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Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46 Page 25 of 35

Figure 12. SEM images of (A) pristine solvate interlayer-modified Li S pellet and (B) interlayer-modified Li S pellet after ten cycles. EDS
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analysis shown in (A) and (B) (yellow rectangle regions). Comparing relative F content (F atoms from LiTFSI and TTE) at different
regions can confirm the possible infiltration of the solvate across the pellet (reproduced with permission from [119] ). (C) TEM cross-
sectional image for the interface of an ALD-Al O -coated SE with Ti protection layer. (D) EELS maps (Al, Li, O, overlap of Al and Li, and
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Ti, respectively) at the interfacial cross section (reproduced with permission from [20] ). Characterization of an SSB with LNTO-coated
LiCoO during the charge (E) and discharge (F) process by EIS, which was carried out after a 1 h charge and a 30 min rest (stacked
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Nyquist plots) and the galvanostatic charge was carried out at 0.1 C (reproduced with permission from [196] ).
discharging, directly provide real-time lithiation/delithiation information and help demonstrate the
structure-performance relationships of ASSLSBs and electrochemical reaction mechanism in real time.

Recently, the interfacial issues in solid-state systems have been studied using in situ/operando
characterization techniques.It has been demonstrated that the electrochemical reaction mechanism in
ASSLSB with a sulfide-based SSE by in-situ SEM characterization . This reveals that homogeneous lithium
[200]
deposition on the SE and the suppression of the dendritic growth are critical to obtaining the highly
reversible lithium deposition and dissolution reaction. A valuable amount of information regarding the
electrochemical behaviors on the interfaces between lithium phosphorus oxynitride and metallic lithium
can be provided by in-situ XPS. Gong et al. constructed a working LiCoO /LLZO/Au ASS battery by using a
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combination of a state-of-the-art chip-based in-situ TEM holder and focused ion beam milling to observe
the structural evolution of electrodes during cycling on an atomic scale with TEM [Figure 13] . These in-
[201]
situ STEM results demonstrated that good interfacial contact between the electrode and electrolyte is very
important to achieving high-performance ASSLSBs.

Advanced in-situ and ex-situ characterization methods can benefit researchers in understanding solid
interfacial reactions, the interfacial structural evolution, the interfacial phase transitions and fundamental
ion-transport mechanisms at interfaces, and overcome interfacial issues, including the interfacial
compatibility, interface stability and interfacial resistance in the solid-state systems, revealing the reaction
kinetics and decay mechanism of ASSLSBs, and affording guidance to design and optimize high-
performance ASSLSBs. Furthermore, a combination of different in-situ and ex-situ characterization
methods is desperately required to further reveal the evolutions at the interfaces.

COMMERCIALIZING PROGRESS IN ASSLSBS
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The high theoretical energy density (~2500 Wh kg ), low cost, natural abundance, environmental benignity