1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Article abstract of DOI:10.1016/j.electacta.2010.03.043

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (-0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g^-1, which the value is near to theoretical specific capacity (170 mAh g^-1). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g^-1) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g^-1 when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.

Full text of DOI:10.1016/j.electacta.2010.03.043

Electrochimica Acta 55 (2010) 4728–4733
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journal homepage: www.elsevier.com/locate/electacta
1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic
liquids as highly safe electrolyte for Li/LiFePO4 battery
Cai Yana, Li Zaijuna,b,∗, Zhang Hailanga, Fang Yinjunb, Fan Xua, Liu Junkanga
a School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
b Zhejiang Zanyu Technology Limited Corporation, 128 Chengtou Xiang Road, Hangzhou 311215, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-
DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and
their electrochemical properties and compatibility with Li/LiFePO4 battery electrodes were investigated
in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16
to 5.2 V vs. Li+/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium
as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based
electrolytes towards lithium anode and LiFePO4 cathode, and enhanced the formation of solid elec-
trolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic
liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conduc-
tivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes,
Li/LiFePO4 battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity
and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is
about 152.6 mAh g−1, which the value is near to theoretical specific capacity (170 mAh g−1). Although the
battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g−1) due
to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g−1
when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results
clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly
safety electrolytes in the future.
Received 3 November 2009
Received in revised form 6 March 2010
Accepted 13 March 2010
Available online 23 March 2010
Keywords:
1-Alkyl-2,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)imide
Ionic liquid
Lithium battery
Safety
LiFePO4
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
as ethylene carbonate and ethyl methyl carbonate commonly used
in lithium ion batteries nowadays.
The realization of large-scale electrochemical devices is strongly
expected all over the world, owing to the demand for energy storage
devices, for example, load-leveling systems. Although many bat-
teries such as sodium–sulfur battery and redox-flow battery have
been in practical use as megawattclass, large-scale batteries up to
now, there has been much concern regarding lithium secondary
batteries for the purpose of achieving high-energy-density and
maintenance-free battery systems [1]. For lithium secondary bat-
teries, metallic lithium is an ideal battery material for the concern of
energy density since its theoretical capacity is as high as 3860 mA/g.
However, one limitation for its practical use is the growth of den-
drites during cycling, which causes non-reversibility and even
safety hazard [2]. Another safety concern is the high flammabil-
ity and high vapor pressure of conventional organic solvents such
Ionic liquids are molten at room temperature, namely coulom-
bic liquids. Ionic liquids have many important specific properties,
including reasonably high conductivity, negligible vapor pressure,
high thermal stability and non-flammability. All these properties
make ionic liquids are very interesting new materials for electro-
chemical uses. Among all possible applications, those directed to
lithium ion batteries are highly appealing. Ionic liquids as elec-
trolyte for lithium batteries have been widely examined in the
recent years, which based on different cation of the ionic liq-
uids such as imidazolium [3–7], phosphonium [8,9], quaternary
ammonium [10], sulfonium [11] and pyrrolidinium [12–15]. The
disadvantages of ionic liquids, higher viscosity, lower conduc-
tivity and poor compatibility with electrodes than conventional
organic solvents, lead to poor electrochemical performance of the
batteries with ionic liquids electrolytes. To resolve the problem,
some organic additives such as ethylene carbonate (EC), viny-
lene carbonate (VC), and special anions of ionic liquids such as
bis(fluorosulfonyl)imide (FSI), etc [16], were used to stabilize and
protect interface between electrodes and ionic liquid phase against
∗
Corresponding author at: School of Chemical and Material Engineering, Jiangnan
University, Wuxi 214122, China. Tel.: +86 5105811863; fax: +86 5105811863.
E-mail address: zaijunli@263.net (L. Zaijun).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.03.043

C. Yan et al. / Electrochimica Acta 55 (2010) 4728–4733
4729
an undesirable irreversible reaction of ionic liquid components. As
the ionic liquids-based imidazolium cation are relatively inexpen-
sive and have been well exploited, these have been reported as ionic
liquid electrolytes for lithium batteries, however, the problem of a
stability limit up to the redox potential of lithium metal has been
pointed out, and there have been very few reports on rechargeable
batteries that use lithium metal as an anode. In recent years, some
research groups tested 2-substituted imidazolium ionic liquid as
electrolyte for lithium secondary batteries [17–19]. The results
revealed introducing an electron-donating methyl into the second
position of the imidazolium cation will promote charge delocaliza-
tion in the imidazolium cation and improve the reduction stability
of imidazolium-based ionic liquids. However, studies devoted on
influence of the ionic liquid’s structure on the electrochemical per-
formance are scarce.
