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Page 8 of 14 Alvarez-Tirado et al. Energy Mater 2023;3:300003 https://dx.doi.org/10.20517/energymater.2022.59

that the low Tg of the iongels and the presence of ethylene oxide segments within the polymeric network
might favor lower activation energies as they provide additional ion mobility pathways to the system (stable
+
[41]
complexes of Li with the ether oxygens) .
+
The contribution of ionic conductivity exclusively resulting from Li was evaluated through the
determination of the lithium transference number (t ), according to the well-known Evans-Vincent-Bruce
Li+
method [Supplementary Figure 7]. The analysis of the iongel electrolytes [Figure 3D] revealed that t
Li+
ranged between 0.10 and 0.25, in accordance with values found in the literature [8,39,42] . Iongel-BETI and
+
Iongel-TFSI had the lowest values (~0.10). TFSI-based IL has been reported to form Li -[TFSI ] negative
-
2
ion pairs, which does not favor ion mobility under an electric field, thus lowering the t of the
Li+
electrolyte . In contrast, the poorer Li solvation ability of DEME-C F SO might lead to a t discreet
+
[39]
Li+
3
3 9
promotion, comparable to the Iongel-FSI value (~0.17). The Iongel-FD electrolyte had the highest t (0.25).
Li+
Overall, the iongels developed in this work possess, to our knowledge, some of the highest ionic
conductivity values found in the literature [8,43-45] and have suitable ion conductive properties for battery
applications. Furthermore, it has been demonstrated that, through the designed ILs, it is possible to achieve
superior ionic conductivities than the more studied [DEME][TFSI]-based electrolytes.
Symmetrical lithium cells
Symmetrical lithium cells containing the iongels were assembled within two lithium metal foils inside a
glove box to determine their stability against lithium. Current densities were increased from 0.01 to
2 mA·cm and cells were cycled three times for each current (1 h half-cycle). The average potential (absolute
-2
value) achieved at each current rate is plotted in Figure 4A. The results showed that the electrolytes with
smaller anion sizes led to lower overpotentials, with cells having a critical current density (where E
WE
exceeds 1 V) of 0.5 mA·cm for Iongel-TFSI (0.33 V) and an excellent 2 mA·cm for Iongel-FSI (0.97 V).
-2
-2
This trend was also found in equivalent cells based on liquid electrolytes [Supplementary Figure 8A] but
with slightly lower overpotentials (e.g., 0.21 V for Liquid-TFSI at 0.5 mA·cm ). At relatively lower and
-2
usually reported current rates (i.e., 0.1 mA·cm ) in literature, the iongel cells exhibited a wide range of
-2
overpotentials with the order of Iongel-FSI (20 mV) < Iongel-TFSI (45 mV) < Iongel-FD (87 mV) < Iongel-
BETI (0.41 V) < Iongel-CFSO (2.9 V). The same tendency was found in cells with liquid electrolytes, with
overpotentials ranging from as low as 9 mV (Liquid-FSI), 50 mV (Liquid-TFSI) and 60 mV (Liquid-BETI).
Due to these promising results, symmetrical lithium cells with DEME-TFSI- and DEME-FSI-based
electrolytes were further cycled (1 h platting and 1 h stripping) in galvanostatic mode at 0.5 mA·cm
-2
[Figure 4B]. Cells with the Iongel-FSI electrolyte showed a polarization as low as 60 mV for 50 h, suggesting
good compatibility between lithium metal and the iongel. On the opposite side, cells with the Iongel-TFSI
electrolyte suffered from dendritic growth in a much earlier step, leading to a “soft” short circuit until the
final hard-short circuit after 23 h . The same trend was found in equivalent cells with liquid electrolytes
[46]
[Supplementary Figure 8B]. Cells with the Liquid-TFSI electrolyte had lower overpotentials (0.17 V), but
they increased rapidly after 14 h of cycling. Cells containing the Liquid-FSI electrolyte had a polarization as
low as 41 mV for at least 100 h. Hence, in comparison with the Iongel-TFSI electrolytes, the Iongel-FSI
membranes were able to potentially suppress dendritic growth more efficiently and cycling was further
pushed to current densities of 1 mA·cm [Figure 4C] and 2 mA·cm [Figure 4D]. At 1 mA·cm , the cells
-2
-2
-2
showed overpotentials of < 0.40 V for 45 h, which increased smoothly with cycling. The stripping/plating
profiles became sharper and less homogeneous under 2 mA·cm (polarization ranging from 0.6 to 1.1 V)
-2
[47]
until the distinctive fingerprint of a short-circuited cell appeared after 41 h .