A review of Functional Separators for Lithium Metal Battery Applications

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3. Limitations of Commercial Separators in Lithium Metal Batteries
Polyolefin is commonly used in commercial separators for LIB applications. In particular, PE and
PP are mostly used, which are produced using wet and dry processes (Figure
4
a–c).
Materials 
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2.9. Preventing Shuttle Effects 
In LSBs, cathode is known to have two plateaus during the discharge process. In the first plateau 
(~2.4 V vs. Li
+
/Li), sulfur is reduced from S
8
to S
42
−
, at which various Li polysulfides (LiPS, Li
2
S
x
) 
dissolve in the electrolyte. In the second plateau (~1.95 V vs. Li
+
/Li), Li
2
S
4
converts into insoluble Li
2
S
2
and Li
2
S [52]. As-formed LiPS diffuses to anode side owing to electrostatic attraction between Li-
metal and charged LiPS, and this phenomenon is designated as shuttle effects [53,54]. Shuttle effects 
lead to several degradations such as loss of active materials, low Coulombic efficiency (CE), and 
passivation of the Li anode [55]. Accordingly, separators have to prevent LiPS from migration. The 
chemical interaction between LiPS and separator can obstruct the diffusion of LiPS to anode side. In 
addition, the separator can enable the reuse of LiPS as active materials [56]. 
3. Limitations of Commercial Separators in Lithium Metal Batteries 
Polyolefin is commonly used in commercial separators for LIB applications. In particular, PE 
and PP are mostly used, which are produced using wet and dry processes (Figure 4a–c). 
Figure 4. 
SEM images of the surfaces of various separators: (
a
) Celgard 2730 (PE), (
b
) Celgard 2500 
(PP), (