A review of Functional Separators for Lithium Metal Battery Applications

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Figure 9. 
(
a
) Schematic of the Li-metal deposition mechanism using the bare PE separator (top) and 
PE/CuTF Janus separator (bottom). Reprinted with permission from [82]. Copyright (2017) WILEY-
VCH. (
b
) Schematic of electrodeposition on a Mg-coated separator. Reprinted with permission from 
[81]. Copyright (2018) Elsevier B.V. (
c
) Schematic of the mechanism with an extended battery life with 
a silica nanoparticle sandwiched separator. Reprinted with permission from [118]. Copyright (2016) 
WILEY-VCH. 
Song and co-workers introduced Mg nanoparticles on one side of a separator [81]. Lithiophilic 
Mg nanoparticles offer the sites for heterogeneous nucleation and produce a strong guiding effect to 
form fixed Li crystal seeds at the initial plating process, and consequently aid in retaining a dendritic-
free and dense Li anode after the long cyclic process. Li nucleation occurs in separator-to-Cu direction 
rather than Cu-to-separator direction; this was confirmed using SEM after a Li|Cu half-cell test 
(Figure 9b). Cui group reported the silica nanoparticle sandwiched tri-layer separators by coating 
SiO
2
nanoparticles between two commercial PE separators [118]. Previous studies focused on the use 
of SiO
2
as a physical barrier because of its thermal stability and high wettability. This study 
emphasized the additional role of SiO
2
: it guides the growth direction of Li-dendrites due to the 
chemical reactions between SiO
2
and Li. They conducted Li|Li symmetric cell tests after making 
pinholes on various types of separators to promote severe Li growth conditions to investigate the 
formation mechanisms. Four types of separators (bare, SiO
2
coated, Si coated, PMMA coated) were 
used for the experiments. The SiO
2
-coated separator exhibited the longest lifespan (
≈
152 h) among 
the others (Figure 9c). 
Yuan group fabricated a ZrO
2
/polyhedral oligomeric silsesquioxane multilayer-assembled PE 
separator, which was synthesized using a simple LBL self-assembly process [119]. This separator 
effectively reduces electrolyte polarization and protects Li-metal anodes from Li dendritic growth, 
and it exhibits excellent electrochemical performance and stability. Xie group reported an interesting 
strategy, which guided the direction of dendrite growth [101]. Their concept was to allow dendritic 
growth from both separator and Li-metal surfaces. These Li layers grew by facing each other, 
resulting in a fused and dense Li formation. This concept was realized by coating a conductive carbon 
layer on the separator surface, which faced the Li-metal anode. This structure enabled dendrites to 
spread widely in a direction parallel to the electrode. The Li-metal electrode exhibited a stable cyclic 
life with a capacity retention of 80% even after 800 cycles.