## 3 Results and Discussion  
### Phenomena and Formation Mechanism of the Multilayer Membrane  
Scheme 1 illustrates the entire fabrication procedure used to create the multilayered porous PVDF-HFP film, which consists of the sequential coating of a PVDF-HFP layer and a PS layer onto the PSS-coated substrate. In the initial step, the PSS solution was coated onto the substrate. Given that the detachment process is crucial to producing a free-standing film from a solid substrate, the PSS coating was used as a water-soluble sacrificial layer to facilitate the transfer process and to enable damage-free detachment. It was shown that polymer membranes with a through-pore structure can be formed on the hydrophilic surfaces, including water, ice, hydrophilic glass, and PSS coatings, whereas a closed one-end structure forms on a solid substrate [38; 39; 40; 41]. Thereby, exploiting the PSS sacrificial layer not only facilitates detachment but also leads to the formation of numerous pores on the backside surface facing the substrate (SI, Figure S3). The higher pore density on the backside surface not only increases the porosity of the membrane but also minimizes the disparity of the porosity across the membrane, leading to a longer cycling lifetime and higher performance of the battery [29]. In the second step, the microporous thin PVDF-HFP film was deposited onto the surface of the PSS-coated substrate by the evaporation-induced phase separation (EIPS) using a simple dip-coating technique. EIPS is the simplest and the most effective method for making microporous separators [42]. Accordingly, a ternary solution containing PVDF-HFP, acetone, and water was dip-coated onto the substrate, followed by a normal air-drying step. Acetone and water were used correspondingly as a solvent and non-solvent pair. Note the water is an environmentally friendly and inexpensive pore inducer. The content of water relative to that of acetone can be as high as 5 vol.%. Because acetone has a higher evaporation rate than that of water, the water content in the solution gradually increases during the drying process. This causes phase separation, thereby giving rise to the porous structure after complete evaporation. Through this approach, the first layer of the PVDF-HFP porous film was generated. Our previous experiences allow us to conclude that other coating methods are associated with several practical and technical problems during the preparation of porous polymer films. For instance, a porous thin film created by the spin-coating method can generate gradient pore sizes from the center to the boundary of the sample, and these films may not be suitable for mass production. With a bar coating, it can be quite difficult to create a thin film with high uniformity and reproducibility. Thus, here, we explored the advantages of the dip-coating method to generate a uniform thin film. Furthermore, this method allows controlling the thickness of the film by regulating the viscosity of the solution or the dipping conditions [32; 33]. In the third step, the PS layer was deposited onto the first layer of the porous PVDF-HFP film by spin-coating a PS solution. In order to produce the next layer of the porous PVDF-HFP, two critical requirements must be fulfilled. First, the pre-prepared PVDF-HFP layer must be stable in the coating solution to become a support for the next PVDF-HFP layer. Second, the surface of the pre-prepared PVDF-HFP layer should be flat because it can affect the structural uniformity of the next PVDF-HFP layer. Therefore, a PS solution in chloroform was used in this work because PS neither dissolves in acetone nor in water but can be quickly dissolved in chloroform. In contrast, PVDF-HFP can be dissolved in acetone but cannot be dissolved in chloroform. Consequently, the porous PVDF-HFP film can be suitably preserved in the PS coating process. The PS coating plays key roles in this process, including preservation of the structure in the acetone solution and the creation of a flat surface for the coating of the next layer of the porous PVDF-HFP.