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ADVANCED MANUFACTURING PROCESSES
FOR LOW COST GREENER LI-ION BATTERIES
Green and safer electrolytes
based on ionic liquids
1
Index
1 Introduction .................................................................................................................. 2
2 Experimental ................................................................................................................. 3
2.1 Synthesis of ionic liquids ....................................................................................... 3
2.1.1 Synthesis of precursor .................................................................................................................... 3
2.1.2 Purification of precursor ................................................................................................................ 4
2.1.3 Synthesis of ionic liquid ................................................................................................................. 4
2.2 Quality control of ionic liquids .............................................................................. 4
2.3 Preparation of ionic liquid electrolytes ................................................................. 6
2.4 Thermal analysis .................................................................................................... 6
2.5 Ion transport properties ........................................................................................ 6
2.6 Rheological properties ........................................................................................... 7
2.7 Electrochemical stability ....................................................................................... 8
3 Results and Discussions ................................................................................................ 9
3.1 Quality control of ionic liquids .............................................................................. 9
3.2 Determination of the optimal ionic liquid mole ratio ........................................... 9
3.3 Determination of the optimal lithium salt mole fraction ................................... 13
3.4 Ion self-diffusion coefficient measurements ....................................................... 16
4 Conclusions ................................................................................................................. 19
5 References ................................................................................................................... 20
6 Contacts and Authors .................................................................................................. 21
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1 Introduction
Ionic liquids (ILs), molten salts at room temperature (or below) constituted by organic cations and
inorganic/organic anions, represent a very interesting new class of room temperature fluids with very
unique properties as flame retardant, negligible vapour pressure in conjunction with ambient or sub-
ambient melting temperature, remarkable ionic conductivity, wide thermal/chemical/electrochemical
stability window, low heat capacity, ability to dissolve inorganic (included lithium salt), organic and
polymeric materials and, in some cases, hydrophobicity [1-3]. In the last years, ILs have been widely
investigating as safe electrolyte components to replace the volatile and hazardous alkyl carbonates
commonly used in commercial lithium batteries [4]. Particularly, ILs based on N-alkyl-N-
methylpyrrolidinium cations, (PYR1A)+ (the subscripts indicate the number of carbon atoms in the alkyl
side chains), and the bis(trifluoromethanesulfonyl)imide, (TFSI)-, and bis(fluorosulfonyl)imide, (FSI)-,
anions have been favourably proposed for this issue [4].
Ionic liquids exhibit specifications, which are basic for the realization of high safety electrochemical
energy storage devices. In addition, one of the most appealing peculiarities of ILs is the possibility of
finely tuning their physicochemical properties by properly designing the cation/anion architecture and/or
incorporating suitable functional groups and/or foreign atoms.
Figure 1: Scheme of IL combining proposal and chemical structure of the selected ionic liquid ions.
However, some requirements are, sometimes, not achievable by single IL materials. Previous work [4] has
shown that suitably combining ILs may lead to synergic effect on the characteristics of electrolyte in terms
of ionic transport, electrochemical and thermal stability and, also, compatibility with electrodes. Our basic
idea (schematized in Figure 1) is to favourably combine two IL materials formed by a common cation, but
sharing a different anion. N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide,
PYR13TFSI, and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide, PYR13FSI, were selected as
the ionic liquid materials due to their wide electrochemical/thermal stability (PYR13TFSI), fast ion
transport properties, low melting point and good ability to form protective layers onto electrodes
(PYR13FSI) [4]. Figure 1 depicts the chemical structure of the PYR13+, TFSI- and FSI- ions.
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2 Experimental
2.1 Synthesis of ionic liquids The PYR13TFSI and PYR13FSI ionic liquid materials were synthesized through a greener and cheaper
procedure route, properly developed at ENEA [5], which uses water as the only process solvent. Also, alkyl
bromide was used in the place of the more expensive, toxic and light/oxygen sensitive alkyl iodide. In
Table 1 are listed the chemicals and equipment required for the synthesis process.
Table 1: Chemicals and equipment for the synthesis process of the ionic liquids.
Chemical Use Supplier Equipment
N-methylpyrrolidine (Mw = 85.15) Reagent Aldrich, 98 wt.% Glass reactor
1-Bromopropane (Mw = 122.99) Reagent Aldrich, 99 wt.% Heating bath
LiTFSI (Mw = 287.08) Reagent 3M, 99.9 wt.% Buckner filtration system
NaFSI (Mw = 216.99) Reagent Solvionic, 99.9 wt.% Oil-free vacuum pump
Activated charcoal Sorbent Aldrich, DARCO G-60 Rotary evaporator
Alumina Sorbent Aldrich, acidic Brockman I Vacuum oven
Deionized water Solvent Standard glassware
A glass, sealed reactor was used for the synthesis of the precursor and the ionic liquid, this avoiding
exposition of the reagent materials to external. The operating temperature of the reactor was controlled
using an oil bath connected to the reactor. The filtration steps were performed using a Millipore vacuum
filter system; an oil-free pump (< 20 mTorr) was used to generate vacuum. The separation of the rinsing
fluid (water) from the ionic liquid was performed by vacuum aspiration. Water was removed using a
Resona Labo-Rota C-311 rotary evaporator connected with an oil-free pump. The final drying step of the
ionic liquids was performed within a dry room (moisture content < 10 ppm) in a BÜCHI-585 glass
vacuum oven connected with an oil-free Schröll pump (10-2 mTorr). The IL materials were finally stored
in sealed glass tubes within the dry room.
