## 2 Materials and Methods
### TR Propagation Barrier Method with Only Insulation Layer Added  
The experiment was carried out in the 4-cell battery module. Three types of heat insulation materials, a, b, and c, with the same size and 3 mm thickness were set between the adjacent cells and secured with a fixture plate. The interface between the cell and the heat insulation material was still arranged with No. 1-8 K-type thermocouples to characterize the temperature distribution on the surface of the cell, as shown in Figure 1e.  
TR propagation between batteries is mainly realized through wall heat conduction. In order to slow down or even cut off such propagation, a scheme involving the addition of a thermal insulation layer between the adjacent cells was proposed. To this end, a variety  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mode** & **Description** & \(\delta\)**(mm)** & \(\lambda\)**(W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\))** & **h** (W\(\cdot\)m\({}^{-2}\cdot\)K\({}^{-1}\))** & \(\epsilon\) \\ \hline \multirow{4}{*}{Heat conduction} & Heater—Battery thermal resistance layer & 0.01 & 0.023 & / & / \\  & Shell—Core thermal resistance layer & 0.01 & 0.023 & / & / \\  & Outer shell—Outer shell thermal & 0.10 & 0.010 & / & / \\  & resistance layer & 0.01 & 0.023 & / & / \\  & Shell—Polar bear thermal barrier & & / & 20 & / \\  & layer & / & / & 0.04 \\ Heat convection & Surface radiant heat transfer & / & / & 0.04 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of the boundary conditions.  
of thermal insulation materials, including a glass aerogel plate and a nano-ceramic fiber board, were considered, and the TR propagation test was carried out to compare the barrier effects. However, the heat insulation measures may lead to the accumulation of heat in the battery module, which is contrary to the demand for heat dissipation in the normal operation of the battery system and thus affects the thermal management of the battery module under conventional working conditions.  
Therefore, a self-developed composite phase change material was selected; it was prepared using nano-ceramic fiber substrate material and phase change endothermic material. The substrate consists of silica nanofiber mats and supporting materials. The phase change materials include one or more silica sol, aluminum sol, and saturated calcium chloride solutions. The flame retardant materials include phosphorus-based flame retardants, such as SCCS, triethyl phosphate, and triphenyl phosphate. Our phase change material contains both phase change materials and insulating substrates, offering a high phase change enthalpy value (1800-2200 J g\({}^{-1}\)). During TR, it effectively absorbs energy through both the thermal insulation properties of the material and the phase transition of the phase change materials, playing a significant role in blocking heat propagation. Nano-ceramic fiber has excellent thermal insulation performance and can withstand high-temperature environments; thus, it is highly suitable for the protection of battery TR. The composite phase change layer used in the TR propagation barrier test carried out in this section absorbs calcium chloride solution, and it was tested and compared with the nano-ceramic fiber and glass fiber aerogel insulation layer; the same 2 mm thickness was used to carry out the TR propagation test and to analyze their TR barrier effects.  
In addition, the main raw materials of the phase change material are nanosilica and silica sol. Studies have shown that nanosilica currently poses no threat to aquatic organisms in surface waters [26]. Silica sol is a purely mineral-based product with adhesive and neutralizing properties for certain pollutants. This makes it suitable for soil stabilization and the neutralization of heavy metal ions and other contaminants in soil [27]. Overall, our phase change material demonstrates excellent environmental friendliness and sustainability.