Study on oxidation and pyrolysis of carbonate esters using a micro flow reactor with a controlled temperature profile. Part I: Reactivities of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate

Carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are widely used as electrolyte solvents in a lithium-ion battery (LIB) and are considered as one of potential fire causes. This study investigates a difference in gas-phase reactivities of DMC, EMC and DEC. Species measurements for oxidation (equivalence ratio of 1.0) and pyrolysis of DMC and DEC at maximum wall temperatures of T_w,max = 700–1300 K and atmospheric pressure were performed using a gas chromatograph connected to a micro flow reactor with a controlled temperature profile (MFR). Measured mole fractions of C₂H₄ and CO₂ in the DEC case started increasing at a low temperature (T_w,max = 750 K), while those of CH₄ and CO₂ in the DMC case started increasing at an intermediate temperature (T_w,max = 1050 K). Gas-phase reactivities of the three carbonate esters were found to be DMC < EMC ≈ DEC through observation of their weak flames. Computational species and heat release rate profiles of DEC showed a three-stage reaction: thermal decomposition, oxidation of decomposition products to CO and CO oxidation to CO₂. This three-stage reaction of DEC is distinct from the one driven by low-temperature oxidation observed with n-heptane and DME in earlier studies. Rate of production analysis indicated that principal fuel consumption reactions of DMC were H-atom abstraction reactions and that of DEC was a thermal decomposition reaction producing C₂H₄. Based on reaction path analysis of DEC oxidation, initial-stage reactions produced few active radicals at the low temperature region. The reactivity difference between DMC and DEC is, therefore, mainly driven by the difference in the primary fuel consumption reactions that relies on fuel molecular structure. Considering the existence of ethyl ester group in EMC, a dominant EMC consumption reaction would be similar to DEC.