) Å, a = 73.0547(13)1, b = 74.4505(12)1, g =
32.477, 36.557, 50.147, 115.950, 119.954, 123.268, 124.648, 76.7560(12)1, V = 1148.67(3) Å3, Z = 2, m(CuKa) = 1.137 mmÀ1
137.085, 148.756, 206.816.
,
Dcalc. = 1.493 g cmÀ3, 80 701 reflections measured (7.36 r 2Y r
133.4), 4047 unique (Rint = 0.0330). The final R1 was 0.0298
(I 4 2s(I)) and wR2 was 0.0721 (all data).
2.2. Salt characterization
Salts were characterized with differential scanning calorimetry
(DSC) on TA Instruments Q200DSC apparatus and with thermo-
2.4. Electrolytes characterization
gravimetric analysis (TGA) on TA Instruments Q600SDT appa- Ionic conductivity measurements were performed using electro-
ratus. The heating rate in both cases was equal to 10 K minÀ1
.
chemical impedance spectroscopy (EIS) in the nearest full 10 1C
1H NMR, 13C NMR and 19F NMR experiments were performed over a melting point up to 70 1C temperature every 10 1C, over a
on Varian Gemini 200. 1H shifts are reported relative to TMS (d = 0), full range of concentration of LiTDI (pure IL, 0.0125, 0.025,
13C chemical shifts are reported relative to CD3CN (d = 1.3). The 0.0375, 0.05, 0.075, 0.1, 0.125 and 0.15 mole fraction, the last
19F shifts are described with CFCl3 as the reference (d = 0).
being the maximum LiTDI solubility in all ILs). Electrolytes were
Elemental analysis was conducted by the combustion method placed into micro conductivity cells with a cell constant equal
with PerkinElmer 2400 series II CHNS/O Elemental Analyzer, using to 0.5 cmÀ1 which were put to a cryostat–thermostat system
imidazole as reference sample, while working in CHN mode. (Haake K75 with the DC50 temperature controller). All impe-
Argon carrier gas was used, providing o0.4% accuracy and dance measurements were carried out on a computer-interfaced
o0.3% precision. Three samples of 10 mg were used for measure- multichannel potentiostat with frequency response analyzer
ment and the results were averaged.
option Bio-Logic Science Instruments VMP3 within 500 kHz–1
Trace moisture content of below 50 ppm level was deter- Hz frequency range with 10 points per decade and with 5 mV a.c.
mined with Karl Fischer titration method. Chloride content of signal. Measurements were repeated with independently pre-
below 20 ppm level was determined using the potentiometric pared samples to improve the consistency of data, obtaining
method.
maximum differences of 5%.
Lithium cation transference numbers (t+) were calculated
using a d.c. polarization method combined with an a.c. impe-
2.3. X-Ray crystallography
Selected single crystals were mounted in inert oil and trans- dance method introduced by Bruce and Vincent.22 Details can
ferred to a cold gas stream of a diffractometer. Diffraction data be found in the ESI.†
were measured at 100(2) K with graphite-monochromated MoKa
Linear sweep voltammetry (LSV) was used to investigate the
(EMImTDI) or mirror monochromated CuKa (LiTDI–PMImTDI electrochemical stability window. Samples of electrolyte were
and LiTDI–BMImTDI) radiation on the Oxford Diffraction sandwiched between the lithium metal electrodes used as both
Gemini A Ultra diffractometer. Cell refinement and data collec- counter and reference electrode and platinum electrode as a
tion as well as data reduction and analysis were performed with working electrode (Pt electrode surface was 0.1 cm2). LSV was
19
CRYSALISPRO
.
