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Alvarez-Tirado et al. Energy Mater 2023;3:300003 https://dx.doi.org/10.20517/energymater.2022.59 Page 5 of 14

methylpropiophene). After UV irradiation for < 2 min on the drop-cast solution, self-standing and
transparent membranes were obtained. In all cases, the LiTFSI salt was dissolved in the ILs to form the ILEs,
which were labeled as Iongel-xx*, where xx is the anion of the IL selected and * corresponds to the use of
LiTFSI as the salt (e.g., Iongel-FSI*, where DEME-FSI and LiTFSI were used as the ILE) [Table 1,
Supplementary Table 1]. In addition to LiTFSI, other salts, such as lithium bis(fluorosulfonyl)imide (LiFSI),
lithium bis(perfluoroethylsulfonyl)imide (LiBETI) and lithium nonafluoro-1-butanesulfonate (LiC F SO ),
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were also used, as shown in Table 1. In all cases, the degree of crosslinking was monitored via FTIR
spectroscopy and conversions of ≥ 94% were reached [Supplementary Figure 4]. The monomer conversion
was examined through the C=C stretching vibration of the acrylic groups (1640-1635 cm ), which
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significantly decreased/disappeared after UV irradiation .
[33]
The thermal and mechanical stability of the iongels was evaluated via thermal gravimetrical analysis (TGA)
and dynamic mechanical thermal analysis (DMTA). From a thermal perspective [Figure 2A], all membranes
using the LiTFSI salt (except for Iongel-FSI*) behaved very similarly and did not present any thermal
degradation until ~310-330 °C. This remarkably high thermal stability was directly attributed to the intrinsic
[17]
properties of the ILs (e.g., a decomposition temperature of ~325 °C for DEME-TFSI and low volatility) .
From this temperature, a one-step degradation occurred for all the iongels analyzed. The thermogram of
Iongel-FSI* showed an earlier thermal decomposition than the other membranes. This was attributed to the
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[31]
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lower stability of the FSI anion compared to the TFSI anion . In summary, all iongel membranes are very
[34]
stable from a thermal perspective, well above typical lithium battery operating conditions (< 100 °C) .
From a mechanical perspective, the iongels containing the LiTFSI salt were first evaluated
[Supplementary Figure 5A]. Overall, the membranes kept their modulus between 10 and 10 Pa at different
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temperatures (from 0 to 100 °C), illustrating their mechanical stability at high temperatures. Interestingly,
the iongels containing the same anionic group in the salt and IL showed a higher modulus (i.e., ~10 Pa for
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Iongel-TFSI* and Iongel-FD*, both containing TFSI anions). Subsequently, Iongel-FSI*, Iongel-BETI* and
Iongel-CFSO* were reformulated by adding LiFSI, LiBETI and LiC F SO salts, respectively, at a 20 mol.%
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concentration and tested [Figure 2B]. The results showed an improvement from 2 × 10 to 4 × 10 Pa for
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Iongel-FSI, from 4 × 10 to 10 Pa for Iongel-BETI and from 4 × 10 to 6 × 10 Pa for Iongel-CFSO.
Furthermore, iongels containing the fluorinated cation (FD-TFSI) at lower ILE contents in the iongel
formulation were also assessed [Supplementary Figure 5B]. The results showed that, as expected, larger
contents of polymer in the iongel formulation led to a higher modulus. In contrast, according to the Tan δ
derivative, the iongels with a smaller anion presented a lower Tg transition, thereby positively enhancing Li
+
transport . Hence, the ranking in terms of Tg was Iongel-FD (-54.1 °C) < Iongel-FSI (-49.7 °C) < Iongel-
[35]
TFSI (-43.2 °C) < Iongel-CFSO (-41 °C) < Iongel-BETI (-26.5 °C). Overall, the low Tg of these iongels,
together with their high thermal and mechanical stability, make these polymer electrolytes interesting
materials for further investigation.
Next, the ionic conductivity (σ) of the iongels containing the LiTFSI salt was evaluated by electrochemical
impedance spectroscopy (EIS) [Figure 3A]. Iongel-FD had the highest σ value (2.48 × 10 S·cm at 25 °C),
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very close to its liquid counterpart (3.24 × 10 S·cm at 25 °C, Figure 3C). Similar to the DMTA results,
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iongels containing the same anionic group in the salt and IL seemed to have higher conductivities.
Consequently, the Iongel-FSI, Iongel-BETI and Iongel-CFSO membranes were further investigated
[Figure 3B]. A significant improvement was found for the Iongel-CFSO and Iongel-FSI formulations, with
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the latter having the highest conductivity of the group (7.8 × 10 S·cm at 25 °C). To the best of our
knowledge, this value is one of the highest ionic conductivities reported in the literature for IL-based solid
electrolytes. In the case of Iongel-BETI, the ionic conductivity was very similar in both cases