The relaxation behavior of high-energy Si/C-graphite anodes in Li-ion cells is systematically investigated as a function of silicon content, SoC, C-rate, and cyclic aging. High-energy anodes (4.6 mAh/cm2) with Si/C-graphite ratios of 30/64, 50/44, 80/14, and 90/4 were assembled in pilot-scale pouch full cells and characterized by operando electrochemical dilatometry under defined contact pressure (0.8 MPa), in-situ optical microscopy, and Post-Mortem analysis. Three coupled relaxation mechanisms were identified: (i) intra-electrode lithium equilibration within and between graphite particles (negligible volume contribution), (ii) time-delayed lithium diffusion from graphite into silicon (anode thickness increase), and (iii) re-intercalation of deposited lithium metal into graphite and silicon (anode thickness decrease). At low C-rates (C/10, C/3) and high SoC (100%), mechanism (ii) dominates, leading to additional volume expansion of up to +0.47 %/Ah during relaxation. At high C-rates and low SoC (30%), mechanism (iii) dominates, with reversible volume contractions of up to -2.80 %/Ah at 3C. Counterintuitively, anodes with lower silicon content exhibit greater volume expansion at high C rates due to higher susceptibility to lithium deposition, linked to their thicker coatings. These results show that relaxation processes strongly affect volume change, lithium redistribution, cell degradation, and offer practical guidelines for electrode design and charging protocol optimization in automotive-grade Si/C-graphite anodes.

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/SiOx 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 degrees C. Cells with low-temperature-induced lithium plating exhibit pronounced safety degradation, with the self-heating onset temperature reduced by up to 50 degrees C regardless of the load profile. Subsequent heating to 45 degrees C for 48 h partially restores safety, leaving the thermal runaway mostly unaffected but increasing the self-heating onset temperature by 25 degrees C-40 degrees C depending on SoH level. These results underscore the importance of lithium plating detection to improve lithium-ion battery safety. Safety behavior of cells aged under different conditionsSafety response of cells with Li plating at various SoH and SoC levelsLi plating can occur already in the first few cyclesStudy of the onset of Li plating via various methods (ARC, DVA, GEIS, GD-OES)Safety of cells with Li plating can partially be improved by heating

Electrolyte-motion–induced salt inhomogeneity (EMSI) is increasingly recognized as a failure mode in fast-charging. However, its generality beyond large jelly-roll cells has remained unclear. In this work, we present this effect for the first time in single-layer pouch cells. EMSI arises whenever two conditions coincide: charge-induced pore-volume reduction with pore filling ratio >1 and strong through-plane salt polarization. We reproduce the reported EMSI fingerprint, a reversible, week-scale rise in ohmic resistance accompanied by rate-dependent capacity loss and use it as a diagnostic marker. Direct mapping by ion chromatography and ATR-FTIR spectroscopy reveals centimeter-scale LiPF₆ gradients, with up to ∼3× center-to-edge differences across 2.5 cm after ∼20 equivalent fast-charge cycles. These gradients require nearly a week to dissipate and coincides with edge-localized Li deposition. Their homogenization is tracked by a reversible drop in high-frequency resistance during the cell rest period. A coupled pseudo-3D electrochemical-fluid model reproduces the experimental trends and illustrates how the resistance evolves. Parameter scans of electrolyte amount, charge/discharge rate, temperature, and silicon content chart the onset conditions and motivate actionable mitigation strategies. Together, these results establish EMSI as a general design and testing challenge across cell formats whenever electrode stacks are mechanically constrained.

This work addresses the limited understanding of how changes in SiO x /graphite anodes and the N/P ratio induced by aging influence the temperature-dependent transition between degradation mechanisms. It provides insight into the potentially evolving risk of lithium plating and its implications for the safe and durable operation of batteries. Commercial 3.5 Ah Li-ion 18 650 cells with SiO x /graphite anodes (∼2.5 wt.% Si) and Ni-rich LiNi 1− x − y Mn x Co y O 2 cathodes were systematically aged in the temperature range from −10 °C to +45 °C at 0.5 C and 0.85 C. Aging rates were evaluated at both beginning-of-life, i.e. 100%–95% state-of-health and mid-of-life (92.5%–87.5% state-of-health) using Arrhenius plots. The V -shaped Arrhenius plots at beginning-of-life exhibited minima, while the minima of the mid-of-life aged cells were either shifted to higher temperatures or even vanished in the investigated temperature range, indicating an increased susceptibility to lithium plating in the aged cells. Post-mortem analyses using FIB-SEM/EDX mapping revealed that SiO x degradation and related side reactions are more pronounced at high temperatures, while most likely lithium plating predominates at low temperatures. Loss of anode active material in the form of SiO x degradation and the resulting decrease in the N/P ratio is most likely the cause of the observed changes in the Arrhenius plots after cycling.

