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Park et al. Energy Mater 2023;3:300005 https://dx.doi.org/10.20517/energymater.2022.65 Page 7 of 13

In addition to LiNO , Li S is a potential additive candidate that forms Li S and Li S . The ionic conductivity
3
2 2
x y
2
-26
-1
of bulk Li S is extremely low (~10 S cm ). However, when Li S exists as a thin layer at the interface, owing
2
2
-5
-1
to the grain boundaries, dislocations, interfaces and amorphous content, its ionic conductivity (10 S cm )
is higher than other common SEI components, such as Li CO (10 S cm ) and LiF (3 × 10 S cm ) . The
-1
-9
-8
-1 [71-73]
2
3
Li S additive also results in a mechanically dense and thick SEI layer on the surface of Li metal, because Li
x y
metal reacts with Li S to form an insoluble component . Note that the passivation layer derived from the
[74]
x y
LiNO additive is less solid. Some synergistic effects can be expected from using dual LiNO -Li S . This
x y
3
3
approach causes flat Li plating without dendritic Li growth in ether-based electrolytes. This feature cannot
be achieved using only LiNO . In a similar manner, dendritic Li growth can be inhibited by using a dual-
[75]
3
layer composed of an organic layer and an inorganic layer. Zhang and co-workers reported a uniform and
compact dual-layer SEI with organic components (e.g., ROLi and ROCO Li) on the top layer and inorganic
2
components (e.g., LiF and Li CO ) on the bottom layer. This organic amorphous polymer layer increases the
3
2
Li-ion diffusivity and avoids damage based on its flexibility and the inorganic LiF-Li CO layer contributes
3
2
to forming the ordered Li nucleation and prevents side reactions by preventing contact between the
electrolyte and Li metal [76,77] .
SEI mechanical properties
When some mechanical damage or breakage occurs at the weak natural SEI of Li metal electrodes, the Li
metal suffers severe loss of its passivation layer, resulting in the degradation of cell performance. In some
cases, thermal runaway can arise at some local points on the electrode. In considering cell design, in
addition to its ionic conductivity, the mechanical properties of the SEI are some of the primary factors.
Recently, Xia and co-workers characterized the SEI layers in carbonate- and ether-based electrolytes with
[78]
the aid of a cryogenic electron microscope . Each organic component was determined during Li plating
and stripping. From density functional theory calculations, a single SEI component derived from different
electrolytes was predicted. The authors found correlations between the SEI components and mechanical
properties and argued that carbonate-based electrolytes are preferable.
Recently, detailed electro-chemo-mechanical modelling was implemented using the finite element method
to provide beneficial information for the SEI. The aim was to determine the correlation between the
mechanical properties of an artificial SEI and Li deposition. The results showed that the mechanical
properties of the SEI are governed by uneven Li deposition, such as in whiskers, tresses, globules and
dendrites . If the ionic conductivity of the SEI is improved, reaching a certain level, mechanical stress
[79]
cannot be concentrated, resulting in even Li deposition. However, Figure 5 shows that, except for
polyvinylidene difluoride, a critical value (σ /σ Electrode ) of > 0.1 was not found. According to their
SEI
calculations, a Young’s modulus (E ) of 4 GPa as a threshold value is a critical point for the deposition of
SEI
uniform Li growth. Therefore, the ionic conductivity of an artificial SEI needs to be improved without
degradation of its mechanical strength. We need to find an artificial SEI to meet the conditions of the
threshold values of σ /σ Electrode > 0.1 and E > 4 GPa. This is a new strategic method of testing artificial SEIs
SEI
SEI
on other metallic anodes .
[80]
Solid electrolytes
In spite of the various attempts to suppress dendritic Li growth in liquid electrolytes, unexpected growth
still constantly occurs. As a principal solution to replacing liquid electrolytes, many studies are underway to
suppress dendritic growth by utilizing solid electrolytes. As shown in Figure 6A, when a liquid electrolyte is
used in a Li metal battery, an SEI is formed due to side reactions, and subsequently, the low ionic
conductivity of the SEI leads to dendritic Li growth. In contrast, using solid electrolytes with adequate
[81]
mechanical properties minimizes dendritic growth . Note that the electrochemical performance of the
battery is inferior to that of the liquid electrolyte due to the lack of ionic conductivity, as shown