## 3 Results and Discussion
### Physical Properties of the Multilayered Porous PVDF-HFP Film  
The developed multilayered porous PVDF-HFP membranes were synthesized and characterized. The obtained results are listed in Table 1. The parameters of commercial Celgard Separator are also given for comparison. As can be seen, the thickness of the developed membranes was controlled to be less than 25 \(\upmu\)m. The standard thickness of the  
Figure 3: Cross-sectional morphologies of the layer-structured PVDF-HFP films (**a**,**c**,**e**) before and (**b**,**d**,**f**) after removing PS. (**a**,**b**) The two-layer film, (**c**,**d**) three-layer film, and (**e**,**f**) four-layer film. commercial polyolefin separators such as the Celgard separator have a standard thickness estimated in the range of 20-30 \(\upmu\)m. It is known that thick separators have high internal resistance, which decreases the cell performance [23; 24; 26]. Therefore, a minimal separator thickness is crucial for high-energy and high-power LIBs. The high uptake amount of the liquid electrolyte plays a key role in improving the ionic conductivity and charge/discharge capability while high electrolyte retention is required for a long cycling lifetime of the LIB cell. Both the electrolyte uptake and the electrolyte retention amount of the layered membranes are much higher than those of the commercial Celgard separator (Table 1). The superior electrolyte uptake and electrolyte retention of the developed layered membrane are attributed to the combination of excellent electrolyte wettability, high porosity, and the unique layered structure of the membrane. Hence, by minimizing the thickness and improving the electrolyte absorption, the developed membrane can demonstrate rapid ionic transport, high ionic conductivity, and promising electrochemical performance when applied to LIBs. High mechanical robustness of the separator is required when assembling the battery cell and to ensure safe battery operation. A tensile tester was used to characterize the mechanical properties of the developed membranes and the Celgard separator. The stress-strain outcomes are shown in Figure 4. Clearly, the tensile strength of the Celgard separator depends on the orientation, as it is produced via a uniaxial stretching process. The Celgard separator has a tensile strength value of 13.4 MPa along the transversal direction. The tensile strength of the layered membranes is close to that of the commercial separator along the transversal direction. It is important to note that high porosity can adversely affect the mechanical strength of these types of membranes; however, the membrane developed here with the layered structure not only has high porosity but also provides exceptional mechanical properties for normal lithium-ion battery applications. Importantly, the layer configuration is believed to enhance flexibility [24; 26]. It should also be noted that the mechanical strength of the multilayer membrane is much higher than those of previously reported single-layer separators using the same PVDF-HFP material [43; 44; 45; 46]. The inset in Figure 4 portrays images of the four-layer membrane and the commercial Celgard separator before and after a heat treatment at 150 \({}^{\circ}\)C for 30 min. As indicated, the Celgard separator did not withstand the elevated temperature well given that melted and formed a fiber-like structure. The high thermal shrinkage of the Celgard material at a high temperature causes internal short circuits between the electrodes. In contrast, the developed four-layer membrane demonstrated relatively good heat resistance with thermal shrinkage of less than 20% under identical conditions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Thickness (\(\upmu\)m)** & **Electrolyte Uptake** & **Electrolyte Retention** \\ \hline One layer & 4.3 & 1.96 & 0.66 \\ Two layers & 13.5 & 2.28 & 0.70 \\ Three layers & 15.2 & 1.57 & 0.61 \\ Four layers & 23.9 & 1.48 & 0.60 \\ Celgard & 27.0 & 0.81 & 0.45 \\ \hline \hline \end{tabular}
\end{table}