## 3 Results and Discussion
### Interfacial Properties of Porous PVDF-HFP Films
Contrary, a shallow porous structure with a smaller pore size is observed on the bottom surface of the membrane (Figure 1e). As clearly seen, the distribution of the pores as well as the pore sizes are irregular. [PERSON] et al. reported that the nonporous skin layers usually form on the surface of a membrane facing the substrate [42]. In order to suppress the adverse effects of the solid substrate on the formation of the porous structure, in our study, a water-soluble sacrificial PSS layer was bar-coated onto the substrate. We found that the PSS coating not only allows easy detachment of the thin polymer membrane from the substrate without any damage but also increases the pore density on the bottom surface (SI, Figure S3). The cross-sectional morphology shown in Figure 1f exhibits the multilayer interconnected symmetric porous structure of the pore array. The formation of a multilayer structure can be explained by the higher density of the non-solvent water compared to that of the acetone solution [39]. The thickness of the fabricated PVDF-HFP porous membrane was estimated to be 4.3 \(\upmu\)m while the depth of the surface pore was \(\sim\)2 \(\upmu\)m (Figure 1f), which is very small when compared to those of commercial separators. Hence, its mechanical integrity is not sufficiently strong for its application as a separator in LIBs. The PS/PVDF-HFP composite film was prepared by spin-coating a 5 wt.% PS solution in chloroform onto the porous PVDF-HFP film. The porous PVDF-HFP membrane was then recovered by selectively washing out the PS solution with chloroform. The effect of the washing process on morphology of the porous PVDF-HFP membrane is shown in Figure 2. It is seen that the porous PVDF-HFP membrane was covered with the PS coating and thereby the composite film demonstrated a relatively flat surface. However, the small holes can still be observed on the surface. Note that the existence of these holes is necessary to provide a strong connection between the two porous PVDF-HFP layers (SI, Scheme SI). After immersing the composite film in chloroform for 30 min, the PS layer was removed from the PVDF-HFP surface while the porous structure of the PVDF-HFP layer was well preserved, as visible in Figure 2b. Evidence of the complete wash-out of the PS from the composite film was confirmed by measuring the water contact angle (WCA) and an FTIR spectroscopy analysis of the composite film before and after the washing process. As can be seen in Figure 2\(c\), the WCA of the composite film before removing the PS was 98\({}^{\circ}\). However, the WCA reached 130\({}^{\circ}\) after removing the PS layer. This WCA value is very closed to that of the as-prepared membrane (129\({}^{\circ}\)). The lower WCA of the composite film before the PS removing can be attributed to the higher inherent hydrophilicity of PS and the lower surface roughness compared to those of the PVDF-HFP film. These results indicate that the PS was completely removed from the composite film and that the porous structure of the PVDF-HFP membrane was well preserved after the washing process. The FTIR spectra of three samples are shown in Figure 2d. The characteristic absorption peaks of PS including peaks at 1600, 1500, and 1400 cm\({}^{-1}\), are recorded for the PS/PVDF-HFP composite film. These picks are attributed to the aromatic C-C bonds and the C-H bonding vibrations of the benzene ring. As can be seen in Figure 2d, the washing leads to the disappearance of PS characteristic peaks meaning that the PS layer was completely washed out from the composite film. Thus, the multilayer composite film was fabricated through the in-series deposition of PVDF-HFP and PS layers. A multilayer porous PVDF-HFP membrane was obtained  
Figure 2: The properties of the PS/PVDF-HFP composite film. SEM plain image of the PS/PVDF-HFP composite film.