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Tao et al. Energy Mater 2022;2:200036 https://dx.doi.org/10.20517/energymater.2022.46 Page 9 of 35

Anode side
Employing lithium metal as the anode in an ASSLSB configuration can result in 70% and 40%
improvements in the gravimetric and volumetric energy density, respectively; however, unstable Li metal
anode/SSE interfaces caused by the poor contact and side reactions during cycling potentially give rise to
high interfacial resistance, capacity fading and battery failure.

Poor interfacial contact
In comparison with liquid electrolytes, owing to the intrinsic rigidity and relatively high stability of SSEs
against Li metal and their mismatched lattice, SSEs cannot easily wet the electrode surface and only point-
to-point contacts between the lithium metal anode and SSEs can be achieved, resulting in the interfacial
[10]
voids and defects, dendrite growth of Li along the grain boundaries and a loss of interfacial contact .
Furthermore, significant volume changes between the anode and SSEs during cycling trigger the interfacial
stress changes, leading to poor physical contact and an increase in interfacial resistance, which is
unfavorable for lithium ion transfer. In comparison with ISSEs, SPEs show higher interfacial compatibility
with lithium metal anodes, but their low ionic conductivity greatly restricts their application in ASSLSBs.

Chemical reactions
In addition to poor contact, side reactions at the Li metal anode/SSE interface can lead to an increase in
interfacial resistance. Since Li metal anodes have an ultralow electrochemical potential, they can directly
react with various kinds of SSEs during the packing process. In addition, the Li metal anode and electrolytes
can offer a high thermodynamic driving force for electrochemical oxidation and chemical reaction to form
interfacial regions during cycling [7,80] [Figures 6 and 7].

The resistance of the formed interface is determined by the chemical composition of the interphases. For
example, harmful by-products, such as Li ionically/electronically insulating Li S/LiX (X = Cl, Br or I)
2
produced by the interfacial reactions of Li metal with sulfide solid electrolytes, contribute to the increase in
the interfacial resistance. According to the thermodynamical stability of the interface, three different types
of interface formation are proposed [Figure 7A-C] . An interface that is electronically insulating and
[81]
ionically conductive could possess high electrochemical stability during cycling, while thermodynamically
unstable interfaces with mixed ionic-electronic conducting interphases keep growing. The Li ionic
conductivity of the interface is very important for the performance of ASSLSBs.

Furthermore, the interphase layer between the Li metal anode and electrolyte arises from the redox
instability of the SSEs. Both a reasonable alignment of the electrolyte bands, including the valence and
conduction bands, relative to the Li chemical potential μ , and a relatively high band gap are two critical
Li
factors affecting the redox stability of the SSEs.

The values of the electrochemical stability window can be obtained by first-principles calculations and
experimental measurements [Figure 7D] . As a result, the calculations predict that sulfide SSEs can only
[82]
afford a narrow voltage stability window, while Li binaries, Li X (n = 1, 2 or 3; X = anion), exhibit an
n
expected trend of extending voltage stability window with increasing anion electronegativity.
Although the formed interphase layer can inhibit dendrite growth, a continuous decomposition and
interphase layer growth are usually contributed by the electron transfer across this mixed interphase layer,
severely increasing the local volume, polarization and interfacial impedance during the cycle. Additionally,
the volumetric expansion can also result in repeated cracking and finally damage the SSE.