Figure 1.1. A schematic showing the construction of cylindrical LIB and the constituent elements used in each component (Kim, et al., 2021).

EOL Management

Most LIBs have a lifespan of between 3 and 8 years (Mossali, et al., 2020), after which they reach their so-called end-of-life (EOL). Individual cells from EOL LIBs, and particularly those from electric vehicles (EVs), can be reused for applications such as backup power and stationary energy storage units (Kim, et al., 2021). At present, this is feasible because the demand for used batteries far outweighs the supply of used batteries (Harper, et al., 2019). However, as the uptake of EVs increases, it is anticipated that the supply of cells (new and reused) will eventually outweigh the demand. At this point, recycling will become a necessity. Furthermore, a percentage of cells from EOL batteries cannot be reused due to irreversible damage that can occur. These have a final fate of recycling, regardless of the route they take to arrive there (Harper, et al., 2019). Currently, only a small percentage of the batteries that are produced are being recycled at EOL (Velázquez-Martinez, et al., 2019), with estimates that 95% of the LIBs that were produced globally in 2016 remaining stockpiled in old devices (Heelan, et al., 2016). This is predominantly due to most people not knowing how to safely manage EOL LIBs and being ignorant of the risks resulting from poor management (Mossali, et al., 2020).
Although recycling is a necessary treatment process for EOL LIBs, it is also a source of secondary raw materials (Velázquez-Martinez, et al., 2019; Harper, et al., 2019). Raw materials of particular interest from recycling include Co, Cu, Ni, Al, and Li, due to their high value (Li, et al., 2018; Zhang, et al., 2021). This is of importance due to the potential of demand outstripping supply for some materials, such as Li (Sonoc, et al., 2015). Additionally, in comparison to the production of virgin Li, in which ± 250 tons of feedstock are needed to produce 1 ton of Li, recycling can recover 1 ton of Li from ± 28 tons of feedstock (Harper, et al., 2019). Furthermore, the production of some virgin materials, such as Co, raise numerous social and ethical concerns (Harper, et al., 2019). More importantly, the recycling of LIBs prevents hazardous materials from entering the environment.
The two commonly used categories of LIB recycling are pyrometallurgical and hydrometallurgical processes (Kim, et al., 2021). Newer direct recycling processes are also under development, but these are not widely operated on an industrial scale yet. Pyrometallurgical processes are the most prevalent because they can handle large volumes of LIBs without need for pre-treatment and regardless of their state-of-charge (SOC) (Zheng, et al., 2018; Kim, et al., 2021; Rouhi, et al., 2021). When using pyrometallurgical processes, Ni, Co, and Cu are recovered from the batteries, but other materials such as the Li, Al and Mn are lost in the slag, while the graphite burns (Yi, et al., 2021). Hydrometallurgical processes are generally better equipped to enable the recovery of materials; however, they require shredding of the LIBs as a first step in the process (Wuschke, et al., 2019). The shredding processes must have a means of damping explosions and preventing fires or else additional stabilisation of the LIBs is required before shredding (Segura-Bailón, et al., 2024).

Discharging (stabilisation)

For LIBs, discharging process refers to processes where the amounts of metallic Li present in the cells are oxidised, preventing any violent reactions which occur when the Li is exposed to oxygen and/or water in the air (Duan, et al., 2022; Yang, et al., 2022). The discharging process (also referred to as stabilisation) forces the Li+ ions to move from being absorbed by the graphite anode back to a stable cathode material. This means that lithium is thermodynamically stable and will not react violently if exposed to air or water (Punt, et al., 2022). This also allows to maximise the recovery of the lithium from the cathode by removing it from the anode. It also prevents short-circuits leading to thermal runaway (TR) events (Perea, et al., 2018; Sahraei, et al., 2012), as this would lead to the self-ignition of the metallic lithium present in the graphite anodes of the cells (Jena, et al., 2021; Zheng, et al., 2023) without the need for an external fuel source or oxygen once they have reached a certain temperature (Sommerville, et al., 2020)

Thermal runaway

Thermal runaway (TR) events in batteries can lead to major dangers, including the formation of hydrogen fluoride (HF) from the polyvinylidene fluoride (PVDF) binder, formation of carcinogenic nickel oxide, and exothermic decomposition of the conducting salt, LiPF6 to form hydrogen fluoride (Hanisch, et al., 2015; Lee, et al., 2023). Nedjalkov et al. (2016) identified 11 different hazardous gases being released from a LIB while in TR. These include styrene, hydrogen fluoride and acrolein.
An oft-neglected fact is that the danger of TR exists even in stockpiled batteries awaiting processing (Harper, et al., 2019). It was reported that 50 % of all recycling and waste fires in the United Kingdom were due to stockpiled LIBs (Brown, et al., 2021). It is therefore important that these stockpiled LIBs be made safe to prevent TR. LIBs that are no longer susceptible to TR have the added advantage of significantly reduced transportation costs (Larouche, et al., 2020).

Stabilisation processes

There are several different techniques used for the stabilisation of LIBs. These techniques are listed in Table 1.2. There is limited research that has been undertaken on the stabilisation of LIBs. Of the 346 articles reviewed by Gao et al. (2024) only 75 even considered the discharge methods utilised as information worth mentioning, and even fewer of these gave more than a cursory mention of these discharge methods. This has led to an incomplete picture of the best methods of stabilisation (Fang, et al., 2022).
Table 1.2. Stabilisation techniques used for LIBs.