The synthesis procedure route was performed through three steps (described in detail as reported below):
i) synthesis of precursor; ii) purification of precursor; iii) synthesis of ionic liquid. The overall synthesis
process is schematized in Figure 2.
2.1.1 Synthesis of precursor - N-methylpyrrolidine (PYR1), e.g., in slight excess (≤ 1 wt.%) with respect to the stoichiometric amount
of 1-bromopropane (1-Br-Prop), is diluted in water (1:1 volume ratio) and loaded in the glass reactor
(PYR1:1-Br-Prop stoichiometric ratio = 1:1);
- 1-Br-Prop is loaded in the reactor. This chemical is immiscible with water and the formation of two
phases is observed, the lower phase being 1-bromopropane since its higher density (1-Br-Prop density =
1.354 g cm-3; PYR1 density = 0.819 g cm-3);
- The chemicals are stirred at 70 °C, allowing to the reaction (1) to take place:
PYR1 (aq.) + 1-Br-Prop PYR13Br (aq.) (1)
to form the N-methyl-N-propylpyrrolidinium bromide (PYR13Br) precursor. The end of the reaction (upon
30minutes stirring), driven until a yield approaching 100 mol.%, is easily detected from the complete
disappearance of the second phase. A yellowish concentrated aqueous solution of PYR13Br was obtained.
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2.1.2 Purification of precursor - The aqueous solution of PYR13Br (housed into the glass reactor), obtained from the step (i), is further
diluted with deionized water to reduce the viscosity. The same water amount used in step (i) was added;
- Activated charcoal (AC) and alumina (Al2O3), used as the sorbent materials to purify the precursor, are
loaded into the reactor. The resulting slurry is stirred overnight at room temperature;
- AC and Al2O3 (solid fraction) are separated from the liquid one (aqueous solution of PYR13Br) by
vacuum filtration with polyamide membrane filter having porosity < 0.2 μm;
- The liquid fraction (clear and colourless) is separately collected for the synthesis of the ionic liquid;
- The solid fraction is rinsed in situ with deionised water (water/sorbents weight ratio = 2:1) in order to
recover the precursor faction trapped through the sorbents;
- The rinsing liquid phase, constituted by a clear and colourless diluted aqueous solution of PYR13Br, is
combined with the liquid fraction obtained upon filtration.
2.1.3 Synthesis of ionic liquid - LiTFSI (or NaFSI), previously dissolved in the minimal water amount, is added (into the glass reactor)
to the aqueous PYR13Br solution (coming from the purification step). The stoichiometric PYR13Br/LiTFSI
(or PYR13Br/NaFSI) ratio is equal to 1:1. However, the lithium salt was used in slight excess (2-3 wt.%) to
enhance the yield of the anion exchange (metathesis) reactions (2) and (3):
PYR13Br (aq.) + LiTFSI (aq.) PYR13TFSI + LiBr (aq.) (2)
PYR13Br (aq.) + NaFSI (aq.) PYR13FSI + NaBr (aq.) (3)
- Immediately upon addition, formation of two separate phases (corresponding to the formation of the
hydrophobic ionic liquid) is observed. The solution is stirred for one hour at room temperature;
- After 30-60 minutes of rest (at 5 °C) complete phase separation takes place. The upper phase, mainly
constituted by an aqueous solution of LiBr or NaBr (by-product), is separated from the lower IL one
(denser);
- The ionic liquid phase is rinsed with water 5-6 times to extract remove the water soluble LiBr (or NaBr)
and the LiTFSI (or NaFSI) excess;
- The PYR13TFSI (or PYR13FSI) ionic liquid is vacuum filtered over a polyimide filter (pore size = 0.2 μm).
A clear, colourless and odourless IL material is obtained;
- The PYR13TFSI (or PYR13FSI) ionic liquid is vacuum (oil-free pump) dried in a rotary evaporator at
90 °C for 2-3 hours;
- The final drying step, performed in dry room (moisture content < 10 ppm), was carried out at 20 °C for
1 hour, then at 60 °C for 3 hours and, finally, at 120 °C for (additional) 18 hours.
2.2 Quality control of ionic liquids The water content in the ionic liquids was measured using the standard Karl Fisher method. The titrations
were performed in the dry room by an automatic Karl Fisher coulometer titrator (Mettler Toledo DL32) at
5
20°C. The Karl Fisher titrant was a one-component (Hydranal 34836 Coulomat AG) reagent provided
from Aldrich.
A screening of impurities in the IL samples was run by X-ray fluorescence spectroscopy using a Philips
PW 2404 spectrometer.
The concentration of specific ion species was checked by atomic absorption analysis (SpetcrAA mod. 220
atomic absorption Spectrometer).
Figure 2: Schematic representation of the synthetic route of the ionic liquid materials in aqueous
environment. AC = activated charcoal. Thick arrows indicate solids or slurries, thin arrows indicate
liquids.
6
2.3 Preparation of ionic liquid electrolytes The ionic liquid electrolytes were prepared within the dry room by blending the IL materials and LiTFSI
(previously vacuum dried at 120 °C for 24 hours) in the appropriate mole ratio and, then, stirring them at
50 °C until lithium salt full dissolution (generally in a few minutes). Two different IL electrolyte sets were
prepared as reported in Table 2. The first one (Set I) is composed by (x)PYR13FSI-(1-x)PYR13TFSI binary
IL mixtures (LiTFSI-free) with the aim to determinate the optimal IL mole composition. The second one
(Set II), constituted by (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte samples, has the
objective to determinate the optimal lithium salt mole fraction. The mole fraction of the electrolyte sample
components is represented by (x), (1-x), (y), (z) and (1-y-z). It is note that the TFSI-FSI (i.e., TFSI coming
from both PYR13TFSI and LiTFSI) mole ratio of Set II samples was fixed equal to 2:3 (see §II.2).
