Katharina Bischof

Thermal runaway poses a substantial safety concern in high-energy lithium-ion batteries. Understanding aging-induced degradation and developing reliable detection strategies are, therefore, essential. This study compares the thermal safety characteristics of commercial 3.5 Ah 18650 cylindrical cells containing nickel-rich cathodes and graphite/SiO _x anodes that had been aged using dynamic WLTP driving cycles and continuous laboratory cycling conditions with and without lithium plating at different SoH and SoC levels. This work focuses on the formation, detection, reversibility, and safety implications of low-temperature-induced lithium plating using accelerating rate calorimetry (ARC), differential voltage analysis (DVA), galvanostatic electrochemical impedance spectroscopy (GEIS), and Post-Mortem analysis with glow discharge optical emission spectroscopy (GD-OES) depth profiling. We already observe lithium plating in the initial cycles and also a continuous growth of plated lithium during continuous cycling with 0.85 C at 0 °C. Cells with low-temperature-induced lithium plating exhibit pronounced safety degradation, with the self-heating onset temperature reduced by up to 50 °C regardless of the load profile. Subsequent heating to 45 °C for 48 h partially restores safety, leaving the thermal runaway mostly unaffected but increasing the self-heating onset temperature by 25 °C–40 °C depending on SoH level. These results underscore the importance of lithium plating detection to improve lithium-ion battery safety.

The development of batteries with new materials typically starts with small active material amounts and consequently with small cells, often with low-loaded electrodes. As development progresses to increasing technology readiness levels, mid-sized and large cells with thicker electrode coatings are mandatory. However, large cells have different electrochemical and physicochemical properties than small lab cells. Herein, we discuss similarities and especially differences in the behavior of small laboratory cells (e.g. coin) and larger cells (cylindrical, pouch, prismatic), regarding cell design, electrode thickness, specific energy on material and cell level, heating behavior due to current flow, impedance, voltage curves, and rate capability. Impedances at 1 kHz are reduced by three orders of magnitude from coin cells to large cells while heating power is increased by a factor of 10 ² to 10 ⁵ . By comparing specific capacities on cathode material level with those on cell level for commercial cells, we find a reduction by a factor of ∼3 to ∼5, depending on the anode coating thickness. The large general gap from pure material to cell and between small laboratory, mid-sized, large pilot, and commercial cells need to be taken into account from the beginning of battery development to avoid potential misinterpretation.

The effect of temperature on aging of commercial Na-ion batteries is investigated quantitatively using commercial 2 Ah 21700-type cells with NFM cathode and hard carbon anode. The cells are cycled at 0.5 C in the temperature range between −10 °C and 45 °C. Interestingly, similar to Li-ion cells, a V-shaped Arrhenius plot of the initial aging rates indicates two different main aging mechanisms below and above 3 °C. Post-Mortem and electrochemical analysis reveal that the dominating aging mechanism for T < 3 °C is most likely Na metal deposition and subsequent reaction of metallic Na metal with electrolyte.

Sodium-ion batteries are emerging as a promising alternative to lithium-ion batteries in the market. However, achieving a competitive battery lifetime is essential for their market acceptance and success. This study systematically examines the cyclic aging behavior of a commercial 1.5 Ah 18650-type sodium-ion cell at low temperatures between −10 °C and ∼26 °C, aiming to understand the mechanism of sodium metal deposition. This was identified as the main aging mechanism under these conditions. Electrochemical characterization of capacity fade evolution, coulombic efficiency, and internal resistance during aging as well as the heating behavior of the cells and the shape of discharge voltage curves demonstrated an increased tendency to sodium deposition after applying lower temperatures or higher end-of-charge voltages. Additional Post-Mortem analyses, including scanning electron microscopy and energy dispersive X-ray spectroscopy, revealed the presence of local sodium metal depositions in new cells and more pronounced depositions in cells aged at room temperature or at 0 °C. Accelerating rate calorimetry tests of cells aged at 0 °C showed reduced self-heating rates at the melting temperature of sodium (∼98 °C). These findings provide a foundation for developing strategies to extend cycle life of sodium-ion batteries under real-world conditions, e.g. by limiting the end-of-charge voltage.

