Novel Pyrrolinium-based Ionic Liquids for Lithium Ion Batteries: Effect of the Cation on Physicochemical and Electrochemical Properties

Article abstract of DOI:10.1016/j.electacta.2017.04.078

Lithium ion batteries (LIBs) are one of the most promising energy conversion/storage systems, but the low thermal stability of the current electrolytes in LIBs should be improved to expand their potential applications. To enhance the safety properties of LIBs, novel pyrrolinium-based ionic liquids (ILs) were proposed as an alternative electrolyte to the current carbonate electrolyte, which have some task-specific functional groups, i.e., a planar C[dbnd]N double bond, a C-O ether linkage, and no unstable C-H bond, designed to improve their electrochemical performances as well as the physicochemical properties. As a result, the pyrrolinium-based ILs exhibited much improved physicochemical and electrochemical properties compared to those of the known ILs. Among the prepared ILs, N-allyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide (A(OMe)Pyrl-FSI, 4) showed the high ionic conductivity (10.2 mS cm^?1), the very good cycling performance (99.3% of retention ratio after 50 cycles) with a LiFePO₄ electrode, and the much improved lithium ion transference number (0.19). IL 4 also had the remarkable rate capability at 5 C-rate with the retention ratio of 81.2% (124.8 mA h g^?1), compared to the initial discharge capacity of 153.7 mA h g^?1 at 0.1 C-rate. In addition, both their high thermal stability and non-flammability were also confirmed.

Full text of DOI:10.1016/j.electacta.2017.04.078

Accepted Manuscript
Title: Novel Pyrrolinium-based Ionic Liquids for Lithium Ion
Batteries: Effect of the Cation on Physicochemical and
Electrochemical Properties
Authors: Hyung-Tae Kim, Oh Min Kwon, Junyoung Mun,
Seung M. Oh, Taeeun Yim, Young Gyu Kim
PII:
S0013-4686(17)30837-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.04.078
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15-4-2017
Please cite this article as: Hyung-Tae Kim, Oh Min Kwon, Junyoung Mun, Seung M.Oh,
Taeeun Yim, Young Gyu Kim, Novel Pyrrolinium-based Ionic Liquids for Lithium Ion
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Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.04.078
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Novel Pyrrolinium-based Ionic Liquids for Lithium Ion Batteries:
Effect of the Cation on Physicochemical and Electrochemical
Properties
Hyung-Tae Kim,1 Oh Min Kwon,1 Junyoung Mun,1,2 Seung M. Oh,1 Taeeun Yim,1,3,*, and
Young Gyu Kim1,*
1School of Chemical and Biological Engineering, College of Engineering, Seoul National
University, Seoul, 08826, Korea
2Department of Energy and Chemical Engineering, College of Engineering, Incheon National
University, Incheon, 22012, Korea
3Department of Chemistry, College of Natural Sciences, Incheon National University,
Incheon, 22012, Korea

Abstract
Lithium ion batteries (LIBs) are one of the most promising energy conversion/storage
systems, but the low thermal stability of the current electrolytes in LIBs should be improved
to expand their potential applications. To enhance the safety properties of LIBs, novel
pyrrolinium-based ionic liquids (ILs) were proposed as an alternative electrolyte to the
current carbonate electrolyte, which have some task-specific functional groups, i.e., a planar
C=N double bond, a C-O ether linkage, and no unstable C-H bond, designed to improve their
electrochemical performances as well as the physicochemical properties. As a result, the
pyrrolinium-based ILs exhibited much improved physicochemical and electrochemical
properties compared to those of the known ILs. Among the prepared ILs, N-allyl-2-
methoxypyrrolinium bis(fluorosulfonyl)imide (A(OMe)Pyrl-FSI, 4) showed the high ionic
conductivity (10.2 mS cm-1), the very good cycling performance (99.3% of retention ratio
after 50 cycles) with a LiFePO4 electrode, and the much improved lithium ion transference
number (0.19). IL 4 also had the remarkable rate capability at 5 C-rate with the retention ratio
of 81.2% (124.8 mA h g-1), compared to the initial discharge capacity of 153.7 mA h g-1 at
0.1 C-rate. In addition, both their high thermal stability and non-flammability were also
confirmed.
Keywords: battery; electrolyte; ionic liquids; pyrrolinium; ether substituent; allyl substituent

