Discussion

The chemical stabilisation of LIBs is a cheap and easily implemented method. Due to them being cheaper than other salts, chloride-based salts, such as NaCl, are the most commonly used. The use of salt solutions for battery stabilisation has largely accepted without serious critical evaluation despite the risks that have been described (Liu, et al., 2019; Rouhi, et al., 2021). Recently, there have been critical evaluations of the use of salt solutions for the stabilisation of LIB cells by authors such as Ojanen et al. (2018), Shaw-Stewart et al. (2019) and Punt et al. (2022). Some of the biggest risks and concerns that have been noted regarding the use of chemical solutions for the stabilisation of batteries are the corrosion risks and the byproducts that could be produced through chemical stabilisation.

Corrosion risks

For the most part, the environmental impact of chemical discharging techniques has been neglected (Xiao, et al., 2020). Fang et al. (2022) observed that in almost all cases, good discharge capability of a solution was tied to corrosion being present. Segura-Bailón et al. (2024) also recognised that a higher corrosion level was linked to a faster discharge rate. The corrosion of a ferrous shell, when using a saline discharging solution, results in the risk of electrolyte leakage into the solution (Xiao, et al., 2020). This could result in a severe threat to the environment and human health if not managed correctly. The correlation could mean that the stabilisation is not being caused by chemical discharge, but rather by a physical process where the electrolyte is removed from the cells or else is contaminated to a point where a current can no longer flow. Li et al. (2016b) showed that the effluent wastewater from a battery stabilisation process using NaCl solutions contained high levels of aluminium, iron, and phosphorus as well as moderate levels of cobalt, lithium, copper, calcium, and manganese. It is probable that these chemicals have been leached out of the battery due to the corrosive nature of sodium chloride solutions. This means a loss of material, which could otherwise be recovered, as well as the need for an expensive effluent treatment facility. An additional hazard reported by Lee et al. was the generation of chlorine gas immediately upon immersion of the cells into a NaCl solution. The LiPF6 from the electrolyte was found to leak from the battery due to the corrosion. According to Wang et al. (2022), the halide salts are the most corrosive, with carbonate and phosphate salts showing lower corrosion risks. Yao et al. (2020) captured the sediment from battery stabilisation processes using various discharge solutions and found that the mass of sediment in the NaCl solution was the highest, pointing to the NaCl solution having the highest degree of corrosion. The MnSO4 solution produced no observable sediment as it has the lowest degree of corrosion. They also considered the concentration of metals in the solution after the discharging process and they found that the NaCl solution contained a significantly higher concentration of Li and Co than the other two solutions, meaning that there was likely a leakage of electrolyte and a leaching of the valuable Co. The corrosion risks destroy the ability to directly reuse materials from the cells and complicates the recycling process (Sommerville, et al., 2020). However, the cheap nature of chemical stabilisation methods mean that these methods are often still preferred despite the risks.

False stabilisation and rebound

Ojanen et al. (2018) showed that the battery terminals corroded “instantly within seconds, hindering the discharging reaction”. They went on to suggest that the reason that Lu et al. (2013) were able to achieve a complete discharge of a battery within 7 minutes in a 10 wt. % NaCl solution was not due to the electrochemical discharge of the battery, but in fact the loss of capacity of the battery due to disintegration of the components thereof. This raises a serious risk that many of the data reported by authors listed in Table 3.1 are inaccurate (Rouhi, et al., 2022). In almost all instances, halide salts have been shown to give the “fastest” discharging time, but this raises the question about whether this faster discharge time is primarily due to the movement of ions between the anode and the cathode within the cell, or due to corrosion of the cell resulting immobilisation of the ions within the cell. Ojanen et al. (2018) found that although it was impossible to bring a cell to a voltage of 0 V by solely using electrochemical discharging methods, it was possible to discharge a cell to below 2 V. Consequentially, their work suggests that all measurements that indicate a reduction of voltage to 0 V are likely erroneous due to corrosion occurring within the cells. In chemical stabilisation methods (section 3.4), the rebound appears to be independent of the discharge medium. Rouhi and co-workers (2021) investigated the possibility of using sequential discharge with multiple stabilisation and rebound cycles. They found that it was possible to lower the voltage output of the rebounded battery to below 2 V, but only after 4 discharge cycles over a period of almost 900 hours. They also investigated a long-term discharge of a battery (340 hours), and found that there was a reduced rebound, but that the rebound did occur. It is therefore important that the true voltage of the cell be measured to ensure sufficient stabilisation, as the voltage generated by a LIB cell can be decreased while undergoing discharge, and after rebound the voltage and SOC can be significantly higher (Rouhi, et al., 2021). One mechanism suggested to avoid the relaxation of the cells after discharging is to short-circuit the terminals once they have been stabilised (Diekmann, et al., 2017). Major concerns with chemical stabilisation are therefore, firstly, whether the perceived discharge is due to a reduction in the amount of lithium ions present within the cell, or a false discharge due to removal of the electrolyte through corrosion, and secondly, whether the reduced voltage achieved through chemical stabilisation will rebound when the cell is removed from the solution.

Emission of hazardous materials

Another major concern is the potential of chemical stabilisation methods to produce hazardous materials which could enter the environment. This is due to changes in the battery materials induced by the discharging process (Wang, et al., 2024). Cells are, after all, chemical reactors, and therefore, as they are pushed beyond their design limits, there are bound to be undesirable chemical reactions occurring, such as the dissolution of copper and subsequent deposition in different areas within the cells as Cu, Cu2O or CuO (Wang, et al., 2024; Kim, et al., 2024). This results in the copper recovery being reduced. Two particularly undesirable cases are the generation of hazardous gases such as HF, and the leakage of per- and polyfluoroalkyl substances (PFAS) into the environment in the leach liquor.

Off-gases

Chemical stabilisation methods have a potential to generate HF particularly if there is contact between the electrolyte (LiPF6) and water (Al-Thyabat, et al., 2013). The resultant gases from any reaction that occurs between these components include HF and POF3 (Punt, et al., 2022; Diaz, et al., 2019). In addition, the LiPF6 can decompose in dry environments to form LiF and PF5. The PF5 will also create HF and POF3 if exposed to water. It is therefore important that the corrosion of the LIB does not result in the contact of the electrolyte with water.

PFAS compounds

In pyrometallurgical recycling processes for LIBs, the initial treatment steps are at temperatures that are sufficiently high to ensure that PFAS substances that are used in the LIBs are destroyed (Rensmo, et al., 2023). However, with chemical stabilisation processes that risk leaking the electrolyte from the cells, there is a risk of the emission of PFAS into the environment, because these are generally very stable at room temperature. This needs to be considered in the development of a cell discharging process using salt solutions, as corrosion of the cell container could lead to the contamination of the stabilisation solution with the electrolyte of other fluorinated liquids. In this case, it is important that the stabilisation solution is correctly treated, to prevent these PFAS from entering the environment.