Table 2: Mole fraction of ionic liquid electrolytes.
Set I Set II
Component I.A I.B I.C I.D I.E I.F I.G I.H I.I I.L I.M II.A II.B II.C II.D II.E II.F
LiTFSI ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- 0.005 0.01 0.02 0.05 0.10 0.20
PYR13FSI 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.600 0.60 0.60 0.60 0.60 0.60
PYR13TFSI 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.395 0.39 0.38 0.35 0.30 0.20
2.4 Thermal analysis Differential Scanning Calorimetry (DSC) measurements were performed using a TA Instruments (Model
Q100). Hermetically sealed Al pans were prepared in the dry room. In order to allow complete
crystallization [6], the IL materials were thermally annealed in the DSC instrument by repeatedly cycling
and/or holding the samples at sub-ambient temperatures for varying periods of time. Successively, the
samples were cooled (10 °C min-1) down to -140 °C and, then, heated (10 °C min-1) up to 150 °C. The
appearance of a cold recrystallization peak during the heating scan was taken as test of uncompleted
crystallization of the sample.
The thermal stability was verified in nitrogen atmosphere by Thermo Gravimetrical Analysis (TGA) using
a Q600 SDT equipment, simultaneous TG-DTA (TA Instruments) with Thermal Solution Software
(version 1.4). The temperature was calibrated using the nickel Curie point as the reference. The mass was
calibrated using ceramic standards provided with the instrument. High purity aluminium oxide was used
as the reference material. During the experiments high purity nitrogen was flown at a relatively high rate
(100 ml min-1) to avoid contamination from the external atmosphere. The experiments were performed on
10-12 mg samples, which were stored, handled and weighed in the dry room. Open platinum crucibles
(cross-section = 0.32 cm2) were used. The thermal stability was initially investigated by heating from
room temperature up to 700 °C at 10 °C min-1. The onset temperature was calculated by thermal analysis
software Universal Analysis version 2.5 as the intersection between the extrapolated baseline weight and
the tangent through the inflection point of the weight vs temperature curve.
2.5 Ion transport properties The ionic conductivity of ionic liquid electrolyte mixtures was determined by a conductivity meter AMEL
160. The IL samples were housed in sealed, glass conductivity cells (AMEL 192/K1) equipped with two
porous platinized platinum electrodes (cell constant equal to 1.0 ± 0.1 cm-1). The cells were assembled in
the dry room. The conductivity tests were performed in the temperature range from –40 °C to 100 °C by a
climatic test chamber (Binder GmbH MK53). In order to fully crystallize the materials, the cells were
immersed in liquid nitrogen for a few seconds and, then, transferred in the climatic chamber at -40 °C.
After a few minutes of storage at this temperature, the solid but amorphous samples relaxed and turned
again liquid. This route was repeated until the samples remained solid at -40 °C. In previous work [6] it
7
was demonstrated that a not careful crystallization process of ionic liquid may generate non-equilibrium
states of the sample, affecting the thermal properties as well as the conductivity results. Finally, after a
storage period at -40 °C for at least eighteen one hours, the conductivity of the IL materials was measured
by running a heating scan at 1 °C h-1.
The pure ionic liquids, PYR13TFSI and PYR13FSI, and their binary mixture (0.6)PYR13TFSI-(0.4)PYR13FSI
(based on the (PYR13+) cation combined with the TFSI and FSI fluorinated anions), have been subjected
to NMR spectroscopy measurements. The chemical structure and atom numbering of the cation and
anions are reported in Figure 3.
Figure 3: Chemical structure and atom numbering of cation (a) and anions (TFSI, b) and (FSI, c).
The 1H and 19F NMR spectra were recorded on a Bruker Avance 500 spectrometer operating at 500.13
MHz proton frequency equipped with a QNP four nuclei switchable probe. The IL samples (0.5-1.0 g each
one) were transferred in 5 mm NMR glass tubes inside the dry room to avoid any contamination from
external. The NMR tubes, equipped with a co-axial capillary containing d6-DMSO as the internal chemical
shift reference, were immediately flame-sealed after transferring the samples. Heteronuclear {1H-19F}HOESY experiments were acquired using the inverse-detected pulse sequence with 256 increments in
the t1 dimension and sixteen scans for each experiment. The mixing time was 50 ms.
Self-diffusion coefficients were measured by pulsed field gradient (PFG) experiments. A pulsed gradient
unit capable of producing magnetic field pulse gradients in the z-direction of 53 G cm-1 was used. All the
experiments were performed using the bipolar pulse longitudinal eddy current delay (BPLED) pulse
sequence. Cation and anion self-diffusion coefficients were measured independently by carrying out PFG
experiments in the 1H and 19F frequency domains, respectively. The duration of the magnetic field pulse
gradients () and the diffusion times () was optimized for each sample in order to obtain complete
dephasing of the signals with the maximum gradient strength. In each PFG experiment, a series of 32
spectra with 16K points were collected. For the investigated samples, δ value was 3 ms for 1H and 19F
experiments whereas the Δ values were in the 0.1-0.33 s. The pulse gradients were incremented from 2 to
95% of the maximum gradient strength in a linear ramp. Variable temperature experiments were
performed in the 300-340 K range. The temperature was controlled with an air flow of 535 l h-1.
