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Page 2 of 14 Alvarez-Tirado et al. Energy Mater 2023;3:300003 https://dx.doi.org/10.20517/energymater.2022.59

INTRODUCTION
Despite all the efforts of the research community in recent decades, the commercialization of high-energy-
density Li-O batteries is still far from being realized. One of the major problems of this technology is
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finding an electrolyte material that can simultaneously provide lithium metal protection and stability
against highly reactive oxygen species (i.e., superoxide) so that the lifespan of Li-O cells can be
2
prolonged . Technologies based on interfacial chemistry regulation (i.e., the stability of the solid
[1,2]
electrolyte interphase, SEI), nanostructured-based solutions (i.e., aligned nanochannels to delay dendrite
growth) or the development of solid electrolytes are plausible strategies to enhance the Li-O cell lifetime by
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protecting the metal anode and reducing safety concerns . For example, Chen et al. developed an effective
[3]
artificial SEI film by impregnating lithium metal with triethylamine trihydrofluoride under controlled
conditions . This process forms a uniform layer of LiF, which is able to minimize side reactions with the
[4]
electrolyte and lithium dendrite growth in both symmetrical and Li /LiFePO cells. Regarding solid
0
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electrolytes, gel polymer electrolytes can provide a tradeoff between high ionic conductivity and improved
safety by encapsulating the liquid electrolyte within a polymeric network . These encapsulated liquid
[5,6]
electrolytes play a key role in ion transport and their components need to be carefully selected for an
optimal set of properties .
[7]
Ionic liquids (ILs) are safer alternatives than conventional flammable organic solvents due to their low
volatility, non-flammability and stability against superoxide radicals . Pyrrolidinium and imidazolium
[8,9]
cations and fluorine-based anions, such as bis(trifluoromethanesulfonyl)imide (TFSI), are the most
recognized IL-based electrolytes [8,10-14] . However, a less well-known IL, N,N-diethyl-N-methyl-N-(2-
methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), has shown promising
properties in Li-O cells, such as low polarization during galvanostatic cycling [15,16] and a good oxygen supply
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[17]
capacity for the oxygen reduction reaction . Beyond the use of advanced catalysts to assist the oxygen
evolution reaction (OER)/oxygen reduction reaction (ORR) kinetics [18,19] , a method to seek higher
performance is to increase the fluorine content within the electrolyte structure, as it has a beneficial effect in
[20]
the formation of a passivating lithium protection layer and/or a tendency to increase oxygen solubility in
the solution (the active material in this type of cell) . A relatively easy method to increase the fluorine
[17]
content in IL-based electrolytes is by tuning their anions and/or cations [21,22] . In contrast, the use of ILs in gel
polymer electrolytes, typically known as iongels, is becoming increasingly more common but has not
[21]
been largely explored for Li-O cells. For example, Zhao et al. developed an ultraviolet (UV)-cured iongel
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containing a 1-ethyl-3-methylimidazolium tetrafluoroborate IL, dimethyl sulfoxide and LiTFSI,
encapsulated in a polymer matrix formed by poly(methyl methacrylate) (PMMA) and a triacrylate .
[23]
Symmetrical lithium cells containing this iongel presented an overpotential of < 0.1 V for 130 h when cycled
at 0.3 mA·h . Other approaches involved the use of poly(vinylidene fluoride-co-hexafluoropropylene), a
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pyrrolidinium-based IL and LiTFSI [24,25] or PMMA as the polymeric matrix .
[26]
In this work, we present a study of highly conductive iongel electrolytes based on five different ILs designed
to improve the Li transport properties and electrochemical performance of Li-O cells. The [DEME] cation
+
+
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-
and [TFSI] anion are tuned to increase the presence of fluorine moieties with a highly delocalized charge.
The synergy between the cations and anions of the IL and the corresponding lithium salts is explored. The
preparation of iongels soft solid electrolytes by fast photopolymerization methods is then pursued. The
mechanical, thermal and electrochemical properties of the obtained iongels are investigated with particular
attention devoted to the optimization of the ionic conductivity. These soft solid iongel electrolytes are first
tested against lithium metal electrodes in symmetrical cells and compared to their liquid electrolyte analogs.
Finally, their performance in Li-O cells with both solid and IL electrolytes is evaluated.
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