Open Access

AbstractLithium‐ion cells connected in series are prone to an electrical safety risk called overdischarge. This paper presents a comprehensive investigation of the overdischarge phenomenon in lithium‐ion cells using operando nondestructive imaging. The study focuses on understanding the behavior of copper dissolution and deposition during overdischarge, which can lead to irreversible capacity loss and internal short‐circuits. By utilizing synchrotron X‐ray computed tomography (SXCT), the concentration of dissolved and deposited copper per surface area is quantified as a function of depth of discharge, confirming previous findings. The results also highlight for the first time a nonuniform distribution pattern for copper deposition on the cathode. This research provides insights for safer battery cell design.

The transport and storage of lithium-ion (Li-ion) batteries — damaged or in an undefined state — is a major safety concern for regulatory institutions, transportation companies, and manufacturers. Since (electro)chemical reactivity is exponentially temperature-dependent, cooling such batteries is an obvious measure for increasing their safety. The present study explores the effect of cryogenic freezing on the electrochemical and physical stability of Li-ion cells. For this purpose, three different types of cells were repeatedly exposed to liquid nitrogen (LN2). Before and after each cooling cycle, electrical and electrochemical measurements were conducted to assess the impact of the individual freezing steps. While the electrochemical behavior of the cells did not change significantly upon exposure to LN2 , it became apparent that a non-negligible number of cells suffered from physical changes (swelling) and functional failures. The latter defect was found to be caused by the current interrupt device of the cylindrical cells. This safety mechanism is triggered by the overpressure of expanding nitrogen which enters the cells at cryogenic temperatures. This study underlines that the widely accepted reversibility of LN2 -cooling on a material scale does not allow for a direct extrapolation toward the physical integrity of full cells. Since nitrogen enters the cell at cryogenic temperatures and expands upon rethermalization, it can cause an internal overpressure. This can, in turn, lead to mechanical damage to the cell. Consequently, a more appropriate temperature condition — less extreme than direct LN2 exposure — needs to be found

Next to performance features, safety aspects of lithium-ion batteries (LIBs) are a crucial research field. The abuse/misuse of a LIB can trigger a chain of exothermic reactions on cell level. Hence, the cell temperature increases dramatically, causing the so-called thermal runaway (TR). Moreover, the TR of one cell can initiate the TR of adjacent cells leading to a TR-propagation. Due to the risk of a TR, special measures need to be applied while handling, storing, and transporting batteries. According to current transport regulations, all different types of lithium-ion and lithium metal cells/batteries (by means of cell format, cathode chemistry, etc.) require the same transport conditions regardless of the intensity of their reaction during abuse tests. To allow more differentiated transport requirements, the United Nations (UN) Subcommittee Transport of Dangerous Goods created an Informal Working Group (IWG) on the topic of a hazard-based classification of LIBs. BAM is one of nine laboratories working on the development of a respective classification scheme including appendant test protocols. Herein, we discuss the latest results of our safety tests on commercial LIB-cells employing the test protocols developed in the UN-IWG. Single cell tests are analysed regarding different hazardous features during the TR, e.g., cell temperature, flame occurrence, and gas amount. Next to the general occurrence of a propagation, the propagation speed is analysed by propagation tests. In total, the presented results are gathered from over 200 tests. Next to the classification of the tested cells, the data set obtained is analysed in respect to the cells’ key features, such as cell energy, state of charge and cathode type. Generally, the presented results can increase the overall understanding of the TR-mechanism supporting the design of advanced safety measures on cell level in the future.

Porous carbon materials play an important role for energy storage and conversion. One (re-)emerging research field is the ability of porous carbons to store sodium metal ions. Current results shows that internal pores – hence, pores which are not accessible for the electrolyte – allow to store large amounts of sodium at low potentials, yielding high energy sodium-ion battery (SIB) anodes. The common synthesis approach to gain carbons with internal pores involves the pyrolysis of a non-graphitizing precursor, resulting in a so-called hard carbon (HC). However, HC-materials frequently show substantial non-reversible initial capacity losses. Commonly, significant losses are associated with the creation of the solid electrolyte interphase (SEI) on the carbon’s surface that occurs during the initial sodium insertion. Intriguingly, large irreversible capacities are often found for samples with experimentally determined low specific surface area.[2] A more comprehensive understanding of the structure-property relations is essential for quantifying and grasping the potential of carbon materials in SIBs. However, the typical synthesis methods do not allow to individually tune the storage properties – mainly connected to the internal properties of the carbons – and the SEI-formation – primarily related to the surface properties. Hence, the objective of the present work is to develop a synthesis route which tackles this challenge. Herein, the main approach is to develop tailor-made core-shell carbon materials consisting of a highly porous carbon core and a quasi-non-porous carbon shell. For the core, two strategies are pursued: A) microporous carbon materials with varied porosity, however, similar chemistry, and B) microporous carbons with tuneable chemistry, but similar porosity. Approach A involves the selection of commercially available activated carbons (ACs). Strategy B is based on the modification of the chemical composition (i.e., amount and type of N-sites) of zeolitic imidazolate framework (ZIF-8) derived carbons. In both cases, the shell is realized by chemical vapour deposition (CVD). Different analytical methods, e.g., powder XRD, gas physisorption (N2, Ar, CO2), XPS, and SAXS are used to thoroughly characterize morphological and chemical features of the core as well as of the core-shell carbons. These features are linked to the electrochemical characteristics of the materials. After CVD-coating, all materials show a significant reduction in detectable surface area (up to a factor of up to 190x) by N2-physisorption. The coating technique is successfully applied to a range of AC-materials, enabling to link porosity features to Na-storage behavior. For the best performing AC-based material, the reversible capacity is increased from ~140 mAhg-1 to ~400 mAhg-1 while irreversible capacity is decreased from ~640 mAhg-1 to ~90 mAhg-1. The results of the coated ZIF-derived carbon reveal that a higher nitrogen content leads to a greater capacity in the sloping region, but to a lower capacity in the plateau region of the voltage profile. Generally, core-shell carbon anodes promise to enable high capacities accompanied with low irreversible losses.

Porosity tuning and “doping” with heteroatoms and/or transition metals are key strategies to improve the performance of porous carbons. Nitrogen doped carbons (NDCs) with macrocyclic functionalities and the respective M-coordinated NDCs (M-N-Cs) containing FeN4 sites, are very promising electrodes for Na-storage and electrocatalytic conversions. Their functional groups are reminiscent of those in biomolecules like e.g. hemoglobin. The materials can carry ions and can be regarded as “ionophoric” carbons.