In this study, several 1-alkyl-2,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)imide ionic liquids which abbrevi-
ated as alkyl-DMimTFSI (see Fig. 1), were designed and examined
as highly safe electrolyte for Li/LiFePO4 battery. We mainly focused
on the study on the influence of the ionic liquid’s structure on their
physicochemical properties, interfacial properties and battery
performance.
temperature. The ionic liquid was dried in vacuum at 80 ◦C for 12 h
and at 120 ◦C for 24 h to remove residual solvent and water. Other
ionic liquids studied were prepared using a similar procedure, with
different bromoalkane replaced by 1-bromobutane, respectively.
The 1H NMR data of five ionic liquids were listed in the follow-
ing (UNITYINOVA 400, 300 MHz, DCCl3): butyl-DMimTFSI: 0.960
(t, 3H), 1.380 (m, 2H), 1.767 (m, 2H), 2.594 (s, 3H), 3.790 (s, 3H),
4.040 (t, 2H), 7.172 (d, 1H), 7.204 (d, 1H); amyl-DMimTFSI: 0.900
(t,3H), 1.326 (m, 4H), 1.788 (m, 2H), 2.588 (s, 3H), 3.786 (s, 3H),
4.029 (t, 2H), 7.174 (d,1H), 7.206 (d,1H); octyl-DMimTFSI: 0.876 (t,
3H), 1.277 (m,10H), 1.788 (m, 2H), 2.600 (s,3H), 3.801 (s, 3H), 4.032
(t, 2H), 7.163 (d,1H), 7.210 (d,1H); isooctyl-DMimTFSI: 0.872 (m,
6H), 1.277 (m, 8H), 1.701 (m, 1H), 2.567 (s, 3H), 3.787 (s, 3H), 3.914
(d, 2H), 7.109 (d, 1H), 7.216 (d, 1H); and decyl-DMimTFSI: 0.866
(t, 3H), 1.285 (m, 14H), 1.794 (m, 2H), 2.587 (s, 3H), 3.788 (s, 3H),
4.020 (t, 2H), 7.155 (d, 1H), 7.198 (d, 1H).
The binary electrolyte (0.2 mol/L solution of LiTFSI in the ionic
liquid) was prepared by dissolving dried LiTFSI in the ionic liquid
in an argon-filled glove box and stirring the solution at room tem-
perature for 48 h. The ternary electrolyte was obtained through the
addition of 5 wt% VC to the above binary electrolyte.
2.2. Test of stability of electrodes in the ionic liquid electrolytes
2. Experimental
2.1. Preparation of ionic liquid electrolytes
Evaluation of the electrochemical stability windows of the ionic
liquids, respectively, were carried out by linear-sweep voltam-
metry (LSV, scan rate of 10 mV s−1) in a two-electrode cell with
glass carbon electrode working electrode (surface area 0.9 cm2)
and lithium foil as both counter and reference electrode. In LSV
experiment, the starting potential is −1 V, and potential scanning
direction is from −1 V to 6 V. The glass carbon was polished before
each potential scanning.
The Li/electrolyte interfacial resistance was measured in
Li/electrolyte/Li symmetric cells through the ac impedance on
Zahner–Elektrik IM6e electrochemical workstation with the fre-
quency ranging from 200 kHz to 10 MHz and an ac signal of 5 mV
in amplitude as the perturbation at room temperature.
Butyl-DMimTFSI was prepared through an usual route [20,21].
First, 1.0 mol of freshly distilled 1-bromobutane (Aldrich, 98%)
and 1.0 mol of 1,2-dimethylimidazole (Aldrich, 98%) were trans-
ferred into a 500 mL of the round bottom flask fitted with a
reflux condenser and nitrogen protecting facilities. 50 mL of toluene
was added to the flask. The mixture was stirred at 70 ◦C for
18 h, to form a transparent viscous liquid product, 1-butyl-2, 3-
dimethylimidazolium bromide. The liquid product was cooled and
transferred to a 2000 mL beaker, and 1.0 mol of LiTFSI dissolved in
500 mL of water was added under stirring, and the mixture was
continuously stirred for 4 h at room temperature. The resulting
organic phase was extracted with dichloromethane, washed with
water, and then purified with activated carbon and acid alumina.
