Ambient lithium-SO2 batteries with ionic liquids as electrolytes

Article abstract of DOI:10.1002/anie.201309539

Li-SO₂ batteries have a high energy density but bear serious safety problems that are associated with pressurized SO₂ and flammable solvents in the system. Herein, a novel ambient Li-SO₂ battery was developed through the introduction of ionic liquid (IL) electrolytes with tailored basicities to solvate SO₂ by reversible chemical absorption. By tuning the interactions of ILs with SO₂, a high energy density and good discharge performance with operating voltages above 2.8V were obtained. This strategy based on reversible chemical absorption of SO ₂ in IL electrolytes enables the development of the next generation of ambient Li-SO₂ batteries. Copyright

Full text of DOI:10.1002/anie.201309539

Angewandte
Chemie
DOI: 10.1002/anie.201309539
Lithium Batteries Hot Paper
Ambient Lithium–SO Batteries with Ionic Liquids as Electrolytes**
2
Huabin Xing,* Chen Liao, Qiwei Yang, Gabriel M. Veith, Bingkun Guo, Xiao-Guang Sun,*
Qilong Ren, Yong-Sheng Hu, and Sheng Dai*
Abstract: Li-SO batteries have a high energy density but bear
serious safety problems that are associated with pressurized
batteries; heretofore, it has been widely used in military,
industrial, and consumer applications requiring long service
life.
2
[
2]
SO and flammable solvents in the system. Herein, a novel
2
ambient Li-SO battery was developed through the introduc-
tion of ionic liquid (IL) electrolytes with tailored basicities to
Despite these favorable attributes, the safety of the Li-SO
2
2
[
2a,5]
system limits its application.
First, liquefied SO2 is
solvate SO by reversible chemical absorption. By tuning the
accumulated in the cell by pressure that necessitates that
the cells can withstand pressures greater than 3.4 atm at 294 K
2
interactions of ILs with SO , a high energy density and good
2
discharge performance with operating voltages above 2.8 V
were obtained. This strategy based on reversible chemical
without leaking toxic SO . Furthermore, volatile acetonitrile
2
is used as the electrolyte solvent, resulting in an uncontrolled
increase in pressure, and the risk of fire, under abuse
conditions. To overcome this limitation, numerous groups
absorption of SO in IL electrolytes enables the development of
2
the next generation of ambient Li-SO batteries.
2
have investigated polar aprotic solvents, such as propylene
[1]
[5a,c]
E
nergy storage is of central importance for modern society.
carbonate, dimethoxyethane, and diglyme,
as well as salts
[6]
The lithium–sulfur dioxide (Li-SO ) battery is a lightweight,
such as lithium tetrachloroaluminate to reduce the reac-
tivity and the vapor pressure of the electrolytes, but the
former were flammable solvents and the latter were sensitive
to moisture.
2
long-life, high energy density primary battery in which cheap,
[
2]
abundant SO2 serves as the active cathode material. In
contrast to the 1.5 V class primary batteries, such as alkaline
[
3]
[4]
batteries and metal–air batteries, the nominal voltage of
the Li-SO battery is 3.0 V. Furthermore, the Li-SO battery
Herein, we have developed a novel ambient Li-SO2
battery using a functional ionic liquid (IL) as an electrolyte
(Figure 1). This strategy involves optimizing the IL chemistry
2
2
À1
has an energy density of up to 330 Whkg , which is 2–4 times
greater than that of alkaline batteries. The unique character-
istics of Li-SO2 batteries has opened up some special
applications beyond the capability of traditional primary
[
*] Dr. H. B. Xing, Dr. Q. W. Yang, Prof. Q. L. Ren
Key Laboratory of Biomass Chemical Engineering of Ministry of
Education, Department of Chemical and Biological Engineering,
Zhejiang University
Hangzhou 310027 (China)
E-mail: xinghb@zju.edu.cn
Dr. H. B. Xing, Dr. C. Liao, Dr. B. K. Guo, Dr. X. G. Sun, Prof. S. Dai
Chemical Sciences Division, Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
E-mail: sunx@ornl.gov
dais@ornl.gov
Dr. G. M. Veith
Figure 1. Representation of the ambient IL-based Li-SO battery.
2
Materials Science and Technology Division
Oak Ridge National Laboratory, Oak Ridge, TN 37831 (USA)
Prof. S. Dai
to capture SO through reversible chemical absorption with
Department of Chemistry, University of Tennessee
Knoxville, TN 37966 (USA)
2
basic ILs, while still maintaining the electrochemical activity
of SO . This kind of new IL-SO electrolyte materials exhibit
Prof. Y. S. Hu
2
2
National Laboratory for Condensed Matter Physics
Institute of Physics, Chinese Academy of Sciences
Beijing, 100190 (China)
extremely high conductivity, low viscosity, and high SO2
loading, which guarantees a high energy density and good
discharge performance of the Li-SO battery.
