Advanced Separators for High-Safety and High-Energy-Density Rechargeable Batteries

The growing dependence of modern society on batteries places higher demands on safety and energy density. Separators are a key component for electrical insulation and structural stability to ensure battery safety. Their inactive volume, ability to regulate solvated ions and transport efficiency also directly impact the energy density of batteries. This review elucidates the relationship between the thermal stability of linear/cyclic polymers and thermal runaway onset, and discusses how organic/inorganic materials enhance safety and energy density by tuning pore structure, thickness, wettability, and mechanical properties. The differences between monovalent (Li⁺/Na⁺/K⁺) and multivalent (Mg²⁺/Zn²⁺/Ca²⁺) ions are compared, and how the polar separator‒electrolyte interaction regulates ion solvation is revealed. Safety-oriented separator strategies, including thermal runaway mitigation, dendrite suppression, ion flux homogenization, overcharge protection, and aging analysis, are evaluated. Recent advances in separators for high-energy-density batteries, such as ultrathin, lightweight, high-voltage-stability, electrochemically active, integrated separator‒electrolyte, are summarized. The challenges of balancing safety and energy density are highlighted, specifically porosity and mechanical strength, safety responses compromising energy density, lab-to-factory translation gap. Forward-looking perspectives are outlined through advanced processing, intelligent design, and integration, paving the way for high-energy-density intrinsically safe batteries.