Anion amphiprotic ionic liquids as protic electrolyte matrices allowing sodium metal plating

Article abstract of DOI:10.1039/c9cc06083a

The sodium-ion battery (SIB) is proposed as a complementary technology to today's commercially dominant lithium-ion battery (LIB). While much know-how can be transferred from LIBs to SIBs, adjustments are still necessary, not the least for the electrolytes employed. Here the use of anion amphiprotic ionic liquid (AAIL) based electrolytes is proposed for SIB application. Two different AAILs, based on organic trifluoromethylsulfonylamide (TFSAm) and inorganic HSO₄^- anions, respectively, doped with NaTFSI salt have been studied, focusing on electrochemical stability and transport properties, complemented by studies of the ion-ion interactions, and final sodium-ion battery performance via stripping/plating vs. sodium metal electrodes.

Full text of DOI:10.1039/c9cc06083a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Anion amphiprotic ionic liquids as protic
electrolyte matrices allowing sodium metal
plating†
Cite this: Chem. Commun., 2019,
55, 12523
Received 6th August 2019,
Accepted 18th September 2019
abc
d
Piotr Jankowski,
Michel Armand,
*
Karolina Matuszek,
Marcel Treskow,ae
f
d
Douglas MacFarlane
and Patrik Johanssonag
DOI: 10.1039/c9cc06083a
rsc.li/chemcomm
The sodium-ion battery (SIB) is proposed as a complementary
The non-aqueous electrolytes, necessary for SIB batteries,
technology to today’s commercially dominant lithium-ion battery can be based on matrices of organic solvents,13–15 polymers16,17
(LIB). While much know-how can be transferred from LIBs to SIBs, or ionic liquids (ILs).18,19 All have different advantages and
adjustments are still necessary, not the least for the electrolytes disadvantages. ILs and organic/IL hybrids in particular offer
employed. Here the use of anion amphiprotic ionic liquid (AAIL) synergies between intrinsic safety and ion transport properties.20–23
based electrolytes is proposed for SIB application. Two different Most attention has so far focused on aprotic ILs, due to their wider
AAILs, based on organic trifluoromethylsulfonylamide (TFSAm) and electrochemical stability windows,24,25 although recently also
À
inorganic HSO4 anions, respectively, doped with NaTFSI salt have protic ILs have been reported with similar transport properties,
been studied, focusing on electrochemical stability and transport but different interactions. Importantly they also offer reduced
properties, complemented by studies of the ion–ion interactions, charge transfer resistance for insertion–extraction of charge
and final sodium-ion battery performance via stripping/plating vs. carriers at the electrode/electrolyte interfaces for both LIBs and
sodium metal electrodes.
SIBs.26–28 The positive effect of the presence of a proton and its
interactions have also been reported for modified anions;
Armand et al. reported a less than fully fluorinated TFSI anion,
i.e. [N(SO2CF2H)2]À, to be beneficial for the cation transport,
especially in PEO based polymer electrolytes, by virtue of
special hydrogen bonds created.29,30 All this taken together
turned our attention back to the concept of anion amphiprotic
ILs (AAILs), which were originally designed as proton conducting
materials for fuel cell application and distinguished by decoupled
transport,31 and we now try to use them as electrolyte matrices
for SIBs.
As 1-ethyl-3-methylimidazolium trifluoromethylsulfonylamide
([EMIm][TFSAm]) previously provided the highest conductivity,31
we have here used this AAIL and compare it with another more
standard type of protic IL that analoguously has a proton in its
anion structure: 1-ethyl-3-methylimidazolium hydrogen sulfate
([EMIm][HSO4], Fig. 1). Both AAILs were carefully dried to reach
a water level o45 ppm and doped with sodium bis(trifluoro-
methanesulfonyl)imide (NaTFSI) to create SIB electrolytes.