2.6 Rheological properties Viscosity measurements were performed (within the dry room) on the ionic liquid electrolytes using a
HAAKE RheoStress 600 Rheometer from 20 °C (i.e., above the melting point of the IL samples) to 80 °C
by running a 0.1 °C min-1 heating scan. The viscosity values were taken at 10 °C steps. The measurements
were run in the 100-2000 s-1 scan speed range in order to verify the Newtonian behaviour of the IL
samples.
The density measurements were performed from 20 to 90 °C by 10 °C step using a density meter (Mettler
Toledo DE40) in the dry room. The IL samples were previously degassed under vacuum at 50 °C
overnight to avoid bubble formation during the cooling scan tests.
8
2.7 Electrochemical stability The electrochemical stability window (ESW) was evaluated by linear sweep voltammetries (LSVs)
performed at 5 mV s-1 and 20 °C. A sealed, three-electrode, glass micro-cell, described in details in [7],
was used for the LSV tests. A glass-sealed, platinum working microelectrode (active area equal to 0.78
mm2) and a platinum foil counter electrode (about 0.5 cm2) were used. The reference electrode was a
silver wire immersed in a 0.01 M solution of AgCF3SO3 in PYR14TFSI ionic liquid, separated from the cell
compartment with a fine glass frit. This reference electrode was proven to be highly reversible and stable
for at least 3 weeks. The potential of this electrode, frequently controlled against a 5×10-3 M solution of
Ferrocene in PYR14TFSI, was +0.39 V vs. Fc/Fc+ (at 20 °C), corresponding to +3.4 V vs. Li/Li+. All
potentials hereafter will be referred to this reference electrode if not mentioned otherwise. High purity
argon (3 ppmv water and 2 ppmv oxygen) was flown over the ionic liquid under investigation for 30
minutes before the start of the test. The gas flow was continued during the experiment. Separate LSV tests
were carried out on each sample to determine the cathodic and anodic electrochemical stability limits.
The measurements were performed scanning the cell potential from the open circuit potential (OCP)
towards more negative (cathodic limit) or positive (anodic limit) voltages. The whole ESW of each tested
sample was obtained by composing two LSV (cathodic and anodic) experiments. Clean electrodes and a
fresh sample were used for each test. To confirm the results obtained, the LSV tests were performed at
least twice on different fresh samples of each IL material. The measurements were performed at 20 °C
using a Schlumberger (Solartron) Electrochemical Interface (model 1287).
9
3 Results and Discussions
3.1 Quality control of ionic liquids The synthesis procedure route described in I.1 allowed to obtain transparent, uncoloured and odourless
PYR13TFSI and PYR13FSI ionic liquids (Table 3) with an overall yield ranging from 85 to 90 mol.%.
The results of the chemical analysis performed on the IL materials are reported in Table 3. The
concentration of Li+ and Br- was found to lower than 2 ppm whereas Fe does not exceed 40 ppm. Other
impurities as Na, Al, Mo, Si, Cl exhibited an overall content below 100 ppm. The humidity content was
found to be lower than 2 ppm was detected.
Table 3: Chemical analysis on the PYR13TFSI and PYR13FSI ionic liquids. A picture of the PYR13TFSI
material (compared with deionized H2O) is reported.
Li
< 2 ppm
Br
< 2 ppm
Fe
< 40 ppm
(Na ,Al, Mo, Si, Cl)
< 100 ppm
H2O
< 2 ppm
3.2 Determination of the optimal ionic liquid mole ratio The first step concerning the development of the ionic liquid electrolyte mixtures was the optimization of
the PYR13FSI/PYR13TFSI mole ratio. Therefore, a set of (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures
(Table 2) was properly prepared and characterized in terms of thermal, physicochemical and
electrochemical properties.
Figure 4 reports the DSC heating trace of the (x)PYR13FSI-(1-x)PYR13TFSI mixture (the thermal cycling
performed to favour crystallization are not reported). The pure IL materials, e.g., PYR13FSI and PYR13TFSI
are reported for comparison purpose. The pure PYR13TFSI (blue trace of Figure 4) and PYR13FSI (red
trace of Figure 4) show an endothermic feature around 12 and -5 °C, respectively, due to melting of the IL
samples [8]. In addition, PYR13FSI exhibits a weaker exothermal peak around -15 °C, ascribable to solid-
solid phase transition, which disappears at x < 0.2. No glass transition feature is observed, indicating that
the IL mixtures were carefully crystallized prior heating scan [6]. The addition of PYR13FSI to PYR13TFSI
results in progressive shift of the PYR13TFSI melting feature (labelled with asterisk) down to lower
temperatures, e.g., for a PYR13FSI mole fraction (x) equal to 0.6 the melting feature is located down to -
20 °C, as a consequence of the fusion temperature decrease of the binary mixture with respect to pure
PYR13TFSI. Conversely, further incorporation of PYR13FSI above x = 0.6 (PYR13FSI-rich samples) leads to
a shift of the endothermic feature towards higher temperatures, indicating progressive raise of the
mixture melting point, until matching that of pure PYR13FSI (x = 1). Summarizing, (x)PYR13FSI-(1-
x)PYR13TFSI mixtures shows lower melting temperature with respect to that of the single ionic liquid
components.
