HYDRA thermal runaway shield acts as a heat sink during normal lithium-ion battery pack operation but also prevents thermal runaway propagation, which is a serious concern for aerospace and defense customers and electric vehicle manufacturers. The HYDRA is a vaporizing thermal capacitor that provides passive prevention of thermal runaway propagation (TRP) in lithium-ion battery packs. Thermal runaway can occur spontaneously in a Li-ion cell due to a short. This can trigger an explosive release of electric energy that ruptures the end cap resulting in a flare and combustion of cell materials. Released heat drives the triggered cell temperatures to > 500°C, causing a dramatic increase in neighboring cell temperatures. Temperatures above the critical 130°C greatly increases the chance for a short in adjacent cells and result in TRP. HYDRA keeps neighboring cell temperatures from rising above 100°C (well below the 130°C threshold) and prevents TRP.