The structures were solved by direct methods performed every time with 1 mV sÀ1 scan rate at ambient
with SHELXS–97.20 Full-matrix least-squares refinement method temperature for PMImTDI and BMImTDI and at 70 1C in case
against F2 values were carried out by using SHELXL–9720 and of EMImTDI (due to its high melting point). The cyclic voltam-
OLEX221 programs. All non hydrogen atoms were refined with metry results of neat ionic liquids are available in the ESI.†
anisotropic displacement parameters. Hydrogen atoms were added
Viscosity experiments were performed with Physica MCR301
to the structure model at geometrically idealized coordinates and Anton Paar Rheometer with CP40 cone tip and thermoelectric
refined as riding atoms. The crystal data and structure refinement heat pump base for thermostating. Each time the 0.4 ml volume
parameters are given in Table S1 (ESI†).
(excess) of the given electrolyte was used, thermostated with
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precision of 0.1 1C at each temperature and measured in a decomposition of the TDI anion18 and the last step is assumed
shearing rate of 10–1000 sÀ1
to be elimination of the alkyl group (ethyl, propyl or butyl,
.
The symbols of the mixture compositions will be used according to the cation). Thus, the main products of the
subsequently for clarity of text throughout descriptions of decomposition are alkanes and their decomposition products
conductivity, viscosity and transference numbers in the Results (not known to be toxic), as well as TDI anion decomÀposition
and discussion. The system of symbols is made as follows: the products. The latter are considered safer than PF6 anion
first letter is the abbreviation of alkyl chain in the cation: decomposition products, if only because of fewer fluorine atoms
E – ethyl, P – propyl, B – butyl; the number means the amount per molecule and no phosphor used. Hence, neither phosphines
of thousandths of LiTDI molar fraction in the mixture of LiTDI nor highly toxic fluorinated phosphines are produced, like in the
and IL, e.g.: P0 means pure PMImTDI, E50 means LiTDI0.05
-
case of LiPF6.
EMImTDI0.95 and B125 means LiTDI0.125BMImTDI0.875. xMIm
means any 1-alkyl-3-methylimidazolium cation.
The electrochemical stability in the case of EMImTDI and
PMImTDI is sufficient for application in all state-of-the-art
electrode materials for lithium-ion cells. The results of linear
sweep voltammetry experiments performed on the 0.1 molar
LiTDI mixtures with ILs are shown in Fig. 3. The stability limits of
these mixtures reach 4.6, 4.6 and 4.0 V vs. Li for LiTDI–EMImTDI,
LiTDI–PMImTDI and LiTDI–BMImTDI, respectively. The main
influence here can be attributed to the anion—the LiTDI salt
has a 4.7 V stability boundary.18 There is lack of other LSV signals
down to 0.6, 0.8 and 0.6 V vs. Li in the case of LiTDI–EMImTDI,
LiTDI–PMImTDI and LiTDI–BMImTDI, respectively, showing
good electrochemical stability and a wide stability window.
Fig. 4 shows a comparison of ionic conductivity plots for pure
IL and LiTDI-doped electrolytes. We can see that the highest ionic
conductivity at a given temperature (70 1C) was measured for
EMImTDI and its mixtures with LiTDI. Pure IL reachs an ionic
conductivity of 28.9 mS cmÀ1 (E0). After addition of a salt
conductivity value change to 18.6–18.7 mS cmÀ1 in the case of
E25 and E37, 18.0 mS cmÀ1 for E75 and as much as 20.3 mS cmÀ1
for E150. To the best of our knowledge, such high (compared to
pure IL) conductivities have never been seen before in ILs after
lithium salt addition, especially at such high molar fractions of
lithium salt. The ionic conductivity curves of LiTDI–PMImTDI
and LiTDI–BMImTDI mixtures show a similar shape to the
LiTDI–EMImTDI one. In their case, though, the conductivity
values at a given temperature are lower. At 70 1C they exhibit
ionic conductivity values of 17.3 and 14.6 mS cmÀ1 for P0 and B0,
respectively. The same result is initially obtained upon addition of
LiTDI, i.e. minor decrease in conductivity. In the case of PMImTDI,
3. Results and discussion
ILs exhibited stability up to about 250 1C, as seen in Fig. 2.