SiOx-graphite composite anodes in Lithium-ion-batteries have emerged as a promising approach to enhance both the specific energy and the energy density of lithium-ion batteries. However, the impact of fast charging on SiOx-graphite composites, particularly with the target to avoid any of lithium metal deposition, remains insufficiently understood. This study aims to identify the key aging mechanisms of SiOx-graphite anodes with a special emphasis on lithium inventory quantification in full cell configuration (SiOx-graphite vs Ni-rich NMC). Our findings indicate that at 25 degrees C the primary cause of degradation is the consumption of cyclable lithium due to excessive SEI growth, which is predominantly driven by lower SOC limits rather than charge rate. Additionally, a reversible increase in cell resistance during fast charging. Post-mortem analysis by SEM/EDX show no particle cracking but a thick SEI layer on the SiOx particle surfaces, corroborating the results of lithium inventory quantification. Our study demonstrates that fast charging-induced degradation can be minimized by avoiding deep delithiation of the anode and sufficient rest periods between individual charging steps. These results offer a more profound comprehension of the aging behavior of SiOx-graphite electrodes and provide guidance for the optimization of fast charging strategies in future lithium-ion batteries. (c) 2025 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, https://creativecommons.org/ licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

A frequently undervalued aspect of lithium-ion battery performance reporting is the specification of the format and area of the tested cells. However, these parameters provide crucial insights into the quality of the electrodes used for cell assembly and the reliability of the data obtained from the investigated systems. Here we focus on the aspects of process standardization and industry collaboration necessary for translating nanoscale electrochemical processes to Ah-scale cells. We examine the role of cell area and format in promoting comparability and standardization in battery research studies with technology readiness levels of 4 or higher. In addition, we discuss the limitations, challenges and expectations associated with measuring and evaluating battery performance exclusively in small cell formats.

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 LiFePO 4 [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.

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 thermal runaway behavior of commercial high-energy 18650 lithium-ion batteries with Si/graphite anodes and Ni-rich NMC cathodes was investigated by ARC-EIS, accelerating rate calorimetry (ARC) in combination with electrochemical impedance spectroscopy (EIS). The cells were cyclically aged at a rate of 1C and at 45 degrees C without Li plating. Aging was characterized electrochemically by Arrhenius plots of initial aging rates and differential voltage analysis (DVA). The effects of state of health (SoH) and state of charge (SoC) in ARC experiments were investigated regarding the onset of self-heating temperature TSH, venting temperature TVent, onset of thermal runaway temperature TTR, and mass loss. With the aim of early thermal runaway detection without temperature measurement, an ARC-EIS method is proposed. This approach is based on impedance changes during thermal runaway at different SoH and SoC. A correlation between TSH, indicating significant exothermic reactions, and different impedance values (Re(Z), phi(Z), and Nyquist integral) was observed for different SoH and SoC levels.

The use of silicon-based secondary anode materials in blend anodes alongside graphite is becoming increasingly prevalent in commercial lithium-ion batteries also used more and more in automotive applications. In addition to the accelerated degradation of silicon due to its significant volume expansion, the crystalline phase transition of fully lithiated silicon results in alterations to the voltage profile during discharging. This study examines the impact of this phase transition on the operation and state estimation of battery cells using such silicon-containing graphite/SiOx blend anodes. A memory effect of trapping lithium in the crystalline phase occurs when the cell is subjected to partial cycling without being fully discharged. However, this effect can be cancelled out by a single deep discharge. To gain further insights, a variation in cycle numbers, state of charge range during cycling, charge and discharge current, and the operation temperature is conducted. In order to validate the findings, a variety of commercial and automotive cells and blend anode half-cells are analyzed.