10
Figure 4: Heating DSC trace of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures. Scan rate: 10 °C min-
1. The features labelled with the asterisk (*) are related to the PYR13TFSI melting.
The ionic conductivity vs. temperature dependence of the (x)PYR13FSI-(1-x)PYR13TFSI ionic liquid
mixtures is illustrated in panel A of Figure 5. Error bars fall within the data markers. Pure PYR13TFSI
(open squares) and PYR13FSI (open triangles) display an onset near -20 °C and -30 °C, respectively,
followed by a substantial conductivity vs temperature slope increase, indicating that the ions are able to
move even if the IL samples are still in solid phase, e.g., in agreement with the DSC results (Figure 4)
which revealed solid-solid phase transitions before melting for PYR13FSI (not detected in the thermal
trace of PYR13TFSI because of the slower crystallization kinetics of this IL material [4,8]. Upon a
progressive rise (from 10-8 S cm-1) of more than four orders of magnitude, the ionic conductivity of
PYR13TFSI and PYR13FSI is seen to approach 10-3 S cm-1 around 12 °C (PYR13TFSI) and -10 °C (PYR13FSI),
respectively, due to the melting of these IL materials (in agreement with the thermal measurements). The
incorporation of PYR13FSI to PYR13TFSI results in relevant conductivity increase at temperatures well
below its melting point, indicating that properly combining ionic liquid materials hinders the
crystallization of the resulting mixture (e.g., once more agreeing the DSC results) and improves the ion
transport properties, especially at the low temperatures. In addition, the presence of anions (TFSI and
FSI) sharing different steric hindrance hinders the ion packing through the IL mixtures. Ion conduction
values close to 10-3 S cm-1 are matched already at -25 °C for mole fraction (x) ranging from 0.6 to 0.8
whereas the single pure IL materials (still in the solid state) do not exceed 10-7 S cm-1. Above 15 °C (IL
samples in the molten state) the difference in conductivity among the (x)PYR13FSI-(1-x)PYR13TFSI
mixtures (and with respect to the single IL components) appears much more moderate.
The effect of the mole composition on the ionic conductivity is more evident in panel B of Figure 5. The
ion conduction of the (x)PYR13FSI-(1-x)PYR13TFSI mixtures is found, at a first sight, to increase with
increasing the mole fraction (Table 4) of the more conductive PYR13FSI component. However, a more
careful observation shows that this trend is true only at temperatures higher than -10 °C, e.g. above the
melting temperature of PYR13FSI. At lower temperatures, conversely with respect to the (x)PYR13FSI-(1-
x)PYR13TFSI mixtures blends which keep liquid, the PYR13TFSI and PYR13FSI materials turn solid and,
therefore, the conductivity vs mole ratio plot follows a bell behaviour, showing a maximum value at
11
PYR13FSI mole fraction ranging from 0.6 to 0.8. The ion conduction value of the (x)PYR13FSI-(1-
x)PYR13TFSI binary mixtures, determined at -20 and 20°C, is summarized in Table 4. Values approaching
10-3 S cm-1 are reached, at -20 °C, in the 0.6 ≤ x ≤ 0.8 PYR13FSI mole fraction range.
Rheological measurements were performed for better understanding the conduction phenomena. In
panels C and D of Figure 5 is depicted the viscosity vs temperature (panel C) and vs PYR13FSI mole
fraction (panel D) dependence of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures. The measurements
were performed at temperatures above the melting point of the IL samples, which have exhibited a
Newtonian behaviour (no viscosity variation with the rotation speed, data not shown). As clearly
evidenced in the figure, the viscosity is seen to decrease with the temperature as well as with increasing
the PYR13FSI mole fraction, showing an inverse trend with respect to that exhibited by the conductivity.
The latter results, therefore, directly correlated with the viscosity, indicating that the ion movement
through the binary IL mixtures is strongly affected by the ion viscous drag/friction (e.g., correlated to the
ion steric hindrance) [4,8].
Figure 5: Ionic conductivity (heating scan) vs temperature (panel A) and vs PYR13FSI mole fraction
(panel B), viscosity (heating scan) vs temperature (panel C) and vs PYR13FSI mole fraction (panel D)
of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures.
12
Figure 6 plots the density vs temperature (left panel) and vs PYR13FSI mole fraction (right panel)
behaviour of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures. The measurements were performed at
temperature above the melting point of the IL materials. The density values are seen to linearly decrease
with the temperature as well as with the PYR13FSI mole fraction (Table 4). Interestingly, the density vs
mole fraction plot shows a knee around x = 0.5, suggesting a rearrangement of the ion structural
organization and/or packing within the binary mixtures. This issue, worth to be better understood, is
actually under investigation in our laboratories.
Figure 6: Density (heating scan) vs temperature (left panel) and vs PYR13FSI mole fraction (right
panel) of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures.
The thermal and electrochemical stabilities of whatever electrolyte component are crucial properties in
view of its application in electrochemical devices. Figure 7 (left panel) plots the variable-temperature TGA
(performed in nitrogen atmosphere) trace of the (0.6)PYR13FSI-(0.4)PYR13TFSI binary mixture. The pure
PYR13FSI (black trace) and PYR13TFSI (blue trace) samples, reported for comparison purpose, are found
to be thermally stable up to 200 and 300 °C, respectively, confirming the superior stability observed for
the TFSI-based ionic liquids [4,8]. Surprisingly, the (0.6)PYR13FSI-(0.4)PYR13TFSI mixture (red trace)
exhibits a thermal behaviour, at least up to about 350 °C, close to that of pure PYR13FSI. Therefore, the
PYR13TFSI fraction does not influence the stability of the binary mixture.
Figure 7: Left panel: TGA trace of the (0.6)PYR13FSI-(0.4)PYR13TFSI binary mixture. The pure
PYR13FSI and PYR13TFSI samples are reported for comparison purpose. The measurements were run
in N2 atmosphere. Right panel: ESW of PYR13FSI and PYR13TFSI at 20 °C. Working and counter
13
electrodes: Pt. A Ag wire immersed in a 0.01 M solution of AgCF3SO3 in PYR14TFSI was chosen as the
reference electrode. Scan rate: 5 mV s-1.
