## 3 Results and Discussion
### Interfacial Properties of Porous PVDF-HFP Films  
Figure 1a shows an optical photograph of the PVDF-HFP membrane fabricated on a PSS/copper substrate using the EIPS method with a simple dip-coating technique. The membrane looks opaque because the light scatters off the microporous structure. The membrane shows a uniform structure over the coating area. XRD patterns of the smooth and porous PVDF-HFP films are given in Figure 1b. As indicated by the diffraction patterns of the smooth film, diffraction peaks observed at 2\(\theta\) = 18.2, 20.9, and 24.3 correspond to the (100), (020) and (110) crystal planes of vinylidene fluoride (VDF) segments [6]. For the porous films, the relative intensity of the peaks is remarkably decreased compared to those of the smooth film. The decrease has appeared with an increase in the number of the PVDF-HFP layers. This result suggests that the formation of the porous structure induces structural reorganization of the PVDF-HFP polymer, leading to a significant decrease in its crystallinity and as consequence, an increase of the amorphous domain in the membranes. This characteristic feature facilitates higher liquid electrolyte absorption and swelling. These findings are supported by the electrolyte uptake capabilities. Wettability is a critical property of the separator due to its influence on electrolyte retention and, thus on the performance and cycle lifetime outcomes of the LIBs. Figure 1c illustrates  
Figure 1: Interfacial properties of a 1-layer porous PVDF-HFP film prepared via EIPS method using a dip-coating technique: (**a**) optical image of a PVDF-HFP film on a copper substrate, (**b**) XRD patterns of flat and porous PVDF-HFP films, (**c**) optical image of a liquid electrolyte dropped onto a commercial Celgard separator and an as-prepared porous PVDF-HFP film, (**d**) SEM plain image of the top surface of PVDF-HFP film facing air, (**e**) SEM plain image of the bottom surface of PVDF-HFP film facing the PSS/copper substrate, and (**f**) cross-sectional SEM image of the PVDF-HFP film. the optical images of the drop shapes of a liquid electrolyte (3 \(\upmu\)L) on the commercial Celgard separator and the as-prepared porous PVDF-HFP film. As can be seen, the electrolyte drop remains upright on the Celgard sample even after 120 s, whereas it quickly penetrates within 5 s into the porous PVDF-HFP membrane (Video 1 in Supporting Information). The apparent contact angles of the Celgard sample with water and with a liquid electrolyte (EC/DEC = 1/4 \(v/v\) containing 1.0 M LiPF\({}_{6}\)) after 5 s are 106\({}^{\circ}\) and 68\({}^{\circ}\), respectively, while the corresponding values for the porous PVDF-HFP are 135\({}^{\circ}\) and zero, respectively. In addition, the contact angle after 120 s was estimated to be 50\({}^{\circ}\). These results indicate that the porous PVDF-HFP film has superior liquid electrolyte wettability while exhibiting high water-repellent capabilities. Thereby, the as-prepared porous PVDF-HFP membrane provides faster electrolyte absorption together with higher electrolyte retention but better moisture tolerance compared to the commercial Celgard sample, which is highly beneficial for developing advanced batteries. In addition, the fast absorption of the liquid electrolyte also facilitates the process of electrolyte filling, leading to easy assembly of the battery. The SEM plain images of the surface of PVDF-HFP membrane facing air and of the bottom surface of PVDF-HFP film facing the PSS/copper substrate are given in Figure 1d,e. The surface morphology of the top and bottom sides of the PVDF-HFP membrane is very different. A uniform and highly porous honeycomb-like structure is observed for the top of the PVDF-HFP membrane. The pore diameter and the interval were found to be 4-6 \(\upmu\)m and \(\sim\)1.5 \(\upmu\)m, respectively. In addition, many smaller pores are visible in subsurface range inside the micropores which were formed due to the interconnected structures of the multilayer pore array.