Synthesis and electrochemical performance of an imidazolium based Li salt as electrolyte with Li fluorinated sulfonylimides as additives for Li-Ion batteries

Article abstract of DOI:10.1016/j.electacta.2019.02.040

Herein, we report the synthesis of a novel imidazolium-based ionic salt, lithium (fluorosulfonyl) ((3-(1-methyl-1H-imidazol-3-ium-3-yl)propyl)sulfonyl) bis(fluorosulfonyl)imide (LiFSMIPFSI) as an electrolyte for the application in lithium-ion battery (LIB). The as-synthesized LiFSMIPFSI exhibited high purity and yield, which was characterized by various spectroscopic techniques. The LiFSMIPFSI electrolyte with a mixed solvent of ethylene carbonate (EC) and dimethyl sulfoxide (DMSO) (75:25 v/v) showed a wide electrochemical stability (ca. 4.5 V vs. Li/Li⁺) and high thermal stability (300 °C), good Li⁺ conductivity (ca. 8.02 mS/cm at 30 °C), and low intrinsic viscosity, which concurrently delivered a specific discharge capacity of ca. 125 mAhg^?1 at 0.1 C with the full LIB configuration of LiFePO₄/electrolytes/graphite. The performance of this LiFSMIPFSI electrolyte was enhanced further by the addition of conventional lithium bis(fluoro-sulfonyl)imide (LiFSI) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) ionic salts (20% each) as additives with the specific discharge capacity of ca. 147 and 139 mAhg^?1, respectively, at 0.1 C. This is mainly due to the additional enhancement of Li⁺ conductivity and its concentrations in the electrolytes induced by the additives. The LiFSMIPFSI electrolyte with LiFSI additive based LIB showed the highest cycling stability (capacity retention ca. 97%) among the electrolytes after 500 charge-discharge cycles. Thus, the present work contributes to the development of new ionic salts and its effects upon the addition of additives on LIB performance.

Full text of DOI:10.1016/j.electacta.2019.02.040

Accepted Manuscript
Synthesis and electrochemical performance of an imidazolium based Li salt as
electrolyte with Li fluorinated sulfonylimides as additives for Li-Ion batteries
Faiz Ahmed, Md. Mahbubur Rahman, Sabuj Chandra Sutradhar, Nasrin Siraj Lopa,
Taewook Ryu, Sujin Yoon, Inhwan Choi, Yonghoon Lee, Whangi Kim
PII:
S0013-4686(19)30283-X
https://doi.org/10.1016/j.electacta.2019.02.040
EA 33631
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 27 November 2018
Revised Date: 8 February 2019
Accepted Date: 8 February 2019
Please cite this article as: F. Ahmed, M.M. Rahman, S.C. Sutradhar, N.S. Lopa, T. Ryu, S. Yoon, I.
Choi, Y. Lee, W. Kim, Synthesis and electrochemical performance of an imidazolium based Li salt as
electrolyte with Li fluorinated sulfonylimides as additives for Li-Ion batteries, Electrochimica Acta (2019),
doi: https://doi.org/10.1016/j.electacta.2019.02.040.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Synthesis and Electrochemical Performance of an Imidazolium
Based Li Salt as Electrolyte with Li Fluorinated Sulfonylimides as
Additives for Li-Ion Batteries
Faiz Ahmed, Md. Mahbubur Rahman, Sabuj Chandra Sutradhar, Nasrin Siraj Lopa, Taewook
Ryu, Sujin Yoon, Inhwan Choi, Yonghoon Lee, Whangi Kim*
Department of Energy and Materials, Konkuk University, Chungju 27478, South Korea
*Author to whom correspondence should be addressed: E-Mail: wgkim@kku.ac.kr (W. Kim).
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Abstract
Herein, we report the synthesis of a novel imidazolium-based ionic salt, lithium
(fluorosulfonyl)((3-(1-methyl-1H-imidazol-3-ium-3-yl)propyl)sulfonyl) bis(fluorosulfonyl)imide
(LiFSMIPFSI) as an electrolyte for the application in lithium-ion battery (LIB). The as-
synthesized LiFSMIPFSI exhibited high purity and yield, which was characterized by various
spectroscopic techniques. The LiFSMIPFSI electrolyte with a mixed solvent of ethylene
carbonate (EC) and dimethyl sulfoxide (DMSO) (75:25 v/v) showed a wide electrochemical
stability (ca. 4.5 V vs. Li/Li+) and high thermal stability (300 ºC), good Li+ conductivity (ca. 8.02
mS/cm at 30 ºC), and low intrinsic viscosity, which concurrently delivered a specific discharge
capacity of ca.
