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Page 16 of 35 Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46

Anode/organic polymer electrolytes
Owing to their poor mechanical strength, Li dendrite growth is still a serious issue for SPEs in the
realization of ASSLSBs, easily causing the internal short circuits of cells. To overcome this problem, various
kinds of inorganic filler, including sulfides and oxides, have been introduced into the SPEs to improve their
mechanical properties, as well as ion conductivities in ASSLSBs, which is believed to be one of the ultimate
choices for suppressing Li dendrite growth and blocking the shuttling of lithium polysulfides [22,64,106-108] .

Anode/inorganic SSEs
Furthermore, different interfacial properties can be observed in the interphases of Li metal anode/inorganic
SSEs, which are demonstrated to be associated with their thickness. For example, a 20 μm interphase layer
[109]
containing Li-Ti/Ge alloys is found at a NASICON-type electrolyte/Li metal anode interface . A passive
layer with a thickness of 2 nm, consisting of Li S species, is formed at the anode/electrolyte interface after Li
2
[110]
metal contact with sulfide electrolytes . Such unstable interphase layers that often block the interfacial
ionic transport eventually lead to increased interfacial resistance. Furthermore, Li dendrite formation in
sulfide electrolytes seriously hinders the development of safe ASSLSBs.

In contrast, a few oxide SSEs, particularly LLZO, exhibit high chemical stability against the Li metal anode
and the resulting passivation layer has the ability to conduct Li ions (e.g., Li O, Zr O and La O ) to restrain
2
3
2
3
the continuous degradation of the SSE/Li metal interface [13,111-114] . However, unlike liquid electrolytes, most
have poor wettability with the Li metal anode because of their rigid and brittle nature, resulting in the loss of
intimate contact and high interfacial resistance between the Li anode and oxide SSEs.
Anode/hybrid electrolytes
Compared to single-component electrolytes (solid-state inorganic electrolyte or solid-state polymer), the
interfacial behavior of polymer-inorganic composite electrolytes, including interfacial compatibility,
chemical stability, safety and mechanical strength, can be effectively improved and the polysulfide shuttle
and Li dendrite growth are suppressed. The interfacial properties of a Li anode/polymer-inorganic
composite electrolyte depend not only on the spatial distribution of each species but also on their own
stability. However, the room-temperature ionic conductivity of polymer-inorganic composite electrolytes is
still low and their interfacial stability against Li metal anodes is very complicated. The following section
presents a detailed discussion of the possible strategies and selection criteria to mitigate these issues for the
development of high-performance ASSLSBs.

STRATEGIES FOR RESOLVING INTERFACIAL ISSUES
Current ASSLSBs still show lower cycle life and rate capability than liquid batteries because of the
previously discussed SSE/electrode interfacial issues. In order to solve these interfacial issues and improve
battery performance, various significant strategies have been conducted, including interfacial engineering,
adding ionic conductive materials, the application of artificial coating layers, reducing the active material
particle size, employing a hot- or cold-press setup, the utilization of fillers and designing composite
cathodes.

Sulfur cathode side
Pure sulfur-based active materials have poor ionic and electronic conductivity, usually leading to high
interfacial resistance and poor battery performance. In order to improve their ionic and electronic
conductivity, large amounts of SSE and electronic conductors need to be added to the composite cathodes
separately. Designing and fabricating composite cathode materials composed of active materials (e.g., sulfur
and sulfur-based constituents), Li-ion conductors (e.g., SSEs), electronic conductors (e.g., carbon and metal)
and/or binders have been demonstrated to be effective strategies for improving the ionic and electronic