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Yang et al. Energy Mater 2024;4:400061 https://dx.doi.org/10.20517/energymater.2023.144 Page 5 of 23

Limiting oxygen index
The limiting oxygen index (LOI) is a significant metric for assessing the flame retardancy of an electrolyte.
[54]
It indicates the minimal concentration needed for a sample to burn steadily in an O /N gas mixture .
2
2
A sample of a certain size is fixed vertically in a combustion cylinder; a mixed gas flow is introduced, the top
of the sample is ignited, and the minimum O concentration required for continuous combustion of the
2
sample is recorded (expressed as the value of the volume percentage occupied by O ). A high oxygen index
2
indicates a good flame retardancy of the material. It is generally recognized that when the LOI of the sample
is < 21%, the material is flammable; 21% ≤ LOI ≤ 28%, the material is combustible; and LOI > 28%, the
[55]
material is nonflammable . The LOI of a polymer correlates with its char formation rate during
combustion, specific enthalpy of combustion and elemental composition. Polymers with lower oxygen and
higher halogen content generally exhibit an increased LOI, indicating improved flame retardancy.

Total heat release and heat release rate
The micro-combustion calorimeter (MCC) is highly effective in assessing the flame retardancy of materials
by measuring their total heat release (THR) and heat release rate (HRR) . This method not only proves to
[56]
be an excellent choice for testing the flame retardancy of materials but also serves as a valuable tool for
evaluating the potential fire hazards associated with these materials.

According to THR and HRR, we can quantitatively determine the fire hazard of the material. Higher values
indicate a greater fire hazard, while lower values indicate a decreased hazard.

Other methods
Currently, the most prevalent technique for evaluating the flammability of materials involves direct ignition
in air, wherein the material is brought into direct contact with a source of ignition to ascertain whether it
ignites [57,58] . If the sample does not ignite, it is categorized as nonflammable; conversely, if it catches fire, it is
deemed flammable. This method is widely employed to assess the flammability of various materials and is
pivotal in determining their fire safety characteristics.

Characterization of thermal stability
Thermal stability, a critical property affecting the safety of electrolytes, is determined by various factors such
as the elements present, molecular bonding energies, structural stability, and intermolecular
interactions [59,60] . To gain a comprehensive insight into the thermal behavior of electrolytes across different
temperature ranges, techniques such as thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) are employed. TGA enables the identification of material losses, intermediates, and
decomposition processes, while DSC provides valuable information on thermal decomposition,
crystallization temperatures (T ), and glass transition temperatures (T ). Additionally, DSC assesses the
c
g
reactivity level through heat uptake and exothermic effects. By analyzing the DSC curve, as illustrated in
Figure 3A, researchers can evaluate the thermal behavior of the electrolyte based on exothermic and
endothermic peaks. The intensity of these peaks serves as an indicator of the extent of the thermal behavior
[61]
exhibited by the electrolyte .
In addition, battery thermal stability is measured by the exothermic rate and cumulative heat release at
different temperatures . Adiabatic accelerated calorimetry (ARC) determines the critical temperature
[62]
[32]
associated with the thermal runaway of the battery [Figure 3B] . Parameters include the self-exothermic