125 mAhg-1 at 0.1 C with the full LIB configuration of
LiFePO4/electrolytes/graphite. The performance of this LiFSMIPFSI electrolyte was enhanced
further by the addition of conventional lithium bis(fluoro-sulfonyl)imide (LiFSI) and lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) ionic salts (20% each) as additives with the specific
discharge capacity of ca. 147 and 139 mAhg-1, respectively, at 0.1 C. This is mainly due to the
additional enhancement of Li+ conductivity and its concentrations in the electrolytes induced by
the additives. The LiFSMIPFSI electrolyte with LiFSI additive based LIB showed the highest
cycling stability (capacity retention ca. 97%) among the electrolytes after 500 charge-discharge
cycles. Thus, the present work contributes to the development of new ionic salts and its effects
upon the addition of additives on LIB performance.
Keywords: Li-ion battery, imidazolium salts, additives, ionic conductivity, specific capacity.
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1. Introduction
Recently, rechargeable lithium-ion batteries (LIBs) have captured an expanding range of
applications as energy storage device for transportation, portable electronics, aerospace, and
biomedical applications [1-2]. This is due to its high specific energy density (150 - 200 WhKg-1)
and efficiency compared to the other rechargeable batteries (e.g., lead-acid battery, nickel metal
hydride battery) [3]. The theoretical energy storage capacity of a LIB is highly dependent on the
ion storage capacity of electrode materials. Electrolytes do not affect the theoretical electron
storage capacity of LIBs. However, an electrolyte can significantly influence the charge transport
kinetics within a battery, which contemporarily affects the deliverable energy amount and the
prolonged lifetime of LIBs [4,5].
Most of the commercial LIBs utilize non-aqueous electrolyte, in which lithium
hexafluorophosphate (LiPF6) is dissolved in a mixture of organic solvents [6,7]. Although the
LiPF6 electrolyte exhibit high ionic conductivity (10.7 mS/cm in a mixture solvent of ethylene
carbonate (EC) with diethyl carbonate (DEC)) and relatively wide electrochemical stability
window (ESW), it suffers from poor chemical and thermal stability [8,9]. Additionally, LiPF6 is
moisture sensitive (hydrolysis at room temperature) and produces harmful byproduct after few
cycling of the battery [10]. Others commonly used non-aqueous electrolyte salts exhibited some
demerits. For example, lithium perchlorate (LiClO4) is moisture sensitive and explosive, lithium
hexafluoroarsenate (LiAsF6) is environmentally unfavorable, and lithium tetrafluoroborate
(LiBF4) show low ionic conductivity (4.9 mS/cm) [11-13].
Nowadays, fluorinated organic compounds have attracted significant interest to develop high
voltage LIB electrolytes with good ionic conductivity and safety. Fluorinated organic compounds
can exhibit wide ESW and high thermal stability due to the presence of a highly electronegative
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fluorine
atom
[14,15].
Lithium
trifluoromethanesulfonate,
lithium
lithium
tris(trifluoromethanesulfonyl)methide
,
lithium bis(perfluoroethylsulfonyl)imide,
nonafluorobutylsulfonyltrifluoromethyl sulfonylimide, lithium bis(fluoro-sulfonyl)imide (LiFSI),
and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) are the commonly used fluorinated
organic compounds as electrolytes for LIBs [16,17]. Most of these imide salts based electrolytes
showed excellent thermal and oxidative stability (>5.0 V) but their ionic conductivities are
generally very low ( 3.0 mS/cm) compared to the LiPF6 [8,18,19]. Whereas, both LiTFSI and
LiFSI imide type electrolytes showed comparable ionic conductivity to LiPF6 [20,21].
Furthermore, both of them exhibited low sensitivity to hydrolysis with enhanced thermal and
ESW compared to the LiPF6 [22,23]. Nevertheless, they are generally used as a co-salts or
additives with other fluorinated slat type electrolytes [22,24]. This is due to their high reactivity
with the Al current collector, which ultimately decrease the lifetime of LIBs.
Recently, ionic liquids (ILs) and Zwitterionic liquids (ZIL) based electrolytes have been
utilized vastly to design and formulation of a high voltage, prolonged lifetime, and safer LIBs
[6,23,25-30]. This can achieve by mixing an optimal amount of LiFSI or LiTFSI salts into these
ILs and ZIL with/without the addition of solvents [25-30]. For example, Kerner et al.
