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the research efforts that employ in-situ/operando spectroscopy, in-situ/operando X-ray techniques, and
in-situ/operando neutron techniques for exploring microscopic information in solid-state polymer batteries
[Figure 4]. These original characterization methods each exhibit unique strengths in studying the
microstructural evolution, ion transport, and lithium dendrite growth processes within SSBs. They
complement each other and collectively propel advancements in solid-state polymer battery technology.

In-situ/operando spectroscopy technique
Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FTIR) technique uses a broadband infrared beam to measure the
amount of light absorbed by a sample. The technique reveals the diverse absorption frequencies by
analyzing the attenuated beam, and processes the detected absorption patterns to generate an infrared
spectrogram. So far, the in-situ FTIR technology has achieved remarkable results in real-time representation
of the microdynamic behavior and structural evolution of materials under various working conditions [60-62] .
He et al. have developed near-field infrared nano-spectroscopy with nanoscale resolution, chemical
selectivity, and surface sensitivity, to probe the graphene/Li/SPE interface information, where a single
graphene sheet uniquely served as both an infrared transparent window and a current collector
[Figure 5A] . To confirm that the nano-FTIR variations across the graphene/SPE interface are
[63]
pronounced, we needed to conduct many control experiments. The nano-FTIR spectra of the graphene/SPE
interface were captured at specific locations [Figure 5B] under room temperature conditions. To visualize
these spectra, each dot corresponds to a unique nano-FTIR spectrum, with the dot’s color serving as a visual
representation of the corresponding spectrum. Nano-FTIR analysis revealed intensity variations in the
-
absorption bands of TFSI and PEO. These variations are attributed to a complex interplay of factors,
-
including changes in the PEO chain structure and its orientation, alterations in TFSI molecular
conformations and orientations, and variations in LiTFSI concentrations [Figure 5C]. Ultimately, this study
revealed that despite its initially atomically flat surface, the graphene was susceptible to transforming into a
heterogeneous surface characterized by newly formed composition and structure. The observed
phenomenon resulted in non-uniform Li-plating at the surface mentioned above, which is distributed at
both the nano- and micro-scales. Wen et al. revealed the oxidation mechanism of a gel bisalt polyether
electrolyte (BSPE) in lithium metal batteries, particularly at high voltage, where polymer electrolytes
[64]
containing -OH, -O-, and -C=O groups are easily oxidized . They utilized in-operando FTIR, combined
with X-ray photoelectron spectroscopy (XPS) technology, to achieve this insight. Calcium fluoride (CaF ),
2
which is transparent to FTIR light, was chosen as the optical prism, and a highly sensitive mercury
cadmium telluride detector was selected and operated in a liquid nitrogen environment. As the battery
charged, a growing vibration peak at 1,727 cm from the NCM/BSPE interface suggested the formation of
-1
O-C=O containing carbonaceous species [Figure 5D and E]. In addition, XPS was employed to validate the
decomposition mechanism of the electrolyte in contact with the NCM811 surface, with ex-operando XPS
results suggesting that ether molecules were prone to transforming into O-C=O based side products at the
interface under a high voltage of 4.8 V.

Raman spectroscopy
Raman spectroscopy technology, utilizing the Raman scattering effect, is a powerful method for analyzing
scattering spectra to extract valuable information on molecular vibration, rotation, and various other
molecular properties [65-67] . Raman spectroscopy is sensitive to Li -solvent interactions and/or anion
+
concentration, allowing it to track the Li-ion transport mechanism and dendrite growth [68,69] . Given the
limitations of conventional Raman spectroscopy, including weak signal and poor temporal resolution,
[57]
Cheng et al. exploited stimulated Raman scattering (SRS) microscopy . This microscopy utilizes two
spatially and temporally synchronized picosecond laser pulse trains [70-72] to in-operando illustrate the Li-ion
migration path in a SSB electrolyte [Figure 6A]. They prepared a cell model using a gel polymer electrolyte