(Invited) Correlation of Degradation Effects and the Thermal Stability of Different Active Materials in Lithium Ion Batteries

Abstract

Three aspects are considered key for active materials in future lithium ion battery applications: High energy density, long cycle life, and enhanced safety properties. Therein, it is of fundamental importance to investigate and understand specific interactions between electrochemical performance, degradation effects, and their influence on the safety properties. In particular, to guarantee safe operation throughout the whole life cycle, it is crucial to know if degradation effects can cause changes in the safety properties of a lithium ion battery. Even though different active materials have been investigated with regard to their thermal stability in the charged and discharged state, the influence of degradation effects on the thermal stability of active materials is widely unknown. Therefore, this study correlates degradation effects and their influence on the thermal decomposition of different layered, spinel-type, and olivine-type based positive electrodes as well as carbonaceous, silicon composite, and spinel-type based negative electrodes at elevated temperatures. Specific active materials suffer from different degradation effects during charge/discharge cycling that can affect the thermal stability. Thus, in a first step, it is crucial to identify and understand degradation effects for each specific active material in order to deduce the influence on the electrochemical performance and the safety properties of the cell. For this purpose, different positive and negative electrode active materials were investigated in the charged and discharged state after charge/discharge cycling. Therein, thermogravimetric analysis (TGA) and adiabatic reaction calorimetry (ARC) was used to investigate changes in the thermal stability of the electrodes. Negative electrodes based on carbonaceous materials are known to be highly reactive in the charged (lithiated) state. However, the introduction of silicon (Si/C composite) or the use of the spinel-type Li4Ti5O12 (LTO) as active material can largely influence the thermal stability. In general, a thermally stable solid electrolyte interphase (SEI) can impede the thermally induced de-intercalation of lithium in the charged state, leading to an increased thermal stability, hence improved safety properties compared to the pristine state. Considering the positive electrode active materials, post-mortem analysis included, amongst others, structural investigations after heat treatment at different temperatures by X-ray diffraction analysis (XRD) to gain insights into the progression of the thermal decomposition of the active materials. Overall, the high thermal stability of spinel-type LiMn2O4 (LMO) is strongly affected by the presence of other transition metals like e.g. nickel in the high-voltage active material LiNi0.5Mn1.5O4 (LNMO). However, the negligible degradation of the spinel-type and olivine-type (LiFePO4) active materials during charge/discharge cycling results in consistent safety properties. In contrast, the degradation effects on the surface of layered transition metal oxides like LiNi x Co y Mn z O2 (NCM, x+y+z=1) can strongly reduce the thermal stability. In addition, the thermal stability of NCM is strongly affected by the state of charge and by an increasing nickel content (0.33 ≤ x ≤ 0.8). In summary, by combining comprehensive post-mortem analysis and thermal analysis, the degradation effects of different positive and negative active materials could be correlated with the thermal stability, hence the safety properties of different electrode active materials.