investigated the performance of LIBs using an ILs (1-ethyl-3-methylimidazolium FSI) with
different composition of LiFSI salt as electrolyte [23]. In another report, Elia et al. demonstrated
the application of a series of pyrrolidinium based ILs (N-butyl-N-methylpyrrolidinium TFSI, N-
butyl-N-methylpyrrolidinium FSI, and N-methoxy-ethyl-N-methylpyrrolidinium TFSI together
with 0.2 M LiTFSI salt as the electrolytes for LIBs [25]. All these electrolytes revealed
exceptionally long cycle-life in LIBs as well as a high safety due to the formation of a stable
solid electrolyte interface (SEI) layer onto the anode. This stable SEI layer hinders the additional
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decomposition of electrolyte salts and stabilizes the anode. Similarly, some other reported IL-
based electrolytes also exhibited long cycle-life in LIBs, including N-propyl-N-
methylpyrrolidinium
TFSI,
N-alkyl-Nmethylpyrrolidinium
TFSI,
N-n-butyl-N-
ethylpyrrolidinium TFSI, 1-(2-methoxyethyl)-3-methylimidazolium TFSI, and 3-(2-(2-
methoxyethoxy)ethyl)-1-methylimidazolium TFSI [6,31-34]. Nevertheless, the ionic
conductivity of these IL-based electrolytes is significantly dependent on temperature, which is
increasing with increasing the temperature [23,25]. This is due to the high viscosity of these ILs
at room temperature. Furthermore, the presence of a small amount of Li-salts into these ILs
electrolytes is another possible reason for their low ionic conductivity [25]. This cannot
overcome by increasing the concentration of Li-salts beyond the optimal concentration. Since a
high concentration of Li-salts into these ILs will increase the net viscosity of electrolytes, which
concurrently affects the net ionic conduction as well as battery performance. Thus, the
preparation of ionic salts based on these imidazolium, pyrrolidinium, and piperidinium ILs is an
attractive approach to develop high-performance LIBs.
In this research, we adopted the concept of the functionalization of an imidazolium ILs (3-
butyl-1-methyl-1H-imidazol-3-ium
bis(fluorosulfonyl)imide,
BMIFSI)
with
lithium
(fluorosulfonyl)imide to synthesize its corresponding lithium salt, lithium (fluorosulfonyl)((3-(1-
methyl-1H-imidazol-3-ium-3-yl)propyl)sulfonyl) bis(fluorosulfonyl)imide, LiFSMIPFSI) as an
electrolyte for LIBs. The LiFSMIPFSI electrolyte based on an EC and dimethylsulfoxide
(DMSO) mixed solvent exhibited a low intrinsic viscosity at room temperature and high Li+
conductivity. The Li+ conductivity and the battery performance of this electrolyte were enhanced
further by the addition of a small amount of LiFSI and LiTFSI as additives.
2. Experimental
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2.1. Materials
Sulfurisocyanatidic chloride, LiTFSI (99.95%), LiFSI (99.9%), DMSO, EC, toluene,
methylene chloride (MC), acetone, tetrahydrofuran (THF), ethyl acetate (EA), hexane, and
benzene were procured from Sigma-Aldrich (St. Louis, MO, USA). 1-methylimidazole, 1,3-
propane sultone, formic acid, hydrochloric acid (37%), and trifluorostibine were purchased from
Alfa Aesar (Ward Hill, Massachusetts, USA). A LiFePO4 coated Al foil (thickness ca. 55 µm), a
CMS graphite coated Cu foil (thickness ca. 0.05 mm thickness), and a celgard separator
(thickness ca. 25 µm) were purchased from MTI corporation (CA, USA). All the chemicals and
reagents were used directly without further purification.
2.2. Instrumentations
Fourier-transform infrared spectra (FTIR) was recorded with a Nicolet iS5 spectrophotometer
(ASB1100426, ThermoFisher Scientific, Massachusetts, USA). The structures of the as-
1
synthesized compounds were analyzed by H-NMR (Bruker DRX, 400 MHz) and 19F-NMR
(JEOL JNM-ECZ400 s/Li, 400 MHz) spectrometers, where tetramethylsilane (TMS) was used as
an internal standard. DMSO-d6, deuterated water (D2O), and deuterated chloroform (CDCl3)
were used as solvents. Thermal properties were measured using a Scinco TGA-N 1000 analyzer
ο
ο
in the temperature range from 30 to 600 C at a heating rate of 5 C/min under inert condition.
The surface morphology and the elemental analysis were obtained with a field-emission scanning
electron microscope (FE-SEM, JEOL 7401 F) and energy dispersive X-ray spectroscopy (EDS,
INCAx-sight7421, Oxford Instruments), respectively. Electrochemical impedance spectra (EIS)
was measured with an IM6ex, Zahner-Elektrik GmbH & Co. KG instrument (Germany) in the
frequency range from 0.1 Hz to 0.1 MHz at open circuit condition with an AC amplitude of 5
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mV. All the EIS spectra were fitted with an appropriate equivalent circuit model using Z-view
software (version 3.1, Scribner Associates Inc., U.S.A.). Linear sweep voltammograms (LSV)
and charge-discharge (CD) plots were measured with a multi-channel potentiostat/galvanostat
(Ivium-n-Stat, Ivium Technologies, The Netherlands). For LSV measurement, conventional
platinum (Pt), an Ag/AgCl (aq. saturated KCl), and a Pt wire were used as the working,
reference, and counter electrodes, respectively. A viscometer (RheoSense, hts-VROCTM) was
used to measure the intrinsic viscosity of electrolytes.