The electrochemical stability window of the pure PYR13FSI (red trace) and PYR13TFSI (blue trace) samples
is depicted in Figure 7 (right panel). The potential values are given both versus the Ag/Ag+ (0.01 M
solution of AgCF3SO3 in PYR14TFSI reference electrode) redox couple. The IL materials (showing a
current density step raise when the anodic or cathodic decomposition process takes place) exhibit similar
anodic breakdown potentials (due to anion oxidation), e.g., from 1.9 V to 2.1 V vs. Ag/Ag+, indicating
nearly stability for the TFSI and FSI anions. No other feature is observed during the anodic scan,
excluding the presence of impurities which are oxidized within the samples. Conversely, PYR13TFSI
displays a cathodic breakdown potential (due to cation reduction) about 500 mV lower than that of
PYR13FSI, suggesting that the cation stability is affected by the nature of the anion. From Figure 8, it
results that the IL samples show an ESW close to 5 V, e.g. suitable for high voltage electrochemical
devices.
Table 4 summarizes the physicochemical properties of the (x)PYR13FSI-(1-x)PYR13TFSI binary mixtures.
The reported results suggest how the (0.6)PYR13FSI-(0.4)PYR13TFSI mixture represents the optimal
compromise among thermal, ion-transport and rheological properties. This allowed to fix the mole ratio
between PYR13TFSI and PYR13FSI equal to 0.4:0.6, corresponding to a TFSI:FSI mole ratio equal to 2:3.
Therefore, the formulation of the LiTFSI-PYR13TFSI-PYR13FSI ternary electrolytes (Table 4) was designed
by fixing the TFSI:FSI mole ratio equal to 2:3.
Table 4: Summary of the physicochemical properties (determined at 20 °C) of the (x)PYR13FSI-(1-
x)PYR13TFSI binary mixtures. The ionic conductivity values are also reported at -20 °C.
Sample Ionic conductivity / S cm-1
-20 °C 20 °C Viscosity / mPa s
Density / g cm-3
x = 0 (4.9 ± 0.3) × 10-8 (2.4 ± 0.1) × 10-3 73 ± 4 1.432 ± 0.001
x = 0.1 (5.2 ± 0.3) × 10-6 (2.6 ± 0.1) × 10-3 67 ± 4 1.427 ± 0.001
x = 0.2 (9.8 ± 0.5) × 10-5 (2.9 ± 0.2) × 10-3 64 ± 3 1.419 ± 0.001
x = 0.3 (2.1 ± 0.1) × 10-4 (3.2 ± 0.2) × 10-3 60 ± 3 1.411 ± 0.001
x = 0.4 (2.7 ± 0.2) × 10-4 (3.4 ± 0.2) × 10-3 n. a. 1.403 ± 0.001
x = 0.5 (4.3 ± 0.2) × 10-4 (3.7 ± 0.2) × 10-3 n. a. 1.394 ± 0.001
x = 0.6 (7.6 ± 0.4) × 10-4 (3.9 ± 0.2) × 10-3 51 ± 3 1.384 ± 0.001
x = 0.7 (8.5 ± 0.4) × 10-4 (4.2 ± 0.2) × 10-3 53 ± 3 1.374 ± 0.001
x = 0.8 (8.3 ± 0.4) × 10-4 (4.4 ± 0.2) × 10-3 n. a. 1.365 ± 0.001
x = 0.9 (3.0 ± 0.2) × 10-4 (4.9 ± 0.3) × 10-3 46 ± 2 1.354 ± 0.001
x = 1 (9.4 ± 0.5) × 10-8 (5.4 ± 0.3) × 10-3 45 ± 2 1.343 ± 0.001
3.3 Determination of the optimal lithium salt mole fraction On the basis of the results obtained for the PYR13FSI-PYR13TFSI binary samples (Table 2), the mole
composition of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte mixtures was designed by
keeping the (y+z)/(1-y-z) mole ratio equal to 2:3. The ternary IL mixtures were characterized in terms of
thermal and physicochemical properties.
The DSC heating trace of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte mixtures are
depicted in Figure 8 (the thermal cycling traces performed to favour crystallization are not reported).
Only an endothermic feature, corresponding to the IL sample melting, was observed in the thermal
diagram of the mixtures (as well as for the PYR13FSI-PYR13TFSI binary mixtures the absence of the glass
transition feature supports for fully crystallized IL samples). The addition of the LiTFSI salt to the
14
(0.4)PYR13TFSI-(0.6)PYR13FSI binary sample (y = 0) results in progressive shift of the melting feature
towards lower temperatures (e.g., the shift of the onset of the melting peak is highlighted by the arrows in
Figure 8). This is due to the presence of cations, i.e., (PYR13)+ and Li+, which, having very different steric
hindrance, hinder the ion packing lowering the melting temperature [4,6]. Therefore, the (y)LiTFSI-
(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte mixtures exhibit fusion temperature around -30 °C,
appealing for low temperature applications.
Figure 8: Heating DSC trace of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte
mixtures. Scan rate: 10 °C min-1. The TFSI/FSI mole ratio was fixed equal to 2:3. The arrows indicate
the on-set of the sample melting feature.
