A non-toxic co-solvent makes batteries safer and more efficient - Chemical Engineering | Page 1

November 1, 2024 | By Mary Page Bailey

The solvent N-methylpyrrolidone (NMP) has been used to coat anodes and cathodes in lithium-ion batteries, but it has been shown to be a reproductive toxin, necessitating controlled handling and recycling within a closed system, which adds complexity and cost to battery production. Today, NMP can be replaced by water for graphite anode production. A new co-solvent for water-based electrode coatings launched by OQ Chemicals (Monheim am Rhein, Germany; chemicals.oq.com), Oxvolt S221, is readily biodegradable and non-toxic — and crucially, helps to improve battery performance by preventing crack formation on the electrode surface. “Oxvolt S221 has a boiling point similar to NMP. As a coalescing agent, it supports the formation of homogenous layers, particularly with high-thickness coating layers. It helps to improve the processability of coated layers and prevents crack formation at higher coating thicknesses. Cracks are not tolerable, and lead to wasted materials,” explains Claudia Fischer, director of global business development at OQ Chemicals. The ability to support thicker coating layers ultimately leads to increased energy density within battery cells, adds Fischer.

Oxvolt S221 lowers the viscosity of the slurry and enhances processing by improving mixing properties and the quality of the electrode after coating and initial drying stages. It is water-soluble and is added to water as a co-solvent at low percentages, completely evaporating during the drying process. In tests conducted at Karlsruhe Institute of Technology (KIT; www.kit.edu), utilizing a coating solution with 3% Oxvolt S221 co-solvent in a roll-to-roll manufacturing process for graphite anodes significantly reduced the formation of cracks on the anode surface (see picture), with a wet coating thickness around 300–340µm and a coating speed of 0.2 m/min.

Oxvolt S221 has also been tested by Germany battery firm CustomCells, demonstrating the potential to increase the overall efficiency of graphite anode production when compared to conventional water-based coatings systems, with no detrimental impact on the electrical properties and lifetime of the battery cells. “Oxvolt S221 is approved by several battery cell producers for graphite anode materials. We have started testing it in other water-based battery processes, such as lithium-iron-phosphate (LFP), and there is still more development needed,” says Fischer. Oxvolt S221 is globally available and currently being produced at the commercial scale by OQ Chemicals in the U.S.