EMImTDI has the DTG decomposition onset at 253 1C,
PMImTDI at 251 1C and BMImTDI at 247 1C. Similar structures
of cations and identical anion are probably the main reason for
such small differences among them. These values are also very
close to the LiTDI decomposition temperature (256 1C). On the
DTG curves three different regions can be observed. The first
one is very slow, with the highest weight percentage loss for
EMImTDI. The second one is the main region of decomposition
with a very similar weight loss rate for all compounds—still
EMImTDI takes the fastest weight loss. Subsequently, one more
decomposition phase takes place which can be observed as a
very narrow DTG signal for EMImTDI, as a widening of the
main signal in case of PMImTDI and as a distinct one for
BMImTDI. EMImTDI has the fastest weight loss in the first two
phases and negligible one in the third phase. The other two
compounds have the same weight loss for the first two signals
and noticeably different weight loss for the third phase—with
BMImTDI taking the most significant weight loss in the last
one. The observations described above are in good agreement
with findings by Chowdhury and Thynell.23 Methyl elimination
from xMIm cations is responsible for the initial weight loss of
about 5% (first DTG peak integral), which is in concert with the
methyl group share of IL weight. The main step is probably the
Fig. 3 Linear sweep voltammetry plots for LiTDI0.1EMImTDI0.9, LiTDI0.1
-
PMImTDI0.9, and LiTDI0.1BMImTDI0.9 mixtures with platinum as working
electrode and lithium metal electrode as reference, all experiments per-
formed with 1 mV sÀ1 scan rate.
Fig. 2 Thermogravimetric analysis TGA and DTG plots of EMImTDI,
PMImTDI and BMImTDI.
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Fig. 5 Viscosity dependence of LiTDI content in (a) LiTDI–EMImTDI
mixture at 70 1C and in LiTDI–PMImTDI mixture in 20–70 1C temperature
range, the line is used only to guide the eye; (b) LiTDI–BMImTDI mixture in
0–70 1C temperature range.
Fig. 4 Ionic conductivity dependence of LiTDI content in (a) LiTDI–
EMImTDI mixture at 70 1C and in LiTDI–PMImTDI mixture in 20–70 1C
temperature range, the line is used only to guide the eye; (b) LiTDI–BMImTDI
mixture in 0–70 1C temperature range.
after addition of a large fraction of LiTDI, at P37 composition, viscosity plots. As can be easily noticed, the viscosity plots show
the conductivity reaches 13.3 mS cmÀ1 at 70 1C. In the case of an almost perfect reflecting picture of the conductivity curves.
BMImTDI, the conductivity reaches 13.3 mS cmÀ1 at 70 1C The viscosity plots are also quite interesting, showing almost-
already at B25 (at the lower concentration of salt). Further addi- plateau areas and even distinct declines in areas of intermediate
tion of LiTDI to PMImTDI results in slow decrease in conductivity local conductivity maxima in all mixtures. The increase of ionic
(down to 5.2 mS cmÀ1 for P125), until it reaches maximum conductivity values in the area of maximum LiTDI solubility
solubility of 0.15 mole fraction. At this point (P150) ionic con- (around 0.15 molar fraction) is reflected by drop of the viscosity
ductivity increases up to 12.8 mS cmÀ1 at 70 1C. BMImTDI after value in case of E150 and P150. Similarly, only a slight increase
LiTDI addition to B25 also suffers ionic conductivity decrease, of viscosity in the case of B150 takes place instead of a typical
but to a less extent. Similar to LiTDI–PMImTDI mixture, LiTDI– dynamic increase (geometric growth) at maximum solubility.
