The Latest Advancements in Battery Technology and Recycling

The widespread use of lithium-ion (Li-ion) batteries in a variety of industrial and private applications amplifies the need to recycle and reuse the constituent components. In addition, increasing energy consumption due to world population growth and the depletion of fossil-fuel resources leads to a strong demand for more alternative renewable energy sources.

A new generation of batteries is under consideration and research is underway to ensure higher energy densities, faster charging capabilities, and longer cycle lifetimes.

While these new generations or modifications can address some of the issues Li-ion batteries have during production and operation, the recycling cannot be directly carried out by established processes. Therefore, new processes and solutions are needed when the already complex mixture of Li-ion batteries expands to new materials and chemistries.

Challenges for lithium-ion batteries

In state-of-the-art manufacturing processes of positive electrodes for lithium-ion batteries, polyvinylidene difluoride (PVdF) is used as binder, which provides excellent adhesion and electrochemical stability. The range of 1-3 wt. percent of binder is used in the composite electrode formulation, and it determines the processing solvent, which must be used to dissolve the binder. In the case of PVdF, N-methyl-2-pyrrolidone (NMP) is the commercially established solvent. However, NMP was classified as a reproductive toxicant in 2009. In 2011, NMP was included in the candidate list for authorization as a substance of very high concern by the European Chemicals Agency (ECHA); its flammability also poses serious risks to the environment and human health. Therefore, complex and expensive safety measures are mandatory to ensure minimal risks during production and handling. The energy-intensive drying process for the removal of NMP from the electrodes and the required solvent recovery also increases the ecological and economical footprint during battery cell production.

Apart from the concerns regarding NMP as the solvent, the binder PVdF itself is considered a potential risk as a per- and polyfluoroalkyl substance (PFA), which is facing an imminent ban in Europe due to its persistency, leading to bio-accumulation with lesser-known long-term health and environment effects. Therefore, alternative processing approaches that exclude NMP as the processing solvent and replace fluorine-containing PFA binders is the goal for future generations of sustainable Li-ion batteries in the European market.

From the recycling perspective, reducing fluorine (and organic) sources has a beneficial aspect. Later processing steps of the black mass, like flotation, can be interfered with by these compounds. Normally, before the flotation step, the black mass is either chemically or thermally treated to get rid of these sources. On the other hand, replacing graphite with silicon on the anode side can pose some challenges. Fluorine compounds are omnipresent and unwanted side reactions, like the formation of silicon tetrafluoride, can occur. Depending on the actual process, formation of silicon-containing slags or black mass may lead to additional refining and cleaning steps.

Pros and cons of next-generation batteries

While some of the next-generation batteries are still being investigated in the lab and are not completely commercialized, they are promising candidates for being dominant battery technologies in the near future. Therefore, early consideration of possible recycling methods and potential problems for these battery types is very important.¹

Sodium-ion batteries

Sodium-ion batteries and similar-based chemistries with naturally highly abundant elements such as sodium, magnesium, or calcium are potential replacements for Li-ion batteries. Among these non-Li batteries, the sodium-ion technology is considered as a drop-in solution. Since it is quite similar to commercial Li-ion batteries, production and recycling need to be heavily adapted. One issue can arise from thiocyanate electrode configuration, whereas during thermal recycling steps, toxic gases can be released. In addition, such battery chemistries with low-cost elements are of little economic interest. Therefore, the recycling of such batteries must be supported by legislation.

All-solid-state batteries

Comparable to the replacement of the fluorine-based binders, with all-solid-state batteries there are fewer safety issues related to the evaporation and the accumulation of harmful fumes. The formation of corrosive hydrofluoric acid can also be neglected. While mechanical pre-treatment seems to be easier, since the applied material is reported to be very brittle, the mechanical separation can be more challenging when small, brittle particles are formed. The hydrometallurgy is also affected since more elements are introduced and the overall chemical composition is increased—especially for systems that include new elements (on both the anion and cation side) that are not incorporated in current cell chemistries. Subsequently, the concentration levels of the applied metal affects the economics of recycling when the concentration of valuable metals is reduced through high levels of low-value metals.

Li-metal

With a higher theoretical capacity compared to graphite (372 mAh g^–1), lithium metal anodes (3860 mAh g^–1) are one of the most promising options to meet these increasing performance requirements. Several of the next-generation batteries apply lithium metal as an electrode material. However, handling is critical during production and recycling. It must be ensured that reaction with water or moisture from air is excluded, which makes this chemistry quite challenging. However, compared to other systems, the lithium content is significantly higher in lithium metal cells. Therefore, the efficient recycling of lithium is mandatory, and hydrometallurgical processes play an important role.

One major issue during recycling is the high reactivity of metallic lithium. There is a high energy release and the formation of hydrogen. Furthermore, dendrite formation during operation increases the risks of short circuits and subsequent thermal runaways. The inconsistent dissolution and deposition of lithium during charge/discharge cycling, leading to the formation of high surface area lithium (HSAL), is considered one of the greatest challenges. Current strategies to develop commercially viable LMBs are focused on the reduction of HSAL growth. Handling, transport, and storage of these cells requires special protections. So far, these points have been addressed at the lab scale, involving a variety of approaches: mechanical crushing in an inert atmosphere, thermal oxidation, cryogenic treatment or discharge through electrical circuits, immersion in salt solutions, or placement in containers filled with steel chips. One promising approach that is used for Li-ion batteries is the extraction of the electrolyte, whereas here, the metallic lithium from the anode is extracted from the cell.

Li-sulfur

With the high theoretical capacity of sulfur (1675 mAh g⁻¹), lithium-sulfur batteries are among the most promising future batteries. Here, a lithium metal anode with the above-mentioned implications and a sulfur/conductive carbon composite cathode are used. With regard to the cathode, formed polysulfides during operation pose a challenge to potential recycling processes. Toxic gases like hydrogen sulfide can occur. So, in combination with the already highly reactive lithium metal anode, the handling and separation of this cell chemistry is challenging for recycling. Despite the differences in composition, the binder, separator, and electrolyte can be recycled by similar procedures that are applied today for Li-ion batteries. These points also complicate the production of lithium-sulfur cells and therefore, the upscaling from the lab approach still needs more investigation and research.

Battery recycling is an emerging field that will likely undergo significant changes as researchers eye new types of batteries for production. Overcoming the challenges of production and recycling will define their success.

References:

1. Neumann, J.; Petranikova, M.; Meeus, M.; Gamarra, J.D.; Younesi, R.; Winter, M.; Nowak, S. Recycling of lithium?ion batteries—current state of the art, circular economy, and next generation recycling. Advanced Energy Materials 2022, 2102917.