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Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46 Page 7 of 35

construction processes and methods, chemical compatibility and non-poisonous. Several kinds of SSEs
have been designed and prepared, including inorganic SSEs (in the three major categories of sulfides (e.g.,
glass and glass-ceramic Li S-P S , argyrodite and thio-LISICON), oxides and LiBH ), organic (polymer) SSEs
4
2 5
2
and hybrid electrolytes [74-78] . However, most SSEs suffer from volume changes and large interfacial
impedances during cycling. In composite systems, inorganic SSEs are expected to be organically combined
with organic SSEs to take advantage of each single-component SSE and reduce their drawbacks.
Nevertheless, it is still difficult to design and develop ideal electrolytes with many advanced properties,
including high ionic conductivity, good mechanical properties, a wide electrochemical window and
excellent thermal and chemical stability.

In most cases, solid sulfur and its composites serve as the cathode for ASSLSBs, which is directly reduced to
form Li S at the plateau of ~2 V during discharge and the dissolution of polysulfides cannot occur because
2
of the employment of SSEs. In the presence of composite electrolytes with a small part of polymer or liquid
electrolytes, the formation and dissolution of intermediate polysulfides can occur at the beginning of
discharge. The two different types of the electrochemical reaction mechanism of ASSLSBs, a solid-solid
single-phase reaction and a solid-liquid dual-phase reaction, inevitably result in unique opportunities and
new challenges for the design and fabrication of electrode and electrolyte materials.

To summarize this section, for the components of ASSLSBs, mainly including anodes, cathodes and
electrolytes, besides their theoretical specific capacity and conductivity, the thermal, structural, chemical
and electrochemical stability, mechanical strength and synthesis method are also critical factors affecting the
comprehensive properties of ASSLSBs. The current operating electrochemical performance of ASSLSBs is
generally unsatisfactory. The poor interfacial compatibility between the electrode and electrolyte is still the
major bottleneck for the development of high-performance ASSLSBs.

Brief overview of issues related to ASSLSBs
Since most of the components in ASSLSBs are solid particles, ASSLSBs are usually assembled by directly
stacking the anode/electrolyte/cathode cell components in a single package, unavoidably leading to the
formation of many interfaces between particles (e.g., anode/electrolyte, cathode/electrolyte,
electrolyte/electrolyte and additive/electrolyte interfaces) [Figures 1 and 5]. Unlike liquid electrolyte-based
LSBs, the nature of the solid/solid interfaces in ASSLSBs can cause serious interfacial issues:

(a) The poor interfacial contact originating from the stress/strain at the SSE/cathode interface induced by
the volume change of active materials, such as Li S or S, during cycling leads to high interfacial impedance.
2
(b) Several kinds of anode/SSE interfacial issues, including the electrochemical instability of SSEs against
metal lithium, Li dendrite penetration and poor interfacial compatibility, contribute to degrading the
performance of ASSLSBs.

(c) The interphase between an electrode (cathode or anode) and electrolyte induced by redox instability of
+
the SSE results in low Li interstitial conductivity.

(d) The electrochemical instability of sulfide-based SSEs against ambient air causes the generation of
harmful by-products. Furthermore, most of these by-products have limited voltage windows and can be
unstable (oxidized at high voltages or reduced at low voltages) at the full voltage range of the electrode
materials, contributing to the formation of interphases.