1. Introduction
Lithium ion batteries (LIBs) have been investigated extensively for energy conversion and
storage systems for applications to large-scale devices due to their moderate rate performance
and long-term cycling life [1,2]. For broader applications of LIBs in the future, many types of
electrode materials with large specific capacities and high working voltages have been
developed to enhance the overall energy density of the cells [3-9]. However, the safety issues
of LIBs restrict the development and applications of advanced materials for the LIBs because
of the flammable nature of conventional carbonate solvents. Once an energy system is ignited
by an internal short or external damage, the flammable carbonate solvents participate in the
combustion reactions as fuel, which could lead to a thermal runaway accompanied with rapid
increase of the cell temperature [10,11]. Therefore, the thermal stability of the electrolytes in
LIBs must be established in order for them to be employed especially for large-scale devices.
In this regard, ionic liquids (ILs) have become a promising candidate to replace the
conventional carbonate solvents due to their non-flammability and non-volatility. Moreover,
the unique properties of ILs such as wide electrochemical window and high ionic
conductivity have made them be more attractive for LIBs as an alternative solvent [12-15].
Nevertheless, their practical applications are still restricted in the current LIBs because of the
relatively high viscosity and redox potential limitation, which seriously affect their overall
electrochemical performance such as the cycling efficiency and rate capability. Note that the
physicochemical and electrochemical properties of ILs are mainly influenced by their
chemical components and structure, both cations and anions. Therefore, it is important to
optimize the chemical structure of ILs for allowing them to be more applicable for the LIBs.
Herein, we wish to propose new pyrrolinium cation-based ILs functionalized by task-
specific chemical structures as an alternative solvent for LIBs (Figure 1) [16-20]. In the

pyrrolinium cation, a C=C or C=N double bond is introduced to improve physical properties
because the planar structure in ILs allows rather facile slip between molecules as shown with
the planar imidazolium-based ILs, resulting in higher ionic conductivity as well as lower
viscosity [13-15,21]. However, such a reactive C-2 proton present in the imidazolium cation
is absent in the pyrrolinium cation. In addition, the C=C double bond present in the alkyl
substituent would help increase the cathodic stability as shown with vinylene carbonate (VC)
or the N-allyl substituted ILs [21-24]. The oxygen atom of the C-O ether substituent would be
beneficial to improve both physical properties and Li+ (lithium ion) migration during
electrochemical charge/discharge processes due to the relatively small ether functional group
and high binding affinity of a Lewis basic oxygen atom to a Lewis acidic Li+ [24-28]. Finally,
a small lipophilic anion, bis(fluorosulfonyl)imide (FSI), would decrease both the melting
point and the viscosity of ILs but increase the stability to water. Note that it is the first
attempt, to the best of our knowledge [16,17], to use the task-specific functionalized
pyrrolinium-based ILs as the only electrolyte solvent for LIBs [18].
2. Experimental Section
2.1. General
Materials were purchased from commercial suppliers and used without further purification.
N-Methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 2-pyrrolidone, 1-bromopropane, allyl
bromide, and dimethyl sulfate were distilled under reduced pressure immediately prior to use.
All experimental glassware, syringes, and magnetic stirring bars were oven-dried and stored
in a desiccator before use. Viscosity measurements were carried out on a Brookfield DV-
II+cone/plate viscometer. Ionic conductivity was determined using a TOA-DKK CM-30R
benchtop conductivity meter. Crystallization and melting points were determined by using
TA Instruments Differential Scanning Calorimeter (DSC) Q1000 under N2 atmosphere.

