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 CO 2 and C 2 H 4 gases leaving lithium propionate (CH 3 CH 2 CO 2 Li) and Li 2 CO 3 as solid residues. The lithium propionate decomposes over the temperature range of 300–600 °C to evolve pentanone leaving Li 2 CO 3 as a residual solid. Finally, the Li 2 CO 3 decomposes over 600 °C to evolve CO 2 leaving Li 2 O 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 Li x PO y F z 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 Li 2 CO 3, resulting in increased complexity of the SEI.