An electrical energy storage device for powering portable devices such as vehicles or consumer electronics includes barriers to minimize migration of thermal energy and propagation of combustion in the rare event that electrical energy storage cells fail, burst and ignite. Thermal energy absorbing materials are contained within the electrical energy storage device. Sacrificial members are provided within the thermal energy absorbing materials. In-situ channels are formed within the thermal energy absorbing materials when the sacrificial members thermally decompose.