Thermograms were recorded during heating from −50 to 100 oC scans at a heating rate of 10
oC min−1 after cooling to −50 oC scans at a cooling rate of 10 oC min−1. Thermal
decomposition temperature was recorded by using TA Instruments SDT Q600 under Ar
atmosphere. Heating rate and terminal temperature were set at 10 oC min−1 and 500 oC,
respectively.
2.2. Synthesis of the pyrrolinium-based ILs.
Although N-propyl-2-pyrrolidone and N-allyl-2-pyrrolidone are commercially available, we
have synthesized them as follows. To a tetrahydrofuran (THF, 200 mL) solution of sodium
hydroxide (1.5 equiv, 150.0 mmol) in a 250 mL round-bottomed flask (RB flask) were added
2-pyrrolidone (100.0 mmol) and alkyl bromide (1.5 equiv, 150.0 mmol) at room temperature.
The reaction mixture was heated under reflux for 24-48 hours. After the reaction was
completed, the solvent in the resulting mixture was evaporated under reduced pressure. To
remove the unreacted sodium hydroxide and other impurity, methylene chloride (MC, 200
mL) was added to the residue and the mixture was cooled down to -5 oC until the solid was
crystallized. Then, the solid was removed by filtration and the filtrate solution was
evaporated under reduced pressure to give crude N-alkyl-2-pyrrolidone. The crude product
was purified by silica gel column chromatography with a solvent mixture (methylene
chloride:methanol = 10:1) to give N-alkyl-2-pyrrolidone as a colorless oil.
A typical procedure for the synthesis of R(OMe)Pyrl-FSI [19]: N-Alkyl-2-pyrrolidone (50
mmol) and dimethyl sulfate (55 mmol, 1.1 equiv) were added to a 100 mL RB flask at room
temperature under nitrogen atmosphere. The reaction mixture was stirred at room temperature
for 12 h. After the reaction was finished, the resulting mixture was washed successively with
diethyl ether (50 mL × 3) and ethyl acetate (50 mL × 3) to remove the unreacted starting
material (the desired product was in the bottom layer, whereas the organic solvent such as

diethyl ether or ethyl acetate was the top layer). After the bottom layer was diluted with MC
(50 mL), the resulting solution was dried over magnesium sulfate (MgSO4) and filtered to
remove residual water. Then, the filtrated solution was concentrated under reduced pressure
to afford N-alkyl-2-methoxypyrrolinium methyl sulfate as a colorless oil.
To an aqueous solution of N-alkyl-2-methoxypyrrolinium methyl sulfate (20 mmol in 100 mL
of distilled water) in a 250 mL RB flask was added lithium bis(fluorosulfonyl)imide (LiFSI,
1.1 equiv, 22 mmol) at room temperature. After the reaction mixture was stirred for 1 h, the
bottom layer containing the desired product was partitioned from the top aqueous layer.
Thereafter, MC (100 mL) was added to separate the organic solution (the bottom layer) from
the aqueous layer. The aqueous layer (top) was extracted with MC (100 mL × 2), and the
combined organic layers (300 mL) together with the bottom layer were washed with distilled
water (150 mL × 3) again. Afterwards, the washed organic layers were dried over MgSO4,
filtered, and concentrated under reduced pressure to give a colorless oil of N-alkyl-2-
methoxypyrrolinium bis(fluorosulfonyl)imide as a colorless oil. In the synthesis of the
pyrrolinium-based ILs, the characterization of the prepared intermediates, such as N-alkyl-2-
pyrrolidone, N-alkyl-2-methoxypyrrolinium methyl sulfate, and N-alkyl-2-
methoxypyrrolinium bis(fluorosulfonyl)imide, was confirmed by 1H, 13C NMR spectra and
elemental analysis, as shown in Supplementary materials.
2.3. Preparation of the electrolytes.
To measure the viscosity, ionic conductivity, lithium ion transference number, and
electrochemical performance of the ILs, each IL was mixed with 1.0 M LiTFSI (lithium
bis(trifluoromethanesulfonyl)imide) as a lithium salt. Then, the resulting mixture was stirred
for 10 min at room temperature and dried at 55 oC in a vacuum oven for 8 h to remove the
water content in the mixture. The water content of the prepared ILs in the present study was

