There is much debate about what a ‘safe voltage’ is in stabilisation processes and researchers have not reached any consensus on what is truly considered to be stabilisation of cells. For example,
Once a cell has been stabilised to low voltages, it should not be recharged under any circumstances, as this could create safety hazards (Afroze, et al., 2023). One of the reasons for this danger is that the copper current collectors dissolve in the electrolyte once the voltage decreases to a certain level, and recharging results in the copper precipitating throughout the cell, resulting in short-circuits and TR (Harper, et al., 2019; Langer, et al., 2021; Hendricks, et al., 2020). Apart from TR, another concern with the deposition of Cu is the contamination of downstream products (Guo, et al., 2016).
Kaas et al. (2023) showed that for the most part, the discharge level of the batteries has no significant impact on the downstream mechanical comminution processes. They did find that there was greater copper dispersion for the cells that had their poles reversed through discharging, which correlates with the findings of Harper et al. (2019). Due to the impact that the discharge level has on the material composition, and therefore the downstream products, Kaas et al. recommended that additional studies into the optimal voltage to which the cells are discharged be undertaken.
Sonoc et al (2015) undertook a safety analysis of the hazards of large-scale dismantling of LIBs for recycling, and how stabilisation mitigates these risks. They opened several stabilised LIBs, with the cells that had only been stabilised to 0.5 V producing small red flames and fumes, but the cells that had been discharged to 0.0 V not only not producing any flames or fumes, but also not showing any evidence of reaction (temperature change or discolouration). However, Sommerville et al. (2020) contradicted these findings, as they showed that the cells from a large-format LMO-NMC battery were stable at 2.5 V when opened in air, and that these cells contained less than 2 % of the total energy that could be held at full charge.

Stabilisation compatibility with direct recycling and reuse

According to Lee et al. (2023), if direct recycling or reuse is intended, LIBs should not undergo stabilisation to 0 V. This is because this level of discharge can cause permanent damage to the LIBs, giving them a poor state-of-health (SOH) and potential swelling (Langer, et al., 2021). Swelling leads to damages to the cells, which would render the battery case, the cells, and the components useless for reuse. Instead, it was suggested that there is an optimum and appropriate discharge voltage that should be attained to keep the parts of the LIBs safe for reuse while preventing the risk of explosion or shock. The challenge with this is that the batteries would need to be manually dismantled while still holding charge. This would need to be done by operators with high-voltage training and insulated tools, or else by expensive robots (Harper, et al., 2019), which adds significant costs.

Battery & cell design

Most LIBs have some form of battery management system (BMS) with a cut-off low voltage. This cut-off voltage prevents the battery from stabilisation and becoming damaged. The various types of cell designs (cylindrical, prism, pouch, etc.) impact the hazardous nature of the cells as well as the best stabilisation technique (Harper, et al., 2019). Prismatic cells, for example, can withstand greater pressures and could be more hazardous in the case of a pressure build-up and rupture. Cylindrical cells, on the other hand, have positive and negative terminals at opposite ends, often with their own fuse. Furthermore, other aspects of battery design that could impact the ease of stabilisation, dismantling, and recycling include how the cells are joined; with this varying from nuts and bolts to welding and the use of potting compounds (Gaines, et al., 2018). Chemical stabilisation techniques are the most flexible for differing cell designs and for whole batteries without the need to disconnect from the BMS, as the terminals of the cells only need to be immersed in the liquid to ensure that the stabilisation can occur.

Safe transport of LIBs

Safety is a major concern in the transport of waste LIBs (Gaines, et al., 2018). Gaines et al. (Gaines, et al., 2018) report that, in the USA, air shipment of waste LIBs is no longer allowed, and rail shippers may also no longer accept EOL LIBs. With the rise of fires and explosions on ships over the last few years, and many of these attributed to LIBs, most shipping lines will also no longer accept EOL LIBs as cargo (NOW Media, 2022; Conway, 2023). As the fire hazard is generally only present so long as there is residual charge in the batteries (Rouhi, et al., 2021), stabilisation may be a means of enabling the shipping of EOL LIBs. However, currently, with the inability to ship EOL LIBs, small, local pre-treatment or recycling plants may need to be preferred.

Voltage Rebound or Relaxation

Voltage rebound or voltage relaxation is a phenomenon of voltage increase when a load is removed from the cell (Fuller , et al., 1994). It is usually studied as a useful phenomenon for extending the operational lifetime at voltages above 2.5 V and a SOC of greater than 0% (Rouhi, et al., 2021). The extent of the voltage rebound is dependent on both the temperature and the electrode materials used in the cells (Reichert, et al., 2013).
Voltage rebound means that the SOC of the battery undergoing stabilisation would be misrepresented, as the battery would contain residual energy even though the voltage readings suggest otherwise (Rouhi, et al., 2021). It is linked to the ohmic-transient behaviour of cells under load (Roscher & Sauer, 2011).