06: The Use of Adhesives in Battery Design

Hi everyone! In this post, I will look at the use of adhesives in designing better batteries for recycling.

Battery Connections

Some examples of joining methods can be physical (clips, screws, springs etc.), metallic (welds and solders), inorganic cements or organic adhesives. Unfortunately, the in-service properties are generally at odds with the end-of-life requirements. In service the joint needs to be durable and non-reactive whereas at end-of-life it needs to be soluble or reactive.

Barriers in Recycling

Most recycling processes start with a disassembly of the battery pack down to either module or cell level. From there, most physical and hydrometallurgical recovery start with shredding to break joints and separate the different phases which results in cross-contamination between components and results in low value product streams.

Dismantling the cell down into individual electrode materials as an alternative to shredding, can significantly improve economics and product purity but it is hampered by the complexity of the component joining techniques. However, because pack, module, and cell design vary, it is hard to achieve fully automated disassembly. Current designs make disassembly complex due to the array of connectors used, the scale and packing of the cells and mechanical and chemical damage to the components during service. One of the biggest barriers to disassembly was the number of screws when disassembling from pack to module and the number of welds and structural adhesives as well as the number of modules when going down to cell level.

Cost savings (with respect to using new material) of up to 20% could be achieved using shredding whereas cell dismantling could recover material with up to 80% cost reduction. The advantages of shredding are that it rapidly reduces the active battery into a safer format. It is a process that is easily scaled, although the atmosphere around the shredder does need to be controlled. However, shredding does not separate aluminum from lithium metal oxide efficiently and attrition milling down to sub-mm scale is required to get reasonable separation. Additionally, impurities incorporated into recycled cathode material can significantly affect the performance of cells.

Battery disassembly is also limited by the use of non-reversible adhesives in products. In some cases, with thoughtful design and strategic placement, non-reversible adhesive bonds can potentially facilitate recycling, where they enable a “path of preferential breakage” which aids material recovery. Finally, moves to make the battery pack a structural element of the vehicle have led to an increased use in structural adhesives and permanent welds to increase pack rigidity.

Designs For Disassembly

Some examples of pack, module and cell design which can be adopted for simplified disassembly and recycling are:

Fewer, but larger cells
Minimal use of thermoset adhesives
Fewer fixing types
Cells that are more easily opened
Electrode binders that can be fully dispersed using water

The polymeric components can be split into two types depending on their applications:

Extracellular: These hold the cells, modules, cooling components and the Battery Management System (BMS) together and are chosen primarily for their strength.
Intracellular: These are chosen largely for their inertness and flexibility and maintain the active material in contact with the current collector.

Extracellular Adhesives

In most pack and module designs currently used in the automotive sector, structural adhesives provide rigidity and strength to the assembly. These are inexpensive to apply and irreversibly provide the strength needed to minimize movement of cells during use. Most of the adhesives used are thermosets, based commonly on epoxides or polyurethanes.

An important design for disassembly would be to avoid using structural adhesives as the only form factor imparting strength to a module or pack. One method could be to create a permanent link between pouch or prismatic cells and strategically place a small amount of adhesive at a point where selected directional movement could physically break the bond. One such arrangement could involve hinging the cells at alternate ends to create a zigzag conformation. This decreases the degrees of freedom that each cell can independently move in and generates levers between the cells. This could significantly decrease the amount of adhesive that needs to be applied to impart structural rigidity.

One possible alternative adhesive is pressure sensitive adhesives. Pressure sensitive adhesives (PSAs) are a versatile class of viscoelastic materials which form bonds using initial pressure and flow, unlike conventional adhesives, which bond once they have hardened through a chemical or physical process. PSAs do not require additional agents such as heat, water, or solvents to activate. The three main characteristics to be considered are ultimate adhesion, shear resistance and initial tack. Ultimate adhesion is the measure of the strength of the fully formed bond once the adhesive has set, shear resistance correlates to the adhesive resisting forces parallel to its surface and initial tack corresponds to the property that controls the instantaneous formation between the adhesive and adherend. Contact adhesives are easier to apply than thermoset resins and their application over large surface areas make debonding slow and necessitates large volumes of solvents.

Comparing Various PSAs

In a test to compare the effectiveness of different types of commercially available PSAs including glue dots, double-sided tape, and Velcro, a peel test with tensile testing apparatus was used to simulate how a real module of cells using the zigzag conformation would be pulled apart. Results are shown below:

Double-sided tape is the strongest adhesive of the set closely followed by Velcro. Both provide viable solutions to holding a zig-zag-configuration cell together, however weight and spacing must be considered alongside the ease of removal of adhesive. In contrast to glue dots or double-sided tape, using Velcro increased the thickness of the sample design by nearly 35%, decreasing the power density. Additionally, a weight calculation was carried out to evaluate each adhesive for the test sample, where it was found that Velcro contributes a relatively high amount of additional weight, while the glue dots contribute almost no weight.

Comparing the ease of removal of the adhesives, the double-sided tape was found to be the most difficult to remove, due to the sticky residue left behind. However, even this residue can be removed relatively easily with an acetone wash or by hand. Both Velcro and the glue dots did not leave residual material behind, thus making them a preferential choice in this respect as they would not require the additional removal step and therefore provide the simplest disassembly procedure. This is particularly useful in the area of pack or module repair and may decrease the proportion of cells being scrapped during production.

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.