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,
- Peng et al. (2019) stabilised cells from spent LIBs to a residual
voltage of less than 2 V,
- Marshall et al. (2020) discharged cells to below 2.5 V,
- Yao et al. (2020) considered a discharge to less than 0.5V as a full
stabilisation, as a cell discharged to this level would release almost
no further charge,
- Yao et al. (2020) also considered stabilisation to 1.0 V (“quick
stabilisation”) as sufficient to keep the dismantling process for
LIBs relatively safe,
- Tao et al. (2023) discharged the cells to below 1 V,
- Lee et al. (2023) showed that significant swelling occurred in pouch
batteries when discharged below 2 V, and they recommended that for
safety, the cells should not be discharged below 2.5 V,
- Kwade & Diekmann (2018) recommended a discharge to a SOC of 0%
(which corresponds to a voltage of less than 2 V),
- Wuschke et al. (2019) recommended a similar 2% SOC.
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).