Presented are thermal management systems with passive quenching sacks for cooling battery assemblies, methods for making/using such systems, and vehicles equipped with such systems. A passive thermal management (PTM) system is presented for cooling a battery assembly, such as a traction battery pack with a battery case containing stacked battery cells. The PTM system includes a fluid container that mounts inside the battery assembly, interposed between the battery case and battery cells. The fluid container stows therein a dielectric coolant fluid and has multiple fluid ports that fluidly connect to the battery cells to dispense thereto the coolant fluid. Thermomechanical plugs, such as wax, film, or smart-material barriers, seal the fluid container ports and passively open (e.g., melt, bend, disintegrate, expand, etc.) at a predefined temperature to thereby unseal the fluid ports such that the coolant fluid is fed from the fluid container into the battery cells.