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Energy applications of ionic liquids
Douglas R. Macfarlane, Naoki Tachikawa, Maria Forsyth, Jennifer M. Pringle,
Patrick C. Howlett, Gloria D. Elliott, James H. Davis, Masayoshi Watanabe,
Patrice Simon, C. Austen Angell
To cite this version:
Douglas R. Macfarlane, Naoki Tachikawa, Maria Forsyth, Jennifer M. Pringle, Patrick C. Howlett, et
al.. Energy applications of ionic liquids. Energy & Environmental Science, Royal Society of Chemistry,
2014, vol. 7, pp. 232-250. �10.1039/c3ee42099j�. �hal-00979082�
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This is an author-deposited version published in : http://oatao.univ-
toulouse.fr/
Eprints ID : 11354
To link to this article : doi:10.1039/c3ee42099j
URL : http://dx.doi.org/10.1039/c3ee42099j
To cite this version : MacFarlane, Douglas R. and Tachikawa, Naoki
and Forsyth, Maria and Pringle, Jennifer M. and Howlett, Patrick C.
and Elliott, Gloria D. and Davis, James H. and Watanabe, Masayoshi
and Simon, Patrice and Angell, C. Austen Energy applications of ionic
liquids. (2014) Energy & Environmental Science, vol. 7 (n° 1). pp.
232-250. ISSN 1754-5692
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Energy applications of ionic liquids
Douglas R. MacFarlane,
*
a
Naoki Tachikawa,
a
Maria For syth,
b
Jennifer M. Pringle,
b
Patrick C. Howlett,
b
Gloria D. Elliott,
c
James H. Davis, Jr.,
d
Masayoshi Watanabe,
e
Patrice Simon
f
and C. Austen Angell
g
Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy
related applications. Cation–anion combinations that exhibit low volatility coupled with high
electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing
ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermo-
electrochemical cells. In the field of water splitting to produce hydrogen they have been used to
synthesize some of the best performing water oxidation catalysts and some members of the protic ionic
liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell
electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of
fuel cells operating in the optimum temperature region above 100
C. Beyond electrochemical
applications, the low vapour pressure of these liquids, along with their ability to offer tuneable
functionality, also makes them ideal as CO
2
absorbents for post-combustion CO
2
capture. Similarly, the
tuneable phase properties of the many members of this large family of salts are also allowing the
creation of phase-change thermal energy storage materials having melting points tuned to
the application. This perspective article provides an overview of these developing energy related
applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities.
1. Introduction
Sustainability of energy supply has become one the great chal-
lenges of our time. The combination of concerns about the
contribution of fossil fuels to greenhouse gas induced climate
change, as well as the long term sustainability of their supply,
has necessitated an increasingly urgent development of alter-
native approaches to energy generation and storage. Similarly,
air quality related environmental concerns, as well as energy
security, is driving the development of alternate energy sources
for transportation including passenger vehicles. As increasingly
larger solar and wind energy capacity is installed around the
world, the intermittency of these sources is creating the need
for large-scale energy storage solutions and variable-load
applications, including fuel generation. At the same time,
there is huge effort being applied to the minimisation of the
impact of fossil fuel use via carbon capture technologies.
In all of this development activity, there is enormous
potential for the discovery and application of new materials that
offer signicant improvements in the way that energy is
generated, stored and delivered. Ionic liquids (ILs) are one such
family of materials that are beginning to have an impact on a
broad swath of energy technologies, offering a range of prop-
erties that can be tuned to optimise their performance in a
variety of contexts. As liquid salts, dominated by strong elec-
trostatic forces between their molecular ions, the key properties
that certain members of this huge family of compounds offer is
low volatility/ammability and high chemical and electro-
chemical stability. This makes them potentially ideal as
solvents and electrolytes, and in electrochemical applications
their intrinsic ionic conductivity is also an important feature.
However, the properties of ILs vary enormously as a function
of their molecular structure and considerable effort has been
devoted to identifying and understanding those that have
superior properties in any given application. Our goal in this
article therefore is to provide a perspective on how, and why,
ionic liquids are impacting on a range of electrochemical
technologies, including advanced batteries, dye sensitised solar
cells, double layer capacitors, actuators, fuel cells, thermo-cells
and water splitting, as well as in non-electrochemical areas
including carbon capture and in emerging thermal energy
a
Australian Centre for Electromaterials Science, School of Chemistry, Monash
University, Clayton, Victoria 3800, Australia. E-mail: douglas.macfarlane@monash.
