05: The Role of Design in Lithium-Ion Battery Recycling

Hi everyone! In this post, I will explore how battery design can be used to improve battery recycling.

Battery Design For Recycling

As the name implies, this takes into account the recycling process when the batteries are designed, which can increase efficiency during recycling compared to conventional methods that involve shredding old batteries in their entirety. I’ve already touched on some issues of current battery structure design in recycling in blog 04.

There are 3 main pillars to battery design: pack and module design, cell design, and material design. 

Pack and Module Design

One example of module (consists of multiple cells) design is standardizing screw connections and junctions between modules or cells to simplify the automated disassembly of cells. Efforts has also been made in providing regulations in battery classification and labeling, which may facilitate automatic sorting processes.

Cell Design

One example of cell design is substituting polymeric organic binders with water-soluble binders, which can potentially be effortlessly eliminated during the recycling process by washing with water. However, water-soluble binders may not provide the same long-lasting adhesion of the batteries’ active substances to metal foils as the current polymeric organic binders.

Material Design

One example of material design is the design of nanostructured hybrid materials based on carbon and metal oxidesm which can improve the kinetics of charge transfer and alleviate structural strain during charge and discharge processes. Additionally, the improvement of the metal oxide-based electrode materials can be obtained by the use of materials with better cycling performance, such as nanosheets, nanorods, and nanospheres.

Ease of Disassembly

Now, let’s take a closer look at some specific design changes that can be used to facilitate recycling processes.

The main issue that I will be looking at is how to open the pack, module and cell easily. Clearly the outer pack design needs to be as robust as possible, so it does not fail in service, but this does not preclude mechanisms which are easier to open. Metallic tools need to be avoided to decrease the possibility of shorting the cell and igniting the contents.

Additionally, pack and module designs vary significantly, even within a manufacturer’s own fleets. It is common to assemble groups of cells into modules but the number of cells and configurations vary. Typically, regardless of the arrangement, cells in a module are permanently affixed to one another and are not intended to be broken down. Thus, any attempt to disassemble these joints are likely to be a destructive process.

Connections between modules are typically more serviceable and may employ technologies such as threaded or torqued connections or bespoke, mechanically constrained push-fit connections. The repeatable functionality of these connections makes the modules both more easily replaceable and simpler to disassemble at the end of life.

At the cell level, the polymer separator between the electrodes can be used as a method to separate the active components of the cell. One example used a vacuum conveyor equipped with pinch grips and a series of skimmers to separate the anode from the cathode and the separator. A Z-fold uses a single sheet of separator wound alternately between the anode and cathode in a cell. As the Z-fold is flattened out the anode and cathode will automatically be partitioned onto opposite sides of the separator. However, these separators can be very fragile and often rip during disassembly.

Recently, Chinese battery manufacturer BYD has developed long, thin cells called blades, which fit into a grid and can impart structural strength to the battery pack. This could remove the need for separate modules and glues, enabling simple disassembly and exchange for cells when faults develop. The optimized pack structure could also enable an increase of 50% in space utilization and lead to safer operation of cells.

Binders

The separation of active from inactive materials such as binder in cathode materials is necessary for economical and sustainable recycling. Organic-based binders, including PVDF, are widely used in cathode sheet production because of good electrochemical stability and adhesive property. Removing these binders can be done via heat-treatment, or toxic organic solvents, such as N-methyl-2-pyrrolidone (NMP). New organic binders with good solubility in mild organic solvents and binder distribution and adhesion forces that are well-characterized will have a direct relevance to recycling. Alternatively, a more straightforward mechanical-method can be used to separate PVDF from active components relying on decreased hardness and modulus of elasticity.

Water soluble binders, including carboxy methylcellulose (CMC), styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), or polyacrylic acid (PAA) are preferred as these are more environmental benign. A recent report discovered that aqueous-processed NCM 811 cathodes at pouch cells exhibited excellent capacity retention through 1000 cycles at C/3 of B70% compared with B76% for NMP processed cells. It also discovered that water-soluble binders boost recovery of NCM523 cathode compounds from spent electrodes using water, and that the regenerated NCM523 exhibited comparable electrochemical performance with that of the original state.

Electrolyte

Replacing flammable carbonates-based electrolytes with recyclable aqueous and solid-state electrolyte could be used to increase recycling safety. Aqueous-based electrolytes provide safe, low cost and scalable energy storage, together with reduced costs in battery recycling. However, compared with more traditional organic electrolytes, large-scale application of aqueous electrolyte is restricted by limited energy density and poor lifespan because of the narrow electrochemical window of water. High concentrate Li salt, especially containing LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), is an important component in these electrolytes. During recycling, these are harmful to the environment because of the high fluorine species. The development of low-concentration, or even fluorine-free aqueous electrolyte with acceptable electrochemical performance would therefore boost sustainability in recycling. Additionally, the use of solid-state electrolytes (SSEs) including, oxides and sulfides, boosts battery safety and separation efficiency, especially sulphide-based SSEs that can be dissolved in low-cost solvents such as alcohols.

Separator

Polyolefin materials are widely used as separator material in LIBs because of excellent mechanical properties, compatibility, chemical stability and low-cost. With traditional recycling of pyrometallurgy and hydrometallurgy, or judicious combination at small and large scale, separators combust to form carbon species. However, it is practical for the separator in spent batteries to be removed and used via direct recycling or re-used following facile treatment.

Currently, we don’t know if there is a difference in electrochemical performance between cells using a fresh or recycled separator. There might be a practical design to modify the separator to boost recycling, based for example on fire-retardant, porous MOF-based,or anti-dendrite, separators.

Sources

Thompson, D. L.; Hartley, J.; Lambert, S.; Shiref, M.; Harper, G.; Kendrick, E.; Anderson, P. A.; Ryder, K. S.; Gaines, L.; Abbott, A. P. The importance of design in lithium ion battery recycling – a critical review. Green Chemistry 2020, 22 (22), 7585–7603. https://doi.org/10.1039/d0gc02745f.

Mao, J.; Ye, C.; Zhang, S.; Xie, F.; Zhou, R.; Davey, K.; Guo, Z.; Qiao, S. Toward practical lithium-ion battery recycling: adding value, tackling circularity and recycling-oriented design. Energy and Environmental Science 2022, 15 (7), 2732–2752. https://doi.org/10.1039/d2ee00162d.