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Page 12 of 32 Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60

Chemical composition and bonding analysis of SEI components
A number of methods have been developed to investigate the evolution of the electrode/electrolyte interface
during cycling. Li detection and quantification can be acquired by in situ X-ray diffraction (XRD), Raman
spectroscopy, Fourier transform infrared (FTIR) spectroscopy, NMR, MRI and neutron depth profiling
(NDP) analysis. The composition of the surface reaction products can be revealed by in situ X-ray
photoelectron spectroscopy (XPS) and cryo-TEM analysis. The salt and Li-ion concentration gradients can
be investigated by holographic interferometry, in situ stimulated Raman scattering (SRS) microscopy and
MRI analysis. In situ XRD [Figure 5A] provides a range of signals, such as some metastable information that
cannot be obtained through ex situ XRD . Shen et al. proved the excellent stability of a LiF-modified Li
[79]
[80]
anode in the 1 M LiPF /EC/DEC electrolyte by in situ XRD . As shown in Figure 5B, at the first cycle, the
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electrolyte in the battery system based on bare Li anode is decomposed to the formation of the SEI film and
Li dendrites. However, the diffraction peak change in the battery system based on the LiF-modified Li
electrode is negligible, indicating the stable SEI.

In situ Raman measurements [Figure 5C] have advantages over in situ XRD due to their applicability to
[81]
analyze poorly crystalline and amorphous compounds . Furthermore, Raman spectroscopy is feasible for
studying the electrolyte, electrode and interface. Qiua et al. found that minute amounts of O in Li-O
2
2
batteries can react rapidly with metallic Li to generate an SEI enriched with Li O, Li O and LiOH species,
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2
which protect the anode from further reacting with salts and solvents . The inhomogeneity of the
[82]
interfacial Li-ion concentration is closely related to the dendritic Li growth; hence, significant studies have
been devoted to the concentration variation of Li ions at the electrode/electrolyte interface. For example, by
+
using in situ Raman spectroscopy measurements, Chen et al. found a Raman band at 741.8 cm , which was
-1
+
attributed to the S-N stretching and coupled CF bending in TFSI , proving the uniform distribution of Li
-
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ions in the MIP-based electrolyte [Figure 5D] . In addition, as a non-linear Raman technique, SRS has
[83]
+
been successfully applied to investigate the variation of the interfacial Li ion concentration, realizing the 3D
visualization of ion depletion and the corresponding dendrite growth . Due to the high measurement
[84]
accuracy, rapid measurement speed, high resolution and detection sensitivity, in situ FTIR spectroscopy has
been used to track electrolyte decomposition and SEI construction. According to in situ FTIR results,
Hu et al. discovered that nitrile compounds are attractive electrolyte candidates with wide electrochemical
windows, outstanding thermal stability and high safety at high voltages [85,86] . However, nitrile compounds are
highly corrosive to Li metal. XPS, which is usually used to characterize the electronic structure of surface
elements and local chemical environments, is an effective means for investigating solid surfaces.
Schwöbel et al. first reported the SEI layer between metallic Li and LiPON using in situ XPS . A more
[87]
fundamental study of the reaction of the active metallic Li with the atmosphere was carried out by exposing
Li to CO by Etxebarria et al. . Yan et al. investigated the influence of the FEC on the SEI constituents by
[88]
2
sputtering XPS, via which it was revealed that the SEI consisted of a dense two-layer structure with the
bottom layer composed of inorganic species, such as Li CO and LiF, and the bottom layer composed of
2
3
organic species, such as ROCO Li and ROLi .
[89]
2
In situ NMR can be used to realize the quantitative evaluation of various Li morphologies because the
radiofrequency can completely penetrate the Li microstructure [Figure 5E] [90-92] . According to the reported
results, the position variation of the Li microstructure could be detected by the changed signal . The
[93]
resonance displacement of NMR shifts with various metallic Li structures, such as the characteristic peaks of
dendritic Li and mossy Li at 270 and 261 ppm, respectively [Figure 5F] . In addition, the intensity of NMR
[94]
varies with the amount of Li deposits. This difference in peak position and intensity enables the deposited Li
to be quantitatively evaluated. For instance, Gunnarsdóttir et al. calculated the amount of ‘‘dead Li’’ from
[92]
the NMR spectra in Figure 5G and H . In addition, 3D matrix images on the timescale of an
electrochemical reaction can be generated by specific chemical shift points, which allows in situ MRI to