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Page 8 of 14 Kühn et al. Energy Mater 2023;3:300020 https://dx.doi.org/10.20517/energymater.2023.07

the R values closely mimics the overvoltage evolution of the stripping/plating experiments: The R values
SEI
SEI
of the BE × DR setup remain higher compared to the BE × GB setup counterparts and both AE-FEC
containing cell setups show similar R values over the first 100 h. The drastic difference in the overvoltage
SEI
evolution of the stripping/plating experiments for AE-VC × DR and AE-VC × GB can also be seen in the
evolution of the R values. Initially, AE-VC × GB experiences a higher SEI resistance, but after 30 h, the
SEI
resistance of the AE-VC × DR starts to rise and matches the shrinking AE-VC × GB values after 50 h. The
R values of AE-VC × DR subsequently reach a local maximum at 80-90 h before they gradually start to
SEI
shrink, as is the case for the overvoltage values of the stripping/plating experiments.

Overall, the stripping/plating experiments and operando EIS measurements show the notable influence of
the ICCA on the electrochemical characteristics of the SEI. In the case of both BE and AE-VC, the ICCA
has a strong impact on the overvoltage profiles and R . However, the influence on the R of FEC
SEI
SEI
containing cells seems to be minimal, although different overvoltage values of AE-FEC × DR and
AE-FEC × GB after FEC consumption indicate different decomposition products depending on the ICCA.
The observed influences could originate from different chemical compositions of the SEI depending on the
ICCA and could result in different stripping/plating homogeneity.

Post-mortem analysis of Li||Li symmetric cells
Electrochemical characterization revealed that the ICCA has an influence on the performance of lithium
metal electrodes. In line with this, post-mortem SEM and XPS analysis after 100 h were performed to
observe the differences in stripping/plating homogeneity and determine how the ICCA influences the
chemical composition of the in situ formed SEI on the lithium metal.

Post-mortem SEM measurements [Figure 4A-F], in combination with optical analysis of the complete
harvested electrode (inlets), enable an evaluation of the stripping/plating homogeneity of lithium metal
electrodes. Electrochemical characterization only revealed a small impact of different ICCAs on the first
100 h in combination with AE-FEC, some impact with BE, and a strong impact in combination with
AE-VC, which can also be observed after cell disassembly. The lower overvoltage profiles and R values of
SEI
BE × GB [Figure 4A] coincide with pronounced stripping/plating (black/gray spots) taking place on the
electrode surface and less concentrated stripping/plating around the edges of the electrode, as is the case for
its DR [Figure 4B] counterpart. In the case of the cell setups containing functional additives, the electrodes
harvested from AE-FEC × GB [Figure 4C] and AE-FEC × DR [Figure 4D] look very similar. The most
substantial differences were observed for the cells containing AE-VC. Electrodes harvested from
AE-VC × GB [Figure 4E] showed indications of stripping/plating over the whole electrode surface, whereas
stripping/plating of AE-VC × DR [Figure 4F] almost exclusively takes place around the electrode edges. A
previous publication suggested a correlation of a lower R value and an increased stripping/plating
SEI
homogeneity , which is in agreement with the findings of this study for BE and AE-VC after 100 h. In the
[7]
case of AE-FEC, the passivating reactivity of FEC appears to suppress the impact of different ICCAs on
stripping/plating homogeneity after 100 h.

The chemical composition of the SEI formed on the lithium metal electrode after 100 h was analyzed via
post-mortem XPS analysis [Figure 4G]. In an attempt to only look at the in situ formed SEI, only spots that
clearly experienced stripping/plating processes (black spots, see Figure 4A-F) were analyzed and the results
were subsequently averaged. For all three electrolytes, considerable differences in the atomic distributions of
the in situ formed SEI were found depending on the ICCA and they all contained the expected mixture of
organic, carbonate, fluoride, and phosphate components [7,39] . For BE, a higher carbon content (BE × GB:
21%; BE × DR: 25%) and a slightly lower fluorine content (BE × GB: 38%; BE × DR: 35%) were observed. In