The reaction of lithiated silicon (LixSi) with 1 M LiPF6 ethylene carbonate-diethyl carbonate(EC:DEC) electrolyte was compared with that of Li0.81C6 and the same electrolyte using accelerating rate calorimetry (ARC). In the presence of LiPF6-containing electrolyte, LixSi and Li0.81C6 showed similar ARC response in cases where the same number of moles of lithium atoms and the same amount of electrolyte were involved. X-ray diffraction results indicate that the major reaction product is comprised primarily of LiF for both LixSi and Li0.81C6. Apparently, the LiF reaction product reduces the reactivity with electrolyte of both LixSi and Li0.81C6. The self-heating rate of LixSi reacting with electrolyte was less than that of Li0.81C6 even though the specific surface area was about twice as large. This suggests composite electrodes using powdered Si-based electrode materials will be as safe as or safer than corresponding graphite electrodes in full-scale Li-ion cells. The reason that the lithiated silicon is safer than Li0.81C6 is because silicon stores more Li per unit volume than graphite, leading to thicker LiF surface layers that slow the reaction with electrolyte. (c) 2006 The Electrochemical Society.