The final product butyl-DMimTFSI was a clear colorless at room
The characteristics of LiFePO4/electrolyte interface were inves-
tigated by electrochemical impedance spectroscopy (EIS) and the
cyclic voltammetry (CV) in a LiFeO4/electrolyte/Li cell with LiFePO4
Fig. 1. Chemical structures of the ionic liquids.

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C. Yan et al. / Electrochimica Acta 55 (2010) 4728–4733
as working electrode and lithium foil as both counter and reference
electrode. The LiFePO4/electrolyte interfacial resistance was mea-
sured in through the ac impedance on Zahner–Elektrik IM6e elec-
trochemical workstation with the frequency ranging from 200 kHz
to 10 MHz and an AC signal of 5 mV in amplitude as the perturba-
tion at room temperature. The cyclic voltammetry of the LiFePO4
electrode in 0.2 M LiTFSI-ionic liquid-VC solution was carried out
in LiFePO4/electrolyte/Li cells (0.02 mV s−1) at room temperature
using Zahner–Elektrik IM6e electrochemical workstation.
properties of neat ionic liquids and corresponding binary elec-
trolytes were well investigated. Fig. 2A represents typical DSC
curves of neat ionic liquids. Fig. 2A indicates clearly there are no
obvious endothermic peak and exothermic peak in the tempera-
ture range between −10 ◦C and 350 ◦C on the DSC curves of these
ionic liquids, which real the type of ionic liquids has excellent heat-
ing stability. Fig. 2B exhibits the conductivity of neat ionic liquid
(red column) and binary electrolyte (yellow column). From Fig. 2B
we can clearly see the conductivity of the ionic liquid is obviously
higher than that of its binary electrolyte. This is the reason that
addition of LiTFSI increases viscosity of the ionic liquid and results
in decreasing the conductivity. Moreover, Fig. 2B also shows ion
conductivity of the ionic liquid seriously depend on the molecu-
lar structure, the conductivity will decrease with increasing carbon
number of alkyl group in the ionic liquid. We think this is mainly
due to rapidly increasing the ionic liquid viscosity with increasing
number of the carbon number in alkyl group (see Fig. 2C). Accord-
ingly, the isooctyl-DMimTFSI and decyl-DMimTFSI are difficult to
be directly applied to lithium batteries due to poor conductivity.
Fig. 2D displays LSV to clarify the anodic and cathodic electrochem-
ical stability of neat ionic liquids and binary electrolytes at room
temperature. Electrochemical windows of the neat ionic liquids and
binary electrolytes (insert in Fig. 2D) were −0.4 to 5.2 V and −0.16
to 5.2 V, respectively. As the anodic limiting current flow observed
at 5.2 V (vs. Li+/Li) corresponds to the oxidation of the TFSI− anions,
while the cathodic limiting current at −0.1 V (vs. Li+/Li) indicated
the decomposition of the alkyl-DMim+ cations. Additionally, CV
exhibits a pair of redox peaks around 0 V (vs. Li+/Li) in the presence
of LiTFSI (no show), which corresponds to the deposition and disso-
lution of lithium on and from the copper substrate. Thus, five new
alkyl-DMimTFSI ionic liquids are of excellent electrochemical sta-
2.3. Cell assemblies and electrochemical measurements
Li/electrolyte/LiFePO4 cells were used for electrochemical mea-
surement. LiFePO4 was purchased from Hunan Shanshan Advanccd
Masterial Co., Ltd. The LiFePO4 surface was particularly modified
and its D50 and BET specific surface area are 5–6 ␮m and 9–11 m2/g.
LiFePO4 cathode electrode was made up of LiFePO4, acetylene black,
and polytetrafluoroethylene binder with a weight ratio of 85:10:5.
The metallic lithium anode, Celgard 7320 separator, the ionic liquid
electrolyte, and LiFePO4 cathode, which the thickness of LiFePO4
cathode is about to 100 ␮m, were assembled in sequence into cells
in an argon-filled dry glove box. The galvanostatic charge/discharge
tests were performed at a C/20 rate and potential of 2.2–3.8 (vs.
Li+/Li) at room temperature under the land CT2001 battery tester.
