Upon reaching certain limits, electric vehicle batteries are replaced and may find a second life in various applications. However, the state of such batteries in terms of aging and safety remains uncertain when they enter the second-life market. The aging mechanisms within these batteries involve a combination of processes, impacting their safety and performance. Presently, direct health indicators (HIs) like state of health (SOH) and internal resistance increase are utilized to assess battery aging, but they do not always provide accurate indications of the battery’s health state. This study focuses on analyzing various HIs obtained through a basic charging–discharging cycle and assessing their sensitivity to aging. Commercial 50 Ah pouch cells with different aging histories were tested, and the HIs were evaluated. Thirteen HIs out of 31 proved to be highly aging-sensitive, and thus good indicators. Namely, SOH upon charging and discharging, Coulombic efficiency, constant current discharge time, voltage relaxation profile trend, voltage–charge area upon discharging, hysteresis open circuit voltage HIs, and temperature difference between the tabs upon charging. The findings offer valuable insights for developing robust qualification algorithms and reliable battery health monitoring systems for second-life batteries, ensuring safe and efficient battery operation in diverse second-life applications.

In this study, commercial 64.6 Ah automotive pouch-cell lithium-ion batteries were used to identify lithium plating (LP) by operando nondestructive electrochemical methods. These methods rely on the analysis of the cell voltage and cell current data, which is usually acquired by a battery management system (BMS). The cells were cycled under specific cycle-rate and temperature boundary conditions (BCs) in order to induce LP. The selected BCs were 0, 10, and 22 °C as test temperatures in combination with 1, 1.5, 2, and 3 C as C-rates. The chosen electrochemical methods are voltage relaxation profile (VRP), differential voltage analysis (DVA), and incremental capacity analysis (ICA). These methods are applied to different sections of charge/discharge cycle voltage and current data to identify the LP. ICA can be applied to the constant current phase of the charge cycle, VRP can be applied during the relaxation time after the charge cycle has finished, and DVA can be applied to the CC-phase of the discharge cycle. These methods are promising, as they do not require additional equipment for data acquisition, proving feasible for online monitoring by the BMS in an electric vehicle. Our investigation shows that in the case of VRP time derivative analysis, with decreasing temperature and increasing C-rate, the plateau/peak position shifts to the right, indicating increased LP during the charge cycle. During the DVA, the characteristic Li-stripping peak shifted to higher capacity values at lower temperatures and higher C-rates. Surprisingly, at a test temperature of 0 °C, the LP is higher at a rate of 2 C than at a rate of 3 C. This effect is attributed to the higher cell polarization at 3 C than at 2 C during the charge phase. In the ICA, at cycling rates of 1.5 C and above, an additional peak was observed at cell voltages above 4 V. This new peak represents an additional reaction taking place at the anode surface, identified as LP. For 0.5 C, no LP was identified in ICA, and LP was only identified at 0 °C at 1 C, whereas LP was always identified at cycling rates of 2 and 3 C at all test temperatures. The described methods and findings open the path toward advanced online LP identification and eventually state-of-health diagnosis of automotive batteries.

Lithium plating (LP), as a specific degradation mechanism in lithium-ion batteries (LIBs), has been thoroughly investigated regarding formation conditions and potential safety hazards, but it is yet unknown how this effect influences the mechanical properties of batteries in the case of mechanical deformation. To address this issue, pouch cells used in EVs were artificially aged (AA) to a state of health of 80–82% in conditions that predominantly cause the formation of LP. These cells were subjected to a mechanical abuse load, and safety-relevant parameters, such as tolerated deformation level, failure force, and the process of thermal runaway (TR), were analyzed and compared with respective fresh (F) and aged cells of the same type. Complementary microscopy analyses were carried out to compare the found changed mechanical response with the different layer morphology caused by LP. The tests did exhibit a significantly different mechanical response of cells in the three states but also clearly altered short-circuiting behavior. The tolerated peak force at discharge state dropped by −28% and at charge state by −37% compared to fresh cells, while the deformation at failure slightly increased by +6% for the AA cells. A clear reduction in stiffness (−16%) of the LP cells was attributed to the formed layer, identified as mossy LP. The significantly stronger voltage drop at failure, seen for the LP cells, was associated with severe exothermal reactions of LP in contact with air and moisture during TR. This study revealed the strong influence of LP on the mechanical properties of LIBs. However, the transferability of the findings to other cell chemistries or formats is unclear, emphasizing the need for further investigations in this research field.

While the effect of ageing has been thoroughly analysed, to improve the cycle life of lithium-ion batteries, its impact on safety in case of a mechanical loading is still a new field of research. It has to be found out how mechanical properties, such as the tolerable failure force or deformation, change over the operational lifetime of a battery. To answer this question, mechanical abuse tests were carried out with pouch cells used in recent electric vehicles in a fresh state and after usage over 160.000 km. These tests were complemented with a detailed component level analysis, in order to identify mechanisms that lead to changed cell behaviour. For the analysed aged cells, a significantly different mechanical response was observed in comparison with the respective fresh samples. The tolerable force was severely reduced (up to −27%), accompanied by a notable reduction in the allowable deformation level (up to −15%) prior to failure, making the aged cells clearly more safety critical. Based on the subsequent component tests, the predominant mechanism for this different behaviour was concluded to be particle cracking in the cathode active material. The found results are partly in contrast with the (few) other already published works. It is, however, unclear if this difference is rooted in different cell chemistries or types, or another battery state resulting from varying ageing procedures. This underlies the importance of further investigations in this research field to close the apparent gap of knowledge.

The performance of LIBs deteriorates over time due to various aging mechanisms, among which lithium plating (LP) is critical. The goal of this research was to analytically approve the presence of LP, prior identified by non-destructive methods.

In this study, the effect of lithium plating (LP) on the thermal properties of lithium-ion batteries (LIBs) was investigated. A large-format pouch 64.6 Ah cell with a graphite-SiO_x/NMC chemistry was artificially aged (AA_LP) in the laboratory under specific conditions to induce LP on the anode. For thermal behavior analysis, temperature ramp experiments were conducted in a nitrogen-filled steel container on the cycled cell, as well as on fresh and real-life aged cells with the same specifications. Characteristic temperatures, such as first venting and safety critical temperatures, were monitored; additionally, the exhaust gas composition was analyzed using Fourier transform infrared spectroscopy (FTIR) and gas chromatography. It was revealed that the voltage decay of the cells started well before any safety-critical temperature, and the first venting of the AA_LP cell was significantly reduced to 112 °C in comparison to the fresh and real-life aged cells, in which it occurred at 130 °C and 134 °C, respectively. The earlier venting of the AA_LP cell was attributed to the reaction of the plated metallic lithium and the electrolyte. The safety-critical temperature rate (>10 °C/min) occurred at 160.9 °C for AA_LP and at around 159.1 °C for the fresh and real-life aged cells. The maximum temperatures reached were 616 °C, 553 °C, and 566 °C for the fresh, real-life aged, and AA_LP cells, respectively. No significant difference was observed in the exhaust gas after the thermal runaway for the tested cells.