Thermal degradation analyses of carbonate solvents used in Li-ion batteries

The thermal degradation of the electrolyte solvents of a Li-ion battery in an inert atmosphere was carried out using an assembly that allowed the quantification of the degradation products. The study of the vapour pressures and DSC showed that, in an inert and confined medium, the linear solvents, dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), become degraded at lower temperatures than those of cyclic solvents: ethylene carbonate (EC) and propylene carbonate (PC). Therefore, they were the ones that were studied under the following operating conditions: initial temperatures of 180 °C, 240 °C and 300 °C for three periods of 30 min, 60 min and 2 h. The thermal decomposition of DMC and EMC was observed under all of the conditions studied. This decomposition is maximum at a temperature of 300 °C and for a pyrolysis duration of 2 h. The species resulting from the pyrolysis of the DMC and EMC in the gas phase correspond to those identified during the global overcharge tests on a study cell containing these two solvents (excluding fluorinated species). The greatest similarity was observed for degradations carried out at 300 °C, whatever the duration. Thus, this study shows the important role of the thermal degradation of volatile solvents.