Thermal barrier assemblies are provided for traction battery packs. An exemplary thermal barrier assembly may be configured to block or even prevent thermal energy associated with a battery thermal event from moving from cell-to-cell, compartment-to-compartment, and/or cell stack-to-cell stack within the traction battery pack. The thermal barrier assembly may include a locator configured to locate the thermal barrier assembly relative to a heat exchanger plate of the traction battery pack. The locator may at least partially fill and seal a slot formed in the heat exchanger plate.