Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li0.81C6 in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF6- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li0.81C6 and that robust passivating layers do not form. By contrast, the cyclic carbonate, EC, creates a robust passivating film that limits the rate of reaction between Li0.81C6 and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF6 to EC/DEC changes the surface film that forms on Li0.81C6 at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li0.81C6 in LiPF6 electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2M LiBOB to EC/DEC greatly improves the thermal stability of Li0.81C6. ARC results for Li0.81C6 in pure and mixed salt LiPF6 and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.