edu
b
Institute for Frontier Materials, Deakin University, Burwood, Victoria, Australia
c
Department of Mechanical Engineering and Engineering Sciences, University of North
Carolina, Charlotte 9201 University City Blvd., Charlotte, NC 28223-0001, USA
d
Department of Chemistry, University of South Alabama, Mobile, Al 36688, USA
e
Department of Chemistry & Biotechnology, Yokohama National University, Yokohama
240-8501, Japan
f
Universit
´
e Paul Sabatier, Toulouse III, CIRIMAT UMR-CNRS 5085, 118 Route de
Narbonne, 31062 Toulouse Cedex, France
g
Department of Chemistry and Biochemistry, Arizona State University, AZ 85287-1604,
USA
storage applications. Since the literature is voluminous we have
only made reference to some of the key, recent and exemplary
papers in each eld, sufficient to provide some background and
in the hope that these will provide a starting point for the
interested reader to follow.
2. Ionic liquids in advanced battery
technologies
Lithium and lithium–ion cells
The attractive properties of ILs, in particular the high electro-
chemical stability of some cation and anion types, lend them-
selves to application in high-energy electrochemical devices
such as lithium batteries. One of the key advantages that they
offer as electrolytes is low volatility and ammability, hence
offering the possibility of enhanced safety and stability. For an
excellent summary of the properties and development of
lithium ion rechargeable batteries generally, the reader is
referred to a recent perspective article by Goodenough and
Park.
1
Early reports of ILs offering sufficient stability for lithium
electrochemistry in quaternary ammonium and phosphonium
[NTf
2
]
ILs
2,3
sparked the interest of battery researchers (struc-
tures and nomenclature of some commonly used ILs are sum-
marised in Fig. 1). Cyclic pyrrolidinium and piperidinium
[NTf
2
]
ILs, which are typically less viscous and slightly more
cathodically stable than the aliphatic cations, were then shown
to support highly efficient lithium cycling.
4
A large number of
publications have subsequently explored variations of IL cation
(dominated by small aliphatic and cyclic ammonium cations)
and anion (dominated by [NTf
2
]
and [BF
4
]
) combinations
with a range of lithium battery anodes and cathodes.
5
Matsu-
moto
6
has reported high rate cycling of a Li|LiCoO
2
cell incor-
porating a pyrrolidinium FSI IL, demonstrating the ability of
this smaller amide anion to promote higher transport rates. In
addition this anion avoided the need for additives
7
to prevent
irreversible interaction of the IL cation at graphitic carbon
electrodes, indicating a probable chemical role in forming a
surface layer on the electrode.
8
Application of small phospho-
nium cations (with [NTf
2
]
) has also been reported,
9
including
their use with high voltage cathodes.
10
Within this relatively
narrow group of ILs, reliable cell cycling with other high-
capacity and/or high-voltage electrodes including sulfur,
11
silicon,
12
polyaniline CNTs,
13
LiMn
1.5
Ni
0.5
O
4
(ref. 14) and LiV-
PO
4
F (ref. 15) has also been reported recently.
Throughout the evolution of ionic liquid electrolytes for
lithium batteries the importance of the fate of the dissolved
lithium ion and its speciation in the bulk liquid has been rec-
ognised, both in terms of its inuence on ion transport and on
interactions at the electrode surface. Design of both the cation
and anion functionality (e.g., alkoxy and cyano functionality) and
the use of additives and diluents (e.g.,vinylenecarbonate)have
been explored as methods to inuence the electrolyte properties
in this regard.
16–20
The formation of protective surface lms (also
known as the solid electrolyte interphase – SEI)
4,21
and the
importance of the role of IL interfacial and bulk structuring on
electrochemical stability, ion transport and charge transfer has
been discussed.
22,23
Lane recently reported a detailed assessment
of the cation reduction reactions occurring at negative electrodes
for the most common ILs.
24
Taken together, these studies (and
others beyond the scope of this article) now provide an under-
standing of the role of the specicinteractionsineachILsystem
(e.g. cation and anion association, speciation, electrochemical
stability, decomposition products and interfacial structuring)
that inuence lithium ion transport and the electrode interac-
tions that occur in an operating lithium cell. However, uncer-
tainty still remains as to the mechanism by which Li salt addition
results in an extension of the cathodic reduction limit.
25
The relative importance of the chemical breakdown of the IL
constituents to form a SEI versus cation–anion interfacial struc-
turing and speciation to exclude reactive constituents, is not
clearly understood and further research in this area is needed to
better inform the design of new IL electrolytes.
The growing need for large-scale energy storage to amelio-
rate the intermittency aspect of renewable energy installations
will provide further stimulus for IL electrolyte development,
with requirements for high energy density becoming less
important compared to safety and robustness, very long cycle
life and an ability to operate at elevated temperatures. This
provides an excellent opportunity for ILs to come to the fore,
with their intrinsic stability offering possible solutions to the
shortcomings of current technology in this regard. Recent
efforts to develop prototype batteries for this type of application
have highlighted the feasibility of IL electrolytes, for example in
combination with photovoltaic panels where thermal stability is
important and rate limitations are less so.
