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Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144 Page 11 of 23

dimethylformamide as a plasticizer. The resulting porous separator demonstrated remarkable stability at
high temperatures, with minimal shrinkage and no significant shape deformation at temperatures reaching
180 °C. In contrast, the commercial polypropylene (PP) separator exhibited pronounced melting under
similar conditions [Figure 4D]. Furthermore, the PA1D1-GPEs displayed a LOI of only 17.0% and a short
SET in ignition tests [Figure 4E], owing to the formation of a dense carbon layer post intense combustion.
This carbon layer effectively insulated flammable gases and heat, preventing the further spread of
combustion. Subsequent studies on pouch cells assembled with PA1D1-GPEs revealed their ability to
operate normally even under bending, cutting, and pinning conditions. In combustion tests, the flexible
pouch cells equipped with PA1D1-GPEs also showed an extremely short SET, highlighting the effectiveness
of the unique phosphate-based backbone in enhancing safety and performance of the gel electrolyte system.

Cyclophosphonitrile-based polymer skeleton
Cyclotriphosphonitrile exhibits excellent flame-retardant properties due to its high phosphorus and
[73]
nitrogen content . Meng et al. innovatively designed a cyclotriphosphonitrile monomer,
butenoxycyclotriphosphazene (BCPN), and introduced it into the polymer backbone for the first time .
[16]
This effectively strengthened the polymer into a gel state, forming a polymer matrix. This innovation led to
the construction of a novel cyclotriphosphonitrile-based gel electrolyte (NGPE), utilizing a LE containing
[16]
fluoromethyl 1,1,1,3,3,3-hexafluoroisopropyl ether as a cosolvent . The NGPE demonstrated exceptional
working performance under extreme conditions, exhibiting long-lasting durability and superior safety.
Notably, in ignition tests, the NGPE displayed complete nonflammability. This was attributed to the ability
of the PO· radicals generated by BCPN to spontaneously capture the hydrogen and oxygen radicals released
from the thermal decomposition of the electrolyte, thus preventing the exothermic chain reaction. In
addition, the generated inert gas N further inhibits the combustion process [Figure 4F]. Furthermore,
2
owing to the physical properties of the NGPE, it remained leak-free during the leakage test. The high
oxidative stability of NGPE effectively prevented the generation of flammable gases due to the continuous
decomposition of the electrolyte, consequently averting an increase in the internal temperature of the
battery. The Li|NGPE|NCM811 pouch cells exhibited no significant temperature changes during the
pinning experiment [Figure 4G]. Under overcharge abuse conditions, the Li|NGPE|NCM811 pouch cells
showed no significant temperature increase and current change during the test [Figure 4H]. Moreover, in
the accelerating rate calorimetry (ARC) test, the Li||NGPE||NCM811 pouch cells demonstrated lower T
1
and T and lower T than batteries using LEs. In addition, NGPE has achieved outstanding progress in
2
3
cycling performance while maintaining a high level of safety. As reported, the assembled Li||NGPE||
NCM811 coin cell demonstrated satisfactory performance following a 0.5 C cycle test. Notably, the battery
exhibited a capacity retention of over 75% after 500 cycles, indicating its long-term stability. This
exceptional performance can be attributed to the effective interaction of NGPE with the lithium metal and
NCM811 electrodes, forming a highly stable passivation layer. Consequently, the battery demonstrates
remarkably stable and durable cycling performance, thereby ensuring reliability in practical applications.
Through enhancing safety and significantly improving cycle life, the advancements of NGPE have
contributed positively to developing cyclic phosphonitrile-based GPEs.

Fluoride for GPEs
The growing interest in the flame retardancy of fluorinated solvents stems from the exceptional properties
of fluorine atoms, which possess excellent oxidation resistance and are inherently resistant to oxidation [74,75] .
When subjected to high temperatures, fluorinated solvents undergo pyrolysis reactions that produce
fluorine radicals. These radicals can interact with the hydrogen radicals within the combustion chain
reaction, effectively blocking the combustion chain reaction, as given in