Information derived from microscopic images of Li-ion cells is the base for research on the function, the safety, and the degradation of Li-ion batteries. This research was carried out to acquire information required to understand the mechanical properties of Li-ion cells. Parameters such as layer thicknesses, material compositions, and surface properties play important roles in the analysis and the further development of Li-ion batteries. In this work, relevant parameters were derived using microscopic imaging and analysis techniques. The quality and the usability of the measured data, however, are tightly connected to the sample generation, the preparation methods used, and the measurement device selected. Differences in specimen post-processing methods and measurement setups contribute to variability in the measured results. In this paper, the complete sample preparation procedure and analytical methodology are described, variations in the measured dataset are highlighted, and the study findings are discussed in detail. The presented results were obtained from an analysis conducted on a state-of-the-art Li-ion pouch cell applied in an electric vehicle that is currently commercially available.

Thermal conductivity (TC) is a parameter, which significantly influences the spatial temperature gradients of lithium ion batteries in operative or abuse conditions. It affects the dissipation of the generated heat by the cell during normal operation or during thermal runaway propagation from one cell to the next after an external short circuit. Hence, the thermal conductivity is a parameter of great importance, which concurs to assess the safety of a Li-ion battery. In this work, an already validated, non-destructive measurement procedure was adopted for the determination of the evolution of the through-plane thermal conductivity of 41 Ah commercially available Li-ion pouch cells (LiNiMnCoO₂-LiMn₂O₄/Graphite) as function of battery lifetime and state of charge (SOC). Results show a negative parabolic behaviour of the thermal conductivity over the battery SOC-range. In addition, an average decrease of TC in thickness direction of around 4% and 23% was measured for cells cycled at 60 °C with and without compression, respectively. It was shown that pretension force during cycling reduces battery degradation and thus minimises the effect of ageing on the thermal parameter deterioration. Nevertheless, this study highlights the need of adjustment of the battery pack cooling system due to the deterioration of thermal conductivity after certain battery lifetime with the aim of reducing the risk of battery overheating after certain product life.

In this paper, tests and analysis of thermal runaway propagation for commercial modules consisting of four 41 Ah Li-ion pouch cells are presented. Module samples were tested at 100% state-of-charge and mechanically constrained between two steel plates to provide thermal and mechanical contact between the parts. Voltage and temperature of each cell were monitored during the whole experiment. The triggering of the exothermal reactions was obtained by overheating one cell of the stack with a flat steel heater. In preliminary studies, the melting temperature of the separator was measured (from an extracted sample) with differential scanning calorimetry and thermogravimetric analysis techniques, revealing a tri-layers separator with two melting points (≈135 °C and ≈170 °C). The tests on module level revealed 8 distinct phases observed and analyzed in the respective temperature ranges, including smoking, venting, sparkling, and massive, short circuit condition. The triggering temperature of the cells resulted to be close to the melting temperature of the separator obtained in preliminary tests, confirming that the violent exothermal reactions of thermal runaway are caused by the internal separator failure. Postmortem inspections of the modules revealed the internal electrical failure path in one cell and the propagation of the internal short circuit in its active material volume, suggesting that the expansion of the electrolyte plays a role in the short circuit propagation at the single cell level. The complete thermal runaway propagation process was repeated on 5 modules and ended on average 60 s after the first thermal runaway triggered cell reached a top temperature of 1100 °C.

Within the scope of developing a multi-physical model describing battery behavior during and after the mechanical load (accelerations, intrusions) of a vehicle’s high voltage battery, an internal short circuit model is of deep interest for a virtual hazard assessment. The internal short resistance and the size of the affected area must be known as a minimum for determining the released heat and, in consequence, the temperatures. The internal short resistance of purpose-built dummy pouch cells, filled with electrolyte-like solvent without conductive salt, has thus been measured in a given short area under various compressive loads. The resistances for different short scenarios obtained are analyzed and described in a mathematical form. Short circuit experiments with dummy cells using an external power source have also been carried out. This set-up allows the measurement of the temperature evolution at a known current and a determination of the actual short resistance. The post-mortem analysis of the samples shows a correlation between the maximum temperatures, released short heat and the separator melt diameter.

In this paper, the direct measurement of the orthotropic thermal conductivity on a commercial Li-ion pouch battery is presented. The samples under analysis are state-of-the art batteries obtained from a fully electric vehicle commercialized in 2016. The proposed methodology does not require a laboratory equipped to manage hazardous chemical substances as the battery does not need to be disassembled. The principle of the measurement methodology consists of forcing a thermal gradient on the battery along the desired direction and measuring the heat flux and temperature after the steady state condition has been reached. A thermoelectric device has been built in order to force the thermal gradient and keep it stable over a long period of time in order to be able to observe the temperatures in steady state condition. Aligned with other measurement methodologies, the results revealed that the thermal conductivity in the thickness direction (0.77 Wm⁻¹K⁻¹) is lower with respect to the other two directions (25.55 Wm⁻¹K⁻¹ and 25.74 Wm⁻¹K⁻¹) to about a factor ×35.

Nail penetration is one of the most critical scenarios for a lithium-ion cell: it involves the superposition of electrical, thermal and mechanical abusive loads. When an electrically conductive nail is introduced into the active layers of a lithium-ion cell, an electric short circuit takes place between the conductive components (electrodes and current collectors). Hence, for this load case, electro-thermal modeling must be performed considering each and every layer of the cell in order to predict the electric quantities and the cell temperature (with numerical models). When standard conic nails are used, as is typical for this class of tests, the electrical contact between conductive components and the nail itself suffers of poor reproducibility mainly due to the separator that interposes between the electrically conductive components. This phenomenon makes it difficult to validate electro-thermal models, since the electrical contact between nail and lithium-ion cell parts cannot be safely determined. In this work, an alternative nail with an optimized ratio between the external surface and volume is presented to overcome this issue. To demonstrate the effectiveness of the designed nail, five tests (with the same conditions) were conducted on five commercial lithium-ion pouch cells, monitoring the tabs voltage and surface temperature. In all tests, thermal runaway was reached within 30 s and the tabs voltage showed comparable behavior, indicating that the short circuit values for all five repetitions were similar. The investigation included the implementation of a detailed layers model to demonstrate how the validation of such model would be possible with the novel data.