3. Results and discussion
3.1. Physicochemical and electrochemical properties
To understand electrochemical behavior of the ionic liquid elec-
trolytes, some important physicochemical and electrochemical
Fig. 2. DSC thermograms (A), conductivity at 25 ◦C (B), viscosity-temperature curves (C) and electrochemical window (D) for different alkyl-MimTFSI ionic liquid, a, b, c, d
and e curves represent butyl-DMimTFSI, amyl-DMimTFSI, octyl-DMimTFSI, decyl-DMimTFSI and isooctyl-DMimTFSI at 25 ◦C, respectively.

C. Yan et al. / Electrochimica Acta 55 (2010) 4728–4733
4731
Fig. 3. Nyquist plots of Li symmetric cell without VC (A and C) and with VC (B and D) at open circuit condition at room temperature after the cell was deposited for 1 day (A
and B) and 5 days (C and D), a, b, c, d and e represent amyl-DMimTFSI, butyl-DMimTFSI, octyl-DMimTFSI, isooctyl-DMimTFSI, and decyl-DMimTFSI, respectively.
bility under charge/discharge conditions. These results confirmed
introducing an electron-donating methyl at the second position
of the imidazolium cation will promote charge delocalization in
the imidazolium-based ionic liquids and remarkably enhance the
stability of the ionic liquid.
low-frequency range after the 1st (blue curve in Fig. 4A) and 25th
cycle (red curve in Fig. 4A). The high-frequency semicircle repre-
sents the resistance of the SEI film, and the inclined line corresponds
to the lithium ion diffusion process, Warburg diffusion. Li/LiFePO4
cell show the size of semicircle at high-frequency is very simi-
lar after and before 1 cycle and slightly increased after 25 cycles,
However, the change value in SEI film resistance with VC is remark-
ably smaller than that of the cell without VC, this results also
demonstrate VC addition can form an improved SEI film. In the test
intercalation/deintercalation of lithium ions into LiFePO4 electrode,
the cyclic voltammetry performance of the LiFePO4 electrode in the
ternary electrolytes was measured in using a LiFePO4/electrolyte/Li
cells at 25 ◦C and the results was shown in Fig. 4B. We see from
Fig. 4B the response evidences in the 1st cycle the occurrence of
some irreversibility, however, the following cycles proceed with
well reproduced peaks without evidence of side reactions. These
results demonstrated better compatibility towards LiFePO4 cath-
ode.
3.2. Compatibility of the ionic liquid electrolytes with electrodes
3.2.1. Metallic lithium electrode
The availability of dimensionally stable and flexible electrolyte
membranes is very desirable for the progress of lithium batteries.
To examine compatibility with metallic lithium anode and LiFePO4
cathode, EIS and CV were used to study on the interfacial proper-
ties of the binary and ternary electrolytes. Fig. 3 exhibits Nyquist
plots of Li symmetric cell without VC (A and C) and with VC (B and
D) at open circuit condition at room temperature after the cell was
deposited for 1 day (A and B) and 5 days (C and D). From Fig. 3A
and B we clearly see the binary electrolyte offers a fairly high and
steep rise of interfacial impedance, and the interfacial impedance
of the ternary electrolytes increased slowly and steadily. Since the
interfacial impedance composes of charge transfer resistance and
SEI film resistance, above results further confirmed the addition of
VC possibly well enhance the formation of SEI and protect lithium
electrode from corrosion. Thus, the ternary electrolyte has excel-
lentcompatibility withmetalliclithiumanode at roomtemperature
under open circuit conditions.
3.3. Battery performance
The charge/discharge performance Li/LiFePO4 cells incorporat-
ing different kind of ionic liquid electrolytes were well examined
and their initial discharge capacities were shown in Fig. 5. It was
found charge/discharge performance obviously depended on the
kind of ionic liquid. Due to high ion conductivity and relatively low
viscosity, Li/LiFePO4 cells which incorporatingbutyl-DMimTFSI and
amyl-DMimTFSI ionic liquids have better charge/discharge capac-
ities than that of cells with other three ionic liquid electrolytes.
