Christin Hogrefe

Understanding the safety profile of aged Li-ion batteries is essential for developing effective battery management and hazard mitigation strategies. However, most safety assessments have focused on fresh batteries, with just a few calorimetry studies on aged batteries with metal oxide positive electrodes. This study provides a broad assessment of commercial 18650-type Li-ion batteries with NCA, NMC, and LFP positive electrodes, both uncycled and aged under conditions that promoted solid electrolyte interphase (SEI) growth as the dominant degradation mechanism. The cells underwent mechanical (nail penetration, crush), electrical (overcharge, overdischarge), and thermal (accelerating rate calorimetry) abuse tests. Safety was rated on general characteristics such as mass loss, maximum temperature, and EUCAR (European Council for Automotive R&D) hazard level, as well as characteristics specific to individual abuse tests. Generally, aged cells with SEI growth exhibited similar or improved safety compared to uncycled cells, contrasting with our previous findings on NCA cells with Li plating as the dominant aging mechanism (Part I of this series). Yet, some tests and characteristics indicated reduced aged cell safety, such as earlier triggering of mechanical failure. These results emphasize the need to examine aged battery safety across diverse empirical techniques, degradation modes, and chemistries.

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.

Li redistribution processes within Si-graphite composite (SiG) electrodes are analyzed using in situ and operando X-ray diffraction (XRD), ex situ light microscopy (LM), in situ optical microscopy of cross-sectioned full cells (CS-IOM), and 3D microstructure-resolved simulations of full cells. First, the lithiation behavior of graphite and SiG full cells (Si content 20.8 wt.-%) is analyzed. The results are used as validation of the methods (XRD, LM, CS-IOM, simulation). Second, the Li redistribution between the graphite component and Si component within SiG electrodes is investigated: By operando XRD measurements during charging in comparison with relaxed cells, a higher lithiation degree in the graphite component is found during charging compared to the relaxed state, indicating Li redistribution from graphite to Si during relaxation. The Li redistribution is directly observed by in situ and ex situ optical microscopy, where the golden LiC₆ phase disappears during a 24 h relaxation period. The results are supported by simulations showing the variation in the Li concentration, not only in graphite but also within the Si component. Furthermore, all methods find that the Li redistribution is more pronounced at a higher C-rate of 0.5 C, suggesting a preference for graphite lithiation over Si lithiation.

The cycle life of Li-ion batteries is limited by aging on the material and electrode level. Prolonging the battery life helps increasing sustainability, reducing the dependency from critical raw materials, and safes costs. The first step in extending battery life is to understand the underlying degradation mechanisms. Post-Mortem analysis of commercial and pilot-line made cells showed that Lithium plating is a very relevant and severe aging mechanism which can lead to high aging rates and decrease safety properties.¹ In this talk, we analyse the aging mechanism of Lithium plating on cell and electrode level. On cell level, capacity fade per time yields aging rates, which are temperature dependent. This temperature dependency is analyzed via Arrhenius plots. The Arrhenius plots for cycling aging show a typical V-shape with a low and a high temperature branch.^2–4 A minimum in the Arrhenius plot corresponds to the maximum cycle life of the battery, while the low temperature branch indicates Lithium plating. Interestingly, the minimum in the Arrhenius plot shifts with the charging C-rate, the battery type,^3,4 and – in case of non-linear capacity fade curves – with aging.⁴ In the last years, our group developed new semi-quantitative methods based on glow discharge optical emission spectroscopy (GD-OES) depth profiling for detection of Lithium plating on graphite⁵ and on Si/graphite composite anodes.⁶ Furthermore, a new in situ optical microscopy method with cross-sectioned full cells allows direct observations of the electrodes during charging and discharging.^7,8 The color change of lithiated graphite in combination with simultaneous electrochemical data gives insight into the charging behavior of anodes. Under slow charging conditions (0.1C), the lithiation of the anode is observed to be homogeneously. Most interestingly, at 1C transport limitations come into play and lithiation fronts moving from the anode surface to the current collector can be directly observed and quantified by image analysis. Our results of both methods^5,6,8 and in full consistence with simulations by others⁹ show that Lithium is mainly deposited on the anode surface, i.e. near the separator (see Figure). Finally, we show that Lithium deposition can be prevented by avoiding anode potentials < 0 V vs. Li/Li⁺ in commercial cells and cells from our pilot-line. Interestingly, Si/graphite anodes in pouch full cells show a lower susceptibility to Lithium plating at low temperatures with increasing Si content and the same anode areal capacity and N/P ratio. This suggests that the transport limitations in the pores of these Si/graphite anodes are reduced due to lower anode coating thickness. References T. Waldmann, B.-I. Hogg, and M. Wohlfahrt-Mehrens, J. Power Sources, 384, 107–124 (2018). T. Waldmann, M. Wilka, M. Kasper, M. Fleischhammer, and M. Wohlfahrt-Mehrens, J. Power Sources, 262, 129–135 (2014). X.-G. Yang and C.-Y. Wang, J. Power Sources, 402, 489–498 (2018). G. Kucinskis et al., J. Power Sources, 549, 232129 (2022). N. Ghanbari, T. Waldmann, M. Kasper, P. Axmann, and M. Wohlfahrt-Mehrens, J. Phys. Chem. C, 120, 22225–22234 (2016). M. Flügel, K. Richter, M. Wohlfahrt-Mehrens, and T. Waldmann, J. Electrochem. Soc., 169, 050533 (2022). C. Hogrefe et al., J. Electrochem. Soc., 169, 050519 (2022). C. Hogrefe, T. Waldmann, M. Hölzle, and M. Wohlfahrt-Mehrens, J. Power Sources, 556, 232391 (2023). S. Hein and A. Latz, Electrochimica Acta, 201, 354–365 (2016). Acknowledgement The authors would like to acknowledge the Federal Ministry for Economic Affairs and Climate Action (BMWK) for financial support of the Structur.e project (03ETE018E), the German Federal Ministry of Education and Research (BMBF) for funding of the project CharLiSiKo (03XP0333A) and the TEESMAT project (Horizon 2020 E.U. Framework Program, Grant Agreement N° 814106, http://www.teesmat.eu). Figure 1

