Page 41 - Read Online

Boaretto et al. Energy Mater. 2025, 5, 500040 https://dx.doi.org/10.20517/energymater.2024.203 Page 13 of 24

-2
whereas Li-Cu|QSPE-3|Li-Cu cells showed the lowest internal resistance of 53 Ω cm , thus confirming the
beneficial effect of LiNO for the plating/stripping process.
3
The effect of LiNO on the plating/stripping process was further examined by cycling in Cu||Li cells. The
3
protocol described in the experimental section (Equation 3) was used to calculate the average coulombic
efficiency of 50 plating/stripping cycles in Cu||Li cells with QSPE-2 and QSPE-3 as electrolytes. Again,
QSPE-3 showed far better performance than QSPE-2, with an average coulombic efficiency of 99.3% ± 0.1%,
compared to 95.4% ± 0.7% of QSPE-2 [Supplementary Figure 8 and Supplementary Figure 9]. The results
further confirm the positive effect of LiNO on the Li plating/stripping process and suggest that the LiNO
3
3
salt additive protects the electrolyte from decomposition at the Li interface. To confirm this hypothesis, XPS
analysis was performed on Cu electrodes recovered from Cu||Li cells, cycled either with QSPE-2 or QSPE-3.
The cells were cycled with a similar profile to the previous cells but were left in the plated state; that is, the
last stripping semi-cycle was not performed. For comparison, XPS spectra were also collected on Cu
-2
-2
electrodes cycled only for three cycles at a capacity of 2 mAh cm and at a current density of 0.1 mA cm
(fully stripped Cu electrodes), and on Cu electrodes cycled for two and a half cycles in the same conditions
(fully plated Cu electrodes). The three cycling profiles are shown in Supplementary Figure 10. The aim was
to study the evolution of the SEI throughout the plating/stripping process and its long cycling stability.
Although the SEI formed on Cu might differ from the one formed on Li , this comparative study was
[59]
useful to assess the effect of LiNO on the electrolyte stability at the anode interface. The XPS spectra
3
collected on the cycled Cu electrodes are shown in Figure 4, whereas those of stripped and plated Cu
electrodes are shown in Supplementary Figure 11 and Supplementary Figure 12, respectively.

The most significant difference between the electrodes cycled with QSPE-2 and QSPE-3 lies in the relative
concentration of FSI anion decomposition products compared to the FSI residuals. As seen in Figure 4,
-
-
FSI residuals are more intense in the Cu electrode cycled with QSPE-3, whereas the signals of the FSI
-
-
decomposition products (e.g., LiF in the F 1s spectrum, SO signal in the S 2p spectrum) are much weaker.
x
Therefore, the addition of LiNO limits the FSI reduction by Li. In addition, the fitting of the B 1s spectrum
-
3
-
of the electrode cycled with QSPE-2 evidences the lack of BOB anion residuals, while other boron oxides
are present, indicating that in the vicinity of the Cu electrode, this anion is completely decomposed during
plating. This finding agrees with the reported reduction potential of BOB at ~1.8 V vs. Li/Li . On the
+
-
contrary, in the case of QSPE-3, a weak signal of BOB is still present, indicating that LiNO partially
-
3
protects this anion from decomposition. The lower reduction of FSI and BOB anions when LiNO is
-
-
3
present in the electrolyte seems to indicate a reduced electrolyte consumption likely by forming a more
effective SEI rich in LiNO decomposition products. Interestingly, no signal from Li N was clearly observed,
3
3
suggesting that this SEI component, if present, might be buried more deeply in the SEI to be visible by XPS.
Similar results were also obtained with Cu electrodes analyzed after three full Li plating/stripping cycles
(stripped Cu electrodes, Supplementary Figure 11) and after two and a half cycles (plated Cu electrodes,
Supplementary Figure 12), except that, in the latter case, the BOB signal was observed as well when QSPE-2
-
was employed, although in minor concentration with respect to the case of QSPE-3. The relative
concentration of the different components is reported in Supplementary Figure 13, also showing similar SEI
composition in the different cycling stages, indicating quite stable chemistry throughout cycling. Larger
differences were observed in the C 1s spectrum, between the electrodes cycled with QSPE-2 and QSPE-3,
which may be caused by the presence of adventitious carbon and the rest of the electrolyte polymer matrix.
Nevertheless, the systematically higher content of carbon bonds in the electrodes cycled with QSPE-2 vs.
those cycled with QSPE-3, especially in form of hydrocarbon, C-O and COO, may denote a more organic
SEI due to higher solvent decomposition. Altogether, the XPS analysis evidenced the effectiveness of LiNO
3