A review of Functional Separators for Lithium Metal Battery Applications

2020
,
13
, 4625
20 of 37
Materials 
2020
, 
13
, x 
20 of 38 
poly(acrylic acid) (PPA) for LSB applications [136]. PP grafted with PPA (PP-g-PPA separator) 
selectively passes Li-ions well, but LiPS is blocked by electrostatic repulsion. Thus, LiPS did not pass 
toward the anode and remained in the separator at the cathode side. In addition, because of the strong 
bonding energy of PP and PPA, consistent and uniform blocking of LiPS during the long cyclic 
process was observed. The LSB exhibited an initial capacity of 800 mAh
g
−
1
, and 580 mAh g
−
1
after 
250 cycles. Yamauchi group reported a double-layered modified separator as a shuttle suppressing 
interlayer for LSBs [137]. They fabricated this double-layered separator combining a microporous PP 
matrix layer and an arrayed PMMA microsphere retarding layer. In this separator, the arrayed 
PMMA microspheres blocked the diffusion of polysulfides chemically and physically. Ester groups 
of PMMA interact with polysulfides and chemically adsorb them. Moreover, the arrayed PMMA 
microspheres are easily formed by self-assembly. Owing to its physical and chemical effects on 
suppressing diffusion of polysulfides, a sulfur with the PP/PMMA separator exhibited an initial 
capacity of 1100 mAh g
−
1
at a current density of 0.1 mA cm
−
2
, which is higher than the first discharge 
capacity of the commercial PP separator (948 mAh g
−
1
). 
Zhang et al. prepared an Al
2
O
3
-modified separator for LSBs and suppressed the shuttle effects 
effectively [138]. In their study, an Al
2
O
3
layer was coated onto a commercial separator using slurry 
coating. In this separator, well-connected voids existed between nanoparticles, aiding Li-ions to move 
freely. Additionally, the entangled structure reduced the shuttle effects. Choi group used permanent 
dipoles in BaTiO
3
(BTO) particles to effectively prevent LiPS from passing through a separator by 
electrostatic repulsion [139] (Figure 11a). A BTO coating increases the mechanical strength of a PE 
separator to prevent thermal shrinkage. Consequently, the BTO-coated separator in a Li|S full cell 
exhibited a high specific capacity and extended cyclic life.