2.3. Synthesis of lithium (fluorosulfonyl)((3-(1-methyl-1H-imidazol-3-ium-3-yl)propyl)sulfonyl)
bis(fluorosulfonyl)imide (LiFSMIPFSI)
Fig. 1 shows the stepwise reaction for the synthesis of LiFSMIPFSI. Prior to the synthesis of
LiFSMIPFSI, different precursors were synthesized with high purity and yield, including
sulfurisocyanatidic fluoride (FSO2NCO), sulfamoyl fluoride (FSO2NH2), 1-methyl-3-(3-
sulfopropyl)-1H-imidazol-3-ium chloride (MSIC), 3-(3-(chlorosulfonyl)propyl)-1-methyl-1H-
imidazol-3-ium chloride (CSPMIC), and 3-(3-(N-(fluorosulfonyl)sulfamoyl)propyl)-1-methyl-
1H-imidazol-3-ium chloride (FSSPMIC). The details for the synthesis of these precursors and
their characterizations are shown in the supporting information (Figs. S1-S4, respectively).
For the synthesis of LiFSMIPFSI, LiFSI (12.5 mmol) was added into a solution of FSSPMIC
(10.4 mmol) in 15 mL acetone and stirred for 3 hours at 25 ºC under nitrogen atmospheric
condition. Then, the product was extracted with water and butyl acetate. The gel-type product
was collected from the water layer after evaporating water, which was recrystallized in the form
of powder with acetonitrile with the yield of ca. 50%. Finally, the as-synthesized white powder
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of LiFSMIPFSI was dried in a vacuum oven for 24 h at 80 °C and stored in a glove box under
the N2 atmosphere for further use.
2.4. Preparation of electrolytes
The LiFSMIPFSI salt exhibit poor solubility in the conventional mixed solvents of EC/DEC
or EC/dimethyl carbonate (DMC). Thus, for the preparation of LiFSMIPFSI (1 M) electrolyte
solution, a mixed solvent of EC and DMSO (75:25 v/v) was used. 10 (0.1 M), 20 (0.2 M) and 30
(0.3M) % of LiFSI and LiTFSI were mixed individually into the as-prepared LiFSMIPFSI (1 M)
electrolyte solutions as additives. All the electrolytes were prepared in a glove box under inert
atmospheric conditions.
2.5. Preparation of dummy cells and coin cells
The asymmetric dummy cells were prepared by sandwiching a LiFePO4 and a carbon black
coated fluorine-doped tin oxide (FTO) electrodes, where Surlyn polymer film (thickness 60 µm)
was used as a spacer and sealant (Fig. S5). The LiFePO4/FTO electrodes were prepared by doctor
blading a homemade paste of LiFePO4 onto FTO electrodes (active area: 0.15 cm2; thickness: ca.
ο
10 µm), which were dried at 90 C in vacuum. The LiFePO4, carbon black, and poly(vinylidene
difluoride) (PVDF) compositions in the LiFePO4 paste was 86, 5, and 9 wt%, respectively.
Similarly, graphite/FTO electrodes were prepared (active area: 0.15 cm2; thickness: ca.12 µm)
using a homemade paste with the graphitic carbon and PVDF compositions of 90 and 10 wt%,
respectively. Prior to the sandwich, both LiFePO4/FTO and graphite/FTO electrodes were dipped
into the electrolyte solutions for 20 min. Then, the electrolyte solutions were injected into the
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sandwiched dummy cells through the pre-drilled hole on one FTO side and sealed with scotch
tape.
For the preparation of coin cells, a graphite coated Cu foil and a LiFePO4 coated Al foil was
used as anode and cathode, respectively. A celgard film was used as a separator. Prior
assembling, both the anode and cathode were soaked into the electrolyte solutions for 20 min.
The cells were assembled under the inert atmospheric conditions in a glove box and directly used
for electrochemical measurements.
3. Results and discussions
3.1. NMR and FTIR characterization of LiFSMIPFSI
1
The chemical structure of LiFSMIPFSI was confirmed by H and 19F NMR, and FTIR
spectroscopy. The 1H and 19F NMR spectra showed that all the protons and fluorine atoms were
positioned correctly without the presence of any impurity peaks (Fig. 2a), indicating the high
purity of the as-synthesized LiFSMIPFSI. The analytical data obtained from the 1H and 19F NMR
are presented as follows:
1H-NMR (DMSO-d6; 298 K): δ = 9.2 (s, H), 7.8 (s, H), 7.7 (s, H), 4.25-4.35 (t, 2H), 3.85 (s,
3H), 2.4-2.6 (t, 2H), 2.0-2.2 (m, 2H) ppm.