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 degrees C and 45 degrees 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 degrees C. Post-Mortem and electrochemical analysis reveal that the dominating aging mechanism for T < 3 degrees C is most likely Na metal deposition and subsequent reaction of metallic Na metal with electrolyte. (c) 2025 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited

Recently, the first sodium-ion cells have been commercialized and have become available for consumers. [1–3] Given, moreover, the exciting announcements by several producers of such battery cells, it is of great interest to analyze these first commercial cells in order to understand which materials are used and how these cells are designed. Herein, the electrodes retrieved from an 18650 cell (1.5 Ah) are characterized through a multi-disciplinary approach: composition, morphology and lattice parameters are investigated via transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, while a comprehensive overview of the active material particle distribution and electrode structure is provided by focused ion beam scanning electron microscopy. X-ray photoelectron spectroscopy is exploited to obtain information about the elements’ oxidation state and the composition of the passivation film formed on the electrode surface. In addition, X-ray diffraction allows for the determination of the crystal structure of the active materials. Basic parameters of the electrodes, including the coating thickness and the active material loading, are provided in view of the investigation of their electrochemical properties carried out in lab-scale half-cells: cyclic voltammetry is used to evaluate the oxidation/reduction processes occurring during dis-/charge, the evolution of the electrode|electrolyte interfaces and interphases are studied via electrochemical impedance spectroscopy, and the cycling performance is assessed via galvanostatic cycling. In addition, the performance of the complete cell under both standard and harsher conditions are investigated, and its specific energy is compared with that of lithium-ion batteries of established configuration. [4] The results obtained herein provide a detailed overview of the chemistry of this commercial sodium-ion technology, enabling a meaningful comparison with the state-of-the-art lithium-ion technology and, thus, the identification of suitable application scenarios. [5] References [1] M. He, A. EL. Mejdoubi, D. Chartouni, M. Morcrette, P. Troendle, R. Castiglioni, J Power Sources 2023 , 588 , 233741. [2] “‘CATL Unveils Its Latest Breakthrough Technology by Releasing Its First Generation of Sodium-ion Batteries’ https://www.catl.com/en/news/665.html,” 2021 . [3] H. Laufen, S. Klick, H. Ditler, K. L. Quade, A. Mikitisin, A. Blömeke, M. Schütte, D. Wasylowski, M. Sonnet, L. Henrich, A. Schwedt, G. Stahl, F. Ringbeck, J. Mayer, D. U. Sauer, Cell Rep Phys Sci 2024 , 5 , 101945. [4] K. Bischof, V. Marangon, M. Kasper, A. Aracil Regalado, M. Wohlfahrt-Mehrens, M. Hölzle, D. Bresser, T. Waldmann, Journal of Power Sources Advances 2024 , 27 , 100148. [5] V. Marangon, K. Bischof, A. Aracil Regalado, M. Keppeler, M. Pogosova, M. Wan, J. Choi, S. Fleischmann, T. Diemant, M. Wohlfahrt-Mehrens, M. Hölzle, T. Waldmann, D. Bresser, J Power Sources 2025 , 634 , 236496.

Li plating significantly contributes to the ageing of lithium-ion batteries (LIBs). An in-depth understanding of Li-ion intercalation kinetics into graphite, still being widely used as anode material, and the subsequent phase formation of LixC6 compounds, is necessary to understand kinetic limits and prevent Li plating. Diffraction and colorimetric studies have explored these processes, noting graphite color changes during (de)intercalation. However, these methods fall short of examining graphite intercalation at a microscopic scale, essential for understanding intercalation kinetics and Li plating onset conditions. This study employs a high-resolution light microscope under inert gas to examine lithiation processes in graphite anodes at the particle level across various C-rates. Qualitative descriptions and quantitative assessments are achieved through colorimetric analysis based on hue and saturation, complemented by machine learning-based segmentation. The results show an increased spatial heterogeneity of lithiation stages both between particles and within individual particles, with increasing C-rate. Notably, up to three stages coexist in one particle, and LiC6 is present at 50% (SOC) state of charge even when lithiated with 0.2C. At 1C charging, 4% and 32.5% of the surface is covered with Li deposits at 30% and 50% SOC, respectively, with underlying graphite particles showing LiC6.

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.

