A review of Functional Separators for Lithium Metal Battery Applications

2020
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Figure 19. 
Preparation of BC/LLTO CGEs. (
a
) Schematic of the effect of (
b
) a commercial separator 
and (
c
) a BC/LLTO CGE on Li deposition in a battery. Reprinted with permission from [165]. 
Copyright (2019) WILEY-VCH. 
As another method of fabricating GPEs, the adaptation of ionic liquids (ILs) has been introduced. 
To realize the uniform Li deposition and effective suppression of dendritic growth, Jin and co-
workers researched the immobilization of ILs in GPEs via ion-dipole interactions [166]. To prepare 
IL immobilized GPEs, they used an imidazolium salt, 1-ethyl-3-methylimidazolium 
bis(trifluoromethylsulfonyl)-imide (EMI-TFSI) as the IL because of its high ionic strength, 
electrochemical stability, and nonflammability. Meanwhile, fluorinated copolymers such as PVDF-
HFP were used as flexible gel polymer matrices because of their high dielectric constant, high 
stability, high thermal endurance, and strong mechanical strength. The absorbed IL into copolymer 
gel electrolyte suppressed dendritic growth by forming a 3D cross-linked network with tethered TFSI 
anions owing to the strong ion-dipole interactions between imidazolium cations and fluorine atoms. 
Moreover, it exhibited a self-healing capability for enhanced ionization of Li salts, promoting the Li-
ion transport. As the interaction between the imidazolium cations and the C-F dipoles on the polymer 
chains was strong, a high self-healing capability was obtained. As-made cells with this GPE exhibited 
a high ionic conductivity, thermal stability, and high mechanical strength, resulting in the effective 
suppression of dendritic growth during the plating/stripping process. 
7. Conclusions 
In this review, we have summarized the recent progress in functional separators for next-
generation batteries. First, existing problems of LMBs are reviewed and various properties that affect 
battery performance are defined. Second, the types of commercial separators in LIBs and their 
limitations for LMB applications are described. For the design of stable separators, materials with 
various compositions and structures have been applied to Li|Li symmetric cells, LMBs, and LSBs. 
Such various materials can be classified into organic, inorganic, carbon-based, and solid electrolyte 
materials. We reviewed their properties, synthetic methods, and positive effects in each section. 
Several remarkable studies were conducted to suppress Li dendritic formation and growth, increase 
mechanical/thermal/chemical stabilities, maximize the use of active materials, and prevent LiPS 
shuttle effects, resulting in excellent LMB and LSB performance. The current problems and prospects 
are categorized according to types of material and summarized below.