A review of Functional Separators for Lithium Metal Battery Applications

2020
,
13
, 4625
14 of 37
Materials 
2020
, 
13
, x 
14 of 38 
Wang group fabricated an ultrastrong nanofiber membrane [106]. A nanoporous membrane was 
fabricated using a poly(p-phenylene benzobisoxazole) nanofiber (PBO-NF) through blade casting 
(Figure 7d). This separator was low cost and had a high strength of 525 MPa and Young’s modulus 
of 20 GPa. The membrane was stable up to 600 °C. In Li|Li symmetric cell, a pure Li-metal surface 
was observed after 700 cycles. It exhibited excellent performance in preventing dendrites growth. 
Kotov group synthesized aramid nanofibers (ANFs) [111]. In a layer by layer (LBL) structure, 
poly(ethylene oxide) (PEO) was applied in the ANFs as an ionic conductor. The tensile strength, 
Young’s modulus, and shear modulus were recorded as 
σ
ICM
= 170 ± 5
MPa, E
ICM
= 5.0 ± 0.05
GPa, and 
G
ICM
= 1.8 ± 0.06
GPa, respectively. The crystallization of PEO, which is known to be detrimental to 
ion transport, can be controlled by the presence of ANF networks. As a result, in Li|Li symmetric 
cell, ionic conductivity was 1.7
× 10
−
4
S
cm
−
1
, which was higher than that of conventional polyolefin-
based separators. 
5.1.3. Strategies for Improving Thermal Stability of Separators 
For the commercial use of LMBs, thermal stability is a very important factor in separators. At 
high temperatures, the ionic conduction is very active, accelerating dendritic growth. 
Simultaneously, the separators lose their mechanical stability at high temperatures [112]. Polyolefin-
based separators are not stable at high temperatures (130–160 °C) [48]. To complement this, a method 
of coating the separator with a ceramic-based substance has been developed. This can provide high 
thermal stability but has the disadvantage that ceramic materials can block the pores in the separator, 
complicating ionic transport and should use polymer binder such as PVDF-HFP [113] and PMMA 
[114]. In this section, we introduce studies that have improved thermal stability. 
Lin group fabricated a sandwich-structured separator composed of PI/PVDF/PI using the 
electrospinning method [115]. This separator had a shutdown function (Figure 8). Because PI has a 
high thermal stability of 500 °C and low shrinkage, it is thermally and mechanically stable. The PVDF 
between the PI layers melted in 10 min at high temperatures above 170 °C. This is approximately 40 
°C higher than that of a PE membrane. In addition, the electrolyte uptake was recorded at 476%, the 
ionic conductivity was 3.46 mS cm
−
1
in a Li|LiMnO
2
coin cell, and the porosity was measured at 83%. 
Because of this, the battery had a high thermal stability, good cyclic life, and 95.1% capacity retention.