Effects of solvents and salts on the thermal stability of LiC 6

Abstract

Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li 0.81C 6 in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF 6- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li 0.81C 6 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 Li 0.81C 6 and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF 6 to EC/DEC changes the surface film that forms on Li 0.81C 6 at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li 0.81C 6 in LiPF 6 electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2 M LiBOB to EC/DEC greatly improves the thermal stability of Li 0.81C 6. ARC results for Li 0.81C 6 in pure and mixed salt LiPF 6 and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.