## 1 Introduction
High voltage lithium-ion batteries (LIB) have attracted enormous amount of attention as one of the most promising energy storage system due to their high specific energy and long cycle lifetimes [18; 19]. The studies have been focused on a major current issue--producing higher power/energy densities while maintaining the safe operation of LIBs [20; 21]. The separator has intimate influences on battery operation, battery performance, and is finally the most prominent in the safety and dependability of LIBs [22; 23; 24]. The separator prevents physical contact between the positive and negative electrodes while serving as the electrolyte reservoir to facilitate ionic transport during the charging and discharging cycles of the battery. Therefore, the separator not only determines the safety but also the performance of LIBs influencing the cell kinetics [25; 26; 27; 28; 29]. With regard to the ion transporting capability, a separator should be highly porous to enable the adsorption of liquid electrolytes, thus supporting the high electrochemical performance of the battery. The porosity of separator is an important parameter due to its influence on the overall impedance and, in turn, the electrochemical performance of the battery cell [29]. A separator with higher porosity provides several potential advantages such as improvement of electrolyte wettability and thermal stability, higher energy density levels, faster ion transport, and longer cycling lifetimes [23; 28; 30; 31]. However, separators with high porosity always have adverse effects including poor mechanical integrity and low structural uniformity while also introducing a high probability of puncture formation during the cell assembly. These issues lead to unreliable operation of LIBs. Therefore, the development of a new highly porous multilayer separator with superior mechanical and structural integrity is a major challenge. The PVDF-HFP membrane is one of the most 5 choices for LIB separator material because of its high dielectric constant, superior affinity to liquid electrolyte solutions, good electrochemical stability, and desirable adhesion with electrodes [25]. In this work, a novel method has been designed to prepare a highly porous PVDF-HFP membrane with a multilayered structure via the combination of evaporation-induced phase separation (EIPS) and selective solvent etching. Membranes with structures consisting of one, two, three, and four layers were synthesized. The multilayered membrane was controlled to be less than the thickness of the commercial Celgard separator. The characteristic properties of the membranes, such as the structural morphology, physical properties, and electrochemical performance capabilities, were characterized. The obtained parameters were compared to those of a reference separator, the commercial Celgard separator. The developed membrane demonstrated superior electrolyte wettability, high porosity, good mechanical properties, and thermal stability. The multilayered membrane yields excellent electrochemical performance due to its significant electrolyte uptake. The obtained results provide an efficient and reliable approach for the construction of high-power and high-energy storage devices.