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Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 7 of 24

where n is the number of short cycles performed, Q is the capacity cycled in the short cycles
c
-2
(0.2 mAh cm ), Q is the capacity of the final stripping half cycle, and Q is the total plated capacity in the Li
T
S
reservoir (2 mAh cm ).
-2
NMC-811||Li-Cu coin cells were prepared with NMC-811 electrode discs (12 mm diameter) and Li-Cu
discs (14 mm diameter) as positive and negative electrodes, respectively. The top side of the QSPE during
casting faced the positive electrode in the full cells. Galvanostatic C-rate and cyclability tests of NMC-811||
Li-Cu coin cells were conducted with a Neware battery tester. During the C-rate capability test, the cells
were cycled between 3.0 and 4.3 V. Two cycles were performed at C/20 (calculated on a nominal capacity of
170 mAh g NMC-811 -1 ), followed by 5 cycles at each of the following C-rates: C/10, C/5, C/3, C/2, and C/1.
Thereafter, the cells were further cycled at C/10 until failure. Cyclability tests and EIS were carried out in
coin cells with a BT-Lab battery tester. In these tests, the cells were cycled between 3.0 and 4.3 V at C/20
(2 cycles) and at C/10 (100 cycles). EIS spectra were collected at the end of discharge (EoD) every 5 cycles.
EIS tests upon cycling in three-electrode cell configuration were carried out with ECC-PAT-core cells (EL-
CELL®), with a VMP3 potentiostat (BioLogic). Cells were cycled between 3.0 and 4.3 V at C/50 (2 cycles)
and C/20 (30 cycles). EIS spectra were collected both at the end of charge (EoC) and discharge. All full cell
tests were performed at 25 °C.

Single-layer pouch cells were manually assembled by stacking alternate layers of the positive electrode,
QSPE film and negative electrode. When the cell stack was assembled, nickel and aluminum tabs (MTI
Corp.) were ultrasonically welded to negative and positive electrode tabs, respectively. Finally, the solid-
state battery was sealed under vacuum in an aluminum laminated pouch bag. The entire assembly was
conducted in a dry room with a dew point of -50 °C. Before cycling, the freshly fabricated solid-state battery
-1
was placed between two metal plates, clamped with a torque of 0.3 N m , and kept for three hours at 25 °C
to ensure the formation of intimate contacts between both electrodes and the solid electrolyte. Finally, the
assembled solid-state pouch cells were cycled at 25 ± 1 °C by BaSyTec cell test system, in accordance with
the protocol described in Table 2.

X-ray photoelectron spectroscopy analysis
The surface chemistry of Li deposits on Cu under striping/platting experiments was analyzed by X-ray
photoelectron spectroscopy (XPS) using a Phoibos 150 XPS spectrometer (SPECS GmbH) installed in an
-10
ultrahigh vacuum (UHV) chamber with a base pressure below 5 × 10 mbar. Samples were transferred in
an argon-filled air-tight container from the glove box to the instrument after cell disassembly. Spectra were
collected in Fixed Analyzer Transmission mode with ~2 mm field of view on the sample using a non-
monochromatic Mg anode source (hν = 1253.6 eV). E = 0.5 eV and E = 90 eV were employed for the
pass
step
survey spectra and E = 0.1 eV and E = 30 eV for the high-resolution regions. The atomic surface species
step
pass
were quantified using tabulated Scofield cross sections, applying corrections to the integrated intensity
considering the energy-dependent analyzer transmission and the variations in the effective attenuation
lengths (EAL) of the collected photoelectrons depending on their kinetic energies using the analytical
[58]
expression proposed by Seah . Data analysis was carried out using CASA XPS software . The binding
[57]
energy scale was calibrated by referencing the aliphatic carbon at 285 eV. The inelastically scattered
photoelectron background was simulated by a Shirley function, and Voigt profiles (30% Gaussian, 70%
Lorentzian) were employed as line shapes for the photoelectron peaks.

RESULTS AND DISCUSSION
QSPE characterization
The chosen composition of the QSPEs results from previous optimization work, aimed at achieving the best