less than 50 ppm, as measured by the Karl-Fischer titration [21,24,29]. It should be noted that
LiTFSI instead of LiFSI was selected as a lithium salt for the electrolyte solution, because (1)
it has a similar chemical structure to LiFSI (compatibility with FSI-based ILs), (2) it is more
resistive to the corrosion of a current collector [30-34], and (3) it is cheaper than LiFSI and
commercially available. The commercial carbonate electrolyte (7, EC/EMC = 3/7 (vol %)
with 1.0 M LiPF6 + 3 wt% VC) was used to compare with the physicochemical and
electrochemical properties of the prepared ILs.
2.4. Lithium ion transference number (tLi+)
The lithium transference number was determined by a dc polarization combined with an
impedance spectroscopy [35-36]. The method consists of applying a small dc pulse to a
symmetrical Li/electrolyte/Li cell and measuring the initial (I0) and the steady-state (Iss)
current that flow through the cell. The cell was also monitored by the impedance
spectroscopy to measure the initial (R0) and the final (Rss) resistance of the two Li interfaces.
Under these circumstances, the lithium transference number, tLi+, is given by
tLi+ = Iss(ΔV − R0I0) / I0(ΔV − RssIss)
(1)
2.5. Electrochemical performance.
For the preparation of a LiFePO4-containing composite electrode, carbon-coated LiFePO4
powder, Super-P (as a carbon additive for conductivity enhancement), and
poly(vinylidenefluoride) (PVdF, as a binder) were mixed in a 8:1:1 wt% ratio, respectively,
which was finely dispersed in N-methyl-2-pyrrolidone (NMP). The resulting slurry was
coated on a piece of an Al current collector, which was dried in a vacuum oven at 120 °C.
After the drying process, the electrode sheet was shaped into a disk with a diameter 1.2 cm
and LiFePO4 loading of 1.05 mg cm-2. Li/LiFePO4 half-cells (2032 coin type) were fabricated
to characterize the electrode performance with the prepared electrolytes. To this end, the

LiFePO4 composite electrode was loaded into the coin cells along with Li foil. A glass fiber
filter (Advantec, GA-55, 0.21 mm thick, 0.6 μm pore) was used as a separator. To investigate
the effect of the prepared electrolytes, each prepared electrolyte was fixed with a 1.0 M
lithium salt concentration. The galvanostatic charge (de-lithiation) and discharge (lithiation)
cycling of the Li/LiFePO4 cells were conducted using a WBCS3000 cycler at 25 °C in a
constant current mode. A pre-cycling step was added to obtain the full activation of LiFePO4.
The galvanostatic charge–discharge processes involve two methods. 1) The cycling of
Li/LiFePO4 cells with different electrolytes was performed at a 1 C current rate with 50
cycles. 2) To investigate the rate capability, each coin cell with a LiFePO4 cathode was
measured at different current densities from 0.1 C to 5 C (0.1 C → 0.2 C → 0.5 C → 1 C →
2 C → 5 C), and the current density was changed after three cycles of each C rate during the
same cut-off voltage (2.5–3.9 V). Linear sweep voltammetry was performed with a CHI660A
electrochemical workstation for the measurement of electrochemical stability window at a
scan rate of 10 mV s−1 with a glassy carbon electrode (7.07 × 10−2 cm2) as a working
electrode. The working electrode was polished before each measurement. A lithium electrode
was used as both a counter electrode and a reference electrode, respectively. LSV tests were
performed on each electrolyte to determine the cathodic and anodic limits, respectively. The
measurements were conducted by scanning the cell potential from the open circuit potential
to 5.25 V (vs. Li/Li+) for determination of the anodic limit and from the open circuit potential
to 0.00 V (vs. Li/Li+) for determination of the cathodic limit. Note that the cut-off density of
current was fixed at 1 mA cm−2 according to the literature [13,19].
3. Results and discussion
Based on the task-specific strategies as described above (Figure 1), four pyrrolinium-based
ILs were synthesized efficiently from commercially available N-alkyl-2-pyrrolidone (Figure

