In this experimental investigation, we studied the safety and thermal runaway behavior of commercial lithium-ion batteries of type 21700. The different cathode materials NMC, NCA and LFP were compared, as well as high power and high energy cells. After characterization of all relevant components of the batteries to assure comparability, two abuse methods were applied: thermal abuse by the heat-wait-seek test and mechanical abuse by nail penetration, both in an accelerating rate calorimeter. Several critical temperatures and temperature rates, as well as exothermal data, were determined. Furthermore, the grade of destruction, mass loss and, for the thermal abuse scenario, activation energy and enthalpy, were calculated for critical points. It was found that NMC cells reacted first, but NCA cells went into thermal runaway a little earlier than NMC cells. LFP cells reacted, as expected, more slowly and at significantly higher temperatures, making the cell chemistry considerably safer. For mechanical abuse, no thermal runaway was observed for LFP cells, as well as at state of charge (SOC) zero for the other chemistries tested. For thermal abuse, at SOC 0 and SOC 30 for LFP cells and at SOC 0 for the other cell chemistries, no thermal runaway occurred until 350 °C. In this study, the experimental data are provided for further simulation approaches and system safety design.

Two different material batches made of random and textured orientated polycrystalline nickel-base superalloy René80 were investigated under isothermal low cycle fatigue tests at 850 °C for a notched specimen geometry. In contrast to a smooth specimen geometry, no significant improvement in fatigue behaviour of the notched specimen could be observed for the textured material. Finite element simulations reveal an area along the notch where high stiffness evolves for the textured material, which lead to nearly similar shear stresses in the slip systems compared to a random orientation distribution and therefore to no distinct differences in the lifetime.

New insights into lithium–ion battery aging behavior beyond a state of health of 80%, as well as for three different aging diagnostics and modeling methods, are obtained through this study conducted on four different cell types. Commercial cylindrical cells of type 21,700 are subjected to calendar aging and cyclic aging with different parameters in a long‐term study. The impact of the aging parameters on the four different cathode materials assessed (lithium–nickel–manganese cobalt oxide (NMC), lithium–nickel–cobalt–aluminum oxide high energy (NCA‐HE), lithium–nickel–cobalt–aluminum oxide high power, and lithium–iron phosphate oxide (LFP)) is examined with a portfolio of diagnostic methods: capacity test, Ohmic resistance test and the core of this study, impedance analysis, together with sophisticated equivalent circuit modeling (ECM). It was found that the NMC cell degraded fastest under all aging conditions, the most durable was in most cases the NCA‐HE cell. Only for one cyclic aging procedure, the LFP cells performs the best. The diagnostics shows that quantitative analysis of Nyquist plots is not sufficient for aging tracking and that some aging effects can only be detected by pulse discharging test. Moreover, capacity and mean Ohmic resistance deduced from electrochemical impedance spectroscopy test allow extrapolation to forecast further aging under diverse aging conditions.

By means of the Heat-Wait-Seek test in Accelerating Rate Calorimeters (ARC) the safety performance of Lithium-ion cells can be estimated. The most interesting measured parameters that can be extracted from these tests are the critical temperatures, such as the onset of self-heating, the venting or thermal runaway temperature. Another essential safety factor is the generated heat during cell failure. In general, the generated heat is calculated by an average heat capacity of the cell and the temperature difference in the experiment, which is not accurate enough especially when facing the challenges in specific volumetric energy density and safety of battery packs. In this study a more accurate way to calculate the generated heat is proposed, which consists of using temperature dependent heat capacity of the cell, based on its individual components. This leads to a significant difference in the calculated generated heat, for instance the specific heat capacity of NMC as a cathode material increases in the temperature range from 298 K to 398 K from 0.83 to 0.96 J/g·K, which is an increase of about 16 %. This leads to a difference in the calculated generated heat in this small range of 7 J/g (or 8 %), if a static specific heat capacity of 0.83 J/g·K and on the other hand a temperature dependent specific heat capacity is used. Moreover, the influence of additional heating steps, venting of the cell, melting of the separator and the overall accuracy of the measurement will be discussed. The investigation of these effects is in the focus of the project AnaLiBa (Analytics of Lithium-ion-Batteries), funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the competence cluster Analytics/quality assurance (AQua). In this project the approach is applied to commercial type 21700 cells. The 21700 type cells are extensively used in battery packs, for instance in electric cars, due to their 20 % higher energy density with respect to the former used type 18650 cells. The cells were measured in an ES-ARC from Thermal Hazard Technologies, UK, using the Heat-Wait-Seek test. The generated heat was compared for either fresh cells and cells after cyclic aging. The cyclic aging was performed at 0 °C and 20 °C with a charging rate of 0.3C for 0 °C and 0.7C for 20 °C and a discharging rate of 2C for 20 °C and 0.5C for 0 °C until the state of health was at 80 % ±2 %. The two different temperatures studied in the cyclic aging were chosen as 0 °C to cause lithium plating and as 20 °C to cause a higher internal resistance due to a thicker solid-electrolyte-interface, which has a high influence on the measured properties in the Heat-Wait-Seek test and therefore on the generated heat. With this approach the generated heat in abuse tests can be calculated more accurately to simulate the propagation of heat during a single cell failure in a pack or to calculate the necessary thickness of a heat barrier for safer battery packs. This helps manufacturers to improve their battery pack design with respect to the specific volumetric energy density and safety.