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Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 17 of 24

The evolution of the three resistances upon cycling is shown in Figure 6D-F. R was approximately
HF
constant upon cycling and similar for the two electrolytes, concordant with the similar ionic conductivity of
the two QSPEs. R was initially 28 and 15 Ω for QSPE-2 and QSPE-3, respectively. R increased steadily
MF
MF
upon cycling, with a more pronounced rise observed for QSPE-2. This trend agrees with the less resistive
and more stable anode interface obtained using LiNO , as observed in Li||Li cells [Supplementary Figure 5],
3
and suggests that R is strongly influenced by the resistance of the anode/QSPE interface. Although XPS
MF
analysis indicated stable SEI chemistry for QSPE-2, the SEI thickness likely increases during cycling due to
the higher propensity of QSPE-2 for reduction by the lithium anode. Finally, R increased steadily upon
LF
cycling, similarly for the two QSPEs, indicating progressive deterioration on the cathode side. This might be
due to various processes, such as progressive oxidation of the electrolyte, degradation of the active material
(e.g., cracking), or loss of contact between the electrolyte and the cathode active material particles upon
repeated lithiation/delithiation cycles. Among the three resistive components, the increase of R was the
LF
most evident, from ca. 1,000 Ω to ca., 5,000 Ω at the end of cycling, thus suggesting that the progressive
capacity decay may be due to the increase of the cathode charge transfer resistance.

In conclusion, the EIS analysis in two-electrode cell configuration suggests that the progressive capacity fade
is mainly associated with the degradation of the cathode interface. In this regard, LiNO has, apparently, a
3
negligible effect. Besides, a progressive increase of R , associated with a progressive deterioration of the
MF
anode interface, was observed in the cell with QSPE-2, while this effect was less obvious in the cell with
QSPE-3. This agrees with the previous results from Cu||Li cells characterizations, indicating a higher
propensity of QSPE-2 to reductive decomposition on the lithium anode.

The effect of LiNO on the cyclability of NMC-811||Li cells was further studied by cycling three-electrode
3
cells at C/20 for 30 cycles, while collecting EIS spectra both at the EoD and at the EoC. The two cycled cells,
with QSPE-2 and QSPE-3 as electrolytes, showed decreasing capacity, as observed in the previous
experiments, with slightly faster fading for the cell with QSPE-3 [Figure 7A]. The coulombic efficiency was
initially higher for the cell with QSPE-2, although it became approximately equal after ca. 15 cycles. So far,
this confirmed the conclusion from previous experiments that the addition of LiNO initially worsens the
3
cyclability, although during long cycling, it has a positive effect due to reduced consumption of Li at the
anode. We then focused our attention on the positive electrode, trying to clarify whether the addition of
LiNO affects cathode aging. Figure 7B shows the open circuit potential of the positive electrode (E +,OCP ),
3
against the lithium metal reference electrode upon cycling time. While the potential at the EoD remained
+
approximately constant, it decreased progressively at the EoC, indicating a decreased ability to extract Li
from the cathode active material upon cycling. This could be attributed to a partial disconnection of the
cathode active material particles upon cycling and is obviously related to the linear capacity decay of the
cycled NMC-811||Li cells. Between the two cells, the decrease of the E +,OCP is faster with QSPE-3, in
agreement with the faster capacity decay, thus suggesting that the cathode aging is accelerated in presence of
LiNO . As mentioned above, EIS spectra were collected both at the EoC and at the EOD. Selected EIS
3
spectra of the positive electrodes are shown in Supplementary Figure 18 (QSPE-2) and
Supplementary Figure 19 (QSPE-3). The spectra show two semicircles in the high/middle frequency range,
clearly separated in the case of QSPE-2 and more overlapped in the case of QSPE-3, followed a large
semicircle at low frequencies. The spectra can be modeled through a ladder circuit encompassing four
resistances and three constant phase elements [Supplementary Figure 20]. The high-frequency series
resistance (R ) is related to the electrolyte ionic resistance, whereas the R (R and R ) is tentatively
HF
MF,2
MF,1
MF
attributed to the planar cathodic interface resistances, namely the interface resistance between the cathode
and the QSPE, and to the contact resistance between the cathode and the current collector. Indeed, planar
interfaces should give rise to impedance features in the middle-/high-frequency range [63,68] , and in the system