Page 201 - Read Online

Page 12 of 23 Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144

Fluorinated additives
Currently, a viable strategy for achieving electrolyte nonflammability is to increase the fluorine content in
electrolyte solvents. To improve the thermal stability of GPEs, it is feasible to introduce fluorinated solvents
into the polymer matrix. Wu et al. have proposed an innovative approach to design a novel fluorinated
[76]
electrolyte embedded in an organic polymer backbone . They prepared this fluorinated electrolyte by
dissolving 1 M LiPF in a mixture of fluorinated solvents, such as fluorinated EC (FEC), 2,2,2-trifluoroethyl
6
carbonate and 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether [Figure 5A]. Subsequently, they
formed fluorinated GPEs (HGPE) through in situ polymerization with the addition of 3 wt% of diethylallyl
phosphate (DAP) and pentaerythritol tetraacrylate (PETEA). In linear scanning voltammetry (LSV) testing,
the oxidation current of this GPE was as low as 5.8 V [Figure 5B]. This is attributed to the high oxidative
stability of the fluorinated solvent, significantly enhancing the stability of this GPE under galvanic corrosion
conditions. During the ignition test, HGPE exhibited zero SET, indicating excellent nonflammability due to
the capture of hydrogen radicals by fluorine radicals, thereby reducing the risk of combustion. Furthermore,
Li||Li RuO (LRO) pouch cells assembled with HGPE showed no voltage and shape changes after aging at
2
3
130 °C for 2,500 s, in contrast to the voltage changes and swelling of the cells assembled with LE starting in
1,964 s [Figure 5C]. Additionally, HGPE exhibited high ionic conductivity (1.99 mS cm ) at room
-1
temperature due to fluorinated solvents, confirming that it combines both high safety and high ionic
conductivity.

Fluorinated polymer backbone
Introducing flame-retardant components into the polymer backbone can improve the thermal stability of
GPEs to some extent. However, the thermal safety of GPEs still needs to be improved due to the use of
conventional organic electrolytes. Hu et al. recently developed an innovative new polymer matrix using the
fluorinated monomer hexafluorobutyl acrylate (HFBA) for in situ radical polymerization with a PETEA
[77]
cross-linker . This approach transferred the conventional liquid-phase addition of fluorinated solvents to
the polymer backbone. It does not require the addition of flame retardants, and possible flammability
caused by liquid organic solvents is eliminated by the gas-phase radical scavenging action of HFBA.
Consequently, the resulting GPEs do not necessitate additional flame retardants. This remarkable
nonflammability phenomenon can be attributed to the fact that at high temperatures, the F· radicals
spontaneously formed by the fluorinated monomer HFBA effectively trap the H· radicals generated in the
LE [Figure 5D]. This trapping effect blocked the chain reaction of combustion, thus terminating the further
propagation of H· radicals. In the ignition test, the GPE exhibited significant nonflammability. Interestingly,
it was also found that the combusted GPE formed a very dense char layer due to the presence of PETEA-
HFBA chains. This highly graphitized char layer effectively isolated the oxygen and heat required for
combustion. This GPE utilizing a synergistic flame-retardant mechanism between the gas and condensed
phases greatly improves the thermal safety of the battery. In the next study, pouch cells with GPEs were
evaluated for safety and tested for various abuse conditions. As shown in Figure 5E, when the battery was
overcharged to 4.5 V, the LE pouch cell experienced significant thermal runaway, while the GPE pouch cell
exhibited only a slight temperature increase. Additionally, the pouch cell with the GPE exhibited minimal
temperature change after pinning compared to LEs [Figure 5F]. This clearly highlights the superior safety
offered by this GPE. In subsequent heating and open flame exposure tests, the pouch cell using the GPE
showed good flame resistance. This series of safety evaluation results further confirms the excellent safety
performance of batteries utilizing this innovative GPE under a wide range of abuse conditions and provides
a strong foundation for its reliability in real-world applications.