2
[
**] The experimental part of this research was supported by the U.S.
Department of Energy’s Office of Basic Energy Science, Division of
Materials Sciences and Engineering; the calculation was supported
by National Natural Science Foundation of China (21222601,
ILs are uniquely qualified for this application because of
their physical properties. Specifically, they have near negli-
gible vapor pressures, are nonflammable, and have wide
[
7]
electrochemical windows. Owing to these attributes, ILs
have been studied as electrolyte materials for various electro-
20936005), and the Zhejiang Provincial Natural Science Foundation
of China (LR13B060001).
[8]
chemical devices, such as electrochemical capacitors, lith-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309539.
[
9]
[10]
ium-ion batteries, Li-O and Li-S batteries, but not Li-SO2
2
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2099

.
Angewandte
Communications
batteries. The structure and properties of the IL can be finely
adjusted by changing the cation or anion of the IL, and
specific functionality can be introduced to the IL. In this
context, a number of functional ILs have been reported to
IL-SO electrolytes also show extremely high conductiv-
2
ity. As shown in Table 1, the presence of SO2 causes an
increase in ionic conductivities of all of the IL-based electro-
lytes with or without lithium bis(trifluoromethanesulfonyl)i-
mide (LiTFSI), while their viscosities decrease significantly
(Supporting Information, Table S1). For example, the con-
ductivity of [Bmim][SCN]-SO electrolytes with a SO molar
[
11]
absorb SO , including guanidinium-based ILs, azole-based
2
[
12]
[13]
ILs, ether-functionalized ILs, and several other anion-
functionalized ILs.
[
14]
2
2
À1
It is these characteristics that we have exploited and
fraction of 0.59 is 16.1 mScm , which is 3.6 times larger than
that of pure [Bmim][SCN]. And the conductivity of SO2-
saturated [Bmim][SCN] electrolyte increases to a high value
tailored in preparing the intrinsically stable Li-SO batteries.
2
In the following, we show how a high SO loading and good
2
À1
discharge performance can be achieved by tuning the
interaction between the ILs and SO2.
of 68 mScm at ambient temperature and pressure, which is
15.4 times larger than that of pure [Bmim][SCN]. The
Four ILs having a different cation, anion, and functional
groups were prepared, and their feasibility as electrolyte
materials in the Li-SO2 battery was explored. These ILs
including three 1-alkyl-3-methylimidazolium-based ILs with
conductivity of SO -saturated [Emim][Cl] electrolyte even
2
À1
reaches 90.1 mScm , which is comparable to that of aqueous
[
15]
electrolytes.
Similarly, the viscosity of SO -saturated
2
[bmim][SCN] electrolyte decrease to 6.8 mPas, which is far
below that of pure IL (54.7 mPas). It is known that when
aprotic ILs are used as electrolytes, the relatively high
viscosity and the low conductivity caused by the intrinsic
strong electrostatic interaction between ions are the main
À
À
À
different anions (SCN , Cl , and TFSI ) that possess tunable
interaction sites and strength with SO , and another IL with
2
a piperdinium cation (Scheme 1). As the capacity of the Li-
SO battery is mainly dependent on the amount of SO in the
2
2
[7d,9a,c]
electrolyte, it is crucial for the ILs to capture as much SO as
drawbacks for ILs.
Therefore, such high conductivities
2
possible under ambient conditions. Indeed, as listed in
Table 1, [Bmim][SCN] shows excellent absorption capacity,
and low viscosities make the IL-SO composites be attractive
electrolyte materials for battery applications.
2
The ambient Li-SO battery with ILs/LiTFSI as electro-
2
lytes was assembled in a glove box under an argon atmo-
sphere (see the Supporting Information). A commercial
3
À1
carbon cloth with a pore volume of 0.49 cm g was used as
the current collector for the cathode. Figure 2a shows the
discharge performance of the Li-SO2 batteries with SO2-
saturated ILs as the electrolytes under a current rate of
À2
2
0 mAcm . These IL-based Li-SO2 batteries have high
working cell voltage above 2.7 V and characteristic flat
Scheme 1. Structures of the ILs in this work.
