Chemistry Reference
In-Depth Information
6.8 POFs for Li-S Battery
Lithium-ion batteries, with the highest energy density among all known recharge-
able battery systems, are considered to be the best candidates for transportation
applications. However, both the energy and power density of the lithium-ion bat-
teries should be enhanced for extending the driving range. Thereby, many pioneer-
ing and effective strategies have been proposed on exploring high energy electrode
materials in the past decades [
31
-
33
]. Because sulfur can react with lithium in the
formation of Li
2
S to generate the highest theoretical capacity of 1,675 mA h g
−
1
,
and with an average voltage of around 2.15 V, sulfur is a very promising cathode
candidate. It is notable that Lithium-ion battery can achieve a high theoretical
energy density of 2,600 W h kg
−
1
.
For Li-S batteries, there are two major problems limiting their practical applica-
tions. One is the electronic insulating nature of the elemental sulfur and the dis-
charge products, resulting in poor utilization of the active material. To resolve this
problem, the effective method is to disperse sulfur into a highly conductive media,
such as porous carbon [
34
] or a conducting polymer [
35
], or to use additive elec-
tron conductors such as acetylene black [
36
]. The other problem is its poor cycle
stability that resulted from the highly soluble intermediate lithium polysulfides.
Alternative electrolytes, including polymer electrolytes, glass-ceramic electrolytes,
and ionic liquid electrolytes, have been attempted to enhance the cycling stability.
In 2013, Guo and Dai et al. demonstrated a simple method to prepare a sulfur
cathode using an electroactive PAF as the host for lithium-sulfur batteries [
37
]. The
PAF-S composite was prepared by loading sulfur via melt diffusion at 155 °C. As a
result, the PAF-S composite exhibited high capacity and excellent cycling stability
in the sulfone electrolyte of 1.0 M LiPF
6
-MiPS and in the ionic liquid electrolyte of
0.5 M LiTFSI-MPPYTFSI. Notably, after 50 cycles in the system of 0.5 M LiTFSI-
MPPY TFSI, the PAF-S composite still retained a capacity of 690 mA h g
−
1
, which is
about 83 % of the initial reversible capacity (Fig.
6.19
). The results suggest that sulfur
Fig. 6.19
Galvanostatic discharge-charge curves (
a
) and cycling performance (
b
) of the
LiJPAF-S cell at a rate of 0.05 C in 0.5 M LiTFSI-MPPY.TFSI at 50 °C. Reprinted with permis-
sion from Ref. [
37
]. Copyright 2013, Royal Society of Chemistry
