Separator
A battery separator is a thin, electronically insulating layer placed between the positive and negative electrodes of a cell. It prevents direct physical contact and an internal short circuit while allowing ions to move through electrolyte-filled pores. DOE Energy Storage Handbook: Lithium-Ion Batteries
What it does
In common liquid-electrolyte lithium-ion cells, separators are often microporous polymer membranes. They do not carry the cell's external electron current and are not an active material; their pores provide the ionic path needed for charge and discharge. Pore structure, electrolyte wetting, chemical stability, mechanical strength, and thermal behavior all affect resistance and reliability.
Some multilayer polymer separators are designed to close their pores as temperature rises, reducing ion transport. This shutdown response is one protective feature, not proof that a cell is inherently safe. Further heating can cause shrinkage or breakdown, while contamination, mechanical damage, and other faults can still defeat physical isolation. ACS Omega: Ceramic-coated separators and thermal shutdown behavior
A solid electrolyte can combine the separating and ion-conducting roles without a conventional porous membrane. For the deeper component interactions, see Cell Chemistry & Components.