Challenges in mechanical recycling of batteries

A series of mechanical processes are carried out during the Crushing and Sorting stages of Li-ion battery recycling. They involve crushing, sieving, magnetic separation, fine crushing, and classification.

Multistage crushing and sieving result in separation of the metal-bearing particles from the waste. The input to this process are deeply discharged batteries.

The outputs are Black Mass (a powder mixture of cathode material and graphite); Aluminium (from battery cases and the foil which carries the cathode material); Copper (from internal connections and the foil which carries the graphite); Separator Plastic (the porous membrane used to separate the anode and cathode); Electrolyte (the fluid used to facilitate the flow of electrodes which creates the energy in the cell).

There are several challenges related to these mechanical processing and separation steps.

1) Challenges originated from the presence of the electrolyte and binder

1.1) Fire and hazardous fumes

The battery electrolyte is the main source of flammable substances. Electrolyte in generally is based on lithium hexafluorophosphate (LiPF₆) but also other Li-salts (LiBF₄, LiClO₄ or LiSO₃CF₃) containing halogens. When overheated during the mechanical treatment the electrolyte will evaporate and will be released. The gases do not need to be ignited instantly. At higher temperature the hydrogen fluoride HF, phosphorus pentafluoride (PF₅) and phosphoryl fluoride (POF₃) can be formed as a consequence of the electrolyte, binder – polyvinylidene fluoride (PVDF) decomposition.

Compounds with fluorine content can also be applied as flame retardants for the components such as electrolyte or separator, even they can be used as the additives for cathode and anode materials, usually in a form of e.g. fluorophosphates. The decomposition of LiPF₆ salt is promoted by the presence of water/humidity according to the following reactions:

LiPF₆ ↔LiF + PF₅ (1)

PF₅ +H₂O ↔ POF₃+2HF (2)

LiPF₆ +H₂O ↔ LiF + POF₃+2HF (3)

The electrolyte organic solvents have very low decomposition temperature (Table 1) and can serve as a potential “fuel” for the fire.

The risk of the fire and presence of hazardous gases from the decomposition has to be controlled. Inert gases such as CO₂, N₂, or Ar are often used to create an inert atmosphere inside of the equipment and as a transfer media that is used to collect the gas decomposition products. Consequently, the gases have to be cleaned and neutralized after removal.

Table 1:

1.2) Bad separation of the foils from the cathode materials due to the binders

Li-ion batteries contain copper and aluminium foils to carry the graphite and cathode material, respectively. To keep the electrode materials on the foils in the batteries a binder is applied.

The most common binder is Polyvinylidene fluoride (PVDF) which keeps powder materials on the foils. It is easiest to remove the copper and aluminium through mechanical processes before hydrometallurgical processing.

However, the aim of the separation is to recover and separate as much of the Black Mass as possible from the foils. The presence of the binder hinders this separation and thus it is very challenging to fully separate those components. This causes some losses of the Black Mass which contain valuable metals such as cobalt or nickel.

2) Challenges originated from the material composition of the batteries

Different material composition (from the chemistry and granulometry point of view) causes several challenges in material recovery and handling.

2.1) Difficult separation of Aluminium and Copper foils

The aluminium and copper foils used in the batteries are very thin and have very similar properties, thus it is very difficult to separate those from each other. Various methods are used to separate the materials such as eddy current separation and methods based on the difference in the density of the materials.

2.2) Dust generation

The Black Mass powder is very fine (<0.25mm). This means that mechanical processing needs to be performed using extensive ventilation and dust-collection systems to collect all the particles from the environment. The cathode material contains heavy metals, which represent a significant health risk for workers if not collected properly.

3) Challenges of material handling, storage, and waste disposal

Due to the volatile nature of many of the components of Li-ion batteries, special care and handling must be taken during the recycling process.

3.1) Battery handling

Batteries must be fully discharged before they can be properly recycled. If a charged battery is damaged or punctured, there is a risk of “Thermal Runaway”; an exothermic chain reaction inside of the cell that leads to a fire.

3.2) Black Mass

The fine particles of the Black Mass powder contain heavy metals which can be harmful. Anyone working around equipment that is processing batteries must wear special Personal Protective Equipment (PPE). This PPE includes a dust suit, ventilated mask, and gloves.

3.3) Electrolyte

The volatile chemicals in the electrolyte can pose a serious risk to humans. Even a small amount of electrolyte can cause serious chemical burns.

3.4) Waste Disposal

While steps are being taken to recycle and reuse as much of the batteries as possible, there are still some materials that must be disposed of. Since many of these materials have been contaminated with the heavy metals in the Black Mass, they must be handled and treated as hazardous waste. This waste presents extensive challenges for Health and Safety (chemicals and materials are dangerous if people are exposed), Logistics (strict governmental regulations regarding the transport of the waste), and the environment (concerns about how the waste can be burned or otherwise disposed of).

Conclusion

These challenges represent a few of the obstacles currently facing the battery recycling industry. As the world changes to rely more heavily on battery power, more and more research and effort will be put into solving these problems and pursuing the opportunities that they create.