BMImTDI is also subject to ionic conductivity increase near the Similar behavior can be observed for LiTDI–EMImTDI and
LiTDI maximum solubility point – 6.1 mS cmÀ1 for B150 vs. LiTDI–PMImTDI mixtures. Starting with 9.1 mPa s at 70 1C for
5.9 mS cmÀ1 for B125, both at 70 1C. Ionic conductivity E0, viscosity is increasing with the first addition of LiTDI, rising
dependence of the salt concentration plot shape is similar at to 11.8 mPa s for E25, then dropping to 10.1 mPa s for E37.
all temperatures for a given ionic liquid. Room temperature Further addition of salt causes the monotonic viscosity to
parameters of the aforementioned points at 20 1C for LiTDI– increase up to 16.0 mPa s for E75. The viscosity of EMImTDI
PMImTDI mixtures exhibit conductivities of 3.2, 2.1, 1.5 mS cmÀ1 mixtures increases, though slowly, but for PMImTDI it descends
for P0, P37 and P150, respectively. At 20 1C, the LiTDI–BMImTDI to 16.4 mPa s at P100. For comparison, P75 exhibits 16.7 mPa s
mixtures show ionic conductivity values of 2.4, 2.2, 1.4 and viscosity value. The values at the highest concentrations are
0.7 mS cmÀ1 for B0, B25, B50 and B150, respectively.
almost the same for both ILs – 26.9 and 27.1 mPa s for E125
Recently, such beneficial conductivity plateau (similar to and P125, as well as 24.0 and 24.4 mPa s for E150 and P150
those presented in Fig. 4), have been explained by our group24 mixtures, respectively. The viscosity plots of BMImTDI mixtures
and assigned to ionic associates formation and cation–solvent exhibit a smaller number of changes. At 70 1C it shows a viscosity
interactions for LiTDI–solvent systems.
value of 12 mPa s in the case of pure IL (B0). After addition of
Fig. 5 displays the results of viscosity measurements. LiTDI, this value increases to 12.3 mPa s for B12 and 13.3 mPa s
The shape of ionic conductivity curve is in agreement with the for B25. After further additions of LiTDI the viscosity value drops
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same concentrations. Thus, the top values of lithium cation
conductivity can be found at low salt concentrations: E25, P37,
B25, as well as at maximum solubility (E150, P150, B150).
These three phenomena—ionic conductivity plots with high
retained conductivity upon lithium salt addition, uncommon
ionic conductivity—viscosity and ionic conductivity, cation trans-
ference number relations, can be explained by the special,
‘‘tailored’’ structure of the TDI anion. It was especially dedicated
to electrolyte applications, which were designed to have the
lowest possible coordinating properties, i.e. uniformed charge
distribution, weakened ionic interactions. Use of the same anion
in lithium salt and in IL may also result in specific interactions
between salt and solvent. Especially interesting is the viscosity
decline at the maximum solubility point, which is also worth
noting, at the same molar fraction for all ILs. It has, to our best
knowledge, never been seen before combined together with a
high transference number, as in the mixtures presented herein.
Our assumption is that, due to the earlier findings,27–30 the
Fig. 6 Lithium cation transference number dependence of LiTDI content
in LiTDI–EMImTDI mixture at 70 1C, in LiTDI–PMImTDI mixture at ambient
temperature and in LiTDI–BMImTDI mixture at ambient temperature. All
values were averaged over three samples.
as low as 11.9 mPa s in the case of B37 mixture. With the maximum solubility ratio is defined by the ratio at which the
increase of salt fraction in the mixture, viscosity keeps growing lithium cation is fully solvated by the IL anions. In that case there
monotonically up to a maximum solubility at B150, at which are two possibilities. One is that 1 :6 ratio (0.143 molar fraction of
the viscosity value equals 37.9 mPa s.
