Battery swelling due to internal gas formation in cells can result in severe degradation of battery performance. Within a battery cell, evolved gas reduces the interfacial area between active material and electrolyte, and thus hinders the electrochemical reactions at the solid electrolyte interphase SEI. Also, the pressure buildup due to gas generation results in mechanical stress inside the electrodes, which has been implicated in local fractures in the electrodes. Cell swelling occurs due to gas generation resulting from the decomposition of the electrolyte and from reactions with impurities in Li-ion batteries. 1-3