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Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 19 of 24

QSPE-3 (27 vs. 53 Ω at the EoC), but the increase is more pronounced for QSPE-2, namely 90% vs. 47%. A
slight divergence between EoC and EoD is observed, especially in the case of QSPE-2. The increase upon
cycling suggests a progressive degradation of the cathode/QSPE interface, or a progressive loss of contact
between the cathode-coated layer and the current collector. The higher resistance of the middle-frequency
component and the higher value of the time constant τ , with QSPE-3, suggest a more sluggish charge
MF,1
transfer across the cathode/QSPE interface. However, the most striking differences are observed in the R .
LF
In the case of QSPE-2, R increases from ~1,500 to ~2,500 Ω after 30 cycles, with similar values at the EoC
LF
and EoD. As for the two-electrode cells, the high values of cathode charge transfer resistance are caused by
the blocking behavior of the cathode in the fully lithiated/delithiated states. For QSPE-3, however, the
increase at the EoC is much more pronounced. R increases from 1,200 to 2,000 Ω at the EoD, and from a
LF
similar initial value to 5,500 Ω at the EoC. This increase suggests a faster degradation of the cathode
interface at high voltages, possibly involving oxidation of the catholyte, and may explain the faster capacity
decay with the LiNO -containing QSPE-3. The EIS analysis in a three-electrode cell configuration confirms
3
the results from LSV and floating current experiments, showing faster electrolyte oxidation for QSPE-3 with
respect to QSPE-2. It is interesting that this effect was not observed in the two-electrode cell, as in that case,
the EIS spectra were collected at the EoD. The study remarks the importance of collecting EIS spectra at the
EoC during cycling, rather than at the EoD, as degradation effects due to high voltage may be visible only in
the charged state.

To sum up, the addition of LiNO improved the coulombic efficiency for the lithium plating/stripping
3
process, delaying sudden cell failure due to lithium depletion at the anode. However, the primary cause for
capacity fade in this type of cell appears to be related to the degradation of the positive electrode or the
related electrolyte interface. In this regard, the addition of LiNO did not show any significant benefit. On
3
the contrary, cells containing LiNO showed slightly lower coulombic efficiency and higher capacity fade
3
rate, possibly due to accelerated electrolyte oxidation at high voltages. In addition, EIS analysis in a three-
electrode cell configuration showed that this accelerated capacity fade is related to a steeper increase of the
cathode charge transfer resistance at the EoC, and thus to a faster degradation of the cathode active material
in the delithiated state, or to a faster oxidation of the electrolyte/catholyte at high voltages. This effect, which
is further confirmed by the previous results from LSV and floating experiments, might be due to several
factors, such as crossover of the LiNO decomposition products from the anode to the cathode, a catalytic
3
activity towards electrolyte oxidation, or LiNO interference with the formation of the LiBOB-borne CEI.
3
Pouch cell performance
To complete the study, a monolayer pouch cell with QSPE-2 as an electrolyte was assembled and cycled,
first at different C-rates, and then at a constant C-rate of C/10. Figure 8 shows the specific capacity and
coulombic efficiency of this cell. The initial specific capacity of this cell (at C/20) was close to 200 mAh g ,
-1
close to the theoretical specific capacity of NMC-811. The higher capacity obtained with this cell, compared
to the previous results in coin cells, is attributed to the use of commercial, optimized NMC-811, while non-
optimized research-grade NMC-811 was used for the study of coin cells. The cell showed also excellent
-1
C-rate capability, with a specific capacity of 150 mAh g at 1C, despite having a cathode with high area
capacity of 2.5 mAh cm . The cyclability was also good, with a capacity retention of 80% reached after ca.
-2
90 cycles. No sudden failure was observed over 160 cycles, possibly thanks to a higher and more
homogeneous pressure distribution in the pouch cell format. The average coulombic efficiency, during the
long cycling at C/10, was over 99.5%.

Altogether, the results confirm the suitability of this QSPE for application in high-voltage quasi-solid-state
lithium metal batteries, although its cyclability must be further improved to meet the requirements for
practical use. The limited cyclability arises from the degradation of the electrolyte/catholyte at high voltages,