Li-ion batteries (LiBs) in electric vehicles (EVs) finish their life with a significant amount of capacity left in them (about 80% of the nominal capacity), which provides a promising avenue for reusing the spent EV-batteries in less demanding second-life applications, such as grid-scale energy storage for peak shaving, EV charging, storage for intermittent energy sources (solar or wind power), backup storage for industries and property owners, and less demanding vehicle propulsion (ferries or forklifts) [1, 2]. However, reusing spent EV batteries in second-life applications is not as straightforward as taking a battery pack from an EV then installing it directly into a second-life application. One must consider the state-of-health (SoH) of the battery packs and hence the modules and cells to avoid any mismatch in terms of capacity, state-of-charge/depth-of-discharge (SoC/DoD). Even within the batteries suitable for reuse, cells must be sorted by similar remaining capacity and identical degradation state, or else the second-life system performance would suffer. The SoH needs careful assessment and ageing conditions evaluated to send heavily degraded batteries to recycling facilities. Whilst assessing the SoH is straightforward [3], identifying the ageing condition is complex, as ageing and degradation of LiBs over time are caused by various factors, including charging/discharging rate (C-rate), operating temperature, lifetime, SoC, and cycling [2]. Moreover, pack design, configuration, cooling methods as well as cell/module’s orientation in a pack can influence the battery degradation.

In the present study, the effect of cell orientation on battery ageing and degradation has been investigated that can have an impact on the life of a battery in second-life applications. Eight large-size pouch batteries from two differently orientated modules from a dismantled first-generation Nissan Leaf retired battery pack have been analysed utilising infrared (IR) thermography and electrochemical impedance spectroscopy (EIS) techniques along with a brand-new second-generation Nissan Leaf battery which has almost the same geometry as batteries from the retired pack. Temperature derivative maps over the battery surface during discharging have been analysed, which show a direct correlation with the battery’s heat generation rates. Obtained results show that the thermal behaviour of brand-new batteries in orientations mimicking aged battery's orientation in the pack during EV life are very similar showing that the temperature derivative map’s hot spot is more towards the edge opposite to gravity vector (Figure 1 left). Also, EIS results (R_CT+R_SEI, charge transfer and solid electrolyte interphase layer resistances) show a wider range over SoCs for rotated-aged than flat-aged cells (Figure 1 right). It is worth noting that cells aged in flat orientation retained higher capacity compared to the cells aged in rotated orientation. These results show that different LiB orientations in EV batteries cause ageing non-uniformities over the battery surface, which would impact their second-life applications [4]. Non-uniform ageing is found to be more pronounced for the rotated module compared with the flat orientation inside the battery pack (Figure 1). Based on the present results, it is clear that avoiding different orientations in the battery pack can be a sustainable design for future EV battery back if reusing of spent EV batteries is envisaged.

This work was part of the ReLiB project (https://relib.org.uk) and was supported by the Faraday Institution (https://www.faraday.ac.uk; grant numbers FIRG005 and FIRG027).

References

[1] ReLiB: Reuse and Recycling of Lithium-ion Batteries, accessed 12 December 2021, <https://relib1.relib.org.uk>.

[2] P.S. Attidekou, Z. Milojevic, M. Muhammad, M. Ahmeid, S. Lambert, P.K. Das, “Methodologies for large-size pouch lithium-ion batteries end-of-life gateway detection in the second-life application,” Journal of the Electrochemical Society, vol. 167, pp. 160534, 2020, DOI: 10.1149/1945-7111/abd1f1.

[3] M. Muhammad, M. Ahmeid, P. Attidekou, Z. Milojevic, S. Lambert, P. Das, “Assessment of spent EV batteries for second-life application”, 2019 IEEE 4th International Future Energy Electronics Conference (IFEEC), IEEE, pp. 1-5, 2019, DOI: 10.1109/IFEEC47410.2019.9015015.

[4] Z. Milojevic, P.S. Attidekou, M. Muhammad, M. Ahmeid, S. Lambert, P.K. Das, “Influence of orientation on ageing of large-size pouch lithium-ion batteries during electric vehicle life,” Journal of Power Sources, vol. 506, pp. 230242, 2021, DOI: 10.1016/j.jpowsour.2021.230242