LiTDI in IL) is the effect of coordinating lithium by six anions,
Fig. 6 shows the results of lithium cation transference number which was suggested by some researchers.31 This is a less
(tLi+) measurements. The investigated electrolytes offer superior probable assumption due to our structural findings presented
values of tLi+ when compared to other ILs.25,26 The highest herein—in the case of nitrogen ligation centers, the coordinat-
measured value is 0.120 for B150, which is indeed an excellent ing number of the lithium cation is four (see below), which is in
value for good-conducting ILs. For comparison, the common concert with other works.27,29 If it is fully coordinated, then for
values of tLi+ for most state-of-the-art ILs are in the every one lithium cation there will be four anions coordinated
0.02–0.04 range.25 LiTDI–BMImTDI has the highest values of with it [Li(TDI)43À] through a direct bonding and three TDI
all three presented ILs for LiTDI molar fractions 0.075 and anions and six IL cations not coordinated directly. As it has
higher. For lower fractions of LiTDI, the best tLi+ performance been shown many times before, IL ions are usually forming
belongs to LiTDI–EMImTDI mixtures, which exhibit tLi+ values agglomerates and it is quite common for them to exist in the
up to 0.099 (E25). Generally, all ILs mixed with LiTDI have the form of triplets—of C2A+ and CA2 type. Hence, it would be
À
same lithium cation transference number plot shapes. They all probable for them to be in such form also in the mixtures
start with quite low values at 0.0125 LiTDI molar fraction, then presented here and to form triplets of, e.g. EMIm2TDI+ form,
after addition to 0.025 the molar fractions increase very quickly keeping the whole electrolyte neutral in the charge terms. Then
to reach maxima. Such extrema are absolute maximum in the the 1 : 6 ratio of saturated LiTDI–IL mixture (0.143 LiTDI molar
case of LiTDI–EMImTDI and second to value at maximum fraction) would be a result of 1 : 4 coordinated lithium cation
solubility in the case of LiTDI–PMImTDI and LiTDI–BMImTDI. and the rest of cations and anions would be forming triplet type
Thus, E25 has the tLi+ value of 0.099, P25 – 0.067 and B25 – 0.086. agglomerates. Such agglomerates would be more immobile
Afterwards, when LiTDI is added, the tLi+ values for all ILs than individual ions and as such, giving the lithium cation a
generally decrease, but reach a local maximum at 0.05–0.075 chance to have a higher transference number as well as
LiTDI molar fraction (tLi+ values: E50 – 0.077, P75 – 0.063 and relatively better mobility. As a result, lower viscosity and higher
B75 – 0.065). After reaching a global minimum at 0.1 molar ionic conductivity would be achieved.
fraction, the tLi+ values increase upon further LiTDI salt addition
Crystallographic analyses were performed on the EMImTDI
to obtain maxima at maximum solubility—0.15 molar fraction. pure IL, as well as LiTDI–PMImTDI and LiTDI–BMImTDI com-
These maxima are global for P and B mixtures, 0.093 and 0.120, plexes. The crystallographic details can be found in the ESI.†
for P150 and B150, respectively, and local for E mixtures—0.040 Single crystals of EMImTDI were obtained by recrystallization
for E150. The usual relationship between tLi+ and ionic conduc- from methanol solutions. Single crystals of compounds LiTDI–
tivity values is more or less reciprocal. What is uncommon in the PMImTDI and LiTDI–BMImTDI suitable for X-ray diffraction
presented mixtures is the combined increase of both these analysis were grown at rt from a LiTDI0.5PMImTDI0.5 and
quantities at low LiTDI concentrations and at the maximum LiTDI0.5BMImTDI0.5 composition. X-Ray crystal structure deter-
solubility of LiTDI. It means the possibility to obtain the highest mination of EMImTDI, reveals planar layers of cations and
values of cell-usable conductivity—lithium cation conductivity, anions interacting through hydrogen bonds (Fig. 7).
sLi+. This quantity is obtained by multiplying the values of the
The TDI anion makes four in-plane hydrogen bonds to methylene
lithium cation transference number and ionic conductivity at the hydrogen atoms of cations at the C9, C10 and C11 positions.
