Page 206 - Read Online

Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144 Page 17 of 23

[86]
through the in-situ copolymerization of tetracyclooxy-cyclosiloxanes and 1,3-dioxolane (DOL) . PTCD
has been found to exhibit remarkable thermal stability, maintaining a stable state even at elevated
temperatures, unlike poly(1,3-dioxolane) (PDOL) which becomes viscous at 70 °C. This thermal
performance improvement highlights the efficacy of organosilicon introduction in enhancing the thermal
safety of GPEs. Moreover, using PTCD in LMBs substantially enhanced cycle stability, enabling a stable
operation for up to 1,000 cycles. An alternative development in this domain was introduced by Chen et al.
by creating polydimethylsiloxane-based GPEs known as flame-retardant GPEs (DSHP) . DSHP was
[87]
synthesized via the in-situ copolymerization of bis(3-aminopropyl)-terminated polydimethylsiloxane with
t o l u e n e 2 , 4 - d i i s o c y a n a t e , c o m p l e m e n t e d b y t h e a d d i t i o n o f F E C a n d l i t h i u m
bis(trifluoromethanesulfonyl)imide (LiTFSI) as plasticizers. Noteworthy for its flame-retardant properties
with an unprecedented SET value of 0, DSHP is completely noncombustible. Additionally, it possesses an
exceptional self-healing capability, enabling it to spontaneously revert to its original form after being cut at
50 °C. Offering remarkable tensile properties and the ability to stretch up to four times its length, DSHP has
demonstrated favorable characteristics for application in GPEs. Encouragingly, Li||LiCoO (LCO) pouch
2
cells integrated with DSHP exhibited proper functionality even after being subjected to 180-degree bending,
underscoring the significant safety benefits that DSHP confers upon LMBs.

Polymer composite separators
The separator plays a crucial role in thermal runaway, in addition to the electrolyte. Particularly, its thermal
stability, which is the carrier of the electrolyte, is fragile, especially in the case of commonly used polymer
separators (e.g., PP and polyethylene (PE) separators). When exposed to high temperatures, these separators
quickly melt, thereby worsening the thermal runaway process. An emerging and noteworthy development is
the introduction of a new thermally stable separator known as the polymer composite separator. This type
of separator has garnered significant attention because it is formed by applying flame retardants or
inorganic particles onto its surface, providing enhanced thermal stability. Moreover, this specialized
separator can exhibit a state akin to GPEs following the absorption of LE. Yang et al. reported their study
results on a new flame-retardant separator, in which they developed a flame-retardant polymer composite
separator (DCPE) by directly applying flame-retardant coatings of decabromodiphenyl ethane (DBDPE)
[88]
and CaO onto a PE separator . The DCPE exhibits a dual flame-retardant mechanism and a remarkable
SET of only 2.8 s upon penetration by LE. Moreover, their research findings show that the pouch cell
assembled with DCPE displays a significantly prolonged thermal runaway process when subjected to a
baking condition of 180 °C. This suggests the effective capability of DCPE in inhibiting thermal runaway
and attenuating the heat accumulation and explosion risk.

Ceramic particles are widely recognized as an effective method for enhancing the thermal safety of
electrolytes. However, their utilization in practical applications has been restricted because of their limited
solubility in the electrolyte. Coating ceramic particles on the separator surface has been found to
successfully overcome this challenge. Roh et al. proposed a novel ceramic particle-coated polymer
composite separator by encapsulating TEP in a copolymer of MMA and ethylene glycol dimethacrylate and
blending it with aluminum hydroxide . This flame-retardant coating was then uniformly applied onto a PE
[89]
separator to develop a distinct flame-retardant polymer composite separator [flame-retardant ceramic-
coated separator (F-CCS)]. The unique characteristic of F-CCS lies in its ability to exhibit a shorter SET
than regular separators during ignition tests, attributed to the synergistic flame-retardant impact generated
by TEP and ceramic particles. Notably, F-CCS retains 80.9% of its area at a temperature of 140 °C. To
evaluate the safety of flexible pouch cells utilizing F-CCS, charge/discharge tests were carried out at 140 °C.
The outcomes indicate that it demonstrates consistent voltage stability, thereby corroborating the
considerable enhancement in battery safety conferred by F-CCS.