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Alvarez-Tirado et al. Energy Mater 2023;3:300003 https://dx.doi.org/10.20517/energymater.2022.59 Page 3 of 14

RESULTS AND DISCUSSION
Synthesis and characterization of ILs and iongels
In this work, four fluorinated ILs, as alternatives to the commercially available DEME-TFSI IL, were
designed to optimize Li-O cell performance [Figure 1]. The [TFSI] anion of DEME-TFSI was replaced by
-
2
different anions, namely, bis(fluorosulfonyl)imide [FSI] , bis(perfluoroethylsulfonyl)imide [BETI] and
-
-
nonafluoro-1-butanesulfonate [C F SO ] . The objective was to modify and increase the fluorinated moieties
-
3
3 9
in the resultant IL and the ionicity of the different ILs with respect to DEME-TFSI. The cation of the
+
DEME-TFSI IL, [DEME] , was redesigned to incorporate a fluorinated pendant group while maintaining
most of its chemical structure (obtaining N,N-diethyl-N-methyl-N-((trifluoroethoxy) ethyl) ammonium,
[17]
FD), following previous works . The IL using this [FD] cation was labeled as FD-TFSI, which was
+
obtained commercially.
+
The [DEME] -based ILs with different anions were synthesized by a typical salt metathesis reaction
[Scheme 1] [17,27,28] . An anion exchange reaction was carried out between a [Br] anion (from N,N-diethyl-N-
-
methyl-N-(2-methoxyethyl)ammonium bromide, [DEME][Br]) and the negative ion of a commercially
available monovalent salt (e.g., X Y , where X = Na , Li or K and Y = [TFSI] , [BETI] or [C F SO ] ). They
-
-
-
+ -
+
+
+
-
+
3 9
3
13
were characterized by H, C and F NMR, as described in the Supplementary Figure 1, as well as by
19
1
attenuated total reflection Fourier transform infrared (FTIR) spectroscopy [Supplementary Figure 2].
DEME-TFSI was also synthesized and compared to a commercial version of this IL to prove the viability of
the synthesis method [Supplementary Figure 2A]. DEME-Br presented one broadband in the FTIR
spectrum at 3450 and 1630 cm corresponding to water and O-H bending , respectively, which
[29]
-1
-1
disappeared in the DEME-TFSI spectrum. The bands between 3000 and 2800 cm were assigned to the
stretching vibrations of the C-H bonds of the alkyl chain of the DEME cation . The characteristic bands of
[30]
+
the fluorinated TFSI anion appeared in the DEME-TFSI spectrum at 651, 741 and 762 cm (attributed to
-1
-
S-N-S symmetric and asymmetric stretching, respectively), 1134 and 1346 cm (-SO asymmetric
-1
2
-1
[30]
stretching), 1058 and 790 cm (-C-S- stretching) and 1165 cm (C-F asymmetric stretching) . Similar
-1
3
bands were observed for DEME-BETI. In contrast, DEME-FSI presented the characteristic bands of the
[FSI]- anion at 1218 and 1365-1383 cm (attributed to -SO symmetric and asymmetric stretching,
-1
2
respectively), 572 cm (δ (O S-N-SO ) + ν(S-F) + δ (SO -F)) and 454-482 cm (δ (SO-F) + δ (OS-N-SO-)
-1
-1
2
ip
ip
sci
2
2
and δ (SO-F) + δ (O S-F), respectively) . DEME-C F SO presented the characteristic bands of the
[30]
3
2
3 9
op
sci
[C F SO ]- anion at 638, 1225 and 518, 1261, 1281 cm (attributed to -SO symmetric and asymmetric
-1
3
3
3 9
stretching, respectively) and 755 cm (C-F symmetric stretching) .
-1
[30]
3
These ILs were further analyzed via differential scanning calorimetry. As shown in
Supplementary Figure 3A-C, DEME-BETI and DEME-C F SO presented small endothermic peaks at -40.5
3 9
3
and -22.1 °C, respectively. The DEME-FSI and DEME-TFSI thermograms were flat, suggesting that their
melting temperatures are probably below -80 °C, outside the equipment range or nonexistent. Literature
works claim only Tg transitions at -94.2 and -111.4 °C for DEME-TFSI and DEME-FSI, respectively .
[31]
Hence, all the ILs are liquid at room temperature, which is a desirable property from a practical battery
perspective.
Regarding the diffusion properties, the mobility of the ions within an electrolyte is directly proportional,
amongst other properties, to the viscosity of the system . Thus, the dynamic viscosity of these ILs was
[32]
measured at 25 and 60 °C [Supplementary Figure 3D]. As expected, the viscosity decreased with
temperature (e.g., up to four times lower at 60 °C for DEME-C F SO ) and the DEME-FSI IL possessed the
3 9
3
lowest values of 71.1 and 26.6 mPa·s at 25 and 60 °C, respectively. The viscosity of an IL is usually
determined by its hydrogen and/or van der Waals forces and is usually affected by the ion size, branching