2, see also Experimental Section). The O-methylation of each N-alkyl-2-pyrrolidone followed
by the anion exchange yielded the corresponding pyrrolinium-based ILs 1~4 in high yields
(Table 1). The two reference ILs, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide
(5, PMPyrd-FSI) and, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (6, EMIm-FSI)
were also prepared as described in a previous report [13]. It should be noted that the
pyrrolinium cations of ILs 1~4 have a partially unsaturated five-membered ring that is an
intermediate structure between the fully saturated pyrrolidinium cation of 5 and the fully
unsaturated imidazolium cation of 6 (Figure 2).
3.1. Thermal and physicochemical properties
The phase transitions of the ILs were measured using differential scanning calorimetry
(DSC), and the melting (Tm) and crystallization points (Tc) of the ILs are summarized in
Table 2 and depicted in Figures 3 and 4. Except M(OMe)Pyrl-FSI (1), three pyrrolinium-
based ILs, E(OMe)Pyrl-FSI (2), P(OMe)Pyrl-FSI (3), and A(OMe)Pyrl-FSI (4), were
colorless liquid at room temperature. Among the pyrrolinium-based ILs, IL 4 with an allyl
group had the lowest melting point. It is interesting to compare the melting points between IL
3 and IL 4 having the same number of carbons substituted at the nitrogen atom. The allyl
group with a planar C=C double bond of IL 4 seems to have some structural effect to
decrease its melting point (Figure 2 and 3). Although its melting point was higher than the
pyrrolidinium-based or imidazolium-based IL, 5 or 6, its crystallization point was not
detected even below -50 oC.
The decomposition temperatures (Td) of the pyrrolinium-based ILs 1~4 were above 150 °C,
which was recorded at 1 wt% loss of the ILs (Table 2). Their Td were lower than that of IL 5
or 6, but they were much more stable than the commercial carbonate electrolyte that started to
evaporate even at 24 oC (at 1 wt% loss, Figure 4). These results were well consistent with our

preliminary study showing the enhanced flame-retarding ability of the mixed electrolytes
containing IL 2 [19], where the self-extinguishing time (SET) results of the mixed
electrolytes were remarkably decreased (for example, the SET value of the mixed electrolyte
with 60 wt% of IL: 5 sec g-1), while the commercial carbonate electrolyte exhibited the
relatively high SET value (103 sec g-1). These results indicate that the use of the pyrrolinium
ILs would be effective for improving safety properties of LIBs. The electrolyte prepared with
IL 2 as the only electrolyte solvent did not ignite at all [19].
The viscosity and ionic conductivity data of the pyrrolinium-based ILs were shown together
with those of IL 5 and IL 6 in Table 3. They were measured at two different temperatures
with or without lithium salt. IL 1 was not listed because it was solid at room temperature. The
viscosity values of the pyrrolinium-based ILs 2~4 were more or less intermediate between
those of IL 5 and IL 6. Among the pyrrolinium-based ILs, IL 4 with an allyl group had the
lowest viscosity but its viscosity was higher than that of IL 6. Based on their structural
differences (Figure 2), both the molecular dissymmetry and the introduction of the C=N
double bond seemed to decrease the viscosity of the pyrrolinium-based ILs. Particularly, the
lower viscosity of IL 4, compared to that of IL 3, seemed to confirm the allyl group effect on
the viscosity of ILs, which was also shown in the previous reports [16-21,24,30]. The
additional C=C double bond present in the allyl group of IL 4 would contribute its lower
viscosity than that of IL 3 (Table 3, entries 3 vs 4). The C=C structure of molecules is known
to allow a facile slip among the molecules, resulting in a low viscosity [13,37-40]. One of the
disadvantages of ILs as an electrolyte solvent is that they are too viscous to facilitate the Li+
migration between electrodes. Therefore, the use of the pyrrolinium cation and the allyl
substituent would be helpful for enhancing other physicochemical properties of ILs. In
addition, the ether group in IL 4 instead of the alkyl group in IL 5 could be beneficial in
improving some physicochemical properties of ILs because it would not only decrease the

van der Waals attraction between the cation and anion but also increase the Li+ mobility as it
readily binds to Li-cations during the electrochemical charge-discharge processes [24-28].
In regard to the ionic conductivity, the ILs with lower viscosity overall had higher ionic
conductivity, which is matched with the Walden’s rule [41-43] (Table 3 and Figure S1). The
highest ionic conductivity was observed with IL 6. All of the pyrrolinium-based ILs 2~4 in
Table 3 had a similar or greater value of ionic conductivity than that of the pyrrolidinium-
based IL 5 with or without lithium salt, which implies that the pyrrolinium cation could be
useful to enhance the kinetic behavior of electrolytes corresponding to the Li+ migration. The
planarity of the cation together with a molecular dissymmetry appeared to have a significant
contribution to the ionic conductivity as well as the viscosity, which was clearly
demonstrated by the imidazolium-based IL 6 having a flat cationic structure (Figure 2). Thus,
the physicochemical properties of ILs seem to be closely related to the structure of the
cations, and the proposed task-specific design of the pyrrolinium cations seems effective for
enhancing the physicochemical properties of the ILs.
3.2. Electrochemical stability
The electrochemical stability of ILs is more important than their physicochemical properties
to apply ILs as practical electrolyte solvents for LIBs. To investigate the electrochemical
stability of the pyrrolinium-based ILs, their electrochemical windows were measured using
linear sweep voltammetry (LSV) as depicted in Figure 5. In the cathodic limit of the LSV
performance in Figure 5(a), the pyrrolinium-based ILs 2~4 did not exhibit significant currents
due to the electrochemical reduction regardless of the substituents on the ILs, which implies
that the proposed pyrrolinium cations have a significantly positive effect on the cathodic
polarization. It should be noted that all the pyrrolinium-based ILs 2~4 were quite stable up to
0.4 V. Although IL 6 with the imidazolium cation had the lowest viscosity and the highest

