A review of Functional Separators for Lithium Metal Battery Applications

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Figure 7. 
(
a
) Comparison of poor wettability (left) and good wettability (right). Reprinted with 
permission from [102]. Copyright (2012) WILEY-VCH. (
b
) Schematic of the fabrication of the highly 
overoxidized PPy paper membrane using heat treatments and sequential base. Overoxidation 
changes the structure of PPy. Reprinted with permission from [104]. Copyright (2018) Elsevier B.V. 
(
c
) Puncture strength according to polymer ratio (HDPE:UHMWPE = 27:3); (1) UHMWPE, Mw = 
240,000,000 and (2) UHMWPE, Mw = 340,000,000. Reprinted with permission from [105]. Copyright 
(2002) Elsevier Science B.V. (
d
) SEM images of a PBO-NF membrane. Reprinted with permission from 
[106]. Copyright (2016) American Chemical Society. 
There is a case of introducing ceramic material to increase ionic conductivity, taking advantage 
of entangled structure of polymer at the same time. Zhang and co-workers
 
synthesized a separator 
using PAN and silica via centrifugal spinning. This cost-effective method developed separators with 
significant ionic conductivity and good wettability owing to the high porous fibril structure of PAN 
[107]. In this separator, PAN provided high ionic conductivity when the electrolyte was absorbed 
and had good thermal stability, with synergetic effects with SiO
2
. Electrolyte uptake was 310% and 
ionic conductivity was 3.6 × 10
−
3
S cm
−
1
in 12wt.% SiO
2
/PAN. They applied SiO
2
/PAN membranes to 
a Li|LFP full cell, which exhibited excellent rate performance with a capacity exceeding 160 mAh g
−
1
. 
5.1.2. Strategies for Improving Mechanical Strength of Separators 
The primary task of a separator is to prevent short circuits between the cathode and anode while 
maintaining ionic conductivity [82]. As described earlier, high mechanical strength is required to 
prevent dendrites from penetrating the separator [108]. Moreover, separators should have good 
electrolyte wettability and proper porosity [109]. In this section, high-modulus and porous materials 
coatings, which help in increasing the mechanical strength of separators, are discussed [61]. 
Ni group reported PVDF-HFP separator cross-linked with Al
2
O
3
as the cross-linker [110]. The 
separator had a high ionic conductivity of 1.37 mS cm
−
1
in a Li|LFP half-cell. Because of the cross-
linking and the presence of Al
2
O
3
, the mechanical strength was significantly increased to 30.4 MPa 
and thermal stability increased up to 180 °C. Kim group fabricated a high-strength separator using 
high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) [105]. 
As the ratio of UHMWPE increased, the mechanical strength increased (Figure 7c). A film with 6wt.% 
of UHMWPE had a tensile strength of 1000 kg cm
−
2
. In addition, it had uniform pores (0.1–0.12 µm) 
and excellent thermal stability that could withstand temperatures up to 160 °C.