Factors Influencing the Thermal Stability of Lithium Ion Batteries - From Active Materials to State-of-Charge and Degradation

摘要

High energy density, long cycle life and enhanced safety properties are considered key for future lithium ion batteries (LIBs). Therefore, it is of fundamental importance to investigate and understand the specific interdependencies between electrochemical performance, degradation effects and the influence on the safety properties of the cell. Considering the large variety of cell chemistries applied in state-of-the-art and future LIBs, each material exhibits different characteristics regarding the electrochemical performance and degradation during charge/discharge cycling. The impact of the degradation effects on the thermal stability, hence the safety properties of LIBs, is widely unknown. Therefore, this study presents fundamental correlations on how state-of-charge (SOC) and degradation effects can influence the thermal stability of specific positive and negative active materials. With regard to different positive active materials, this study focuses on LiNixCoyMnzO2 (NCM, x+y+z=1) materials with varying nickel contents (0.33 ≤ × ≤ 0.8). The electrodes were cycled in coin cells using the same carbonaceous counter electrode and electrolyte to provide a direct access to the evaluation of the influence of different nickel contents on the degradation and thermal stability. The investigations on the thermal stability were performed on electrode level and in presence of electrolyte in the charged and discharged state. Therein, thermogravimetric analysis (TGA) was combined with accelerating rate calorimetry (ARC) experiments in the heat-wait-search (HWS) mode and comprehensive post-mortem analysis of the electrodes after charge/discharge cycling. Thus, this study aims on extending the knowledge about the thermal stability of NCM materials which are known to be less stable, i.e. more prone to structural decomposition and oxygen release, in the delithiated state and beyond that, exhibit an intrinsically reduced thermal stability at higher nickel contents. Apart from the NCM materials, investigations on the high voltage spinel-type LiNi0.5Mn1.5O4 (LNMO) showed that the presence of nickel strongly affects the thermal stability, especially in the charged state. In addition to the different positive active materials, the thermal stability of negative electrodes were investigated in the charged and discharged state. In LIBs the thermally induced deintercalation of lithium from the graphitic host structure leads to the onset of exothermic reactions at elevated temperatures. Therein, the analysis of changes in the reactivity due to the presence of fresh or aged electrolyte can lead to a more detailed understanding of the influence of aging on the thermal stability of LIBs. Moreover, the influence of silicon on the thermal stability of Si/C composite electrodes for high-energy applications was evaluated by comparison with pure carbonaceous electrodes. Furthermore, despite the numerous advantages of Li4Ti5O12 (LTO) for high-power applications, the thermal stability of this active material and the influence on the overall safety properties of LIBs has been barely investigated, yet. In summary, an in-depth knowledge of the factors influencing the thermal stability of specific LIB components is essential to develop cells with superior energy density, cycle life, and most importantly enhanced safety properties.