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Page 4 of 24 Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203

plasticizer to the PVdF-HFP/acetone solution. The resulting solution was then cast on Mylar® films by a
doctor blade and dried at room temperature and reduced pressure (500 mbar) for 16 h. Membranes with
2
thicknesses ranging from 50 to 100 µm and an area of ~200 cm were produced in this way. With
unsupported QSPEs, three plasticizers were tested: tetra(ethylene glycol) dimethyl ether (TEGDME), PC,
and EC. LiFSI was used as a lithium salt.

Supported QSPEs were prepared inside a glovebox by solvent casting on a microporous polyolefin separator
(Celgard® 2500). MEK was used as a processing solvent. In a typical preparation, the lithium salts (LiFSI,
LiBOB, and LiNO ) were dissolved in EC in the chosen concentration (see Table 1), while 0.4 g of PVdF-
3
HFP was dissolved separately in 6 mL of MEK. Then, 1.6 g of the EC solution was added to the PVdF-HFP/
MEK solution, and the resulting mixture was stirred overnight at room temperature. The solution was then
2
cast with a doctor blade on a 10 × 10 cm Celgard® 2500 sheet, which was placed previously onto a
poly(ethylene terephthalate) (Mylar®) foil substrate. The casting was conducted at a speed of 1 mm s and
-1
with a blade gap of 300 µm. The casting solvent was finally removed by keeping the membrane under Ar
flow overnight at room temperature. Three QSPE compositions were prepared, with LiFSI only, binary
LiFSI/LiBOB mixture (in a 4:1 molar ratio), and ternary LiFSI/LiBOB/LiNO mixture (in a 4:1:1 molar
3
ratio). The composition of the prepared QSPEs is reported in Table 1.

QSPE characterization
Thermogravimetric analysis (TGA) was performed with a TGA 209 F1 Libra analyzer (Netzsch). The
temperature scans were performed from room temperature up to 600 °C, at a heating rate of 10 °C min ,
-1
and under argon (60 mL min ). Differential scanning calorimetry (DSC) was performed with a DSC 2500
-1
differential calorimeter (TA Instruments). The scans were conducted in the temperature range between -80
and 100 °C, with a heating rate of 2 °C min . The samples were cycled twice between -80 and 100 °C, and
-1
the second heating scan was used for the analysis. For the sample preparation, 5-10 mg of each sample was
placed in sealed aluminum pans under argon atmosphere.

Ionic conductivity was determined by electrochemical impedance spectroscopy (EIS). The EIS spectra were
collected with a Solartron 1260A Impedance/Gain-Phase Analyzer in the frequency range between 32 MHz
and 1 Hz (20 points per decade), with a signal amplitude of 10 mV, and in the temperature range between
20 and 80 °C (with 10 °C step). Additionally, the ionic conductivity was measured at 25 °C. For the
conductivity measurements, the membranes were placed in coin cells CR2032, with three stainless-steel (SS)
plates of 0.5 mm thickness. The temperature was controlled with a Binder KB23 cooling incubator. The
ionic conductivity was calculated using:

where σ is the ionic conductivity, R is the measured resistance, A is the electrode surface area, and L is the
membrane thickness. The latter was measured with a digital micrometer after the experiment. The
measurement was repeated on three different cells for each composition, and the values of resistance and
conductivity used in the analysis are the average values of the three measurements.

The morphology of a QSPE cross-section, obtained by an ion milling technique (Hitachi 4000 Plus), was
analyzed by field emission scanning electron microscopy (FESEM, APREO 2 S HiVac FESEM). QSPE
samples were transferred between the glovebox, the ion milling and the scanning electron microscope
(SEM), under argon atmosphere, with a transfer device. Cross-sectioning was performed at -70 °C with a
cryogenic module.