11422 | Phys. Chem. Chem. Phys., 2014, 16, 11417--11425
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Fig. 7 Thermal ellipsoid plot of ions in the EMImTDI asymmetric unit with an
atom numbering scheme and hydrogen bonds motif constituting individual
layers. HÁÁÁA distance: H9ÁÁÁN21 2.40 Å; H10ÁÁÁF3 2.49 Å; H10ÁÁÁN1 2.35 Å;
H11ÁÁÁN43 2.57 Å. Symmetry codes: (1) +x, +y, 1 + z; (2) +x, +y, À1 + z; (3) 1 + x,
À1 + y, +z; (4) À1 + x, 1 + y, +z.
Namely, a short, bifurcated contact between the hydrogen atom at
C10 and N1/F3 atoms, a hydrogen bond between H9 and imidazole
nitrogen atom N2, and short contact methylene hydrogen atom
H11 and nitrile nitrogen N4. The layers are stacked one above the
other in the ABAB fashion through ionic interactions between
cations and anions from adjacent layers.
Lithium electrolytes formed by addition of LiTDI salt to
PMImTDI (2) and BMImTDI (3) ILs crystallize in the triclinic
%
space group P1 as colorless plates. In both cases, single crystal
X-ray analysis showed a novel 1D ladder-like assembly of the
nÀ
[Li(TDI)2]n polyanions as depicted in Fig. 8.
Lithium cations adopt distorted tetrahedral geometry and
are linked via two bridging dicyanoimidazolato ligands forming
characteristic dimeric units with a ten-membered Li(NCCN)2Li
ring.32 The central ring is planar with r.m.s. deviations from
planarity of 0.012 Å for both 2 and 3. Dimeric units are linked to
each other through the spanning TDI anions which are coordi-
nated in an unsymmetrical fashion through the nitrogen atom
of the imidazole ring and one of the cyano groups. Consequently,
the ladder-like 1D coordination polymer propagates in the direc-
tion of the X axis, with the Li–Li distance along the chain being
Fig. 9 Ion packing in the crystal structure of 3 (LiTDI–BMImTDI): (a) pre-
sentation of the channels occupied by imidazolium cation molecules shown
in space-filling mode, view along the b-axis; (b) space-filling model of three-
dimensional formation of the channels. Color code: gray C; green: F, blue: N,
cyan: Li.
equal to the a cell parameter, 8.2837(2) and 8.4241(1) Å for 2 and
3 respectively. Polyanions are arranged in column-type fashion
with perpendicular channels occupied by imidazolium cations as
shown in Fig. 9.
The coordination polymers in structures 2 and 3 are
characteristic of and consistent with observations made by
Henderson31 for LiTFSI–xMImTFSI system (coordination
number = 6). It is worth stressing that in the case of LiTDI–
xMImTDI system the coordination sphere is completely different,
due to the presence of nitrogen donor centers. The oordination
number in our case is 4 (with one additional fluorine dative
bond). Unlike LiTFSI–xMImTFSI, in the case of LiTDI–xMImTDI
system the TDI anion enforces formation of dimeric moieties
(Li2TDI2) which are subsequently linked via bridging TDI
nÀ
Fig. 8 Perspective view of the ladder-like polyanionic [Li(TDI)2]n chain
observed in the crystal structure of 2 (LiTDI–PMImTDI) and 3 (LiTDI–BMImTDI). anionic ligands.
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˙
6 L. Niedzicki, M. Kasprzyk, K. Kuziak, G. Z. Zukowska,
This allows implying that addition of the IL effects in this
coordination polymer [Li(TDI)2nÀ] the dissociation. At first, due
to coordination polymer interaction with TDI anions, the
dimeric structure is retained after fulfilling the coordination
M. Marcinek, W. Wieczorek and M. Armand, J. Power
Sources, 2011, 196, 1386.
7 L. Niedzicki, S. Grugeon, S. Laruelle, P. Judeinstein,
M. Bukowska, J. Prejzner, P. Szczecinski, W. Wieczorek
and M. Armand, J. Power Sources, 2011, 196, 8696.