The ionic conductivity vs temperature dependence of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI
ternary electrolyte mixtures is illustrated in Figure 9. Error bars fall within the data markers. Apart the
LiTFSI-free sample (x = 0), the ternary IL mixtures display an onset near -40 °C, followed by a substantial
conductivity vs temperature slope increase, once more indicating that the ions are able to move even if the
IL samples are still in solid phase (e.g., solid-solid phase transition features are not revealed in DSC traces
because of the slow crystallization kinetics of IL samples). Upon a progressive rise (from 10-7 S cm-1) of
more than three orders of magnitude, the ionic conductivity of the ternary mixtures is seen to overcome
10-4 S cm-1 already below -25 °C because of the melting of these IL materials (in agreement with the
thermal measurements) and to approach 10-3 S cm-1 at -20 °C.
Figure 10 plots the density vs temperature behaviour of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI
binary mixtures. The measurements were performed at temperature above the melting point of the IL
mixtures. The density values (Table 5) are seen to linearly decrease with the temperature, but to increase
with the LiTFSI mole fraction, sharing very similar slope not depending on the lithium salt content and
temperature, respectively. Conversely to that observed in PYR13FSI-PYR13TFSI binary mixtures, no
rearrangement of the ion structural organization and/or packing within the LiTFSI-containing electrolyte
mixtures was detected.
15
Table 5 summarizes the physicochemical properties of the (y)LiTFSI-(z)PYR13TFSI-(1-y-z)PYR13FSI
ternary electrolyte mixtures. On the basis of the reported results, the optimal range of LiTFSI mole
fraction is 0.02 ≤ x ≤ 0.1, allowing high conductivity values even at low temperatures (≤ -25 °C). It is to
note that a LiTFSI mole faction ≤ 0.1 is able to prevent crystallization of the IL electrolyte, even in
presence of high lithium concentration gradients [6], and results in moderately low viscosity values [4].
However, LiTFSI mole fractions < 0.1, corresponding to a LiTFSI molar concentration < 0.4 M, are
regarded to not sufficiently assure an efficient Li+ reservoir at interface with electrodes. Therefore, the
optimal LiTFSI mole fraction was fixed equal to 0.1.
Figure 9: Ionic conductivity (heating scan) vs temperature dependence of the (y)LiTFSI-
(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte mixtures. The TFSI/FSI mole ratio was fixed equal
to 2:3. Scan rate: 1 °C hour-1.
Figure 10: Density (heating scan) vs temperature dependence of the (y)LiTFSI-(z)PYR13TFSI-(1-y-
z)PYR13FSI ternary electrolyte mixtures. The TFSI/FSI mole ratio was fixed equal to 2:3.
16
Table 5: Summary of the physicochemical properties (determined at 20 °C) of the (y)LiTFSI-
(z)PYR13TFSI-(1-y-z)PYR13FSI ternary electrolyte mixtures. The TFSI/FSI mole ratio was fixed equal
to 2:3. The ionic conductivity values are also reported at -20 °C.
LiTFSI mole fraction (y) Ionic conductivity / S cm-1
-20 °C 20 °C Density / g cm-3
0 (7.6 ± 0.4) × 10-4 (2.2 ± 0.1) × 10-3 1.384 ± 0.001
0.005 (6.1 ± 0.3) × 10-4 (1.6 ± 0.1) × 10-3 1.386 ± 0.001
0.01 (5.8 ± 0.3) × 10-4 (1.5 ± 0.1) × 10-3 n. a.
0.02 (5.5 ± 0.3) × 10-4 (1.4 ± 0.1) × 10-3 1.390 ± 0.001
0.05 (4.6 ± 0.3) × 10-4 (1.2 ± 0.1) × 10-3 1.398 ± 0.001
0.1 (3.8 ± 0.2) × 10-4 (1.1 ± 0.1) × 10-3 1.413 ± 0.001
0.2 (1.6 ± 0.1) × 10-4 (6.9 ± 0.4) × 10-4 1.443 ± 0.001
3.4 Ion self-diffusion coefficient measurements NMR heteronuclear NOE correlation experiments (HOESYwere performed to gain information on cation-
anion interactions, aggregation motives and ions mobility. The 2D HOESY experiments, run on pure
PYR13TFSI as well as on the (0.4)PYR13TFSI-(0.6)PYR13FSI mixture, show dipolar contacts among
selected protons of the cation and the fluorine nuclei of the TFSI anion (Figure 11).
The pure IL material and the binary mixture show a similar NOE pattern with weaker NOE contacts
between the CF3 group of the TFSI anion and the H(2) - H(3) protons belonging to the cation alkyl chain.
The most intense cross peaks are detected among the N-methyl protons and the anion fluorine groups.
Also, the pyrrolidinium ring strongly interacts with the anion. No significant difference are observed in
the cation-TFSI anion structural organization between pure PYR13TFSI and the (0.4)PYR13TFSI-
(0.6)PYR13FSI mixture.
The ion self-diffusion coefficient values (D) for the PYR13+ cation were determined, in the 300-340 K
temperature range, for the pure IL materials (PYR13TFSI and PYR13FSI) and their (0.4)PYR13TFSI-
(0.6)PYR13FSI mixture by using pulsed field gradient spin-echo (PFGSE) techniques. The results are
reported in Table 6. It is interesting to compare the D values of the PYR13+ cation among the pure
components and the binary mixture. The observed trend is DPYR13(FSI) > Dmix > DPYR13(TFSI) as expected
because of the lower viscosity of PYR13FSI (Figure 5). The activation energy value of the ion diffusion
motion has been calculated from the D vs. temperature dependence, shown in Figure 12, accordingly to
the Arrhenius law.
Table 6: Experimental self-diffusion coefficients (D), determined at 305 K, for the pure ionic liquids
and the (0.4)PYR13TFSI-(0.6)PYR13FSI mixture.