Among five new ionic liquid electrolytes, the amyl-DMimTFSI has
best electrochemical properties, its initial discharge capacity is
about 152.6 mAh g−1. The value is near to theoretical specific capac-
ity (170 mAh g−1) and much higher than that of PP14TFSI ionic
liquid electrolyte [13]. Although the cell with isooctyl-DMimTFSI
3.2.2. LiFePO4 electrode
The investigation of the electrode behavior in the ternary elec-
trolyte was extended to the LiFePO4 case (see Fig. 4). Fig. 4A
presents typical Nyquist plots of the EIS measurements for
Li/LiFePO4 cells incorporating two kinds of electrolytes during dif-
ferent cycles when charged to 3.8 V. The electrolyte exhibits a
semicircle in the high-frequency range and an inclined line in the

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C. Yan et al. / Electrochimica Acta 55 (2010) 4728–4733
Fig. 4. Nyquist plots of Li/electrolytes/LiFePO4 cell after special charge/discharge was carried out for 1 (A) and 25 cycles (B). (C) Cyclic voltammetry of Li/LiTFSI + amyl-
DMimTFSI + VC/LiFePO4 cell.
ionic liquid electrolyte exhibited smallest initial discharge capac-
ity (8.1 mAh g−1), it well be dramatically added to 129.6 mAh g−1
when 10% propylene carbonate was introduced into the ternary
electrolyte as diluent, which demonstrate the viscosity of the ionic
liquids is main factor to effect on the cell performance for this type
of ionic liquids.
Typical galvanostatic charge/discharge and cyclic performance
for Li/LiFePO4 cell were shown in Figs. 6 and 7 with amyl-DMimTFSI
ionic liquid electrolyte at 0.05 C rate, respectively. Fig. 6 indicates
the cell with the electrolyte offer one charge and discharge plateau
around 3.4 V, i.e. within the stability domain of the electrolyte solu-
tion. Fig. 7 exhibited the discharge capacities of the cell become
148 mAh g−1 after 120 cycles, which is about to 97% of the initial
capacity. The reversibility is obviously better than that of classi-
cal 1-alkyl-3-dimethylimidazolium type ionic liquid electrolytes.
Because substituting the chemically stable methyl group reduces
the high reactivity and controls the side reaction with the metallic
lithium electrode, a high reversibility was achieved in the bat-
tery systems that used the alkyl-DMimTFSI ionic liquids. Moreover,
the capacity approaches the theoretical value and thus, we may
conclude that also the LiFePO4 electrode behaves in the ionic liquid-
based solution as well as in the common organic electrolytes.
Fig. 5. Initial discharge capacity of the cell with different ionic liquid ternary elec-
trolyte, in which a, b, c, d and e column represent the cell with amyl-DMimTFSI,
butyl-DMimTFSI, octyl-DMimTFSI, decyl-DMimTFSI and isooctyl-DMimTFSI ternary
electrolyte, respectively. Ternary electrolytes were prepared using the method
described in Section 2.1.
Fig. 6. Galvanostatic charge/discharge for Li/LiFePO4 cell with the amyl-DMimTFSI
ternary electrolyte for 1st cycle (1) and 120th cycle (2). The amyl-DMimTFSI ternary
electrolyte was prepared using the method described in Section 2.1.

C. Yan et al. / Electrochimica Acta 55 (2010) 4728–4733
4733
oped for Li/LiFePO4 batteries as highly safe electrolytes. Although
another three ionic liquids are not suit as electrolytes for Li/LiFePO4
battery as electrolyte due to relatively low conductivity and big
viscosity, these can be used as additive of electrolytes for various
batteries to improve conductivity and safety
Acknowledgements
The authors acknowledge the financial support from the
National Natural Science Foundation of China (Nos. 20771045 and
20676052), the Natural Science Foundation of Zhejiang Province
(No. Y4080404) and Qing Lan Project.
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3.4. Conclusions
In the paper, five new 1-alkyl-2,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)imide ionic liquids were designed
and investigated as highly safe electrolyte for Li/LiFePO4 battery.
The results show the type of ionic liquid electrolytes have very
wider electrochemical window (−0.16 to 5.2 V vs. Li+/Li) and are
theoretically feasible as an electrolyte for batteries with metallic Li
as anode. Substituting the chemically stable methyl group reduces
the high reactivity and controls the side reaction with the metallic
lithium anode and LiFePO4 cathode under charge/discharge condi-
tions. This results in improvement of the electrochemical stability
and compatibility with electrodes. The electrochemical properties
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conductivity, viscosity, charge/discharge capacity and reversibil-
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