Safety is the key-feature of large-size lithium-ion batteries and thermal stability of the electrolytes is crucial. We investigated the thermal and flammability properties of mixed electrolytes based on the conventional ethylene carbonate–dimethyl carbonate (1:1wt/wt)–1M LiPF6 and the hydrophobic ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). The results of thermogravimetric analyses and flammability tests of mixed electrolytes of different compositions are reported and discussed. An important finding is that though the mixtures with high contents of ionic liquid are more difficult to ignite, they burn for a longer time, once they are ignited.