Rechargeable lithium-ion batteries (LIBs) have emerged as a key technology to meet the demand for electric vehicles, energy storage systems, and portable electronics. In LIBs, a permeable porous membrane (separator) is an essential component located between positive and negative electrodes to prevent physical contact between the two electrodes and transfer lithium ions. Among several types, microporous polyolefin membranes have dominated the commercial separator market for LIBs operated with liquid electrolytes, favored for their chemical and electrochemical stability, high mechanical strength, uniform pore size, and inexpensive manufacturing and materials cost. In this review, we summarize the principles and theoretical background underlying conventional manufacturing processes and newly emerging microporous polyolefin separators. Based on their mechanical and physical properties, as collected from the literature, we introduce a number of processing type-dependent characteristics and universal correlations among their properties. This will provide a macroscopic view on the subject and a guideline for the development of next-generation separators.

In this study, a simple method for quantifying the porous nature based on the permeability of the thin ceramic coating on microporous polyolefin flims used as separators in lithium-ion batteries is demonstrated. The air permeability of the ceramic coating was determined via the ideal laminate theory (ILT), which is widely accepted for multilayered films or well-defined laminates. This study mainly aims at introducing an ILT-based resistors-in-series model with the Gurley method and its potential applications in battery separators. We investigated a few previously reported examples of ceramic-coated separators to compare the air permeability of various ceramic coatings along with their base polyolefin films.

Solid polymer electrolytes (SPEs) have attracted significant attention as promising electrolyte systems for all-solid-state batteries due to their excellent interfacial contact and flexible processability. However, their practical application is still limited by low ionic conductivity and insufficient mechanical stability. To address these limitations, the incorporation of inorganic nanofillers into polymer electrolytes has been widely investigated. Nevertheless, conventional filler materials often require complicated synthesis processes and involve high production costs, limiting their large-scale application. As a sustainable alternative, natural clay minerals have recently emerged as attractive nanofillers for SPEs because of their low cost, eco-friendliness, abundant surface active sites, and diverse structural characteristics. In this review, clay mineral-based nanofillers are systematically classified according to their structural morphology into 1D, 2D, and 3D structures. The effects of each morphology on ionic conductivity, lithium-ion transference number, interfacial resistance, electrochemical stability, and mechanical properties are comprehensively discussed. In addition, this review highlights that electrolyte performance is strongly influenced not only by filler morphology but also by interfacial engineering, including surface modification, filler dispersion, and interactions with polymer matrices. Based on these insights, major challenges and future strategies are discussed, such as hybrid structural design, interface control, and filler content optimization to overcome the limitations of morphology-based approaches. Overall, this review provides deeper insight into the structure–performance relationship of clay mineral-based SPEs and offers practical design guidelines for next-generation all-solid-state battery electrolytes.