19F-NMR (D2O; 298 K): δ = 52.04 (s, 2F), -126.47 (s, F) ppm.
Fig. 2 b shows the FTIR spectra of LiFSMIPFSI. For comparison, we also measured the
FTIR spectra of LiFSI and LiTFSI (Fig. 2b). All the compounds showed their common FTIR
characteristics peaks for S-N overtone and O=S=O (symmetric) at ca. 1640 and 1190cm-1,
respectively [35]. The broad peaks in the range of 3000-3700 cm-1 for all the compounds indicate
the presence of adsorbed water molecules and N anions [36]. LiFSMIPFSI displayed the
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additional FTIR peaks of =C-H, C=N, and C=C stretching at ca. 3104, 1580, and 1472 cm-1,
respectively [35], indicating the successful synthesis of LiFSMIPFSI.
3.2. Physicochemical properties
Fig. 3a illustrates the TGA plots of LiFSI, LiTFSI, and LiFSMIPFSI. Both LiFSI and
LiFSMIPFSI exhibited two-step degradation. The fist-step weight loss for LiFSI was 6% in the
temperature range from 200 to 300 ºC, while it was 40% for LiFSMIPFSI from 240 to 370 ºC.
Afterwards a sharp weight loss of 80% was occurred for LiFSI up to 475 ºC, while it was ca.
20% for LiFSMIPFSI up to 650 ºC. This two step weight loss for LiFSI and LiFSMIPFSI can be
attributed to the carbonization decomposition with slow kinetics of certain decomposition
processes [37]. LiTFSI exhibited signle-step weight loss and highest stability with the
decompisition at 320 ºC, which is consistent with a previous report [23]. The weight loss for
LiTFSI was ca. 88% at 430 ºC. This indicates that LiFSMIPFSI has little lower thermal stability
ο
compared to LiFSI and LiTFSI. The thermal stability up to 240 C for LiFSMIPFSI is high
enough for the development of thermal resistive LIBs by considering the normal operation range
(-40 85 οC) of LIBs. After the thermal decomposition of LiFSI, LiTFSI, and LiFSMIPFSI, the
remaining residues weigh percent was ca. 11, 12, and 40%, respectively, which is possibly due to
the formation of residual carbon species, which requires further depth investigation. The
percentage of residual species is much higher for LiFSMIPFSI compared to LiFSI and LiTFSI.
This can be ascribed to the high percentage of atomic carbon in the structure of LiFSMIPFSI
compared to LiFSI and LiTFSI, which concurrently exhibit high percentage of residue.
ο
Additionally, we measured the melting temprature of LiFSMIPFSI, which was 177±2.5 C,
while it was 132-145 οC and 234 οC, respectively for LiFSI and LiTFSI [37,38].
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Fig. 3b shows the dynamic viscosity vs. temperature plots of electrolytes in the range
between 30 to 80 ºC. For comparison, we also measured the dynamic viscosities of LiFSI and
LiTFSI in the same temperature range as shown in Fig. S6. The dynamic viscosity of
LiFSMIPFSI (1 M) was ca. 2.20 cP at 30 οC, while it was ca. 0.75 cP at 80 ºC. In contrast, both
LiFSI (1 M) and LiTFSI (1 M) exhibited little lower viscosity compared to the LiFSMIPFSI,
which were ca. 1.91 and 2.05 cP, respectively, at 30 οC and ca. 0.45 and 0.55 cP, respectively, at
80 ºC. This decreasing viscosity with increasing temperature can be attributed to the reduction of
the intermolecular forces between LiFSMIPFSI molecules. This is well agreed with the reported
literature [25]. The dynamic viscosity for LiFSMIPFSI+LiFSI and LiFSMIPFSI+LiTFSI
electrolytes with 20% LIFSI and LiTFSI was little higher (ca. 2.50 and 3.80 cP, respectively)
than the pure LiFSMIPFSI electrolyte at 30 ºC. While they were ca. 1.30 and 1.95 cP,
respectively, at 80 ºC. This low dynamic viscosity of all the electrolytes in a wide temperature
range indicates that they are capable of conducting Li+ ions with high conductivity.