Table 1: SO loading in ILs (q, mol SO per mol IL) and conductivities (s,
2
2
À1
mScm ) of ILs and IL-SO composites with different SO loading at
2
2
2
96.3 K.
[
a]
ILs
ILs with low SO loading SO -saturated ILs
2 2
s
s
q
s
q
s
s
[
b]
[b]
[d]
[b]
[b]
[
[
[
[
Bmim][SCN] 4.4 3.3 0.59
16.1
13.8
9.5
2.91 68.1 41.6
2.75 90.1 78.4
1.33 17.3 10.3
1.31 9.0 5.3
[
c]
[c]
Emim][Cl]
–
–
0.62
[
b]
b]
Bmim][TFSI] 3.9 2.6 0.67
[
Bmpp][TFSI] 1.2 0.6 0.63
4.5
[
a] The ILs were saturated at a SO pressure of 1 bar and 296.3 K. [b] The
2
ILs contain 0.5m LiTFSI. [c] [Emim]Cl is solid at 296.3 K. [d] The IL
contain 0.3m LiTFSI.
and it can capture 2.91 mol SO per mol IL through multiple-
2
site interactions under ambient conditions (see below for
a detailed absorption mechanism), that constitutes about
5
0% of the electrolyte weight. For comparison, in conven-
tional organic-solvent-based Li-SO2 cells, SO2 constitutes
about 70% of the electrolyte weight; however, the SO
2
[
2]
Figure 2. a) Discharge curves of ambient IL-based Li-SO batteries
2
pressure can reach as high as 3.4 atm at 294 K. Furthermore,
the temperature has a significant effect on the capacity of ILs.
As the temperature increases from 286.1 to 316.1 K, the
with different ILs as electrolyte materials under a constant current
À2
density of 20 mAcm at 296.3 K. b),c) EIS of IL-SO batteries with
2
different ILs after 24 h discharge at a constant current density of
À2
capacity of [Bmim][SCN] decreases from 3.52 to
2
0 mAcm : b) [Bmim][SCN] and [Emim][Cl], c) [Bmim][TFSI] and
À1
1
.85 molmol (Supporting Information, Figure S1).
[Bmpp] [TFSI].
2
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discharge curve, which are same as that of standard Li-SO
2
[
2]
battery using an acetonitrile-based electrolyte. This ambient
Li-SO2 battery also demonstrated a good continuous dis-
charge performance for at least over 20 days (Supporting
Information, Figure S2), and about 90% of solvated SO still
2
remained in the [Bmim][SCN]-based electrolyte after 20 days
discharge. The open-circuit voltage of the Li-SO batteries
2
with IL electrolytes is 3.0 Æ 0.5 V, indicating that SO is the
2
active cathode material. Figure 2a also shows the effect of the
structure of the ILs on the discharge performance of the Li-
SO batteries. Different kind of anions investigated in this
2
work had a slight effect on the discharge voltage, and the cell
with [Bmim][SCN] as electrolyze has the highest discharge
voltage of 2.83 V. When the cation is changed from 1-butyl-3-
methylimidazolium to N-butyl-N-methylpiperdinium, the
À
discharge voltage of the IL-SO battery with TFSI anion
2
decreases from 2.80 to 2.71 V.
Figure 2b and c show the electrical impedance spectros-
copy (EIS) results of the four IL-based Li-SO cells after 24 h
2
À2
discharge at a constant current density of 20 mAcm . The
intercept at high frequency indicates the bulk resistance that
mainly relates to the electrical conductivity of electrolyte and
electrodes, and the strongly superimposed semicircle at high
and medium frequencies is usually assigned to the solid-
electrolyte interface resistance and charge-transfer resis-
Figure 3. a) Discharge curves of an ambient [Bmim][SCN]-based Li-SO
2
battery with different SO loadings under constant current density of
2
À2
[10b,16]
20 mAcm and at 296.3 K; red curve: SO
2
-saturated [Bmim][SCN];
tance.
As shown in Figure 2b and c, it is clear that the
blue curve: [Bmim][SCN]-0.69SO . Insets: the interaction and the
2
cells with [Bmim][SCN] and [Bmim][Cl] as the electrolytes
have a relatively small intercept at high frequency, which is
consistent with the ionic conductivity data shown in Table 1.
The semicircle radii of the [Bmim][SCN]-based cell is
obviously smaller than those of other IL-based cells, indicat-
ing faster charge-transfer kinetics and a better discharge
performance in the [bmim][SCN]-based cell.
enthalpy of [Bmim]SCN-3SO (b) and [Bmim]SCN-SO (c); O red,
2
2
S yellow, N blue, C gray. d) Discharge voltages of an ambient [Bmim]-
SCN]-based Li-SO battery under different current densities at 296.3 K.