ionic conductivity, there was a continuous electrochemical decomposition from 0.68 V (vs.
Li/Li+) caused by the irreversible electrochemical reduction of the imidazolium cation.
According to the previous studies, the acidic proton at the C-2 position of the imidazolium
ring was demonstrated to affect the electrochemical reduction in the cathodic limit, which
resulted in the poor electrochemical stability of the imidazolium-based ILs [13,29,44-50].
These results indicate that the proposed pyrrolinium cation contributes to enhancing the
overall cathodic stability of the electrolytes because of the absence of a reactive proton in the
pyrrolinium-based ILs. The similar cathodic stability was also observed with the
pyrroldininium-based IL 5 that also had no such a reactive proton as that in IL 6.
The anodic stability of the pyrrolinium-based ILs 2~4 was also measured to confirm their
electrochemical oxidative potentials. As shown in Figure 5(b), the decomposition of the
commercial carbonate (7) and IL 6 began at 5.12 V and 5.20 V (vs. Li/Li+), respectively, and
their oxidation currents related to the electrochemical decomposition gradually increased. On
the other hand, all the pyrrolinium-based ILs 2~4 exhibited stable performance without the
significant oxidative decomposition compared with that of IL 6 and commercial carbonate
(7). They were all electrochemically stable at the end of measurement of LSV. Interestingly,
the pyrrolidinium-based IL 5 also showed the similar stable electrochemical behavior at the
anodic polarization. In the previous study, it was shown that the pyrrolidinium-based ILs
would enhance the anodic stability of electrolytes because they could provide an effective
protection layer on the electrode surface, which was responsible for slowing down the
electrolyte decomposition [51]. In this regard, the stable anodic stability of the pyrrolinium-
based ILs might be attributed to the increased surface stability via the formation of a
protection layer on the electrode.
3.3. Electrochemical performance

The charge-discharge processes of the prepared ILs as electrolytes with 1 C-rate were
measured in the Li/LiFePO4 half coin cells, and the results are shown in Figures 6,7 and S2,
and the values of initial discharge specific capacity and retention ratio were summarized in
Table 4. The initial discharge capacity of the cells with the pyrrolinium-based ILs 2~4 as
electrolytes was much higher than that with IL 5 and comparable to that of commercial
carbonate (7). For the cycling retention, all the half cells with the pyrrolinium-based ILs 2~4
as electrolytes showed much higher retention ratios than that with IL 6, and also comparable
to that with commercial carbonate (7). Thus, all the pyrrolinium-based ILs seemed to have
comparable electrochemical performance to that of commercial carbonate (7) in terms of both
the initial discharge capacity and the cycle life.
The rate performance of the cells with a LiFePO4 cathode was evaluated from 0.1 C- to 5 C-
rates (Figure 8 and Table 5). It is apparent that the discharge capacity of the cell with each
electrolyte was decreased with an increase of the C-rate. All the cells with the pyrrolinium-
based ILs 2~4 as electrolytes exhibited much higher discharge capacity with an increase in
the current rate than that with IL 6, and comparable to that with commercial carbonate (7).
Among all the cells with the ILs or commercial carbonate (7) as electrolytes in Table 5, the
rate performance with IL 4 having both a pyrrolinium ring and an N-allyl substituent
exhibited the best results without a significant decrease in the discharge capacity, which
should be compared with that of IL 3 or 5 having the similar cationic structure and
substituents. Note also that the retention capability of IL 4 at 5 C-rate (81.2%) was
remarkable compared to that of commercial carbonate (7) (70.1%). It was also observed that
the Li+ transference numbers (tLi+) of the two pyrrolinium-based ILs 2 and 4 were higher than
that of the pyrrolidinium-based IL 5, which indicates the better mobility of Li+ during the
electrochemical processes (Table 6). Note also the favorable allyl group effect in IL 4 for the
enhanced kinetic behavior of the Li+ migration by comparing the structural differences