3.3. Electrochemical stability and ionic conductivity
Fig. 4a shows the LSV plots of LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI
electrolytes with 20% LIFSI and LiTFSI at a Pt working electrode in the potential range from 0
to 7.0 V (vs. Li/Li+). For comparison, we also measured the LSV plots of LiFSI and LiTFSI
electrolytes (1 M each) as shown in Fig. S7. LiFSMIPFSI electrolyte displayed the oxidative
stability up to ca. 4.6 V. The current flow above 4.6 V indicating the degradation of
LiFSMIPFSI. LiFSMIPFSI+LiFSI and LiFSMIPFSI+LiTFSI electrolytes remained stable up to
ca. 5.50 and 5.70 V, respectively. While the electrochemical oxidative stability for LiFSI and
LiTFSI electrolytes were ca. 4.65 and 4.75 V, respectively. This designates that both LiFSI and
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LiTFSI can act as an effective inhibitor for the decomposition of LiFSMIPFSI. The
electrochemical
oxidative
stability
enhancement
for
LiFSMIPFSI+LiFSI
and
LiFSMIPFSI+LiTFSI electrolytes compared to their parent compounds can be ascribed to the
widening of the energy gap between the oxidation and reduction potential of electrolyte [39] induced
by the synergistic effect of LiFSMIPFSI and additives (LiFSI or LiTFSI). The oxidative
potentials of all the electrolytes are well above the working potential of the high-performance
LiCoO2 and LiFePO4 (ca. 4 and 3.5 V, respectively, vs. Li/Li+) cathode [40,41]. Thus,
LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI are promising electrolytes for the
development of high voltage LIBs.
The Li+ conductivity of all the electrolytes was measured by EIS analyses with the
asymmetric dummy cells (Fig. S5) in the temperature range between 30 to 80 ºC. The Li+
conductivity was calculated according to the equation S1. Fig. 4b tabulates the ionic
conductivities of LiFSMIPFSI, LiFSMIPFSI+LiFSI and LiFSMIPFSI+LiTFSI electrolytes with
20 wt% LIFSI and LiTFSI and the corresponding EIS spectra are presented in Fig. S8. All the
EIS plots were fitted with an appropriate equivalent circuit model as shown in Fig. S8. The
details for the EIS spectra can be found in the supporting information. The ionic conductivity of
LiFSMIPFSI electrolyte was ca. 8.02 mS/cm at 30 ºC with the Li+ transference number (tLi+) of
ca. 0.40, which is higher or comparable to many other conventional high-performance
electrolytes, including LiPF6 (10.7 mS/cm), LiBF4 (4.9 mS/cm), LiClO4 (8.4 mS/cm), and imide
based electrolytes [11-13,20,21]. The ionic conductivity of LiFSMIPFSI electrolyte was
increased with increasing temperature and it was ca. 8.16 mS/cm at 80 ºC. This is consistent with
the variation of dynamic viscosities of LiFSMIPFSI electrolyte with temperature. The observed
high Li+ conductivity of LiFSMIPFSI electrolyte can be explained further by the charge
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delocalization of FSMIPFSI− anion (Fig. S9). The charge delocalization of FSMIPFSI anion can
induce to stabilize it, which concurrently decrease the association constant of ion pairing and
increased the conductivity of Li+ [42].
In contrast, both LiFSI and LiTFSI additives (20% each) enhanced the ionic conductivity of
LiFSMIPFSI electrolyte. It was ca. 8.92 and 8.25 mS/cm for LiFSMIPFSI+LiFSI and
LiFSMIPFSI+LiTFSI electrolytes, respectively, at 30 ºC with the tLi+ of 0.48, and 0.45,
respectively. The ionic conductivities of LiFSMIPFSI+LiFSI and LiFSMIPFSI+LiTFSI
electrolytes were increased with increasing temperature (ca. 9.17 and 8.45 mS/cm, respectively,
at 80 ºC), which is consistent with the variation of their dynamic viscosities with temperature.
The ionic conductivity for LiFSMIPFSI+LiFSI electrolytes with 10 and 30% LiFSI was ca. 8.10
and 8.75 mS/cm, respectively, while it was ca. 8.0 and 8.15 8.75 mS/cm for
LiFSMIPFSI+LiTFSI electrolytes with 10 and 30% LiTFSI, respectively. Hereafter, 20% LiFSI
and LiTFSI is considered as optimized additives concentrations and used for further studies. The
improvement of ionic conductivity of LiFSMIPFSI electrolytes induced by the LiFSI and LiTFSI
additives can be ascribed to the net enhancement of tLi+ into the resultant electrolytes, and it is
consistent with the reported results [43,44]. LiFSMIPFSI+LiFSI electrolyte showed higher ionic
conductivity enhancement compared to LiFSMIPFSI+LiTFSI electrolyte. This can be attributed
to the lower viscosity of LiFSMIPFSI+LiFSI electrolyte compared to LiFSMIPFSI+LiTFSI
electrolyte. Additionally, the smaller size of FSI anion compared to the TFSI anion can also
facilitate the faster ion movement and concurrently increase the Li+ conductivity of
LiFSMIPFSI+LiFSI electrolyte. The activation energy (Ea) for the LiFSMIPTFSI,
LiFSMIPTFSI+LiFSI, and LiFSMIPTFSI+LiTFSI electrolytes were ca. 3.80, 460, and 415
J/mol, respectively, which were calculated from the Arrhenius plots of the measured ionic
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conductivities of the electrolytes as shown in the inset of Fig. 4(b). The low Ea of all the
electrolytes compared to the reported other electrolytes [45,46] could be attributed to the weak
binding energies between the Li+ with the corresponding anions of the electrolytes and facilitates
to obtain high ionic conductivity [46].
