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Page 24 of 35 Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46

[193]
which is added to the SSE to reduce the interfacial resistance between the Li metal anode and SSE .
Adding Li La Zr Ta O to a polymer matrix can contribute to an improvement in interfacial
12
0.6
3
6.4
1.4
compatibility and Li-ion conductivity because of the bonding of the PEO polymer matrix to the anode and
rigid Li La Zr Ta O particles with PEO chain segments .
[191]
1.4
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6.4
0.6
3
Composite electrolytes are enlightening in improving the electrochemical performance of ASSLSB cells
because of the combined advantages of both ceramics and PEO polymers. In the composites, a host PEO
polymer matrix filled with inorganic Li-ion conductors presents new recrystallization kinetics of the
polymer chains and increases more flexible local chains in the amorphous phase, resulting in enhanced
electrochemical stability and interfacial compatibility. However, the relatively high operating temperature of
[194]
composite solid electrolytes restricts their practical application, which needs to be further reduced .
ADVANCED ANALYTICAL CHARACTERIZATION
In-situ and ex-situ characterization methods have been widely used for characterizing all-solid-state Li
batteries [195-201] , including X-ray absorption spectroscopy, XRD, Raman spectroscopy, nuclear magnetic
resonance spectroscopy, neutron diffraction, transmission X-ray microscopy, SEM, TEM and atomic force
microscopy (AFM). These characterization techniques are powerful tools to investigate and understand
interfacial issues in solid-state batteries and can afford deep insights into their electrochemical behavior,
electronic state, structure and morphology at the atomic level .
[202]

Ex-situ techniques
Ex-situ characterization is a relatively low-cost and convenient method to reveal the evolutions at the
interfaces of electrodes. SEM and TEM have been used for characterizing the interfacial properties of
electrodes/SSEs, including the structural changes and chemical instability, and are useful tools suitable for
comprehensively understanding the role of interfaces in solid‐state lithium batteries at the atomic level.

SEM/EDS analysis of the solvate interlayer-modified Li S pellet has revealed that the use of a solvate can
2
enhance the wettability of the cathode and the solvate diffuses into the void spaces of the cathode and grain
boundaries of the SSE during cell cycling, resulting in a decrease in the interfacial impedance of the cell
[Figure 12A and B] . TEM imaging and electron energy-loss spectroscopy mapping demonstrated that the
[119]
SSE surface with the Al O coating has good wettability with Li metal. Al can permeate into the SSE surface
3
2
and form a lithiated-alumina transition layer [Figure 12C and D] [20,158] . Furthermore, electrochemical
impedance spectroscopy (EIS) is a powerful tool to investigate the interfacial behavior between the electrode
[195]
and SSE and afford useful information on Li-iondiffusion in all-solid-state Li batteries . It has been
reported that the interfacial behavior, including stability and resistance between electrodes and SSEs during
cycling, can be assessed by the changes of semicircles in the low- and high-frequency ranges
[Figure 12E and F] [197,203] . Neutron depth profiling is used to diagnose short circuits in symmetric solid-state
batteries and analyze the Li-ion transport behavior of solid-state batteries .
[198]

In-situ techniques
Although most characterization methods for investigating all-solid-state Li batteries are ex-situ experiments,
in order to further understand the electrochemical phenomenon and structural evolution of the interfaces of
the solid-state systems during the cycling process, more advanced in situ/operando characterization
techniques are required. These techniques will benefit researchers attempting to reveal the interfacial
reaction kinetics and decay mechanism of solid-state batteries in real time. In comparison with ex-situ
characterization, in situ/operando characterization techniques including S/TEM can dynamically detect the
interfacial structural evolution and phase transitions in a nonequilibrium state during charging and