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.

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.

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.

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.

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.).