We also investigated the stability of electrolytes by measuring their ionic conductivities with
the number of EIS scan at 30 οC. Each of the EIS spectra was measured after cyclic voltammetric
(CV) measurement of the dummy cells for 10 cycles at a scan rate of 25 mV/s in the potential
range between 0 to 5 V (vs. Li/Li+). The variation of ionic conductivities with the number of EIS
scan is summarized in Fig. S10. The ionic conductivities were decreased only ca. 2.75, 4.60, and
8.30% for LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes,
respectively. This little variation of ionic conductivities of all the electrolytes indicating their
high electrochemical stability, which is promising for the development of high voltage LIBs.
3.4. Electrochemical performance of electrolytes
The LIB performance of electrolyte was investigated with a full coin cell with the device
structure of LiFePO4/electrolytes/graphite. Fig. 5 (a-c) shows the CD profiles of all LIBs at 0.1,
0.2, and 0.3 C, respectively, in the potential up to 4 V. All the LIBs showed a lower difference in
the CD potential or overpotential, which is favorable to obtain high energy efficiency with good
reversibility [47]. The discharge specific capacity of these LIBs were calculated based on the
cathode materials LiFePO4 (LFP). The discharge specific capacity of LiFSMIPFSI,
LiFSMIPFSI+LiFSI, and LiFSMIPFSI +LiTFSI electrolytes based LIBs were ca. 125, 147, 139
mAhg-1, respectively, at 0.1 C. While it was ca. 120, 142, and 134 mAhg-1, respectively, at 0.2 C
and ca. 116, 136, and 128 mAhg-1, respectively, at 0.3 C. The discharge specific capacity of all
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the electrolyte based LIBs were ca. 85, 102, and 94 mAhg-1, respectively, at 1 C (Fig. S11). For
comparison, we also prepared the LIBs using LiFSI (1M) and LiTFSI (1M) in the EC/DMSO
solvent (75:25 v/v) with the same device structures, which revealed the discharge specific
capacity of ca. 126 and 123 mAhg-1, respectively, for LiFSI and LiTFSI at 0.1 C (Fig. S12). This
indicates that LiFSMIPFSI electrolyte can exhibit comparable electrochemical performance in
LIBs. The discharge specific capacity of LiFSMIPFSI electrolyte based LIB was much lower
than the theoretical capacity of LFP (170 mAhg-1) [48]. This can be ascribed to the lower Li+
ionic conductivity with significantly low tLi+ of LiFSMIPFSI electrolyte [44]. Nevertheless, the
discharge specific capacity of pure LiFSMIPFSI electrolyte based LIBs are higher or comparable
than the reported others LIBs as summarized in Table S1. The specific capacity of LiFSMIPFSI
electrolyte based LIBs was enhanced upon the addition of LiFSI and LiTFSI additives and it is
more effective for LiFSI additive. This is well matched with the variation of Li+ ion
conductivities, tLi+, and viscosities of the corresponding electrolytes. In can be noted that the
discharge voltage of all the electrolyte based LIBs were much lower (ca. 2.75 V) than the normal
operating voltage of LFP (3.2- 3.3 V) based LIBs [49]. This is possibly due to the low tLi+ of all
the electrolytes (<0.5), which indicate a substantial contribution of anions motion in their total
ionic conductivities [44]. Therefore, a concentration gradient is developed within the LIBs
induces by the migration of anions and Li-ions in the opposite direction during charging and
discharging. This creates a concertation overpotential and concurrently limits the operating
voltage and the thickness of the electrodes [44,50]. All of these effects are responsible for low
discharge specific capacity of these electrolytes in LIBs.
Fig. 5d shows the variations of the specific capacities and coulombic efficiencies vs. CD
ο
cycle number of all the electrolytes based LIBs at 0. 1 C and 30 C. The discharge specific
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capacity for LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes based
LIBs was decreased of ca. 21 (99 mAhg-1), 4 (140 mAhg-1), and 17% (119 mAhg-1), respectively,
after 500 CD cycles. This specifies that LiFSI is an effective additive for the LiFSMIPFSI
electrolyte to obtain high electrochemical performance as well as stability. The coulombic
efficiency for LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes based
LIBs was ca. 82, 94, and 90.0%, respectively, in the first CD cycle, which was monotonically
increased to ca. 89, 95, and 97%, respectively, after CD cycling. This monotonic increased of
coulombic efficiency with increasing the number of CD cycling can be attributed to the
formation of a stable SEI layer onto the graphite anode induced by the electrolytes [51,52].
