Required properties of separators for lithium-ion batteries: A systematic review

• Systematic review of Li-ion battery separator properties (2015–2025). • Critical comparison of composite, coated, and nanofibrous separators. • Evaluates EHPT and advanced fabrication strategies for performance enhancement. • Analyzes trade-offs among ionic conductivity, mechanical strength, and thermal stability. • Assesses surface modification approaches including alumina, AlOOH, and LAGP coatings. • Identifies scalability challenges and industrial translation barriers. • Proposes future directions including multifunctional and AI-guided separator design. In response to growing environmental issues and the shift in the global movement to renewable energy sources and electromobility, lithium-ion (Li-ion) batteries are a key source of energy in a broad spectrum of applications. Nevertheless, commercially available Li-ion batteries have continued to be plagued by a number of issues such as capacity degradation, poor thermal and ionic conductivity, low energy transfer efficiency, and safety concerns. Among the various components that influence battery performance, the properties of the separators are key among the other elements that determine the level of performance of the battery. This systematic literature review (SLR) investigates the main properties required for effective separators in Li-ion batteries. The results indicate that composite-material-based separators are superior in performance due to their excellent mechanical, thermal, and electrochemical properties. Moreover, surface treatments, including alumina layers on polyethylene (PE) separators, AlOOH coatings, electron beam physical vapor deposition (EB-PVD), and advanced fabrication techniques like electrospinning–hot pressing technique (EHPT) and EHPT combined with Styrene–isoprene–styrene and polyacrylonitrile (SIS/PAN), show great potential for improving battery performance and safety. This review contributes to the development of next-generation separators for highly efficient Li-ion batteries by analysing the current best practices and outlining key areas for future research.