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Kühn et al. Energy Mater 2023;3:300020 https://dx.doi.org/10.20517/energymater.2023.07 Page 5 of 14

The electrodes were rinsed with ethyl methyl carbonate (EMC; E-Lyte Innovations; 3 × 500 µL) and
subsequently dried in an antechamber for > 15 min. They were then transported to the workstations or
measurement devices using in-house-built (SEM, ATR-FTIR) or commercially available (XPS, Vacuum
Transfer module by Thermo Scientific) sample transfer holders, preventing any contact with the outer
atmosphere and/or moisture.

Scanning electron microscopy
Scanning electron microscopy (SEM) measurements were conducted at a Carl Zeiss Auriga Modular
Crossbeam workstation utilizing a Schottky field emission gun with a Gemini column as an electron source.
Images were taken at an accelerating voltage of 3 kV (SEM) using an in-lens secondary electron detector.
The aperture of the lens was 20 µm and the working distance was 3.0 mm.

X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) measurements were carried out at an angle of emission of 0° and a
pass energy of 20 eV using a monochromatic Al Kα source (Ephoton = 1,486.6 eV) with a 10 mA filament
current and a filament voltage source of 12 kV. The analyzed area was approximately 300 µm × 700 µm. In
order to compensate for the charging of the sample, a charge neutralizer was used. The F 1s peak at 684.8 eV
(LiF) was taken as an internal reference for the adjustment of the energy scale in the spectra. CasaXPS
software was used for fitting and peak assignment in accordance with known literature values [38,39] .

Attenuated total reflection Fourier transform infrared spectroscopy
Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis of the CEI on
NMC811 electrode surfaces was conducted on a Bruker ALPHA II FT-IR spectrometer with a platinum
ATR unit (diamond crystal) and a DLaTGS detector inside an argon-filled glovebox (H O and O < 5 ppm).
2
2
ATR-FTIR measurements were conducted at multiple spots in the center, the middle and the edge of the
-1
harvested electrode [Supplementary Figure 3]. The spectra were acquired with a spectral resolution of 4 cm
at an incidence angle of 45°. Each spectrum was obtained by accumulating 32 and 64 interferograms for
background and sample spectra, respectively. The spectra are presented in the form of absorbance and were
processed by subtracting the spectrum of pristine NMC811, ATR correction, and concave rubber band
correction (10 iterations, straight lines).

Quantum chemistry calculations
Different quantum chemistry methods were used to interpret experimental results and to propose a new
decomposition mechanism for FEC on NMC811 electrode. Density functional theory (DFT) calculations of
putative decomposition products were performed at the B3LYP/6-311+G (3df, 2p) level of theory [40-42] using
Grimme’s empirical D3 dispersion correction with Becke-Johnson damping . In addition, reaction
[43]
energies and free energies, as well as the corresponding barriers, were calculated using the highly accurate
G4MP2 composite method . All calculations were performed using the Gaussian 16 software using the
[44]
SMD solvation model with acetone parameters .
[45]
RESULTS AND DISCUSSION
Stripping/Plating and operando EIS performed in Li||Li symmetric cell setups
Initial electrochemical characterization to determine the influence of the ICCA on lithium metal electrodes
was conducted by stripping/plating experiments [Figure 2] in Li||Li symmetric cells at a constant current
-2
density of 0.5 mA cm (0.5 mAh cm ) until the cell reached a set limit of ±0.3 V. The eventual increase in
-2
overvoltage values originates from a reduced Li transport in the electrolyte due to the consumption of Li
+
ion transporting components. Utilizing a Li||Li symmetric cell setup allows the effect of the different ICCAs
to be narrowed down to the impact on the lithium metal electrode and excludes the influences of