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Page 14 of 23 Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144

[78]
final product was produced by in-situ polymerization [Figure 5G] . The resulting fluorine-phosphonate-
based gel electrolyte (FRGE) is reported to be translucent and exhibits zero SET, indicating its high flame
retardancy [Figure 5H]. Notably, introducing phosphates has increased the decomposition temperature of
FRGE compared to conventional liquid ester electrolytes, supporting its high-temperature stability. The
FRGE, attributed to the synergistic effect between the fluorine groups and the polymer backbone, shows
excellent electrochemical window and electrode-interface compatibility. Moreover, it effectively reduces
parasitic reactions of the electrolyte on the electrode interface. The polymer backbone contributes to the
high Young’s modulus of FRGE, which physically inhibits the Li dendrite growth, thus enhancing safety.
Experimental validation demonstrates that the FRGE-assembled pouch cells maintain normal operation
under severe conditions such as bending, pinning, and ignition. These results emphasize the high safety
features resulting from the synergistic interaction among fluorine and phosphate groups and the polymer
backbone. Furthermore, experimental validation shows that the Li||Li||LiFePO (LFP) battery assembled
4
with FRGE maintains a high-capacity retention rate of 94.6% after 700 cycles at 0.5 C [Figure 5I and J]. This
demonstrates the excellent cycling performance of the FRGE in Li||LFP batteries, reflecting its capabilities in
ensuring battery safety and stability.

Nitrile for GPEs
Nitrile solvents, such as succinonitrile (SN), have gained extensive attention and application in flame
retardancy in Les owing to their exceptional thermal stability, high dielectric constant, and broad
electrochemical window . These solvents, when subjected to high temperatures, release non-flammable
[79]
inert gases, such as N , which play a crucial role in suppressing combustion propagation and thereby
2
averting thermal runaway incidents. Nonetheless, butanedinitrile, in particular, presents several challenges
due to its lowest unoccupied molecular orbital (LUMO) and poor compatibility with Li anodes, thus
restricting its applicability in lithium batteries . The issue with butanedinitrile is expected to be addressed
[80]
in GPEs. When the nitrile groups are incorporated into the polymer backbone, it is expected to significantly
alleviate the parasitic reaction between the nitrile groups and the Li anode, thereby enhancing the suitability
of such solvents in LMBs.

Based on this concept, Sun et al. successfully embedded fumaronitrile (FN) into a polymer backbone
constructed from MMA and ethoxylated trimethylolpropane triacrylate (ETPTA) . The free nitrile groups
[81]
were grafted onto the polymer chains by reacting with C=C unsaturated bonds [Figure 6A]. Subsequently, a
lithium difluoro(oxalato)borate (LiDFOB)-containing LE was introduced to regulate the compatibility of
the electrode/electrolyte interface. It was reported that this GPE (FN-based GPE (FGPE)) not only
effectively prevented the parasitic reaction between the nitrile groups and the Li anode, but also realized the
flame-retardant properties of the electrolyte, exhibiting a prolonged period of 12 s of nonflammability in the
ignition test. In addition, FGPE displays a low glass transition temperature, indicating that Li ions can
+
migrate more rapidly. Pouch cells assembled with this electrolyte show excellent safety when subjected to
extreme conditions such as cutting and repeated twisting, even after being cut for 12 h. Moreover,
subsequent gas production experiments were conducted to evaluate its thermal stability. After a week of
storage at 80 °C, the pouch cells showed little volume change and exhibited excellent thermal stability
compared to the results obtained with the LE. These results demonstrate the excellent thermal stability of
FGPE. In addition, the FGPE also exhibits good cycling performance [Figure 6B and C]; Li||LFP batteries
assembled with this electrolyte could cycle stably at 0.5 C for 200 cycles at 60 °C and also had good capacity
retention in the rate performances [Figure 6D].

In addition, the nitrile group can be introduced into the polymer backbone as a cross-linking agent.
Zhang et al. chose vulcanized nitrile butadiene rubber (v-NBR) as a monomer and incorporated ionic liquid
(IL) as a plasticizer and a unique triallyl cyanurate (TAC) as a cross-linking agent to develop the