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Page 2 of 23 Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144

INTRODUCTION
The rapid surge in interest in lithium-ion batteries (LIBs) has been driven by the increasing demand for
[1-4]
energy storage devices, particularly in the context of electric vehicles and other emerging fields .
Nevertheless, due to the limited energy density of LIBs, they can no longer meet the escalating demand. In
light of this, lithium (Li) metal has emerged as a promising alternative owing to its high energy density
-1
(3,860 mAh g ) and low overpotential (-3.04 V vs. the standard hydrogen electrode). Consequently, lithium
[5-9]
metal appears to be a suitable candidate for replacing the graphite anode in LIBs , and lithium metal
batteries (LMBs) are being hailed as the candidate for next-generation batteries [10-15] . However, the
development of LMBs is impeded by a series of safety concerns [16-19] , particularly under extreme conditions
such as thermal abuse [20-22] (e.g., high-temperature exposure), mechanical abuse [23-25] (e.g., nail penetration),
and electrical abuse [26-29] (e.g., short-circuit, overcharge and overdischarge). These conditions can potentially
elevate the internal temperature of the battery, leading to a chain of exothermic reactions and gas
generation, ultimately resulting in uncontrollable overheat - a phenomenon known as thermal runaway of
the battery [30-32] . Hence, while LMBs offer promising potential, addressing the safety concerns is crucial to
ensure their successful development and commercialization.

The process of solving the thermal runaway problem in LMBs begins with a thorough consideration of the
specific steps involved in thermal runaway. When thermal runaway occurs in a battery, it initiates with the
decomposition of the solid electrolyte interphase (SEI), subsequently leading to electrolyte parasitic
reactions at the anode and cathode, and culminates with the melting of the diaphragm, resulting in a short-
circuit due to direct contact between the cathode and anode [33-35] . This subsequent rupture of the cell and
leakage of electrolyte, exacerbated by the heat buildup, ultimately presents a flammable and explosive risk
due to the leaked organic molecules [36,37] . Consequently, a comprehensive understanding of the thermal
runaway mechanism highlights the critical role of the electrolyte within the entire process. Therefore,
addressing the issue of LMB thermal runaway hinges on the imperative step of enhancing the safety of the
electrolyte. Improving and optimizing the electrolyte can effectively impede the thermal runaway by
preventing SEI decomposition, inhibiting parasitic reactions, and averting diaphragm melting. By
employing innovative technologies to elevate electrolyte safety, the risk of thermal runaway in LMBs under
extreme conditions can be reduced, thereby providing a more dependable foundation for large-scale
applications.

Currently, the high mobility of conventional liquid electrolytes (LEs) poses a risk of leakage, despite the
addition of flame retardants such as phosphoric groups, fluorine, and cyclophosphonitrile, which has
improved the thermal safety [17,38,39] . Solid-state electrolytes (SSEs) have attracted attention due to their
excellent electrode compatibility and superior mechanical properties, yet they suffer from relatively low
ionic conductivity at room temperature [40,41] . In response to this challenge, gel polymer electrolytes (GPEs)
have emerged as a promising solution, as they form by adding LE to the polymer matrix. This approach not
only addresses the leakage-prone problem but also maintains high ionic conductivity, positioning them as
[42]
the next-generation electrolyte to drive the commercialization of LMBs . However, the flammability of LE
poses a potential safety hazard for GPEs [43,44] . To enhance the safety of GPEs, two approaches are currently
being pursued. Firstly, nonflammability is achieved by adding flame-retardant additives to reduce the fire
[45]
risk . Secondly, flame-retardant functional groups are grafted onto the polymer backbone of GPEs through
molecular design to enhance the overall safety and pave the way for commercializing high-performance
batteries .
[18]

As shown in Figure 1, this review focuses on the essential role of understanding the thermal runaway
mechanism in ensuring the safety of LMBs. Thus, a comprehensive discussion of this mechanism is