A review of Functional Separators for Lithium Metal Battery Applications

2. Properties of the Separator
The separator a
ff
ects cell performance and safety. Therefore, understanding their characteristics
and requirements for better performance is important. This section overviews commercial separators,
specific parameters and analysis of LMB separators, and characteristics of separators for LMBs (Figure
1
).

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Materials
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2. Properties of the Separator 
The separator affects cell performance and safety. Therefore, understanding their characteristics 
and requirements for better performance is important. This section overviews commercial separators, 
specific parameters and analysis of LMB separators, and characteristics of separators for LMBs. 
(Figure 1) 
Figure 1. 
Illustration of the key properties of separators. 
2.1. Thickness 
Uniform thickness of the separator promotes homogeneous ion distribution, leading to the 
uniform use of the active materials present in the electrode layer and induces flat Li-metal formation 
by suppressing the growth of Li-dendrites [41]. Commercial separators have a thickness ranging 
between 20–25 µm [42]. Thin separators can maximize the energy density of batteries by providing 
more space for electrodes. However, thin separators increase the possibility of punctures and short 
circuits. In contrast, if a separator is too thick, it causes high resistance and decreases the energy 
density. In the case of LMB application, an optimal thickness should be systematically determined to 
prevent the growth of sharp Li-dendrites. 
2.2. Porosity 
Optimum porosity enables the electrolyte to be thoroughly wetted into the pores and provides 
facile ionic conduction. Generally, commercial separators with pores of 1 µm
or less have a porosity 
of ~40% [42]. A high porosity reduces the mechanical strength of the separator and increases the 
possibility of punctures. In contrast, if the porosity is too low, electrolyte wettability decreases and 
the internal resistance increases. Therefore, an optimum thickness should be determined and the 
pores of the separator should have even morphology and pore size distribution. The homogeneous 
pore size distribution facilitates uniform ion distribution. Pore size should be sufficiently large to 
absorb the electrolyte and enable Li-ions to pass; however, it should be smaller than the size of the 
particles of the electrode material [41]. In the case of LMBs, a sub-micrometer pore size has proven to