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Page 14 of 35 Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46

CURRENT KEY CHALLENGES OF SOLID/SOLID INTERFACES BETWEEN ELECTRODES
AND SSES
As discussed earlier, the major issues result from the interfaces between SSEs and electrodes in typical
ASSLSBs. Various kinds of SSEs have been applied to ASSLSBs, including organic polymers, inorganic
materials and organic-inorganic hybrids, and their reactivity with electrodes contributes considerably to the
internal resistance of the battery. Several parameters or properties of an electrolyte play a critical role in
determining the performance of interfaces, namely, ionic conductivity, interfacial properties, mechanical
properties, electrochemical stability and compatibility with the electrodes during battery cycling tests
[Figure 9] [94-97] . ASSLSBs employing different SSEs could face different interfacial challenges because of their
different electrochemical reaction mechanisms at the electrode/SSE interfaces.

Issues facing cathode/electrolyte interfaces
The cathode materials in ASSLSBs have been intensively reported. The cathode/electrolyte interfacial issues
induced by the electrochemical potential occur during cycling and are significantly different for various
+
electrolytes. However, owing to the low operation potential (< 2.8 V vs. Li/Li ) of sulfur-based cathodes,
their interfacial electrochemical reaction is negligible for SSEs. Therefore, the main concern at the cathode
sides of ASSLSBs could be the chemical stability of SSEs in contact with cathodes (S or Li S based) [98-101] . The
2
following sections summarize and discuss the relationship between interfacial issues and categories and the
interfacial reactions and interphase formation between the cathode and electrolyte.

Cathode/organic polymer electrolytes
SSPEs possess several special advantages, including good interfacial compatibility, high safety, easy
preparation, flexibility, lightweight and the possibility of scalable roll-to-roll manufacturing processes, and
they have been considered as promising candidates for the development of safe ASSLSBs. However, several
issues hinder the development of SSPEs in ASSLSBs, such as low room-temperature ionic conductivity and
poor thermal stability. In particular, a high diffusion of lithium polysulfides in the organic matrixes often
fails to inhibit their formation and shuttle and a S-rich passivation layer on the Li metal anode surface could
be formed and observed after cycling . In order to block the polysulfide dissolution and shuttle for high-
[102]
performance solid-state polymer-based ASSLSBs, S-based composite electrode materials combined with
SSPEs have been proposed, as presented in the following sections.

Cathode/inorganic SSEs
Inorganic SSEs can be further classified as oxide or sulfide-based SSEs. Although remarkable progress in the
development of oxide SSEs has been made, there remains a challenge to adequately integrate oxide SSEs,
including perovskite-type Li La (2/3)x TiO (LLTO), garnet-type Li La Zr O (LLZO), NASICON-type
2
3
3x
12
2
3
Li Al Ti (PO ) (LATP) and Li Al Ge (PO ) (LAGP), into ASSLSBs because of the obvious differences
x
1+x
2x
4 3
4 3
x
1+x
2x
between the thermal and chemical properties of oxide SSEs and S-based cathodes, the poor interfacial
compatibility and large grain boundary resistance [4,13,15,20] .
In addition to oxide SSEs, sulfide-based SSEs with high room-temperature ionic conductivities of 10 S cm
-1
-2
have also been considered in the design of favorable SSEs for inorganic SSE-based ASSLSBs, such as glass
sulfide, glass-ceramic and crystalline sulfide SSEs. Compared with oxide SSEs, sulfide-based SSEs exhibit
low grain boundary resistance; however, several major shortcomings of the single-component sulfide-based
SSEs still limit their application in ASSLSBs, including high chemical instability against moisture and
narrow electrochemical stability windows (in the range of 1.5-2.5 V). Normally, inorganic SSEs are coupled
with liquid electrolytes, ionic liquids or polymer-based SSEs for the improvement of their interfacial
behavior.