A shutdown separator is designed to stop ion transport between the anode and cathode once the cell reaches a defined temperature, acting as a passive, intrinsic protection layer before a cell reaches thermal runaway. This is typically achieved with a multi-layer separator where one layer (often polyethylene) melts and closes its pores at a lower temperature, shutting down current flow, while a higher-melting-point layer (often polypropylene) maintains physical separation between the electrodes.
Shutdown function has to be reliable and distinct from simple thermal failure of the separator: uncontrolled or inhomogeneous shrinkage of a separator at high temperature is itself a trigger for internal short circuit rather than a protective response, so separator behaviour under heat is both a safety feature and a potential failure mode depending on how it is engineered.
Related: Current Interrupt Device, Positive Temperature Coefficient, Separator