Nevertheless, the coulombic efficiency of LiFSMIPFSI electrolyte based LIB is only ca. 89%
after CD cycling, which is much lower than the coulombic efficiency of LiFSMIPFSI+LiFSI,
and LiFSMIPFSI+LiTFSI. This is possibly due to the low capability of the formation of SEI
layer onto the graphitic anode induced by LiFSMIPFSI electrolyte.
The cell interface stability of LIBs based on these electrolytes was further investigated by
analyzing the surface morphologies of the graphite anodes. Fig. 6 (a) shows the FE-SEM image
of bare graphite and (c-d) shows the FE-SEM images of graphite anodes of LIBs based on
LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes, respectively. All the
LIBs graphite anodes were recovered after 500 CD cycling of the cells at 0.1 C. It revealed that
all the electrolytes are capable of forming a dense and stable SEI layer onto the graphite anode.
Nevertheless, the SEI layer formed onto the graphite anode enabled by the LiFSMIPFSI+LiFSI
electrolyte is much thicker than the SEI layer formed by LiFSMIPFSI, LiFSMIPFSI+LiTFSI
electrolytes. This SEI layer can induce to extend the stability of the electrode/electrolyte
interface, which concurrently improved the stability of LIBs [10]. This is consistent with the
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electrochemical cycling stability and coulombic efficiencies of the LIBs based on these
electrolytes. The formation of the SEI layer was verified further by measuring the EDS elemental
mapping of the corresponding graphite anodes as shown in Figs. S13-S15. The EDS elemental
mapping clearly indicates the existence of high density of elemental C, O, F, S, and N in all the
graphite anodes and it is higher for LiFSMIPFSI+LiFSI.
4. Conclusions
The aim of this research is to functionalized an imidazolium ILs (BMIFSI) with lithium
(fluorosulfonyl)imide to prepare its corresponding ionic salts LiFSMIPFSI as an electrolyte for
LIBs. The LiFSMIPFSI electrolyte showed comparable ionic conductivity with many other
conventional electrolytes, high electrochemical and thermal stability, which concurrently
delivered a specific discharge capacity of ca. 125 mAhg-1 at 0.1 C in a LIB. The electrochemical
performance of this electrolyte was enhanced additionally by the addition of LiFSI and LiTFSI
additives, which was maximized for LiFSI additive in terms of ionic conductivity, discharge
capacity, and cycling stability. Despite the good electrochemical performance of this
LiFSMIPFSI electrolyte without and with additives, the safety of these electrolyte based LIBs is
still a big concern for commercialization. This required further studies, which is underway of
investigation in our laboratory. The present study will give a breakthrough for the development
of high-performance electrolytes for LIBs based on the functionalization of others imidazolium,
pyrrolidinium, and piperidinium ILs based ionic salts.
Acknowledgements
This work was supported by Konkuk University in 2018.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/xx.xxxx/j. electacta.20xx.xx.xxx.
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Figure captions
Figure 1. Schematic of the stepwise reactions for the synthesis of Lithium (fluorosulfonyl)((3-
(1-methyl-1H-imidazol-3-ium-3-yl)propyl)sulfonyl) bis(fluorosulfonyl)imide (LiFSMIPFSI).
1
Figure 2. (a) H NMR spectra (inset shows the 19F NMR spectra) and (b) FTIR spectra of
LiFSMIPFSI.
Figure 3. (a) TGA plots of LiFSMIPFSI, LiFSI, and LiTFSI. (b) Viscosity plots of LiFSMIPFSI
(1 M) without and with LiFSI (0.2 M) and LiTFSI (0.2 M) additives in EC and DMSO (75:25
v/v) solvent.
Figure 4. (a) LSV plots (scan rate of 25 mV/s) and (b) Li+ conductivity vs. temperature plots of
LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes. Inset of (b) shows the
Arrhenius plots of the measured ionic conductivity.
Figure 5. Voltage profiles of LiFePO4/electrolytes/graphite based LIBs coin cells at (a) 0.1 C, (b)
ο
0.2 C, and (c) 0.3 C at 30 C. (d) Variation of the discharge capacity and the coulombic
efficiency as a function of CD cycling number of the corresponding LIBs at 0.1 C and 30 οC.
Figure 6. FE-SEM images of (a) bare graphite anode and (b-d) graphite anodes based on
LiFSMIPFSI, LiFSMIPFSI+LiFSI, and LiFSMIPFSI+LiTFSI electrolytes contained LIBs,
respectively. All the graphite anodes were recovered after 500 CD cycles of LIBs at 0.1 C and
30οC. Insets of all images show the corresponding magnified FE-SEM images.

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Figure 1. W.G. Kim et al.

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Figure 2. W.G. Kim et al.

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Figure 3. W.G. Kim et al.