IL sample D cation / m2 s-1 Ea / kJ mol-1
PYR13TFSI 3.32 × 10-11 29.4
(0.4)PYR13TFSI-(0.6)PYR13FSI 3.89 × 10-11 25.2
PYR13FSI 4.22 × 10-11 25.7
The PYR13+ cation diffusivity (Figure 12) in the (0.4)PYR13TFSI-(0.6)PYR13FSI binary mixture seems to be
the average of the values measured for the pure IL components. The same does not hold for the diffusion
activation energy (Ea), indicating that the Ea value observed for PYR13+ in the binary mixture is nearly
equal to that measured in PYR13FSI. This behaviour seems to indicate that the ion diffusion in a ionic
liquid mixture is a complex phenomenon to be investigate with a more sophisticated approach. Indeed, a
new methodology is currently being used to understand the behaviour of IL mixtures and
17
Figure 11: Contour plot of HOESY experiment on PYR13TFSI with 1H spectrum (top) and 19F spectrum
(left).
Figure 12: PYR13
+ cation self-diffusion coefficient vs temperature dependence, reported as Arrhenius
plot, for pure PYR13TSFI, PYR13FSI and their mixture (0.4)PYR13TFSI-(0.6)PYR13FSI.
The complex interplay of factors affecting the association phenomena and/or the local order. The
approach is based on variable diffusion time experiments, which have been started on the mixture shown
18
above and on the pure IL components. Such a class of experiments provides the means squared
displacement (MSD) of the ions and its time dependence. Preliminary results point out the existence of
anomalous diffusion regimes (non-Stokesian behavior), especially in the ILs mixture, providing
experimental data on a novel aspect in the transport properties of ILs so far not extensively present in
literature.
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4 Conclusions
Innovative ionic liquid (IL)-based mixtures as safer electrolytes for lithium-ion battery systems were
properly developed. The basic idea is to combine different IL materials with the aim to obtain mixtures
with improved properties with respect to single ionic liquids. PYR13TFSI and PYR13FSI were selected
because of their wide thermal/electrochemical stability (PYR13TFSI) in combination with high ionic
conductivity, low melting point and good film-forming ability (PYR13FSI).
Particular care was devoted to the synthesis and purification routes of the ionic liquid materials. Only
water was used as the process solvent. This safer, cheaper and environmentally friendly procedure route
allowed of obtaining high purity, anhydrous ionic liquids.
The IL electrolyte mixtures exhibited fast ion-transport properties even at low temperatures (< -20 °C) in
combination with good thermal and electrochemical stability. Conversely, pure PYR13TFSI and PYR13FSI
are still in the solid state. This clearly demonstrated the feasibility to properly combine different ILs for
preparing electrolytes with superior characteristics. On the basis of the obtained results, the TFSI:FSI
mole ratio = 2:3 was seen as the optimal one whereas the most promising LiTFSI mole fraction was 0.1.
Therefore, the (0.1)LiTFSI-(0.3)PYR13TFSI-(0.6)PYR13FSI ternary mixture was selected as the most
appealing electrolyte.
Two NMR tools were mainly used to understand, at molecular level, the interactions and the transport
properties of IL mixtures amenable to be used in lithium-ion battery (LIB) systems as safer electrolytes:
hetero-nuclear NOE and diffusion measurements. In particular, diffusion experiments seem to be the
appropriate experimental tool for the characterization and selection of IL mixtures suitable for application
in LIB, especially by exploiting the variable diffusion time approach.
20
5 References
1. J.R.D. Rogers, K.R. Seddon, Ionic Liquids: Industrial Application to Green Chemistry (ACS
Symposium Series 818), American Chemical Society, Washington (2002).
2. C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. 18 (2005) 275.
3. Electrochemical Aspects of Ionic Liquids, Ohno, H.; Ed., John Wiley & Sons Inc., Hoboken, New
Jersey (2005).
4. G.B. Appetecchi, M. Montanino, S. Passerini, Ionic Liquid-based Electrolytes for High-Energy
Lithium Batteries in Ionic Liquids: Science and Applications, ACS Symposium Series 1117, A.E.
Vissser, N.J. Bridges, and R.D. Rogers editors, Oxford University Press, Inc., American Chemical
Society, Washington, DC, USA (2013).
5. M. Montanino, F. Alessandrini, S. Passerini, G.B. Appetecchi, Electrochim. Acta 96 (2013) 124.
6. W.A. Henderson, S. Passerini, Chem. Mater. 16 (2004) 2881.
7. S. Randström, G.B. Appetecchi, C. Lagergren, A. Moreno, S. Passerini, Electrochim. Acta 53
(2007) 1837.
8. G.B. Appetecchi, M. Montanino, M. Carewska, M. Moreno, F. Alessandrini, S. Passerini,
Electrochim. Acta 56 (2011) 1300.
21
6 Contacts and Authors
Main contact
Dr Giovanni Battista Appetecchi, ENEA (Italian National Agency for New Technologies,
Energy and the Sustainable Economic Development), Rome, Italy.
Authors
Dr Giovanni Battista Appetecchi (ENEA): [email protected]
Dr Maria Carewska (ENEA): [email protected]
Dr Maria Montanino (ENEA): [email protected]
Dr Margherita Moreno (ENEA): [email protected]
Prof. Andrea Mele (Politecnico of Milan): andrea [email protected]
Dr. Franca Castiglione (Politecnico of Milan): [email protected]
Prof. Stefano Valdo Meille (Politecnico of Milan): [email protected]
Prof. Guido Raos (Politecnico of Milan): [email protected]
Prof. Antonino Famulari (Politecnico of Milan): [email protected]
