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Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 15 of 24

Figure 5. Galvanostatic cycling of NMC-811||Li-Cu coin cells with QSPE-2 and QSPE-3, performed at room temperature, in the voltage
-2
range between 3.0 and 4.3 V, and with cathode active material loading of ~13 mg cm . (A) Specific discharge capacity during the first
150 cycles; (B) coulombic efficiency during the long cycling at C/10; (C) Capacity retention during the long cycling at C/10, normalized
on the first cycle at C/10 after C-rate test.

cycled for over 200 cycles without showing sudden failure, despite with a continuous capacity fade
[Supplementary Figure 14]. The trends in the capacity fade correlate with some slight differences observed
in the coulombic efficiency during long cycling. Initially, the cell with QSPE-2 has higher coulombic
efficiency, concordant with the lower capacity fade rate. However, during the subsequent cycling, the
coulombic efficiency decreased continuously for QSPE-2, while increasing for cells with QSPE-3. The
coulombic efficiency dropping below 99% for QSPE-2 coincides with the sudden capacity drop observed in
this cell. This is probably caused by the depletion of the cyclable Li in the anode , exacerbated by the low
[62]
amount of Li reservoir on the anode (~20 µm). This effect was not observed with QSPE-3 for over 200
cycles, which is attributed to the higher coulombic efficiency for the lithium plating/stripping process in
presence of LiNO . To summarize, the aging behavior of the analyzed cells is characterized by two distinct
3
regions. In the first region, the cells experience linear capacity fade, slightly more accelerated for cells with
QSPE-3 than cells with QSPE-2. In the second region, which is dominated by anode depletion, the cells
undergo sudden capacity fade. The onset of this second aging regime is delayed in cells with QSPE-3 as an
electrolyte.

EIS study in two- and three-electrode cell configurations
To understand the origin of the different behavior with the two electrolytes, two other cells were cycled at a
constant C-rate of C/10 (after a formation cycle at C/20). In addition, EIS spectra were collected at the EoD,
every 5 cycles, in the frequency range between 10 kHz and 10 mHz. The specific discharge capacity and
coulombic efficiency of the two cells are represented in Figure 6A. The discharge capacity in the initial
cycles was similar, namely 164 and 161 mAh g at C/20 and 142 and 138 mAh g at C/10 for QSPE-2 and
-1
-1
QSPE-3, respectively. In this experiment, no sudden failure was observed, presumably because cycling was
stopped before this mode of failure occurred. However, as in the previous experiment, the two cells showed
a constant capacity decrease, with slightly better cyclability for the cell with QSPE-2. The capacity fade rate