State-of-the-art lithium ion batteries (LIBs) contain graphite as negative active material. It combines comparatively high energy and power density with low cost and a long cycle life. However, the energy density of the negative electrode, and thus of the complete battery, can be further increased by substituting graphite by silicon (Si). The distinctly higher specific capacity of Si by a factor of nearly ten compared to graphite makes it a promising candidate for high-energy LIBs.
However, enormous volume changes of Si during lithiation/delithiation induce mechanical stress and degradation of active material particles leading to poor cycle life of Si-based negative electrodes. As a consequence, research efforts focus on the improvement of battery performance and lifetime, while safety properties are only rarely investigated. With regard to battery safety, the thermal stability of both the individual components and of the complete cell at different states of charge and health is considered the most relevant safety factor.
In this study, the thermal stability of LIBs with graphitic as well as with Si-based negative electrodes was investigated. Initially, analyses focused on electrodes in combination with electrolyte. Therein, differential scanning calorimetry (DSC) and heat-ramp tests in an accelerating rate calorimeter (ARC) were performed to gain information on onset temperatures and heat release as well as pressure and gas analysis for the heat-ramp tests. Finally, the thermal behavior was examined on cell level with both negative active materials in an ARC.
It was shown that Si-based negative electrodes cause distinctly stronger exothermic reactions compared to graphite-based electrodes with the same capacity. This increased reactivity led to a faster thermal runaway of Si-based cells, depicted by a much shorter period of time between the first detectable exothermic reactions and thermal runaway of the cell. Furthermore, the differing temperature profiles of cells with Si-based negative electrodes and graphite-based electrodes, respectively, indicated different reaction mechanisms for both cell chemistries.
The combination of various investigation methods revealed that solely thermal analysis by DSC with low amounts of active material is not sufficient to draw comprehensive conclusions on the thermal stability of Si-/graphite-based electrodes. In contrast, the combination with pressure and gas analysis enabled crucial additional conclusions on the thermal behavior. Therein, Si-based negative electrodes showed a different gas composition after thermal treatment in presence of electrolyte compared to graphitic electrodes.
In summary, this study compares the thermal behavior of both Si-based and graphite-based LIBs. It was shown that the utilization of Si as the negative active material leads to stronger exothermic reactions, and thus, the corresponding cells enter thermal runaway more quickly. In addition to only thermal analysis, pressure and gas analyses were revealed to support understanding the thermal behavior of LIBs since gaseous components can also be decisive for occurring exothermic reaction mechanisms.