A promising approach to increase the energy density of lithium ion batteries (LIBs) is the substitution of the state-of-the-art negative active material graphite by silicon (Si) due to the notably higher specific capacity of Si in comparison to graphite. However, Si exhibits enormous volume changes during lithiation/delithiation, which lead to high mechanical stress and degradation of active material particles. Consequently, Si-based negative electrodes suffer poor cycle life. Amongst others, the reduction of the active material particle size, e.g. the utilization of Si nanoparticles can mitigate degradation effects of the active material to some extent resulting in enhanced capacity retention. Besides an increased energy density, reduced costs and a long cycle life, safety, and thus a high thermal stability need to be guaranteed for future battery technologies to ensure safe operation in all states of charge and health. However, there is only little research on safety and thermal stability of LIBs with Si-based negative electrodes as most research focuses on the improvement of the energy density and electrochemical performance.
In this study, the thermal stability of LIBs with Si-based as well as graphitic negative electrodes was investigated on electrode level by differential scanning calorimetry (DSC) as well as on cell level by accelerating rate calorimetry (ARC). For state-of-the-art LIBs, the thermally induced deintercalation of lithium from the graphitic host structure was revealed to cause the onset of exothermic reactions within the cell at elevated temperatures. Thus, the composition and thermal behavior of the solid electrolyte interphase (SEI) is indicated to play a major role for the onset of exothermic reactions.
Similarly, it was also shown for LIBs with negative electrodes based on Si nanoparticles that exothermic processes at the negative electrode are responsible for the initial exothermic thermal decomposition reactions of investigated cells. The thermal behavior of both active material systems was investigated to elucidate the mechanisms initiating exothermic reactions, which is crucial to enhance the overall thermal stability, hence, the safety properties of graphite-/Si-based LIBs. Above 200 °C, rapidly increasing self-heating rates up to the occurrence of a thermal runaway could be assigned majorly to thermal processes at the positive electrode, which originated from highly exothermic reactions between electrolyte and released oxygen from the positive active material.
In summary, this study reveals the occurring processes during the onset of exothermic reactions in LIBs. It was shown that despite the notably higher specific capacity of Si compared to graphite and the high surface area of Si nanoparticles, LIBs with negative electrodes based on Si nanoparticles exhibited similar reactions at the onset of exothermic reactions as state-of-the-art LIBs with carbonaceous electrodes.