We started our analysis with the pure ILs and found that the
AAILs have superior transport properties, as illustrated by the
In the last three decades the lithium-ion battery (LIB) technology
has risen in popularity to the point that today it totally
dominates the market for modern rechargeable batteries, being
used for portable electronics, electric cars and grid storage.1–5
However, continuing with lithium based technologies might
not be the best solution for the future. The relatively limited
resources of lithium and uneven global distribution cause high
cost, large price fluctuations of the raw material and a depen-
dency on a few countries willingness to share their natural
resources.6–8 Therefore, development of complementary battery
technologies to the LIB is crucial. Among the possibilities,
sodium can be considered as a natural candidate to replace
lithium, since these two elements are chemically very similar,
paving the way for sodium-ion batteries (SIBs).9–12
a Department of Physics, Chalmers University of Technology, Gothenburg, Sweden
b Department of Energy Conversion and Storage, Technical University of Denmark,
Kgs. Lyngby, Denmark. E-mail: pioja@dtu.dk
c Faculty of Chemistry, Warsaw University of Technology, 00-664 Warsaw, Poland
d Monash University, School of Chemistry, Clayton, Victoria, Australia
e Evonik R&D, Frankfurt am Main, Germany
f
˜
CIC Energigune, 01510 Minano, Spain
g Alistore – European Research Institute, CNRS FR 3104, Hub de l’Energie, Amiens,
France
† Electronic supplementary information (ESI) available: Experimental and com-
putational details, spectroscopic and computational analysis. See DOI: 10.1039/
c9cc06083a
Fig. 1 Chemical structures of the AAILs.
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 12523--12526 | 12523

View Article Online
Communication
ChemComm
Fig. 2 Walden plot of the studied ILs, with data for EMImTFSI from ref. 39.
Fig. 4 Electrochemical stability of the two AAILs. WE: Pt, RE: Ag, CE: SS.
Potential of Ag calibrated vs. ferrocene, EAg = À0.18 V vs. Fc+/Fc0.
Walden plot (Fig. 2), allowing us to consider them as superionic
ILs.32 The origin of this has been analysed before by us and was
postulated to be related to a proton exchange equilibrium.31
Our present DFT calculations confirm the existence of this
critical process, with the equilibrium shifted much more
towards the singly-charged anion structure for [TFSAm]À than
for [HSO4]À (Fig. 3). This has a huge impact on the electro-
chemical stability; the singly protonated structures are pre-
dicted to have stabilities against reduction of À1.17 and
À0.76 V vs. Na+/Na0, respectively, while the doubly protonated
structures are predicted to have poorer stabilities of 1.41 and
0.90 V vs. Na+/Na0, respectively, and the latter is for both
structures related to the hydrogen evolution reaction (HER).
Indeed, [EMIm][HSO4] exhibits the lower electrochemical stability
and is reduced at ca. 1.0 V vs. Na+/Na0 (Fig. 4), having a two orders
of magnitude higher concentration of the doubly protonated
form – as derived from the calculated equilibrium constants.
To further test the stability, adding a small piece of sodium metal
into this IL leads to formation of bubbles at the metal surface. The
analogous test with the [EMIm][TFSAm] IL gives no bubble
formation and the CV demonstrates stability at the potential of
sodium metal, with a small process at ca. 1 V vs. Na+/Na0, possibly
related to the formation of a passivation layer, similarly to that of
highly concentrated electrolytes based on TFSI.33,34
likely related to different activation energies of the ion trans-
port, and thus possibly different ion transport mechanisms.
