Chemical stabilisation processes

There are two distinct uses of chemical stabilisation in literature: namely, the use of stabilisation to prepare batteries or cells for research activities, and the use of stabilisation as a pre-treatment for industrial recycling activities. However, in both cases the purpose is to prevent TR incidents, explosions and fires.

Research activities

In most research-based use of chemical stabilisation, the process is undertaken manually and on a small scale. There is often little consideration of the scaling of the process or the treatment of effluent. However, more details are provided on the composition of the discharge solution, the length of time of immersion and the temperature of operation.

Water as the discharge solution

Kang et al. (2010) placed LIBs in distilled water to discharge them after passing them through a roller press to contact the cathode and anode. These batteries were left in the water for a full day before removal, drying and downstream processing. Kim et al. (2014) also used distilled water as a discharge solution. According to Zhao et al. (2022), the immersion of charged batteries in water during the separation of the components is safe, efficient, and profitable. However, Sonoc et al. (2015) and Raj et al. (2022) pointed out that the difficulty and major danger associated with the stabilisation of LIB with pure water is the initial voltage of a charged cell will be higher than the electrolysis voltage of water, leading the splitting of the water molecules to produce oxygen and hydrogen gas These gases could lead to explosion risks if not handled safely with proper ventilation (Punt, et al., 2022).

Addition of salts

A salt, such as NaCl, is added to the water to avoid electrolysis of water in the chemical stabilisation of batteries (Hantanasirisakul & Sawangphruk, 2023) (Xiao, et al., 2020). The addition of the salt increases the electrolysis voltage of the solution and enables a faster discharge than can be achieved with water as the discharge solution. Even with the addition of salts, the discharge process can take more than 24 hours to complete (Or, et al., 2020). Although the addition of salts results in increased conductivity and electrolysis voltage, resulting in a more rapid discharge of the batteries, it also causes metal corrosion, which could result in the leakage of the electrolyte solution, leading to secondary pollution (Xiao, et al., 2020; Lee, et al., 2023). With the addition of NaCl, there is also a risk of chlorine gas generation (Kim, et al., 2021) with its associated toxicity risks (Sommerville, et al., 2020). NaCl solutions are the most used for stabilisation in research studies due to the high perceived discharge rate achievable and the low costs (Wu, et al., 2022). Other salts that have been proposed and investigated include FeSO4(aq) (Yao, et al., 2020), Na2S (Torabian, et al., 2022), MgSO4(Torabian, et al., 2022) and MnSO4 (Xiao, et al., 2020). FeSO4 is seen as a more environmentally friendly option, but it comes at a higher cost than NaCl. It also gives slower discharge rates, although Yao et al. (2020) showed that the performance of FeSO4 solutions would be comparable to that of NaCl solutions if a cut-off safety voltage of 1V was used. MnSO4 solutions give a slower discharge rate than NaCl solutions, but there is reduced galvanic corrosion, and this prevents the leakage of the electrolyte solution into the discharge solution (Xiao, et al., 2020). Most literature that mentions of the use of chemical stabilisation methods (see Table 3.1) give few specifics on how the stabilisation procedures were developed or tested (Ojanen, et al., 2018). As stated by Garg et al. (2024), much of “the literature concerning the electrochemical discharge of LIB was centred around the simplistic statement of: “batteries can be discharged in salt solutions” where the salt solutions were mainly NaCl or Na2SO4”. A majority of the experimental papers in the literature do not investigate the performance of the specific salt that is used, and therefore are not able to comment on the suitability of the salt in comparison to water or other salt solutions. Only a few authors such as Ojanen et al. (2018), Shaw-Stewart et al. (2019), and Yao et al. (2020) have considered the impact that the salt used in the discharge solution has on the stabilisation process.
Table 3.1. Literature data on the solutions used and time of the chemical stabilisation reported