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Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 9 of 24

residual MEK was detected by either NMR or FTIR. In the proton NMR spectra [Figure 1A], MEK gives an
intense signal at 2.1 ppm, which was not detected in the spectra of the QSPEs. The most intense peak in the
QSPE spectra is due to EC at ca. 5 ppm. FTIR spectra of QSPE-2, processed either with acetone or MEK, are
reported in Figure 1B. The spectra are similar, and most importantly, the intense C=O stretching peak of
MEK at 1,718 cm was not observed, confirming absence of residual process solvents. In the carbonyl
-1
-1
stretching region, only the doublet related to EC, at 1,774 and 1,801 cm , is visible in both spectra.
The thermal properties of the three QSPEs were analyzed by TGA and DSC. The TGA profiles
[Figure 1C and D] show a slow mass loss up to at least 100 °C, attributed to EC evaporation, followed by a
rapid mass loss at ca.150 °C, which is caused by faster EC evaporation and possibly salt degradation. A final
mass loss is observed at 500 °C, corresponding to the thermal degradation of the polymer matrix. The initial
mass loss is slightly faster for QSPE-1, for which the 95 wt% threshold is reached at 97 °C. For comparison,
the same threshold is reached at 137 and 122 °C in QSPE-2 and QSPE-3, respectively. The mass loss of the
three QSPEs at 300 °C corresponds approximately to the content of EC, namely 60 wt%. The DSC profiles
[Figure 1E] show some differences between the three QSPEs. The QSPE-1 has a glass transition temperature
(T ) of ca. -72 °C and two melting transitions, at -25 and 22 °C, preceded by a weak crystallization at -50 °C,
g
with enthalpy of ca. 1 J g . The SPEs with binary and ternary salt mixtures, QSPE-2 and QSPE-3, exhibit T
-1
g
of -67 and -62 °C, respectively. Both show a melting transition at 20 °C, which is attributed to the melting of
the EC. The different crystallization/melting behaviors with the binary and ternary salt mixtures possibly
arise from the increased disorder, caused by the presence of various anions, whereas the increase of the T
g
-
-
-
might indicate a lower mobility of BOB and NO , with respect to FSI . Nonetheless, the ionic conductivity
3
of the three QSPEs is remarkably similar, slightly higher than 1 mS cm at room temperature and
-1
-1
~3 mS cm at 80 °C [Figure 1F, Table 3]. Interestingly, the ionic conductivity is close to that of unsupported
QSPEs [Supplementary Figure 1B], although the addition of the separator, with a 55% porosity, was
expected to lower the ionic conductivity. This is probably due to the large thickness of the QSPEs,
dampening the effect of the microporous separator, which has a thickness of 25 µm. Altogether, this
conductivity is expected to be high enough to allow cell cycling at room temperature. The lithium-ion T at
+
room temperature was determined by the potentiostatic polarization method and is also reported in Table 3.
+
T is close to 0.1 and increases slightly from QSPE-1 (0.09 ± 0.01) to QSPE-3 (0.14 ± 0.01). The low value of
+
T indicates that most of the ionic conductivity is due to the motion of the anions and is attributed to the
strong coordination of Li by the EC molecules. The slight increase observed from QSPE-1 to QSPE-3 is
+
-
attributed to the partial substitution of FSI with less mobile BOB and NO anions. Some representative
-
-
3
chronoamperometric profiles and correspondent EIS spectra are reported in Supplementary Figure 5. In the
chronoamperometry profiles, the steady state is reached within a few seconds (less than one minute in most
cases), owing to the high ionic conductivity of the QSPEs. Additionally, the EIS spectra were used to
estimate the Li|QSPE area-normalized interface resistance (R , Table 3). This was calculated by multiplying
*
int
R by the electrode area and dividing the resulting value by two (thus accounting for the presence of two Li
int
electrodes). In this case, the effect of the salt mixture composition is significant, with R decreasing from
*
int
+
2
240 ± 50 to 107 ± 5 Ω cm for QSPE-1 and QSPE-3, respectively. The less resistive interface and higher T of
QSPE-3 result in a correspondingly higher steady-state current [Table 3] and may result in better Li plating/
stripping performance for this electrolyte.
The oxidative stability of QSPEs was analyzed by LSV and floating tests, as described in the experimental
section. The use of cc-Al working electrodes allowed studying the oxidative stability of the SPEs in
conditions resembling the operational cell conditions, as cc-Al is used as a current collector for the NMC-
811 cathodes. A higher oxidative current and lower oxidative stability were expected due to larger
electrochemical surface area and possible catalytic activity of carbon coating. Furthermore, possible