Christiane Rahe

The spectroscopic electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) methods individually play an important role in unravelling operation and degradation mechanisms of batteries. As the methods probe complementary spin information, the correlation of both methods promises to enhance the knowledge gained about the battery cell, as demonstrated for lithiated graphite. So far, in operando cells were developed for both methods individually. We present an in operando battery cell that successfully works in both EPR and NMR instruments, enabling correlative experiments. The cell is demonstrated in a full-cell battery configuration. The design employs metal sheets as current collectors, closely mimicking a real battery. We present in operando cycling of a LiCoO2 (LCO) vs graphite battery with electrode components from a disassembled commercial battery.

Lithium (Li) plating on graphite is a significant degradation mechanism in Li-ion batteries. While numerous experimental techniques have been used to study Li plating in laboratory cells, investigations of commercial high-energy cells often rely on electrochemical methods. Here we present and classify various methods for detecting Li plating on a commercial A123 pouch cell. In a round robin study across multiple battery research laboratories, Li-plated graphitic electrode material was analyzed using electrochemical, microscopic, and spectroscopic methods capable of detecting metallic Li deposits. After cell opening, their overall distribution on the anode surface was examined using a flatbed scanner to ensure comparability of the samples. Optical and electron microscopy provided detailed surface and, in combination with a focused ion beam, subsurface structure and morphology. Spectroscopic methods confirmed the presence and onset of plated Li with varying sensitivity. Moreover, spectroscopic and imaging techniques were combined correlatively where possible. Availability and measurement duration of each technique was compared. Optical methods are fast and easy to use; thus, they are recommended for most samples, with spectroscopic confirmation reserved for reference samples. This multimodal study demonstrates a range of methods that can be used alone or in combination to qualitatively or quantitatively detect Li-plating.

The effect of the electrolyte on lithium-ion battery degradation is a subject of current research. In this publication we present the investigation of a total of 16 cells divided into 4 groups based on the anode material (graphite and SiOx-graphite composite) and the electrolyte volume (3.5 ml as recommended by the manufacturer and 2.8 ml). Cells were aged with a low-current cyclic ageing regime at elevated temperatures of 45 °C. Cells with SiOx-containing anodes suffer substantial loss of active material during the initial cycles regardless of electrolyte volume, whereas degradation of cells with graphite-anodes depend heavily on the electrolyte volume. Cells with 2.8 ml exhibit a knee-point after about 100 cylces, whereas those with 3.5 ml exhibit a linear capacity fade until well over 300 cycles. The knee-point coincides with substantial loss of lithium, and loss of active anode material.

Sodium‐ion batteries offer a promising alternative to lithium‐ion batteries by addressing ecological and economic challenges. However, to assess the applicability of these cells for different sectors, understanding aging behavior, including degradation modes, is crucial. This work presents a comprehensive aging analysis of 67 commercial sodium‐ion batteries under various temperatures, C‐rates, and depths of discharge. We analyzed the initial cell‐to‐cell variance and the aging trajectories regarding capacity fade and resistance increase. We demonstrated that the cycling rate does not significantly influence the aging trajectories, whereas smaller depths of discharge significantly reduce degradation. The degradation gradients for 25 °C and 40 °C were similar; for −10 °C, we observed rapid capacity fading that can be attributed to irreversible sodium plating. Furthermore, we identified the degradation modes for four different aging categories. Since some aging tests stopped due to gas‐induced current interrupt device triggering at low current rates and states of charge, we proposed two hypotheses for the gassing under specific conditions, suggesting inadequate gas consumption in cathode‐electrolyte side reactions or solid electrolyte interphase instability as potential causes. Overall, this work provides a valuable in‐depth analysis of the aging behavior of a commercial sodium‐ion battery as a function of temperature, C‐rate, and depth of discharge, with data made available for further research.

Calendaric lifetime testing of lithium‐ion cells is time‐consuming and resource‐intensive. As Dubarry et al.^[1] state, testing is often limited to a few aging conditions. This paper presents the results of a long‐term aging study on lithium‐ion cells with a nickel‐rich NCA cathode and a graphite‐silicon anode. 69 cells were stored at 5 different voltages and under 4 different temperatures for 5 years. Regular reference performance tests (RPT) provide insights for State of Health (SoH) calculation and further analysis through differential voltage analysis. The results are verified against post‐mortem analyses. The long aging period enables accurate determination of aging rates. Our results demonstrate that the storage voltage level strongly influences the degradation rate, with temperature playing a minor role. The identified aging effects include loss of active material on the cathode side and loss of lithium inventory. Initial degradation follows a ‐trajectory but is caused by overhang effects. The long‐term aging is rather linear.