Testing has verified lithium iron magnesium phosphate technology (LiFeMgPO4) as one of the most inherently safe type of lithium batteries available.
LiFeMgPO4 cells under puncture or short circuit conditions are much less likely to experience thermal runaway than (for example) Lithium metal oxide. The Exponent testing report, which compares selected lithium-ion battery chemistries, is available here Report file in pdf format.
Punctured or short circuited lithium metal oxide cells will cause heating, making the oxygen bonds prone to breaking resulting in the release of oxygen and thermal runaway. This causes a fire with temperatures reaching up to 1000 degrees and self perpetuated by released oxygen from within the metal oxide cathode materials.
With lithium metal oxide cells the bond with oxygen is much weaker than the oxygen bond with phosphorous in a phosphate thus permitting lithium metal oxide thermal runaway at much lower temperatures.