In Li-Ion batteries, the graphite anode undergoes chromatic changes from grey (unlithiated graphite) to blue (LiC₁₈) to red (LiC₁₂) to gold (LiC₆) during lithiation.¹ When charged with high current rates, lithiation gradients are observable due to mass transport limitations within the anode.² Due to high lithiation degrees at the surface, the anode potential decreases locally and the possibility of Li deposition is increased.³ We introduce a new in situ optical microscopy set-up, which allows a direct observation of the cross-section of Li-ion full cells in combination with simultaneous recording of electrochemical data (Figure 1). Extensive analysis of the chromatic changes from the graphite electrode surface to the current collector gives insights into the lithiation processes on electrode and particle level. The set-up was validated by comparing the electrochemical results with data from coin full cells. The propagation of lithiation fronts for LiC₁₂ (~ 3300 µm² min^-1) and LiC₆ (~1260 µm² min^-1) at 1C through the graphite electrode coating were determined as well as the estimation of apparent solid-state diffusion coefficients in the order of 10^-10 cm² s^-1 from analyzing the phase propagation within single particles at C/10. Additionally, the expansion of all components of the whole cell can be observed individually. We found that the graphite contributes mainly to the cell expansion, both irreversibly (4%) and reversibly (4-13%). Directly observing the described phenomena in the full cell can give insights into aging mechanisms of the materials. Figure 1 Exemplary image from the video at the end of charge of an in situ measurement (cycling at C/10) of a graphite-NMC 622 full cell cross-sections. References P. Maire, A. Evans, H. Kaiser, W. Scheifele and P. Novák, J. Power Sources, 155(11), A862 (2008). M. Weiss, R. Ruess, J. Kasnatscheew, Y. Levartovsky, N. R. Levy, P. Minnmann, L. Stolz, T. Waldmann, M. Wohlfahrt-Mehrens, D. Aurbach, M. Winter, Y. Ein-Eli and J. Janek, Adv. Energy Mater., 11(n/a), 2101126 (2021). T. Gao, Y. Han, D. Fraggedakis, S. Das, T. Zhou, C.-N. Yeh, S. Xu, W. C. Chueh, J. Li and M. Z. Bazant, Joule (2021). Figure 1

A new in situ optical microscopy set-up is introduced which allows direct observation of cross-sections of Li-ion full cells in combination with simultaneous recording of electrochemical data. The method is validated by comparison of electrochemical data from coin full cells. Color changes give insights into processes on the electrode and particle level, such as lithiation behavior and electrode thickness changes. Our observations allow the evaluation of (i) the speed of lithiation fronts for LiC₁₂ and LiC₆ through anode coatings, (ii) estimation of apparent diffusion coefficients from analysis of the color distribution in single graphite particles, as well as (iii) electrical de-contacting and re-contacting of single graphite particles in connection with (iv) electrode thickness changes. Furthermore, our direct observations from the inside of full cells give indirect insights into aging phenomena such as Li plating and SEI growth.