[
2
is in the physical regime. Thus, during the discharge process
accompanied with SO desorption, the interaction between
2
The interactions between ILs and SO were analyzed with
ab initio calculations at the B3LYP/6-31 ++ G(d,p) level.
The minimum energy structures of the complexes with
the SO and [Bmim][SCN] changes gradually from a physical
interaction to a chemical interaction because of the increase
in absorption enthalpy.
2
2
[17]
different ratios of IL and SO are shown in Figure 3b,c, and
It is thus an important issue whether all of SO captured
2
2
in the Supporting Information, Figures S5–S7. The cation of
by ILs through chemical and physical absorption are available
for electrochemical reduction reaction. In this work, the
discharge performance of Li-SO2 batteries with [Bmim]-
[SCN]-0.69SO and [Bmim][TFSI]-0.64SO as electrolytes,
[
Bmim][SCN] was omitted for clarity in Figure 3b and c. As
seen in these figures, the anions are the main sites to capture
SO . The calculated absorption enthalpies of SO for [Bmim]-
2
2
2
2
À1
[
TFSI]-SO was À17.9 kJmol , indicating it was a physical
respectively, were evaluated and the results were compared
2
absorption. As for [Emim][Cl], the SO uptake at 1 bar was
with that of SO -saturated cells. As shown in Figure 3a, the
2
2
2
.75 mol per mol of IL, and the absorption enthalpies of SO2
[Bmim][SCN]-0.69SO2 cell also shows characteristic flat
discharge curves, although the working voltage is slightly
for [Emim][Cl]-SO and [Emim][Cl]-2SO complexes were
2
2
À1
À1
À64.7 kJmol and À49.0 kJmol , respectively. Thus one
below that of SO -saturated [bmim][SCN], revealing that all
2
mol of SO was trapped in [Emim][Cl] by chemical absorption
of the SO , regardless of chemical absorption or physical
2
2
À1
(
absorption enthalpy < À50 kJmol ). This chemical absorp-
absorption, are capable of discharging. Similar discharge
behavior was observed in the [Bmim][TFSI]-0.64SO2 cell
(Supporting Information, Figure S3). Therefore, with
[Bmim][SCN] as an example, the aforementioned studies
clearly show that it is a practical approach to simultaneously
tion mechanism was confirmed by a single-crystal X-ray
[
18]
diffraction experiment.
In the case of [Bmim][SCN], almost 3 mol of SO were
2
captured by the IL through multiple-site interactions (Fig-
ure 3b, and Supporting Information, Figures S7). The absorp-
tion enthalpies of SO for the [Bmim][SCN]-3SO , [Bmim]-
achieve high SO loading and satisfactory discharge perfor-
2
mance in a Li-SO2 battery using an IL electrolyte with
2
2
[
SCN]-2SO2, and [Bmim][SCN]-SO2 complexes were
multiple-site SO interactions.
2
À1
À1
À1
À35.4 kJmol , À44.1 kJmol , and À58.6 kJmol , respec-
tively. According to the calculated results, the interaction of
SO2 and IL in [Bmim][SCN]-SO2 is mainly a chemical
interaction; however, the interaction in [Bmim][SCN]-3SO2
The ambient Li-SO batteries based on IL electrolytes not
2
only have good discharge performance but also have good
rate capability. The working voltage of the Li-SO batteries
with [Bmim][SCN] (Figure 3d) and [Bmim][TFSI] (Support-
2
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ing Information, Figure S4) as the electrolytes decreased
slightly with the increase of discharge current density. For
chemically and physically captured SO2 with absorption
À1
enthalpy less than À64.7 kJmol have good electrochemical
example, the working voltage in the [Bmim][SCN]-SO cell
activity during discharge. Furthermore, a high energy density
and good discharge performance with working voltage over
2.8 V was achieved using an IL electrolyte with multiple-site
2
decreases from 2.83 to 2.74 V as the current density increases
À2
from 38 to 159 mmcm .
The cathode discharge products were examined with
infrared (IR) spectroscopy and X-ray photoelectron spec-
troscopy (XPS). Figure 4a shows the IR spectra of the
SO interactions. This strategy based on reversible chemical
2
absorption of SO in IL-electrolytes opens a new avenue to
2
improve the safety of Li-SO batteries and is also helpful for
2
cathode products from the Li-SO cell with [Bmim][TFSI]
the development of next generation of ambient Li-SO2
batteries.
2
Received: November 2, 2013
Published online: January 20, 2014
Keywords: absorption · ionic liquids · lithium ·
.
primary batteries · sulfur dioxide
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