Are Na-ion batteries safer than Li-ion batteries? Such a binary question together with a binary answer arises from shortcomings in comprehending the multicriteria aspects of safety applied to batteries. Herein, results from a comparative safety assessment of 20 Ah Na-ion and Li-ion (using LiFePO₄ [LFP] as cathode material) prismatic cells are depicted. Specific cell behaviours upon a series of abuse tests (overcharging, overdischarging, nail penetration, external short-circuiting and thermal stability) are described in detail, and illustrate a fraction of the safety characterisations necessary prior homologation of industrially produced battery cells. For this test series, the hazard potential of the Na-ion cells investigated is found in general higher than for the LFP cells, with a larger release of thermal and electrical energy, smoke, sparks and particles depending on the test considered. Interestingly, the initial rates of temperature and voltage changes are found higher for the LFP cells than for the Na-ion cells, but the changes are more limited in magnitude. This suggests the presence of self-limiting mechanisms against the propagation of hazardous processes at electrode level in the LFP cells that are absent in Na-ion cells.

Extending the lifetime of lithium-ion batteries is essential to maximize resource efficiency and minimize environmental impact. Therefore, understanding the aging mechanisms that batteries undergo in their first life is critical to ensure safe operation in second-life applications. This study focuses on a comprehensive safety assessment of commercial 18650-type lithium-ion batteries with graphite||NCA chemistry. The safety of aged cells with the aging mechanism of lithium plating was tested using thermal (ARC), electrical (overcurrent, overcharge, overdischarge), and mechanical (nail penetration) abuse tests. New cells without lithium plating serve as control samples for comparison of the different safety test types and for the cells with lithium plating. The presence and absence of lithium plating is confirmed by electrochemical tests and Post-Mortem analyses (SEM, GD-OES). The cells with lithium plating exhibit significantly lower onset of self-heating temperatures, a tendency to higher maximum thermal runaway temperatures and increased EUCAR hazard levels. The results highlight potential hazards associated with lithium plating in lithium-ion batteries and the necessity to detect and avoid lithium plating in first life in order to safely reuse them in second life applications. This is part one of two papers dealing with safety testing aspects of aged cells with different degradation mechanisms.

Lithium deposition on anode surfaces can lead to fast capacity degradation and decreased safety properties of Li-ion cells. To avoid the critical aging mechanism of lithium deposition, its detection is essential. We present workflows for the efficient detection of Li deposition on electrode and cell level. The workflows are based on a variety of complementary advanced physico-chemical methods which were validated against each other for both graphite and graphite/Si electrodes: Electrochemical analysis, scanning electron microscopy, glow discharge-optical emission spectroscopy and neutron depth profiling, ex situ optical microscopy, in situ optical microscopy of cross-sectioned full cells, measurements in 3-electrode full cells, as well as 3D microstructurally resolved simulations. General considerations for workflows for analysis of battery cells and materials are discussed. The efficiency can be increased by parallel or serial execution of methods, stop criteria, and design of experiments planning. An important point in case of investigation of Li depositions are rest times during which Li can re-intercalate into the anode or react with electrolyte. Three workflows are presented to solve the questions on the occurrence of lithium deposition in an aged cell, the positions of lithium deposition in a cell, and operating conditions which avoid lithium depositions in a cell.

Calendar aging of Li-ion batteries with Si/graphite electrodes was investigated within this study. A total of 121 single-layer pouch full cells with either graphite or Si/graphite (3.0 wt−%, 5.8 wt−% and 20.8 wt−% Si) anodes and NMC622 cathodes with the same N/P ratio were built on pilot-scale. Calendar aging was studied at SoC 30%, 60%, and 100%, as well as temperature (25 °C, 45 °C, 60 °C) and time dependence. The aging data was analyzed in terms of capacity fade and a square-root behavior was observed. Differential voltage analysis (DVA) has been performed as a function of aging time. The observed temperature and time dependence is best described by time dependent, 3D Arrhenius plots. Post-Mortem analysis (SEM, EDX, GD-OES) is applied to investigate the changes on electrode and material level. Conclusions are drawn on the main aging mechanisms for calendar aging of Li-ion cells with Si/graphite anodes and differences between Si/graphite and pure graphite anodes are discussed. The Si-containing cells show a combination of lithium inventory loss and a loss of accessible Si active material, both caused by SEI growth.