between ILs 3 and 4 (Figure 2). An additional C=C double bond present in the allyl group of
IL 4 would allow a more facile slip among the molecules, compared to the saturated propyl
group of IL 3, thus leading to its low viscosity and high conductivity [50,52-55]. Although IL
4 shows a lesser value of tLi+ than carbonate, probably because more ions are present in the
IL-based electrolytes than those in the carbonate-based electrolyte [15,36,56-59], the
relatively higher tLi+ value of IL 4 could be attributed to its unique structure.
Additional experiments for investigating the effect of IL 4 on Li metal provided some
evidence to explain the improved electrochemical performance of IL 4. In the EIS results for
the cell after storage test (Figure S3 in Supplementary materials), the cell assembled with IL
4 exhibited lower resistance (RSEI: 33.1 Ω and RCT: 142.6 Ω) than that fabricated with the
carbonate electrolyte (RSEI: 188.4 Ω and RCT: 792.1 Ω) at 1 h. The difference in the
interfacial resistances was larger in the cells after a storage test for 24 h. The cell stored with
IL 4 exhibited a relatively low resistance (RSEI: 93.6 Ω and RCT: 197.9 Ω), and the cell stored
with carbonate 7 showed a higher resistance (RSEI: 254.9 Ω and RCT: 1140.0 Ω). This finding
implies that IL 4 is effective for improving the interfacial stability of the Li metal. Note that
IL 4 is composed of some task-specific functional groups, such as the C=C double bond and
FSI anion. According to the previous studies, the C=C functional group is effective in the
formation of a solid electrolyte interphase (SEI) layer on the electrode surface by
electrochemical reactions, which is responsible for the long-term reliability of LIBs [60-64].
In addition, the use of the FSI anion seems beneficial in forming an effective SEI layer
through the chemical or electrochemical reaction of the FSI anion [36,65-68]. Based on these
considerations, the Li metal surface seems stabilized by the task-specific local structure of IL
4, thus leading to the enhanced interfacial stability of the Li metal electrode. The above
results indicate that the structural changes in the cation and the substituents of ILs can
enhance their electrochemical performance.

4. Conclusion
Novel task-specific pyrrolinium-based ILs 1~4 with the multi-purpose functional groups
were designed and efficiently synthesized (Figure 1 and Table 1). Their physicochemical and
electrochemical properties were characterized, and compared with the well-established ILs
such as the pyrrolidinium-based IL 5 and the imidazolium-based IL 6, and also with
commercial carbonate (7). Three pyrrolinium-based ILs 2~4 were liquid at room temperature
and their decomposition temperature ( 150 oC) was much higher than that of commercial
carbonate (Table 2 and Figure 4). Among the pyrrolinium-based ILs, A(OMe)Pyrl-FSI (IL 4)
exhibited the improved physicochemical and electrochemical properties; i.e., the lower
viscosity (30.0 cP) and the higher ionic conductivity (10.2 mS cm-1) than those of the
pyrrolidinium-based IL 5 (40.0 cP and 8.8 mS cm-1), and the wider electrochemical window
(Table 3 and Figure 5).
With regard to its electrochemical performance measured with a LiFePO4 electrode, the cell
with IL 4 as an electrolyte had the higher initial discharge capacity (151.0 mA h g-1 at 1 C-
rate) than that with IL 5 (127.5 mA h g-1), which was comparable to that with carbonate 7
(145.3 mA h g-1) or IL 6 (153.0 mA h g-1) (Figures 6 and 7, and Table 4). The cell with IL 4
also had the very good cycling retention (99.3% after 50 cycles) compared to that with IL 6
(77.6%), which was similar to that of carbonate 7 (99.4%) and the highest Li+ transference
number (tLi+, 0.19) among the cells with the ILs in Table 6. In addition, the cell cycled with
IL 4 showed the remarkable rate capability even at 5 C-rate (81.2%) compared to that of
carbonate 7 (70.1%) (Figure 8 and Table 5).
In the present study, it was shown that the structure of the cations in ILs played an important
role to affect not only their physicochemical properties but also electrochemical performance.
The proposed pyrrolinium cation having the C=N double bond and the methyl ether