26
However, despite such extensive study and demonstrated
wide applicability of a variety of different ILs in various Li cell
congurations of different energy and power capabilities, no
commercial application of batteries incorporating IL electro-
lytes has yet been developed. It would seem this is due in part to
the prohibitive expense of ILs in comparison to the conven-
tional carbonate solvents, in particular of some of the uori-
nated amide anions. If we accept that the inherent cost of
current IL electrolytes is prohibitive for the development of
commercial devices, then strategies to either reduce cost or
increase value are required. The most obvious strategy is to
continue to explore and develop lower cost cations and anions;
the recent demonstration of Li cells incorporating a pyrrolidi-
nium dicyanamide IL, Fig. 2, demonstrates some progress in
this direction by avoiding the use of uorinated anions.
27
Another avenue will emerge from the inherent stability of ILs,
aiming to develop batteries capable of very large numbers of
charge–discharge cycles, particularly under adverse thermal
conditions. This has been initially demonstrated for devices
based on stable intercalation electrodes (i.e.,Li
4
Ti
5
O
12
and
LiFePO
4
) where substantial efforts have been made to demon-
strate prototype devices with high cycle stability.
28
The other signicant factor limiting application relates to
the relatively low rate capability displayed by IL based batteries,
owing mainly to the greater viscosity of the electrolyte, partic-
ularly aer Li salt addition which contributes to stronger ion
association in the electrolyte. Progress on this issue has
emerged recently from the FSI family of ILs, where very high
lithium ion contents have been found to support substantial
charge–discharge rates,
6
although this has come at the cost of
reduced stability.
29
Clearly these concerns about thermal
stability and safety of ILs of this anion need to be further
investigated.
29,30
More recently, to overcome both the issues of cost and rate
capability (particularly at low temperature), studies which
investigate blended systems of ILs with conventional carbonate
based electrolytes have become increasingly common.
31–33
In
this case, the ILs are incorporated to increase the ionic strength
of the electrolyte and hence manipulate ion dynamics, interfa-
cial stability and, most importantly, ammability.
Recently, research focus has turned to high-energy electrode
combinations such as Li–O
2
and Li–S rechargeable cells to
address the limitations of current Li–ion devices. Particularly in
the case of Li–O
2
, problems associated with electrolyte decom-
position at the cathode, and the need for high e ffi ciency Li
cycling at the anode requires signicant focus on electrolyte
development.
34
Mizuno et al.
35
assessed the stability of a range
of electrolytes based on the calculated Mulliken charge for each
atom. It was determined that a piperidinium [NTf
2
]
IL should
possess stability towards the O
2
radical anion and this was
conrmed by demonstrating a cell with reduced cathodic
polarization compared to a carbonate electrolyte. Higashi et al.
36
subsequently showed improved performance by the incorpora-
tion of an ether functionalised cation (N,N-diethyl-N-methyl-N-
(2-methoxyethyl) ammonium, [DEME], Fig. 1). Allen et al.
37
reported a strong correlation between the oxygen reduction
reaction products and the ionic charge density of the IL cation,
which was rationalised in terms of the Lewis acidity of the
Fig. 1 Common ionic liquid ion families appearing in energy applications and their commonly used acronym systems. In acronymns such as
[C
n
mpyr]
+
the subscript “n” indicates the number of carbons in the alkyl substituent.
![Fig. 3 Reversible cycling of sodium in a [C4mpyr][NTf2] ionic liquid electrolyte (from ref. 45).](/figures/fig-3-reversible-cycling-of-sodium-in-a-c4mpyr-ntf2-ionic-1qx1izr8.webp)
![Fig. 2 Charge–discharge profiles at the 10th, 50th, and 100th cycle of a Li|LiFePO4 cell using a [dca] IL at 80 C (redrawn from ref. 27).](/figures/fig-2-charge-discharge-profiles-at-the-10th-50th-and-100th-r6zim2h8.webp)
![Fig. 6 Schematic of a thermo-electrochemical cell using the [Fe(CN)6] 3 /[Fe(CN)6] 4 redox couple in aqueous electrolyte.](/figures/fig-6-schematic-of-a-thermo-electrochemical-cell-using-the-3trv7kdj.webp)
![Fig. 7 Dependence of the Seebeck coefficients on (ZOx 2 ZRed2)/r for various iron and chromium complexes in [C4mpyr][NTf2].108](/figures/fig-7-dependence-of-the-seebeck-coefficients-on-zox-2-zred2-36klclh6.webp)