A review of Functional Separators for Lithium Metal Battery Applications

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5.2. Separators for LSBs 
As described earlier, currently used polyolefin-based separators cannot prevent LiPS crossover 
during the LSB cycles. Therefore, several studies on blocking LiPS in a separator are being conducted 
[126]. Establishing a multi-functional separator that increases the conductivity of the Li-ions and 
effectively blocks LiPS is necessary. Electrostatic repulsion and chemical trapping of LiPS are actively 
investigated approaches to achieve this aim. In this section, we introduce studies on suppressing the 
shuttle effect chemically and physically using various materials. 
5.2.1. Strategies for Suppressing Shuttle Effects by Chemical Methods 
Polymers have an advantage in that they can be prepared in multiple layers. Therefore, a 
separator that exhibits the synergistic properties of each polymer can be manufactured. For example, 
Lee group used multi-walled carbon nanotube (MWCNT)-wrapped polyetherimide nanomats as 
top/bottom layers and poly(1-ethyl-3-methylimidazolium) bis(trifluoromethanesulfonyl)imide 
(PVIm[TFSI])/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as a middle layer [127] 
(Figure 10a). In this structure, TFSI ions chemically trapped LiPS, alleviating the shuttle effect. In 
addition, PVIm[TFSI] in the middle layer improved the mechanical properties as a pillar and the 
PEI/MWCNT in the top/bottom layers acted as a current collector. These multiple layers promoted 
the redox reaction of the cathode but prevented the shuttle effect. Xu group conducted a study on 
blocking LiPS using functional groups on the separators [128]. They prepared a gum arabic (GA) and 
conductive carbon nanofiber (CNF) mixture. GA is composed of branched polysaccharides, which 
are composed of alactose, ramnose, arabinose, and hydroxyproline side chains. Thus, it traps LiPS 
using hydroxyl, carboxyl, and ether functional groups. The LSB with this separator exhibited 
excellent performance: 94% capacity retention even after 250 cycles.