07: Battery Designs for Recycling at the Cell Level

Hi everyone! In this post, I will explore some battery designs that can be used to improve battery recycling at the cell level.

Intracellular Polymer Binders

Once the cell is opened and the electrodes are separated, the next challenge is to separate the active material from the current collector and the polymeric binder. Polymeric binders provide adhesion and interconnectivity between electrode components, but they cause significant issues when left as a residue within battery waste streams obtained via shredding, known as the ‘black mass’. Interactions between the binder and the other electrode components occur during slurry mixing via two mechanisms: direct binding, where the binder is physically adsorbed to adjacent particles forming interparticle bridge and indirect binding, where the polymer forms a chemically inert network which constrains the particles.

Sufficient dispersion of particles is also imperative in the formation of homogeneous slurries and is dependent on numerous factors, such as the density, flexibility and polarity of polymers, to promote electrostatic repulsion. Usage of appropriate solvents for a given polymer facilitates dissolution and aid in dispersion of particles within the slurries. For instance, the conventional polyvinylidene fluoride (PVDF) binder possesses a high dipole moment necessitating the use of polar solvents, such as N-methyl-2-pyrrolidone (NMP), to dissolve the polymer and resist flocculation within the electrode manufacturing process.

Binders also play an important role in electrochemical performance as key attributes of the binders such as flexibility and oxidation/reduction resistance, can dictate the degree of structural changes and chemical decomposition, impacting the amount of capacity fade and consequently the lifetime of these batteries.

Alternative Binders

Recently, alternative water miscible binders have been the focus of research to reduce the usage of toxic solvents used in conventional electrode slurries, such as NMP. However, implementation of alternative binders is also essential to facilitate simplified and low energy separation of the electrode materials during battery disassembly. Fluorinated binders, such as PVDF, require high temperature pyrolysis to be removed, which produces toxic gaseous products such as HF during decomposition. In-service breakdown products from PVDF, such as HF, are capable of reacting with transition metal oxides within the cathode active materials, decreasing their capacity.

These conditions could be significantly improved if an alternative binder is used in manufacturing. At present, the use of alternative binders has been largely limited to current and next generation anodes, with the most common example being carboxymethyl cellulose (CMC)/styrene butadiene rubber (SBR), due to CMC being dispersible in water and SBR possessing good thermal stability, flexibility and adhesion. Additionally, other water miscible binders, such as guar gum, gelatin, sodium alginate and chitosan have been investigated, showing similar properties to the CMC/SBR binders, with the possibility to be further enhanced via modification. These water dispersible binder systems eliminate the need for the intensive conditions required for conventional battery separation steps, allowing facile separation of the active material and current collector, promoting the production of higher purity waste streams and simplifying subsequent recycling procedures.

Incorporation of alternative binders into the cathode, as well as the anode, would simplify subsequent recycling procedures, minimize the use of harmful solvents, additives and high-power consuming processes, as well as attaining better recovery of the cathode active materials, which currently make up the majority of the value of end-of-life batteries.

Environmental Impact

To assess the effect alternative electrode binders have on battery disassembly, a ultrasound delamination technique was used. This has already been shown to have a beneficial technoeconomic analysis compared to many hydrometallurgical processes. Only delamination was assessed, so that the environmental impact of replacing the conventional binders can be emphasized. To obtain the input materials for this process from the opened module, the cells acquired in the previous step would have to be opened before the cell components, i.e. electrodes, separators, electrolyte and packaging, are separated into distinct waste streams.

Two scenarios were investigated for electrode delamination, one being a reference scenario, using PVDF and CMC/SBR as the cathode and anode binders respectively. This scenario was compared to a hypothetical pouch cell using water miscible binder systems in both electrodes. Electrode delamination of the cathode and anode will result in the formation of four distinct waste streams; the separated anode/cathode active materials, and their respective current collector foils.

Figure 2 (a) shows a significant reduction in the GWP of ultrasonic delamination when alternative binders are utilized within the electrodes, with the recovery of both types of active material reducing their environmental impact. Figure 2 (b) and (c) show the power requirements and the GWP associated with solvent usage of ultrasonic delamination. This allows determination of whether the changes to the power output of the ultrasound or the solvents and additives used will reduce GWP the most. Since the anode already uses a water miscible polymer, CMC/SBR, the reduction in GWP is not as extensive as that seen for the cathode, where PVDF is utilized. Although the impact that changing the anode material has on GWP is smaller than for the cathode, the elimination of additives and associated manufacturing routes can be significant, when dealing with the considerable amount of battery waste which will be seen in the coming years.

While ultrasonic delamination is effective at removing the active material from the current collectors, the binders are still adhered to the active material particles, requiring high temperature processing in order to remove the binder. Overall, when comparing the reference scenarios to the best alternative scenarios, i.e. using the glue dot and ‘water miscible’ scenarios, the % reduction in GWP in producing the separate anode and cathode material is 150% and 173%, respectively. A similar reduction will also be observed in processing costs of battery recycling, when comparing the reference and alternative scenarios. It is reasonable to assume that novel cell designs, new structural adhesives and water miscible binders will minimize recycling processing costs.

Sources

Scott, S.; Islam, Z.; Allen, J. K.; Yingnakorn, T.; Alflakian, A.; Hathaway, J.; Rastegarpanah, A.; Harper, G.; Kendrick, E.; Anderson, P. A.; Edge, J.; Lander, L.; Abbott, A. P. Designing lithium-ion batteries for recycle: The role of adhesives. Next Energy 2023, 1 (2), 100023. https://doi.org/10.1016/j.nxener.2023.100023.