Verification of the modified degradation mode identification technique by employing electrochemical impedance spectroscopy and differential voltage analysis

For retired automotive lithium-ion batteries, state of health (SoH) is currently utilised to grade them for a second-life application. However, researchers previously challenged this and expressed that, in addition to SoH, the actual degradation mechanism, also known as degradation mode (DM), should be considered for grading, for efficient second-life operation. To date, there is little evidence to support this. A validated DM detection technique for cell/module grading does not exist. A modified DM detection technique by tracking and indexing the incremental capacity (IC) curves was previously proposed by the authors; nevertheless, it was difficult to validate. Researchers previously proposed DM identification using Electrochemical Impedance Spectroscopy (EIS) and Differential Voltage (DV) analysis. With a direct comparison of the techniques made exploiting IC, DV, and EIS, a correlation can be made, which is presented in this article. The correlation suggests that cells identified as having the same (or different) DM by the proposed technique also identified as having the same (or different) DM growth by EIS technique proposed by other researchers. Likewise, DV analysis suggests that the DV peak’s standard deviation of similar DM cells is smaller than that of the different DM cells

Modification of degradation mechanism identification technique for cell grading

The service life of the retired automotive lithium-ion battery (LIB) can be extended through reusing without major changes and reusing through remanufacturing. Reusing through remanufacturing at the module level likely outweigh remanufacturing at the cell level and reusing without major changes. To remanufacture a battery pack, cells/modules are required to be graded and cells/modules in a remanufactured pack needs to be at identical state. The current approach utilises cell/module capacity i.e. State of Health (SoH), to grade the cells/modules at the end of first life. However, the only capacity may not be enough to match the cells/modules; how the cells/modules have been degraded (degradation mechanism, DM) may play a vital role in defining their performance in second life. However, the existing DM detection techniques are not designed to grade the cells/modules at the end of the first life. In this paper, a modified DM detection process is proposed. Here, the proposed modified DM detection process is verified by applying them to the degradation experimental results. Employing the proposed modified DM detection method, cells at same SoH at the end of first life can be graded based on their DM

Assessing the impact of first-life lithium-ion battery degradation on second-life performance

The driving and charging behaviours of Electric Vehicle (EV) users exhibit considerable variation, which substantially impacts the battery degradation rate and its root causes. EV battery packs undergo second-life application after first-life retirement, with SoH measurements taken before redeployment. However, the impact of the root cause of degradation on second-life performance remains unknown. Hence, the question remains whether it is necessary to have more than a simple measure of state of health (SoH) before redeployment. This article presents experimental data to investigate this. As part of the experiment, a group of cells at around 80% SoH, representing retired EV batteries, were cycled using a representative second-life duty cycle. Cells with a similar root cause of degradation in the first life (100–80% SoH) exhibited the same degradation rate in second life after being cycled with the same duty cycle during the second life. When the root cause of degradation in the first life is different, the degradation rate in the second life may not be the same. These findings suggest that the root cause of a cell’s first-life degradation impacts how it degrades in its second life. Postmortem analysis (photographic and SEM images) reveals the similar physical condition of negative electrodes which have similar degradation rates in their second life cycle. This demonstrates that cells with a similar first life SoH and root cause of degradation indeed experience a similar life during their second life. The experimental results, along with the subsequent postmortem analysis, suggest that relying solely on SoH assessment is insufficient. It is crucial to take into account the root causes of cell degradation before redeployment