To facilitate the electrification and decarbonization of the transport sector, there is a growing demand for battery cells that can provide satisfactory lifetime performance, cost efficiency, and improved energy and power density. By harnessing the enhanced specific capacity of silicon and the structural stability of graphite, the silicon-graphite negative electrode exhibits promise in achieving the aforementioned characteristics at the material level. However, the intrinsically different lithiation principles of both materials not only result in different active potential windows during charge/discharge reaction, but also lead to uneven lithium distribution at higher current densities [1, 2]. Taking this effect into account, this work focuses on investigating the “Li-deposition absent” aging phenomenon upon fast charging of the state-of the-art silicon oxide (hereafter SiO x )-graphite blend electrode at different state of charge (SOC) windows. Through post-mortem analytical and electrochemical techniques, lithium inventory quantification is, to the best of our knowledge, applied for the first time to the silicon-graphite full cells. The results reveal that the irreversible loss of over 20% of cyclable lithium after 400 equivalent full cycles is the main aging mechanism in this system, irrespective of the charging profile. Rather than the charging rate, cycling in the lower SOC range appears to be the most critical factor contributing to the degradation of the negative electrode. The study shows that the increased charge rate does not significantly contribute to the loss of cyclable lithium or electrochemically active materials, which can even be eliminated by limiting the lower SOC. In conclusion, the study provides a quantitative indication of the suitable operating conditions for fast charging of SiO x -graphite blend electrodes, which serves as a fundamental guideline for their industrial integration in automotive battery cells. References [1] J. Knorr et al 2024 J. Electrochem. Soc. 171 080512. [2] S. Friedrich et al 2024 J. Electrochem. Soc. 171 100503.

The present article addresses the following research question: What are core aspects of a sustainable lithium‐ion (Li‐ion) battery system? In a multidisciplinary approach, the article describes key aspects of the battery cell safety assessment (accelerating rate calorimetry, nail penetration, overcharge) for 2nd life on the example of field‐aged cells from an automotive application, details of the stationary application, and the design of a sustainable battery system. For battery system recycling, an alternative expanded recycling process with economical aspects for the plastic recycling is described. Finally, the article summarizes CO 2 emission for Li‐ion batteries in general and gives details about life cycle aspects replacing an aluminum battery tray with a plastic battery tray.

Commercially available 18650-type cylindrical sodium-ion battery (SIB) cells with a nominal capacity of 1.5 Ah are comprehensively investigated, yielding in-depth insights into the cell design, the chemical composition of the electrodes and the electrolyte composition. In addition, the performance of single electrodes as well as the complete cell as such - under both standard and harsher conditions - are investigated. The results reveal superior charge storage kinetics at the NaxNiyFezMn1-y-zO2-based cathode and rather sluggish kinetics at the hard carbon negative electrode, while the analysis of the already formed interphase indicates the presence of functional additives in the organic carbonate-based electrolyte. As such, this study reports a multi-disciplinary approach to assess the most relevant characteristics of commercial(-type) cells from the macro-scale to the micro-scale.

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.

To meet the carbon neutrality goal set by the International Energy Agency (IEA), battery cells have attracted significant attention due to their potential to electrify the transport sector and thereby reduce the concomitant emissions. To accelerate the market penetration of electric vehicles, there is a growing demand to address the trilemma of typical battery cells, namely their energy, power, and lifetime. By leveraging the enhanced specific capacity of silicon and the structural stability of graphite, silicon-graphite electrodes have sought to achieve a satisfactory balance of the aforementioned characteristics. This has resulted in a superior commercial viability of this type of electrode in recent years. However, the intrinsically different lithiation principles of both materials not only result in different active potential windows during charge/discharge reaction, but also lead to uneven lithium distribution at higher current densities [1, 2]. Taking this effect into account, this work focuses on investigating the “Li-deposition absent” aging phenomenon upon fast charging of the state-of the-art silicon oxide (hereafter SiO x )-graphite blend electrode at different state of charge (SOC) windows. Through post-mortem analytical and electrochemical techniques, lithium inventory quantification is, to the best of our knowledge, applied for the first time to the silicon-graphite full cells. The results reveal that the irreversible loss of over 20% of cyclable lithium after 400 equivalent full cycles is the main aging mechanism in this system, irrespective of the charging profile. Rather than the charging rate, cycling in the lower SOC range appears to be the most critical factor contributing to the degradation of the negative electrode. The study shows that the increased charge rate exerts a minimal impact on electrode, which can be eliminated by limiting the lower SOC. In conclusion, the study provides a quantitative indication of the suitable operating conditions for fast charging of SiO x -graphite blend electrodes, which serves as a fundamental guideline for their industrial integration in automotive battery cells. References [1] J. Knorr et al 2024 J. Electrochem. Soc. 171 080512. [2] S. Friedrich et al 2024 J. Electrochem. Soc. 171 100503.