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

market demand for lithium-ion batteries, which have high energy density and good long-term cycle
performance. A lithium-ion battery is mainly composed of a cathode, such as lithium cobalt oxide, lithium
manganese oxide, lithium nickel manganese cobalt oxide or lithium iron phosphate, an electrolyte, a
separator and an anode, such as graphite. At present, traditional Li-ion batteries cannot meet the desired
storage requirements due to their limited theoretical specific capacity and safety issues . Thus, the
[1]
development of novel energy storage energy systems has become a significant challenge. Lithium-sulfur
batteries (LSBs) have received substantial attention due to their high energy density, environmental
friendliness and low cost. However, liquid electrolyte LSBs are limited by low Coulombic efficiency and fast
capacity degradation because of the polysulfide dissolution and shuttling. The toxicity and flammability of
liquid electrolytes also cause safety concerns. All-solid-state lithium-sulfur batteries (ASSLSBs) are regarded
as potential substitutes. ASSLSBs contain a metallic lithium-based anode, solid-state electrolyte (SSE) and
sulfur-based cathode. The use of a nonflammable SSE can suppress the polysulfide dissolution, eliminate the
[2,3]
shuttle effect and restrain the formation of lithium dendrites . Furthermore, ASSLSBs show additional
advantages, including reduced wasted volume and net weight, low self-discharge, high operating voltages
[4-6]
and excellent thermal stability . A comparison between liquid electrolyte-based LSBs and ASSLSBs is
shown in Table 1.

A variety of SSEs, including organic polymer electrolytes (PEs), inorganic solid-state electrolytes (ISSEs) and
PE and ISSE composites, have been investigated extensively [11,12] . The fabrication of ideal SSEs should
consider their mechanical strength, electrochemical window, chemical stability, cost and ionic conductivity.
Generally, ISSEs can be classified into two categories: oxide solid electrolytes and sulfide solid electrolytes.
PEs include solid polymer electrolytes (SPEs) and gel polymer electrolytes. Each type of SSE has its own
relative merits. For example, ISSEs show higher ion conductivities at operating temperature, while PEs
exhibit lower hardness, better interfacial contact and lighter weight. Thus, novel composite electrolytes may
be enlightening in improving the performance of ASSLSBs because of their potential multifold benefits.

After decades of research, it is now clear that the main obstacle facing ASSLSB development is no longer
improving the ionic conductivity of the SSEs but has instead shifted towards increasing the interfacial
compatibility between the SSE and electrodes. The different interfacial nature of the electrode/SSE interface,
in comparison with the electrode/liquid electrolyte interface, easily causes significant interfacial
resistance [13-16] . A good electrode/solid electrolyte interface should have excellent chemical and mechanical
stability, high ion transport and maximum contact area during cycling. Currently, the electrode/electrolyte
interface remains an important challenge that is limiting the possible commercialization of ASSLSBs. The
resistance at the interfaces between the electrodes (anode and cathode) and SSEs is usually large. Favorable
interfacial compatibility, contact and chemical stability are critical to improving the electrochemical
properties of ASSLSBs [Figure 1].

Although a number of impactful reviews have summarized the progress and prospects of ASSLSBs,
including SSEs, metallic lithium-based anodes and sulfur- and Li S-based cathodes, few reviews have
2
specifically focused on the electrode/SSE interfaces in ASSLSBs [18-23] . Therefore, to improve the
understanding of the role of the solid/solid interfaces in these systems, this review provides a comprehensive
summary and analysis based on the key interfacial challenges of electrode/SSE interfaces, including metallic
lithium-based anodes/SSEs and sulfur- and Li S-based cathodes/SSEs, and SSE/SSE interfaces. The advanced
2
strategies employed to resolve these interfacial issues are also introduced.

Evolution and classifications of ASSLSBs
Evolution
In order to understand the role of solid/solid interfaces in ASSLSBs, the evolution and classifications of