The thermal stabilities of 1M LiPF 6 /EC+DEC and with electrodes were studied by calorimetry. The results show that both the electrolyte–Li 0.5 CoO 2 and electrolyte–Li x C 6 system have lower decomposition onset temperatures than either the separate electrolyte or electrodes. The electrolyte is oxidized by Li 0.5 CoO 2 , while its reaction with lithiated graphite occurs because the solid electrolyte interphase (SEI) breaks down at 57°C. The 1M LiPF 6 /EC+DEC in air is less stable than in argon, but the reaction is similar.