substituent together with an N-allyl group as shown with IL 4 would be effective for well-
balanced physicochemical and electrochemical properties of ILs. We believe this attempt
would be valuable for understanding some principal physicochemical and electrochemical
features of ILs, which could be utilized for developing new ILs for broader applications.
Acknowledgement
This work was supported by the BK 21 Plus Program, Institute of Chemical Processes,
Agency for Defense Development, and Samsung Electro-Mechanics Co., Ltd. and was
partially supported by the National Research Foundation of Korea (NRF) grant funded by the
Korea government (MSIP) (No. 2016R1C1B1009452). We thank Nippon Shokubai Co., Ltd.
for generous supply of LiFSI. Korea Electronics Technology Institute (KETI) is also
acknowledged for the measurement of thermal properties and electrochemical stability of the
prepared ILs.
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Tables
Table 1. Preparation of the pyrrolinium-based ILs
Yield (%)
1st step
2nd step
Ionic liquids
(O-alkylation)
(Anion exchange)
M(OMe)Pyrl-FSI (1)
E(OMe)Pyrl-FSI (2)
P(OMe)Pyrl-FSI (3)
A(OMe)Pyrl-FSI (4)
91
92
90
93
93
94
92
95

Table 2. Thermal properties of the prepared ILs
Ionic liquids
Tm (oC)a
Tc (oC)a
Td (oC)a,b
M(OMe)Pyrl-FSI (1)
78
9
69
167
E(OMe)Pyrl-FSI (2)
P(OMe)Pyrl-FSI (3)
A(OMe)Pyrl-FSI (4)
PMPyrd-FSI (5)
-15
-31
NDc
-29
-36
159
165
150
257
273
21
4
-10
-7
EMIm-FSI (6)
aTm, Tc, and Td: melting point, crystallization point, and decomposition temperature,
respectively. bat 1 wt% loss, cNot detected

Table 3. Physicochemical properties of the prepared ILs at 25 and 50 oC
Electrolytes without lithium salt
Electrolytes with lithium salt
Ionic
Ionic
Viscosity
Viscosity
conductivity
conductivity
(, mS cm-1)
Electrolytes
(η, cP)
(η, cP)
(, mS cm-1)
25 oC 50 oC 25 oC 50 oC 25 oC 50 oC 25 oC 50 oC
E(OMe)Pyrl-FSI (2) 32.0
P(OMe)Pyrl-FSI (3) 40.0
17.0
19.0
13.0
8.4
16.8
13.4
52.0
56.0
22.0
26.0
5.3
4.8
11.3
8.3
A(OMe)Pyrl-FSI
30.0
14.0
10.2
16.3
12.7
42.0
21.0
5.2
10.9
(4)
PMPyrd-FSI (5)
EMIm-FSI (6)
40.0a
19.0b
21.0
11.0
8.8a
59.0
30.0
26.0
16.0
4.7
8.4
15.8b
19.1
10.3
15.7
o
aThe reported values: 40 cP and 8.2 mS cm-1 at 25 C.13 bThe reported values: 18 cP and 15.4
mS cm-1 at 25 oC.13

Table 4. The initial discharge capacity and retention ratio of each electrolyte in the Li/LiFePO4
half cell at 1 C-rate and 25 oCa
Initial discharge capacity
Retention ratio after 50 cycles
Electrolytesa
(mA h g-1)
147.3
(%)
99.4
98.6
99.3
99.8
77.6
99.4
E(OMe)Pyrl-FSI (2)
P(OMe)Pyrl-FSI (3)
A(OMe)Pyrl-FSI (4)
PMPyrd-FSI (5)
EMIm-FSI (6)
144.8
151.0
127.5
153.0
Carbonate (7)
145.3
aAll of the electrolytes except 7 were with 1.0 M LiTFSI. Carbonate (7) with 1.0 M LiPF6.