Thermal Decomposition of the Solid Electrolyte Interphase (SEI) on Silicon Electrodes for Lithium Ion Batteries

Thermal behavior of the solid electrolyte interphase (SEI) on a silicon electrode for lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50–300 °C, LEDC decomposes to evolve CO2 and C2H4 gases leaving lithium propionate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propionate decomposes over the temperature range of 300–600 °C to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600 °C to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of LixPOyFz at 300 °C and the polyimide binder at 550 °C. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propionate and Li2CO3, resulting in increased complexity of the SEI.