Doping of [EMIm][TFSAm] with NaTFSI causes a decrease in
conductivity, connected with aggregation of the system, as
observed for most ILs,35 while for [EMIm][HSO4] the introduc-
tion of NaTFSI increases the conductivity. For both the NaTFSI
doped AAILs a local maximum in ion conductivity is observed
for x = 0.15 at 2.7 and 1.5 mS cmÀ1, respectively, corresponding
to concentrations of ca. 0.7–0.9 mol kgÀ1 – quite typical of
standard LIB (and SIB) electrolytes. A further increase in salt
concentration, into the highly concentrated electrolyte (HCE)
regime, results in another two maxima in ion conductivity: at
4.0 mS cmÀ1 for (NaTFSI)0.5([EMIm][TFSAm])0.5 and 6.1 mS cmÀ1
for (NaTFSI)0.4([EMIm][HSO4])0.6, respectively. At higher NaTFSI
concentrations in [EMIm][HSO4] a huge decrease in ion conduc-
tivity at lower temperatures is observed as the material partly
crystallizes (melting point ca. À20 1C, Fig. S4, ESI†). This also
affects the glass transition temperature (Tg) decreasing from
ca. À65 to À90 1C. Much more of a glassy behaviour was recorded
for the NaTFSI–[EMIm][TFSAm] system where no melting/crystal-
lization is observed up to x = 0.4 and with a glass transition
temperature more or less fixed at ca. À72 1C (Fig. S4, ESI†). The
appearance of crystalline phase (Tm = 160 1C) for x = 0.5 decreases Tg
down to À84 1C, that also can explain the higher ion conductivity,
while a further increase in NaTFSI concentration increases the Tg to
À62 1C, nicely correlated with a reduced ion conductivity.
Looking at the pure ILs, the lowest ion conductivity is
observed for [EMIm][HSO4] – around half order of magnitude
lower – likely primarily due to the much higher viscosity
(Fig. 5a). In contrast, [EMIm][TFSAm] exhibits a similar ion
conductivity as that reported for the ‘‘standard’’ [EMIm][TFSI]
IL.18 However, in Fig. 5a there are different behaviours observed,
Clearly the organic AAIL based system outperforms the
inorganic for properties important for battery application.
The [HSO4]À based system indeed has some crucial drawbacks:
low electrochemical stability, low ion conductivity, and ample
crystallization, why only the TFSAm based system is targeted for
SIBs. A detailed analysis show significant changes in the
structure in this system upon salt addition (ESI†). Aggregation
around Na+ leads to the formation of a NaxTFSAmy phase, as
seen also by DSC (Fig. S4, ESI†), together with a weaker
intermolecular H-bonding, related to an increasing competi-
tion with Na+ coordination to the oxygen atoms of the anion
(Fig. S6, ESI†). This is also confirmed by MD simulations, with a
major contribution from TFSAm in the Na+ coordination and
formation of densely packed NaxTFSAmy clusters.
Fig. 3 Proton exchange reactions in the AAILs; equilibrium constants and
reduction potentials obtained from DFT calculations.
12524 | Chem. Commun., 2019, 55, 12523--12526
This journal is ©The Royal Society of Chemistry 2019

View Article Online
ChemComm
Communication
Fig. 5 Conductivity curves for neat ILs (a), AAILs doped with NaTFSI (b), and corresponding isotherms for (NaTFSI)x([EMIm][TFSAm])1Àx (c) and
(NaTFSI)x([EMIm][HSO4])1Àx (d). Data for EMImTFSI taken from ref. 18.
Finally, some plating/stripping tests in Na|Na cells were per-
The results presented here show on possibilities for success-
formed for the electrolytes xNaTFSI = 0.15 and 0.5 at 60 1C, a ful application of AAILs as matrices for Na-salts to create SIB
temperature used to compensate for the overall low ion conductiv- electrolytes. The perhaps expected restricted diffusion of the
ities (Fig. 6). For the former electrolyte, the cycling was less stable, relevant charge carrier and limited electrochemical stability
with significant potential increases during single cycles suggesting window, due to H-bonding related increases in viscosity and
quite low sodium transference numbers, while the latter showed proton reduction directly at the anionic structure, were both
lower overpotentials that only slightly increased during subsequent shown to be overcome due to a unique amphiprotic anion
cycles. This is a behaviour similar to other HCE IL based sodium design. The pure [EMIm][TFSAm] AAIL shows ample electro-
conducting systems, where sodium conductivity has been inferred chemical stability together with adequate transport properties
to occur through highly aggregated clusters.36–38 We speculate that and moreover, after addition of NaTFSI the electrolyte contains
the introduction of a proton in the anion structure may further sodium cation based aggregates, NaxTFSAmy clusters, as shown
support/facilitate this ion transport inside the cluster by a proton – spectroscopically and supported both by DFT and MD simula-
sodium exchange at the basic oxygen centres enabling tions. This results in high ion conductivity, as for other HCEs,
de-coordination of sodium and further improve the Na+ mobility. especially for the equimolar mixture of [EMIm][TFSAm] and
Fig. 6 Na|Na cycling of (NaTFSI)0.15([EMIm][TFSAm])0.85 (a) and (NaTFSI)0.5([EMIm][TFSAm])0.5 (b) at 60 1C.
