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Page 2 of 24 Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203

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pouch cell with a cathode area capacity of ca. 2.5 mAh cm . This cell delivered an initial capacity close to
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200 mAh g at C/20, 150 mAh g at C/1, and 80% capacity retention after 100 cycles at 25 °C. The results
demonstrate the viability of supported QSPEs, based on poly(vinylidene-fluoride-co-hexafluoropropylene),
ethylene carbonate, LiFSI and LiBOB, for application in high-voltage quasi solid-state lithium metal batteries.
Keywords: Lithium metal batteries, gel polymer electrolytes, quasi solid-state electrolytes, high voltage, NMC-811,
LiNO 3

INTRODUCTION
Gel polymer electrolytes, or quasi-solid polymer electrolytes (QSPEs), are considered as a promising
[1-5]
alternative to commercial organic liquid electrolytes for application in lithium-metal batteries . The basic
composition of QSPEs includes a polymer host matrix, possibly crosslinked, one or more lithium salts as
charge carriers, and liquid organic solvents (sometimes called plasticizers) to help dissociate the lithium
salts and increase the ionic conductivity. In some cases, non-soluble inorganic fillers are also added to
improve the mechanical properties (passive fillers) or further increase the Li transport properties (Li-ion
[6-8]
+
conducting fillers) [9-12] . Typical polyether-based fully solid polymer electrolytes (SPEs), which do not rely on
the addition of solvents, have relatively low ionic conductivity (<< 10 S cm at room temperature) and
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[13]
cannot sustain low-temperature operation or cycling at high cathode loading and high C-rates . QSPEs, on
the contrary, exhibit ionic conductivity comparable to that of liquid electrolytes, enabling similar cycling
[14]
performance . In addition, many polymer host matrices used in QSPEs, such as poly(vinylidene fluoride)
and its copolymers, poly(acrylonitrile), polycarbonates, etc., have higher oxidative stability than
poly(ethylene oxide) and other polyethers used in SPEs [15,16] . Consequently, QSPEs are assuming an
increasingly important role, especially in high-voltage lithium metal batteries (HV-LMBs) featuring 4V- and
5V-class cathode active materials. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) is one of
the most widely used polymer matrices for QSPEs [17-24] . Combined with liquid electrolytes, even in
concentrations below 50 wt%, it forms mechanically strong and flexible membranes with high ionic
conductivity, comparable to liquid electrolytes impregnated in microporous separators. It has high oxidative
stability (which in QSPEs is then limited by the liquid electrolyte components) [16,25] , high thermal and
mechanical stability (with melting temperature at ca. 140 °C), and, depending on the nature of the
plasticizers, high fire resistance . Among the liquid electrolyte solvents used as plasticizers in QSPEs,
[26]
carbonates provide combined high ionic conductivity and electrochemical stability. While mixtures of linear
and cyclic carbonates are routinely used in in situ crosslinked QSPEs [27-29] , the high volatility of linear
carbonates hinders their use with QSPEs processed by solvent casting routes. On the contrary, cyclic
carbonates, such as propylene carbonate (PC) [30-32] , ethylene carbonate (EC) [33-35] , and fluoroethylene
carbonate (FEC) , can be used as plasticizers in solvent-processed QSPEs owing to their higher thermal
[36]
stability. Regarding lithium salts, several options are available, with LiPF being commonly used in
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combination with mixtures of linear and cyclic carbonates. It provides electrolytes with high ionic
conductivity and electro-oxidative stability, but relatively low thermal stability. For SPEs and QSPEs, amide-
based salts, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium
bis(fluorosulfonyl)imide (LiFSI), are the most common choices. Besides, other lithium salts are used as
additives to protect the electrolyte from electrochemical degradation. Lithium bis(oxalate)borate (LiBOB)
and lithium difluoro(oxalate)borate (LiDFOB) are among the most common cathode electrolyte interface
(CEI)-forming salt additives [1,37,38] , whereas LiNO is a common solid electrolyte interface (SEI)-forming
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additive [39-51] . LiNO is mostly used with ether-based electrolytes in Li-S batteries, in which it forms a
3
multilayer SEI rich in LiN O and protects the lithium anode from direct contact with the soluble
y
x
polysulfides . On the contrary, the low solubility in carbonate-based solvents has limited its use in HV-
[52]
LMBs. Despite this limitation, the beneficial effects of LiNO in high-voltage batteries have been reported in
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