4À
sphere. As the effect of that we assume formation of Li2TDI6
species (double triplets). The unusual conductivity plot could
be a consequence of the existence of such dimers in a more
concentrated solution. During further dissolution one might
´
8 M. Bukowska, P. Szczecinski, W. Wieczorek, L. Niedzicki,
B. Scrosati, S. Panero, P. Reale, M. Armand, S. Laruelle and
S. Grugeon, WO Pat., 023 413, 2010; M. Bukowska,
expect the occurrence of equilibria with lithium mononuclear
3À
´
P. Szczecinski, W. Wieczorek, L. Niedzicki, B. Scrosati,
complexes LiTDI4
.
S. Panero, P. Reale, M. Armand, S. Laruelle and S. Grugeon,
Fr. Pat., 2 935 382, 2009.
9 D. W. McOwen, S. A. Delp, W. A. Henderson, Meeting
Abstracts 2013, MA2013-02, 1182.
10 P. Ribiere, S. Grugeon, M. Morcette, S. Boyanov,
S. Laruelle and G. Marlair, Energy Environ. Sci., 2012,
5, 5271.
Noteworthy, almost identical crystal structures are observed
for different organic cations. This fact proves the stability of the
anionic coordination polymers formed. Thus, the observed
crystal structure assembly of the described species is directed
by the acidic lithium center interacting with TDI anions.
11 R. Renner, Environ. Sci. Technol., 2001, 35, 410A.
12 A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton,
S. Sheff, A. Wierzbicki, J. H. Davis and R. D. Rogers, Environ.
Sci. Technol., 2002, 36, 2523.
4. Conclusions
We have presented three new ILs based on our anion ‘‘tailored’’
for electrochemical applications: EMImTDI, PMImTDI and
BMImTDI. All three ILs are thermally and electrochemically
stable. They also contain less fluorine than the industry standard
(PF6À), which is bound in a more stable way than with P–F or B–F
bonds. The lithium electrolytes formed by the addition of LiTDI
salt to these ILs reach high ionic conductivity—over 3 mS cmÀ1
at 20 1C. The new IL-based electrolytes exhibit good lithium
cation transference numbers (over 0.1) overlapping with high
ionic conductivity. Negligible change of the ionic conductivity
upon lithium salt addition (less than 10% instead of few-fold
decrease)33 combined with high ionic conductivity has never
been seen before. As a result, we introduce a safe, effective and
in many aspects environmentally friendly completely new class
of electrolytes.
´
13 U. Domanska, in Ionic liquids in chemical analysis,
ed. M. Koel, CRC Press Taylor & Francis Group, 2008,
ch. 1.
14 J. P. Hallet and T. Welton, Chem. Rev., 2011, 111,
3508.
15 N. Plechkova and K. Seddon, Chem. Soc. Rev., 2008,
37, 123.
16 M. Armand, F. Enders, D. MacFarlane, H. Ohno and
B. Scrosati, Nat. Mater., 2009, 8, 621.
17 A. Fernicola, F. Croce, B. Scrosati, T. Watanabe and
H. Ohno, J. Power Sources, 2007, 174, 342.
˙
´
18 L. Niedzicki, G. Z. Zukowska, M. Bukowska, P. Szczecinski,
S. Grugeon, S. Laruelle, M. Armand, S. Panero, B. Scrosati,
M. Marcinek and W. Wieczorek, Electrochim. Acta, 2010,
55, 1450.
Acknowledgements
19 CRYSALISPRO Software system, Agilent Technologies UK Ltd.,
Oxford, UK, 2012.
20 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, 64, 112.
21 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
and H. Puschmann, OLEX2: A complete structure solution,
refinement and analysis program, J. Appl. Crystallogr., 2009,
42, 339.
This work was financially supported by Warsaw University of
Technology. This work has been supported by the European
Union in the framework of European Social Fund through the
Warsaw University of Technology Development Programme,
realized by the Center for Advanced Studies.
22 P. Bruce and C. Vincent, J. Electroanal. Chem., 1987, 225, 1.
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