Any moving electric charge will produce an electromagnetic field.
I wonder whether the container which serves as fire/leak shield for Li-ion batteries may be reflecting back into the battery the (strong) electromagnetic fields which are produced by the movement of lithium ions within it. Depending on the 3D geometry of the container, such retro-reflection may become amplified and focused on a local point within the battery. If such is the case, this point may become susceptible to heat damage and maybe ultimately lead to thermal runaway.
Is this shielding container metallic in the first place? If yes, it is designed to preclude retro-reflection of electromagnetic radiation produced internally by the battery?
[Edited 2013-01-18 07:59:30]