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 12523--12526 | 12525

View Article Online
Communication
ChemComm
14 A. Ponrouch, E. Marchante, M. Courty, J.-M. Tarascon and M. Rosa
NaTFSI, which also provides acceptable sodium plating/strip-
ping performance. Altogether this proves that the strategy to
apply AAIL based battery electrolytes is promising and calls for
further development and improvement.
´
Palacın, Energy Environ. Sci., 2012, 5, 8572–8583.
15 G. G. Eshetu, S. Grugeon, H. Kim, S. Jeong, L. Wu, G. Gachot,
S. Laruelle, M. Armand and S. Passerini, ChemSusChem, 2016, 9,
462–471.
˙
16 A. Bitner-Michalska, G. M. Nolis, G. Zukowska, A. Zalewska,
All calculations were carried out at the Wrocław Centre for
Networking and Supercomputing, Grant 346. The support by
ALISTORE-ERI and Chalmers Area of Advance Energy to
P. Jankowski are both gratefully acknowledged. P. Johansson
acknowledges the financial support from the Swedish Energy
Agency (contract #37671-1, Next Generation Batteries) and
Chalmers Battery Initiative, part of the profile Materials for
Energy Applications jointly managed by the Areas of Advance
Materials Science and Energy at the Chalmers University of
Technology. The stimulating discussions within the ALISTORE-ERI
network are also highly appreciated. D. MacFarlane acknowledges
the financial support of the Australian Research Council through
the Australian Laureate Fellowship program.
M. Poterała, T. Trzeciak, M. Dranka, M. Kalita, P. Jankowski,
L. Niedzicki, J. Zachara, M. Marcinek and W. Wieczorek, Sci. Rep.,
2017, 7, 40036.
17 A. Boschin and P. Johansson, Electrochim. Acta, 2015, 175, 124–133.
´
´
18 D. Monti, E. Jonsson, M. R. Palacın and P. Johansson, J. Power
Sources, 2014, 245, 630–636.
19 I. Hasa, S. Passerini and J. Hassoun, J. Power Sources, 2016, 303,
203–207.
20 S. Menne, R.-S. Ku¨hnel and A. Balducci, Electrochim. Acta, 2013, 90,
641–648.
21 R.-S. Ku¨hnel and A. Balducci, J. Phys. Chem. C, 2014, 118, 5742–5748.
22 C. V. Manohar, K. Anish Raj, M. Kar, M. Forsyth, D. R. MacFarlane
and S. Mitra, Sustainable Energy Fuels, 2018, 2, 566–576.
´
23 D. Monti, A. Ponrouch, M. R. Palacın and P. Johansson, J. Power
Sources, 2016, 324, 712–721.
24 S. A. Mohd Noor, P. C. Howlett, D. R. MacFarlane and M. Forsyth,
Electrochim. Acta, 2013, 114, 766–771.
25 D. Kundu, E. Talaie, V. Duffort and L. F. Nazar, Angew. Chem., Int.
Ed., 2015, 54, 3431–3448.
26 T. Vogl, S. Passerini and A. Balducci, Electrochem. Commun., 2017,
78, 47–50.
