TECHNOLOGY FOCUS - ELECTRONICS - Preferred Device: In the battery assembly, the battery has an 18650 form-factor. The intumescent material is selected from the group of intumescent materials consisting of graphite-based intumescent materials, thermoplastic elastomers, ceramic-based intumescent materials, vermiculite/mineral fiber based intumescent materials, and ammonium polyphosphate based intumescent materials. The intumescent material has a start expansion temperature in the range of 1000 to 300 degrees C. The intumescent material has a start expansion temperature in the range of 200 to 300 degrees C. The intumescent material is biologically inert. It further comprises a sleeve (301), where the sleeve (301) surrounds the intumescent material, and where the sleeve (301) is spaced apart from the intumescent material. The sleeve (301) is fabricated from a material with a yield strength of at least 75 MPa. The sleeve (301) is fabricated from a material with a yield strength of at least 150 MPa. The sleeve (301) is fabricated from a material with a yield strength of at least 250 MPa. It further comprises at least one spacer ring, where the at least one spacer ring separates an interior surface of the sleeve (301) from an exterior surface of the intumescent material prior to the thermal runaway event. It further comprises a second layer of the intumescent material, the second layer covering the cell case (101) bottom, where the second layer inhibits the escape of hot, pressurized gas from within the battery through a perforation formed in the cell case (101) bottom during the thermal runaway event. The second layer of the intumescent material is integral with the layer of the intumescent material surrounding the outer surface of the cell case (101). Preferred Method: Preventing the formation of a perforation in an outer surface of a battery case (101) further comprises the step of surrounding the layer of the intumescent material with a sleeve (301), the surrounding step further comprises the step of separating the layer of the intumescent from an interior surface of the sleeve (301) by a preset distance. The covering step further comprises the step of coating the outer surface of the battery case (101) with the intumescent material to form the layer. The covering step further comprises the steps of forming a strip of the intumescent material and wrapping the strip of the intumescent material around the outer surface of the battery case (101). It further comprises the step of bonding the strip of the intumescent material in place. The covering step further comprises the step of covering an exterior bottom surface of the battery case (101) in the layer of the intumescent material. EXAMPLE - No example given.