Karli (2021) investigated the optimum temperature ranges for the
stabilisation of prismatic cells to improve the recovery and quality of
the cathode material from spent batteries. They found that slightly
elevated temperatures of between (318.15 and 323.15) K were optimum for
stabilisation using a salt discharge solution. According to Al-Thyabat
et al. (2013), the time needed to achieve a sufficient stabilisation is
dependent on several process parameters, including the conductivity of
the solution, the temperature of the solution, and the state of charge
of the LIB. There are therefore several variables that must be
considered in developing a chemical stabilisation process. According to
Harper et al. (2019), the stabilisation of batteries in salt solutions
is not suitable for high voltage modules and packs due to the high rate
of electrolysis and evolution of gases that would occur. It is however
the cheapest in terms of operational costs (Sommerville, et al., 2020).
Although for the most part, the objective of chemical stabilisation is
solely to reduce the voltage of the cells, there has also been some
research into using the process to produce valuable materials. Rouhi et
al. (2022) pointed to the electrolysis of ammonium-based solutions as a
means of discharging batteries, as solutions such as ammonium carbonate
and ammonium hydroxide were reported by Lu et al. (2019) to be less
severe than other solutions. The process of electrolysis of ammonium
salt solutions was envisioned by Boggs and Botte (2009) to produce
hydrogen for fuel cells.
Multicomponent & complex
solutions
Ali et al. (2022) suggested the use of ascorbic acid as an accelerant
for the discharge process. This was also explored by Song, et al.
(2015). The oxidation reaction that takes place with the ascorbic acid
is anticipated to accelerate the discharge process. However, the
ascorbic acid cannot be regenerated, making its use costly. For this
reason, it has not been explored further.
Authors such as Ojanen et al. (2018) have proposed the use of a complex
discharge solution containing solids rather than a simple salt solution.
They showed that the addition of iron or zinc powder to the discharge
solution could accelerate the discharge by up to 90%. Nan et al. (2005)
placed EOL LIB into a stainless-steel container which was filled with
water and ‘electric iron powder’. The solution was then stirred, and the
batteries were discharged completely after 30 minutes. The biggest
challenge with this is the sacrificial oxidation of the powder,
increasing the operational costs.
Garg et al. (2024) used a Fe(II)-Fe(III) redox couple electrolyte (more
specifically, a 5 wt. % solution of potassium hexacyanoferrate and
potassium hexacyanoferrate) for the chemical stabilisation of LIB to a
voltage of 2.0 V after a rebound had occurred. This method ensured that
there was no corrosion of the battery casing observed, ensuring that
there was no leakage of electrolyte into the discharge solution. Gas
evolution was still, however, observed in this discharging process, and
this was attributed to the electrolysis of water molecules and the
formation of H2 and O2. To get the
voltage to below 2.0 V, a periodic discharge regime was necessary, as
the voltage did not drop below 2.4 V with a single immersion despite
immersion for up to 167 hours.
Mikita et al. (2024) proposed a so-called redox shuttle (RS) in the form
of ferrocene and phenothiazine solution introduced into the spent cell
through a small bore in the shell. The potentials of redox reactions
induced by the RS must be between the positive and negative electrode
potentials. The positive electrode is then reduced with electron
acceptance from the RS and lithium acceptance from the electrolyte
solution. In parallel, the negative electrode is oxidized with electron
donation to the RS and lithium donation to the electrolyte solution.
Electrons are shuttled from the negative to the positive electrode,
resulting in an internal short circuit of the cell.
Agitation
Ojanen et al. (2018) showed that agitation of the solution was a
promising option for industrial applications. However, the battery
connector poles were rapidly corroded, leading to discharge and leakage
of the internal battery components.
The discharge of the cells can be further accelerated by ultrasound
cavitation effects or magnetic fields (Ali, et al., 2022). These methods
increase the movement of the ions and therefore enhance the discharge of
the cells. Yu et al. (2021) showed that the discharge rate can by
increased by as much as 90% through subjecting the cells and discharge
solution to ultrasound.
Cell vs module
Most commonly the module or the entire battery is immersed in the
solution, as the intention of the stabilisation process is to make the
battery safe to dismantle and extract the modules or the cells. However,
the investigations of Xu et al. (2017) showed that the immersion of a
battery pack results in higher risk of a high current electric arc
forming, which could result in battery poles or shells fusing, resulting
in the leakage of electrolyte, which could be ignited by the arc and
burn on the water surface.
Industrial processes
There are several commercial/industrial processes that make use of
chemical stabilisation techniques to prepare the batteries for
recycling. This contradicts the claim of Amalia et al. (2024), who
stated that there are “no reported commercial applications of battery
discharging using the submersion method in an electrolyte solution”.
The details of the companies that make use of chemical stabilisation
methods, together with a few details about the processes are given in
Table 3.2. The names of companies in brackets are related technologies
(previous names, subsidiaries, joint venture participants, etc.).
Table 3.2. Chemical stabilisation processes used in existing industrial
LIB recycling.