How does EMF cause lithium batteries to experience thermal runaway and thus spontaneously combust? One of two main ways:
#1 - Overcharge/Discharge
In this dissertation, they found that external magnetic fields significantly increased charge and discharge rates in individual lithium battery cells; like shown in the following image. They also found that smaller batteries were impacted to a greater degree than larger ones. This is likely a reason Tesla is switching to larger battery cells in addition to employing magnetic attenuating honeycomb sandwich battery pack.