Conflicts of interest
27 T. Vogl, S. Menne, R.-S. Ku¨hnel and A. Balducci, J. Mater. Chem. A,
2014, 2, 8258–8265.
There are no conflicts to declare.
28 T. Stettner, P. Huang, M. Goktas, P. Adelhelm and A. Balducci,
J. Chem. Phys., 2018, 148, 193825.
Notes and references
˜
29 H. Zhang, U. Oteo, H. Zhu, X. Judez, M. Martinez-Ibanez, I. Aldalur,
1 M. Winter, B. Barnett and K. Xu, Chem. Rev., 2018, 118, 11433–11456.
2 D. Deng, Energy Sci. Eng., 2015, 3, 385–418.
3 M. Li, J. Lu, Z. Chen and K. Amine, Adv. Mater., 2018, 30, 1800561. 30 U. Oteo, M. Martinez-Ibanez, I. Aldalur, E. Sanchez-Diez, J. Carrasco,
E. Sanchez-Diez, C. Li, J. Carrasco, M. Forsyth and M. Armand,
Angew. Chem., 2019, 131, 7911–7916.
˜
4 B. Diouf and R. Pode, Renewable Energy, 2015, 76, 375–380.
5 G. E. Blomgren, J. Electrochem. Soc., 2017, 164, A5019–A5025.
6 M. Bini, D. Capsoni, S. Ferrari, E. Quartarone and P. Mustarelli, in
M. Armand and H. Zhang, ChemElectroChem, 2019, 6, 1019–1022.
31 M. Treskow, J. Pitawala, S. Arenz, A. Matic and P. Johansson, J. Phys.
Chem. Lett., 2012, 3, 2114–2119.
Rechargeable Lithium Batteries, ed. A. A. Franco, Woodhead Publishing, 32 C. A. Angell, Y. Ansari and Z. Zhao, Faraday Discuss., 2011, 154, 9–27.
2015, pp. 1–17.
7 J.-M. Tarascon, Nat. Chem., 2010, 2, 510.
33 N. Nakatani, K. Kishida and K. Nakagawa, J. Electrochem. Soc., 2018,
165, A1621–A1625.
8 C. Grosjean, P. H. Miranda, M. Perrin and P. Poggi, Renewable 34 L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang
Sustainable Energy Rev., 2012, 16, 1735–1744.
9 C. Delmas, Adv. Energy Mater., 2018, 8, 1703137.
10 J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Chem. Soc. Rev., 2017, 46,
3529–3614.
11 C. Vaalma, D. Buchholz, M. Weil and S. Passerini, Nat. Rev. Mater.,
2018, 3, 18013.
and K. Xu, Science, 2015, 350, 938–943.
35 H. Yoon, A. S. Best, M. Forsyth, D. R. MacFarlane and P. C. Howlett,
Phys. Chem. Chem. Phys., 2015, 17, 4656–4663.
36 F. Chen, P. Howlett and M. Forsyth, J. Phys. Chem. C, 2018, 122,
105–114.
37 A. Basile, F. Makhlooghiazad, R. Yunis, D. R. MacFarlane, M. Forsyth
and P. C. Howlett, ChemElectroChem, 2017, 4, 986–991.
38 M. Forsyth, H. Yoon, F. Chen, H. Zhu, D. R. MacFarlane, M. Armand
and P. C. Howlett, J. Phys. Chem. C, 2016, 120, 4276–4286.
12 K. Kubota and S. Komaba, J. Electrochem. Soc., 2015, 162,
A2538–A2550.
13 A. Plewa-Marczewska, T. Trzeciak, A. Bitner, L. Niedzicki, M. Dranka,
˙
G. Z. Zukowska, M. Marcinek and W. Wieczorek, Chem. Mater., 2014, 39 C. Schreiner, S. Zugmann, R. Hartl and H. J. Gores, J. Chem. Eng.
26, 4908–4914.
Data, 2010, 55, 1784–1788.
12526 | Chem. Commun., 2019, 55, 12523--12526
This journal is ©The Royal Society of Chemistry 2019